Question

C6
224
Chemical
synthesis
C6: Chemical Synthesis
Why study chemical
synthesis?
We use chemicals to preserve food, treat disease, and decorate our homes. Many of these
chemicals do not occur naturally: they are synthetic. Developing new products, such as
drugs to treat disease, depends on chemists who synthesise and test new chemicals.
What you already know
• Atoms are rearranged during chemical reactions.
• The number of atoms of each element stays the same
in a chemical reaction.
• Raw materials can be used to make synthetic
materials.
• Alkalis neutralise acids to make salts.
• Chemical reactions can be represented by word
equations and balanced symbol equations.
• Some substances are made up of electrically charged
particles called ions.
• Data is more reliable if it can be repeated.
Find out about
• the importance of the chemical industry
• a theory to explain acids and alkalis
• reactions that give out and take in energy
• techniques for controlling the rate of chemical change
• the steps involved in the synthesis of a new chemical
• ways to measure the efficiency of chemical synthesis.
The Science
Chemists who synthesise
new chemicals need practical
skills and an understanding
of science explanations.
They must control reactions
so that they are neither too
slow nor too fast. They must
calculate how much of the
reactants to use to make the
amount of product required.
Chemists also take into
account any energy changes.
Acids are important reactants
in synthesis. Ionic theory
explains the characteristic
behaviours of these chemicals.
Ideas about
Science
Chemists make sure they use
the right grade of chemical for
a reaction. Technical chemists
test chemicals from suppliers
to check the purity. They take
measurements and make
sure the data they collect is
as accurate and reliable as
possible. They can then make
the best estimate of the true
value of the purity.
225
A
The chemical industry
Find out about
D the chemical industry
D bulk and fine chemicals
D the importance of
chemical synthesis
Thechemical industryconvertsrawmaterials,suchascrudeoil,
naturalgas,minerals,airandwater,intousefulproducts.Theproducts
includechemicalsforuseasfoodadditives,fertilisers,pigments,dyes,
paints,andpharmaceuticaldrugs.
Theindustrymakesbulk chemicalsonascaleof thousandsoreven
millionsof tonnesperyear.Examplesareammonia,sulfuricacid,
sodiumhydroxide,chlorine,andethene.
Onamuchsmallerscale,theindustrymakesfine chemicalssuchas
drugs,herbicides,andpesticides.Italsomakessmallquantitiesof specialitychemicalsneededbyothermanufacturersforparticular
purposes.Theseincludesuchthingsasflameretardants,foodadditives,
andtheliquidcrystalsforflat-screentelevisionsandcomputerdisplays.
the chemical industry converts raw
materials into pure chemicals,
which are then used in synthesis to
make a wide range of products.
the range of products made by the chemical industry in the UK by value of sales.
industrial chemists work in the
plant and the laboratory.
Key words
D chemical industry
D bulk chemicals
D fine chemicals
D plant
D pilot plant
D scale up
226
Thepartof achemicalworksthatproducesachemicaliscalledaplant.
Someof thechemicalreactionsoccuratahightemperature,soasource
of energyisneeded.Also,alotof electricalpowerisneededforpumps
to movereactantsandproductsfromonepartof theplanttoanother.
Sometimestheenergytoproducethispowercanbesuppliedby
chemicalreactionsthatgiveoutenergy.
Sensorsmonitortheconditions,suchastemperatureandpressure,inall
areasof theplant.Thedataisfedtocomputersinthecontrolcentre,
wherethetechnicalteamcontrolstheplant.
C6: ChemiCal SyntheSiS
People in the chemical industry
Peoplewithmanydifferentskillsareneededinthechemicalindustry.
Researchchemistsworkinlaboratoriestofindnewprocessesand
developnewproducts.
Theindustryneedsnewprocessessothatitcanbemorecompetitiveand
moresustainable.Theaimistousesmalleramountsof rawmaterials
andenergywhilecreatinglesswaste.
Peopledevisingnewproductshavetoworkcloselywithpeopleinthe
marketingandsalesdepartment.Theyareabletosayif thenovel
productiswanted.If thenewproductispromising,itmayfirstbetried
outbymakingsmallamountsof itinapilot plant.
Aspartof themarketresearch,possiblenewproductsaregivento
customersfortrial.Atthesametime,financialexpertsestimatethe
valueof thenewproductinthemarket.Theythencomparethiswith
the costof makingtheproducttocheckthatthenewprocesswillbe
profitable.
Chemicalengineershavetoscale uptheprocessanddesignafull-scale
plant.Thiscancosthundredsof millionsof pounds.
Somechemicalsfromtheindustrygodirectlyonsaletothepublic,but
mostof themareusedtomakeotherproducts.Transportworkerscarry
thechemicalstotheindustry’scustomers.
Questions
1 Classify the following as
raw materials or products
of the chemical industry:
air, ammonia, aspirin,
water, crude oil, polythene.
2 Give the name and chemical
formula of a bulk chemical.
3 list these chemicals under
two headings: ‘bulk chemical’
and ‘fine chemical’.
• the drug aspirin
• the hydrocarbon ethene
• the perfume chemical
citral
• the acid sulfuric acid
• the herbicide glyphosate
• the alkali sodium
hydroxide
• the food dye carotene
Everychemicalplantneedsmanagersandadministratorstocontrol
the wholeoperation.Therearealsopeopleinservicedepartmentswho
lookaftertheneedsof thepeopleworkingintheplant.Theseinclude
medicalandcateringstaff,andtrainingandsafetyofficers.
Plant operators monitor the processes from a control room.
maintenance workers help to keep the plant running.
A: the ChemiCal inDUStry
227
B
Acids and alkalis
Find out about
D acids and alkalis
D the pH scale
D reactions of acids
Acids
Thewordacidsoundsdangerous.Nitric,sulfuric,andhydrochloric
acidsareverydangerouswhentheyareconcentrated.Youmusthandle
themwithgreatcare.Theseacidsarelessof ahazardwhendilutedwith
water.Dilutehydrochloricacid,forexample,doesnothurttheskinif youwashitawayquickly,butitstingsinacutandrotsclothing.Itisin
factpresentinourstomachswhereithelpstobreakdownfoodandkill
bacteria.Ourstomachsarelinedwithaprotectivelayerof mucus.
Notallacidsaredangeroustolife.Manyacidsarepartof lifeitself.
Biochemistshavediscoveredthecitricacidcycle.Thisisaseriesof reactionsinallcells.Thecycleharnessestheenergyfromrespirationfor
movementandgrowthinlivingthings.
Organic acids
Key words
D acid
D alkali
Organicacidsaremolecular.Theyaremadeof groupsof atoms.Their
moleculesconsistof carbon,hydrogen,andoxygenatoms.Theacidityof theseacidsarisesfromthehydrogeninthe—COOHgroupof atoms.
Citricandtartaricacidsareexamplesof solidorganicacids.Ethanoic
acidisaliquidorganicacid.
acetic acid (chemical name: ethanoic acid) is a
liquid. it is the acid in vinegar. most white
vinegar is just a dilute solution of acetic acid.
Brown vinegars have other chemicals in the
solution that give the vinegar its colour and
flavour. most microorganisms cannot survive in
acid, so vinegar is used as a preservative in
pickles (e260).
228
Citric acid (a solid acid) is found in citrus fruits like oranges
and lemons. the human body processes about 2 kg of citric
acid a day during respiration. Citric acid and its salts are
added to food to prevent them reacting with oxygen in the
air, and to give a tart taste to drinks and sweets.
C6: ChemiCal SyntheSiS
Mineral acids
Sulfuric,hydrochloric,andnitricacidscomefrominorganicormineral
sources.Thepureacidsareallmolecular.Sulfuricacidandnitricacid
areliquidsatroomtemperature.Hydrogenchlorideisagas,and
becomeshydrochloricacidwhenitdissolvesinwater.
Alkalis
Pharmacistssellantacidsintabletstocontrolheartburnand
indigestion.Thechemicalsinthesemedicinesarethechemicalopposites
of acids.Theyaredesignedtoneutraliseexcessacidproducedinthe
stomach–hencethename‘antacids’.
Antacidsinmedicinesareusuallyinsolubleinwater.Otherchemical
antacidsaresolubleinwaterandgiveasolutionwithapHabove7.
Chemistscallthemalkalis.Commonalkalisaresodiumhydroxide,
NaOH;potassiumhydroxide,KOH;andcalciumhydroxide,Ca(OH)2.
Thetraditionalnameforsodiumhydroxideiscausticsoda.Theword
causticmeansthatthechemicalattackslivingtissue,includingskin.
Alkaliscandomoredamagetodelicatetissuesthandiluteacids.Caustic
alkalisareusedinthestrongestovenanddraincleaners.Theyhaveto
beusedwithgreatcare.
Sulfuric acid, h2SO4, is
manufactured from sulfur, oxygen,
and water. the pure, concentrated
acid is an oily liquid. the chemical
industry in the UK makes about
2 million tonnes of the acid each
year. the acid is essential for the
manufacture of other chemicals,
including detergents, pigments,
dyes, plastics, and fertilisers.
Questions
1 From the pictures of
molecules work out the
formulae of:
a acetic acid
b citric acid.
2 What are the formulae of
these antacids?
a magnesium hydroxide
made up of magnesium
ions, mg2+, and
hydroxide ions, Oh–.
b aluminium hydroxide
made up of aluminium
ions, al3+, and
hydroxide ions, Oh–.
hydrogen chloride forms when
concentrated sulfuric acid is added
to salt (sodium chloride) crystals.
hydrogen chloride, hCl, is a gas that
fumes in moist air and is very soluble
in water.
Oven cleaners often contain
caustic alkalis.
B: aCiDS anD alK aliS
229
Indicators and the pH scale
Indicatorschangecolourtoshowwhetherasolutionisacidicor
alkaline.Bluelitmusturnsredinacidsolutionandredlitmusturnsblue
inalkalis.Specialmixedindicators,suchasuniversalindicator,showa
rangeof coloursandcanbeusedtoestimatepHvalues.
pHvaluescanalsobemeasuredelectronicallyusingapHmeterwithan
electrodethatdipsintothesolution.Themetercanbereaddirectlyfrom
thedisplayoritmaybeconnectedtoadataloggerorcomputer.
ThetermpHappearsonmanycosmetic,shampoo,andfoodlabels.
It is a measureof acidity.ThepH scaleisanumberscalethatshows
the acidityoralkalinityof asolutioninwater.Mostlaboratorysolutions
haveapHintherange1–14.
the ph scale.
a ph meter can be used to
measure ph values.
hydrangea flowers contain natural indicators – they are blue if grown on acid soil
and pink on alkaline soil. note that this is the opposite of the litmus colours.
Reaction of acids
Acids with metals
Using a feather to brush away
hydrogen bubbles while etching
a metal plate with acid.
Acidsreactwithmetalstoproducesalts.Theotherproductis
hydrogen gas.
acid
1 metal
Forexample: 2HCl(aq) 1 Mg(s)
salt
1 hydrogen
MgCl2(aq) 1
H2(g)
Notallmetalswillreactinthisway.Youmayrememberthelistof metalsinorderof reactivityinC5,G.Metalsbelowleadinthelistdonot
reactwithacids,andevenwithleaditishardtodetectanychangeina
shorttime.
230
C6: ChemiCal SyntheSiS
Acids with metal oxides or hydroxides
Anacidreactswithametal oxideorhydroxidetoformasaltand
water.Nogasforms.
acid
1 metaloxide
(orhydroxide)
Forexample: 2HCl(aq) 1
salt
1 water
Thereactionbetweenanacidandametaloxideisoftenavitalstepin
makingusefulchemicalsfromores.
Acids with carbonates
Acidsreactwithcarbonatestoformasalt,water,andbubblesof carbon
dioxidegas.
salt1water1carbondioxide
Geologistscantestforcarbonatesbydrippinghydrochloricacidonto
rocks.If theyseeanyfizzing,therockscontainacarbonate.Thisis
likelytobecalciumcarbonateormagnesiumcarbonate.
Thewordequationis:
hydrochloric
calcium
1
acid
carbonate
calcium
carbon
1water 1
chloride
dioxide
Thebalancedequationis:
2HCl(aq)
1 CaCO3(s)
D pH scale
D metal oxide
D indicators D metal
hydroxide
D metals
D carbonates
D salts
MgCl2(aq) 1H2O(l)
MgO(s)
acid1metalcarbonate
Key words
CaCl2(aq) 1 H2O(l) 1
CO2(g)
Thisisafoolproof testforthecarbonateion.Sotheterm‘theacidtest’
hascometobeusedtodescribeanywayof providingdefiniteproof.
Questions
3 a pattern can be etched
onto a zinc plate using
hydrochloric acid to react
with the zinc, forming
soluble zinc chloride, ZnCl2.
Write a word equation
and a balanced symbol
equation for the reaction.
4 magnesium hydroxide,
mg(Oh)2, is an antacid
used to neutralise excess
stomach acid, hCl. Write a
word equation and a
balanced symbol equation
for the reaction.
5 there is a volcano in
tanzania, africa, whose lava
contains sodium carbonate,
na2CO3. the cooled lava
fizzes with hydrochloric
acid. Write a word equation
and a balanced symbol
equation for the reaction.
6 limescale forms in kettles
where hard water is heated.
limescale consists of
calcium carbonate. three
acids are often used to
remove limescale:
citric acid, acetic acid (in
vinegar), and dilute
hydrochloric acid. Which acid
would you use to de-scale an
electric kettle and why?
testing for carbonate using hydrochloric acid.
B: aCiDS anD alK aliS
231
C Salts from acids
Find out about
DD an ionic explanation for
neutralisation reactions
DD salts and their formulae
What makes an acid an acid?
Chemists have a theory to explain why all the different compounds that
are acids behave in a similar way when they react with indicators,
metals, carbonates, metal oxides, and metal hydroxides.
It turns out that acids do not simply mix with water when they dissolve.
They react, and when they react with water they produce hydrogen ions
(H1). For example, hydrochloric acid is a solution of hydrogen chloride
in water. The HCl molecules react with the water to produce hydrogen
ions and chloride ions.
HCl(g)
water
H+(aq) 1 Cl–(aq)
The theory of acids is an ionic theory. Any compound is an acid if it
produces hydrogen ions when it dissolves in water.
All acids contain hydrogen in their formula. Nitric acid, HNO3, and
phosphoric acid, H3PO4, both contain hydrogen. But not all chemicals
that contain hydrogen are acids. Ethane, C2H6, and ethanol, C2H5OH,
are not acids.
In an organic acid it is only the hydrogen atom in the —COOH group
that can ionise when the acid dissolves in water.
What makes a solution alkaline?
Alkalis such as the soluble metal hydroxides are ionic compounds.
They consist of metal ions and hydroxide ions (OH–). When they
dissolve, they add hydroxide ions to water. It is these ions that make
the solution alkaline.
NaOH(s)
water
Na+(aq) 1 OH–(aq)
Neutralisation
Hydrogen chloride dissolves in
water to make hydrochloric acid.
The HCl molecules react with water
to form ions.
Key words
DD hydrogen ions
DD hydroxide ions
DD neutralisation reaction
232
Sodium hydroxide and hydrochloric acid react to produce a salt (sodium
chloride) and water.
Na+(aq) 1 OH–(aq) 1 H+(aq) 1 Cl–(aq)
Na+(aq) 1 Cl–(aq) 1 H2O(l)
During a neutralisation reaction the hydrogen ions from an acid react
with hydroxide ions from the alkali to make water.
H+(aq) + OH–(aq)
H2O(l)
The remaining ions in the solution make a salt.
C6: Chemical Synthesis
Salts
Salts form when a metal oxide, or hydroxide, neutralises an acid.
So every salt can be thought of as having two parents. Salts are related
to a parent metal oxide or hydroxide and to a parent acid.
Salts are ionic (see C4, J: Ionic theory). Most salts consist of a positive
metal ion combined with a negative non-metal ion. The metal ion comes
from the parent metal oxide or hydroxide. The non-metal ion comes from
the parent acid.
Questions
1 Write equations to show
what happens when these
compounds dissolve in
water:
a nitric acid
b
sulfuric acid
c calcium hydroxide.
Dilute sodium hydroxide solution neutralises dilute hydrochloric acid, forming a
neutral solution of sodium chloride. Water molecules not involved in the reaction
are shown in a paler colour.
It is possible to work out the formulae of salts knowing the charges
on the ions. Remember that all compounds are overall electrically
neutral (see C4, K: Ionic theory and atomic structure). Some nonmetal ions consist of more than one atom. The table below includes
some examples. In the formula for magnesium nitrate, Mg(NO3)2, the
brackets around the NO3 show that two complete nitrate ions appear
in the formula.
Non-metal ions that consist of more than one atom
Symbols
carbonate
CO32–
hydroxide
OH–
nitrate
NO3–
sulfate
SO42–
2 Write down the name of
the salt produced in the
following reactions:
alithium hydroxide with
hydrochloric acid
bcalcium carbonate with
nitric acid
cmagnesium oxide with
sulfuric acid.
3 Use the tables of ions in C4
Section K and on this page
to write down the formulae
of these salts:
a potassium nitrate
b magnesium carbonate
c sodium sulfate
d calcium nitrate.
4 Use the tables of ions in C4
Section K and on this page
to write down the charge
on the metal ion in each of
these salts:
aCuCO3
bPbBr2
cFe2O3.
C: Salts from acids
233
D
Purity of chemicals
Find out about
D purity
D titrations for testing
purity
Grades of purity
Thereactionsof acidswithmetals,oxides,hydroxides,andcarbonates
canbeusedtomakevaluablesalts.Forusessuchasfoodormedicines,
thesesaltshavetobemadepuresothattheyaresafetoswallow.
Chemicalsdonotalwayshavetobepure.Calciumcarbonate,for
example,isusedinablastfurnacetoextractironfromitsores.Theiron
industrycanuselimestonestraightfromaquarry.Limestonehassome
impuritiesbuttheydonotstopitfromdoingitsjobinablastfurnace.
Suppliersof chemicalsofferarangeof gradesof chemicals.Inaschool
laboratoryyoumightuseoneof thesegrades:technical,general
laboratory,andanalytical.Thepurestgradeistheanalyticalgrade.
label on a bottle of laboratory grade
calcium carbonate. the term ‘assay’
tells you how pure the chemical is. the
calcium carbonate is 99% pure with the
small amounts of impurities shown.
Key words
D titration
D burette
D end point
Questions
1 Uses of sodium chloride
(salt) include:
i flavouring food
ii melting ice on roads
iii saline drips in hospitals.
Put these in order of
the grade of sodium
chloride required, with
the purest first.
2 From 'steps involved in a
titration' in which step is:
a a solution made?
b a pipette used, and
what is it used for?
c the end point reached,
and how does the
technician know?
234
Purifyingachemicalisdoneinstages.Eachstagetakestimeandmoney,
andbecomesmoredifficult.Sothehigherthepurity,themoreexpensive
thechemical.Manufacturersthereforebuythegrademostsuitablefor
theirpurpose.
Whendecidingwhatgradeof chemicaltouseforaparticularpurpose,it
isimportanttoknow:
• theamountof impurities
• whattheimpuritiesare
• howtheycanaffecttheprocess
• whethertheywillendupintheproduct,andwhetheritmattersif theydo.
Testing purity
Medicinescontainanactiveingredient.Otheringredientsareincluded
tomakethemedicinepleasanttotasteandeasytotake.Thismeansthat
thepharmaceuticalcompaniesthatmakemedicinesneedsweeteners,
foodflavours,andotheradditives.
Thecompaniesbuyinmanyof theiringredients.Technicalchemists
workingforthecompanieshavetomakesurethatthesuppliersare
deliveringtherightgradeof chemical.
Citricacidisoftenaddedtosyrups,suchascoughmedicines,tocontrol
theirpH.Technicianscancheckthepurityof theacidusingaprocedure
calleda titration, whichmeasuresthevolumeof alkalithatitcan
neutralise.Thetechnicianhastoknowtheaccurateconcentrationof the alkali.
C6: ChemiCal SyntheSiS
Steps involved in a titration
1
the technician fills a burette with a
solution of sodium hydroxide. She
knows the concentration of the alkali.
Questions
2
the technician weighs out a sample
of citric acid accurately.
3 a 1.35 g sample of impure
citric acid was dissolved in
water and titrated with
sodium hydroxide solution
of concentration 40 g/dm3.
the average titre was
found to be 20.6 cm3.
a Use the following
equation to find the
mass of citric acid in
the sample:
mass of citric acid (g) 5
average titre (cm3) 3
0.064.
b Use the following
equation to calculate
the percentage purity of
the sample:
percentage purity 5
mass of citric acid
_____________
3 100 %
mass of sample
3 the technician dissolves the acid in
pure water. then she adds a few
drops of phenolphthalein indicator.
the indicator is colourless in the acid
solution.
4
the technician adds alkali from the
burette. She swirls the contents of the
flask as the alkali runs in. near the end she
adds the alkali drop by drop. at the end
point all the citric acid is just neutralised.
the indicator is now permanently pink.
4 a technical chemist
measures the purity of
tartaric acid by titration,
and obtains these results:
98.7%, 99.0%. 105.4%,
80.0%, 98.8%, 98.5%
a Suggest reasons why
the results are not
exactly the same.
b Which two values
should be checked?
c Calculate the mean
value of purity after
discarding the two
outlying values.
d Suggest the limits
between which the true
value is likely to lie.
Thetechnicianwillrepeatthetitrationseveraltimes.If thereareany
resultsthatdiffergreatlyfromtherest,andthereisreasontodoubt
theiraccuracy,theywillbediscarded.Theremainingvalueswillbe
averagedtofindthemean.
D: PUrit y OF ChemiCalS
235
E
Energy changes in chemical reactions
Find out about
D reactions that give out
energy and reactions
that take in (absorb)
energy
Exothermic and endothermic reactions
Mostchemicalreactionsneedasupplyof energytogetthemstarted.
Somealsoneedenergytokeepthemgoing.ThatiswhyBunsenburners
andelectricheatingmantlesaresocommoninlaboratories.
Mostreactionsgiveoutenergyoncetheyaregoing–combustion
(burning),andneutralisationof acidstomakesaltsarecommon
examples.Sulfuricacidisanimportantchemicalinindustry.Oneof thekeyreactionsinmakingthisacidhastobecarefullycontrolled
becauseitgivesoutagreatdealof energy.
Reactionsthatgiveoutenergytothesurroundings(thesurroundings
gethotter)arecalledexothermic.Therearealsoreactionsthatabsorb
energyfromthesurroundings(thesurroundingsgetcooler).Theseare
calledendothermic.
Itispossibletotellwhetherachemicalreactionisexothermicor
endothermicbymeasuringthetemperatureof thereactantsbeforethe
reactionstarts,andthetemperatureof theproductsafterithasfinished.
If thetemperaturehasrisen,thereactionisexothermic.If ithas
dropped,thereactionisendothermic.
Energy changes in the chemical industry
Scientistsworkinginthechemicalindustryneedtoknowwhethera
reactionisexothermicorendothermicwhenitispartof asynthesis.
Thereareseveralreasonsforthis:
• Ittakesfuel,whichcostsmoney,toprovidetheenergyinputfor
endothermicreactions.
• Theenergygivenoutbyexothermicreactionscanbeusedelsewhere
intheplant,toproduceelectricity,forexample.
• Atemperatureincreasemakeschemicalreactionsgofaster
(see SectionF)–areactionthatgivesoutheatenergywilltendto get
fasterandfaster,andmay‘runaway’,possiblycausingan explosion.
Both exothermic and endothermic
reactions have practical uses.
an exothermic reaction provides
the energy for welding. a cold
pack absorbs energy from an
injured muscle using an
endothermic reaction.
236
Sucha‘runaway’reactionmayhavebeenthecauseof amajordisaster
atachemicalplantinBhopal,India,in1984.Between2000and15000
peoplemayhavediedandover25 yearslater,thehealthof manypeople
isstillbeingaffectedbytheincident.Properunderstandingandcontrol
of theenergychangesinthereactionsconcernedmighthaveprevented
thedisaster.
C6: ChemiCal SyntheSiS
Energy-level diagrams
Allchemicalreactionsgiveoutortakeinenergy.Understandingenergy
changeshelpschemiststocontrolreactions.
• Inanexothermicreaction(whereenergyisgivenout),theproducts
musthavelessenergythanthereactants.
• Inanendothermicreaction(whereenergyisabsorbed),theproducts
musthavemoreenergythanthereactants.
Key words
D exothermic
D endothermic
D energy-level diagram
Chemistskeeptrackof thechangesinenergyinchemicalreactions
usingenergy-level diagrams.Theseshowtheenergiesof thereactants
andproducts.Noticethatenergyisplottedverticallyupthediagram.
Reactantsareontheleftandproductsontheright.Theseareusually
shownusingabalancedequation.
an electric heating mantle,
used to heat up reactions
without a naked flame.
an exothermic reaction.
Question
Theenergy-leveldiagramforthereactionof magnesiumand
hydrochloricacid,whichisexothermic.Energyisgivenoutinthis
reaction,sotheproductshavelessenergythanthereactants.
an endothermic reaction.
Energy-leveldiagramforthereactionof citricacidandsodium
hydrogencarbonate,whichisendothermic.Energyisabsorbedinthis
reaction,sotheproductshavemoreenergythanthereactants.
1 if you have ever had a
plaster cast for a broken
bone, apart from the pain of
the injury, you might
remember the pleasant
feeling of warmth as the wet
plaster begins to set. if you
put a sherbet sweet on your
tongue you will feel your
mouth getting cold. Both of
these are chemical reactions.
a Which reaction is
exothermic and which
is endothermic?
b Draw an energy-level
diagram for each reaction.
Use ‘reactants’ and
‘products’ for the names
of the chemicals involved.
E: enerGy ChanGeS in ChemiCal reaC tiOnS
237
F
Rates of reaction
Find out about
D measuring rates of
reaction
D factors affecting rates
of reaction
D catalysts in industry
D collision theory
Controlling reaction rates
Somechemicalreactionsseemtohappeninaninstant.Anexplosionis
anexampleof averyfastreaction.
Otherreactionstaketime–seconds,minutes,hours,orevenyears.
Rustingisaslowreactionandsoistherottingof food.
Itisthechemist’sjobtoworkoutthemostefficientwaytosynthesisea
chemical.Itisimportantthatthechosenreactionshappenata
convenientspeed.Areactionthatoccurstooquicklycanbehazardous.
Areactionthattakesseveraldaystocompleteisnotpracticalbecauseit
tiesupequipmentandpeople’stimefortoolong,whichcostsmoney.
Measuring rates of reaction
Yourpulserateisthenumberof timesyourheartbeatseveryminute.
Theproductionrateinafactoryisameasureof howmanyarticlesare
madeinaparticulartime.Similarideasapplytochemicalreactions.
Chemistsmeasurethe rate of a reactionbyfindingthequantityof productproducedorthequantityof reactantusedupinafixedtime.
Forthereaction
an explosion is an example of a
very fast chemical reaction.
Mg(s)12HCl(aq)
MgCl2(aq)1H2(g)
theratecanbemeasuredquiteeasilybycollectingandmeasuringthe
hydrogengasproduced.
changeinthevolumeof hydrogen
averagerate5_________________________________
timeforthechangetohappen
Inmostchemicalreactions,theratechangeswithtime.Thegraphon
theleftisaplotof thevolumeof hydrogenformedagainsttimeforthe
reactionof magnesiumwithacid.Thegraphissteepestatthestart,
showingthattherateof reactionwasgreatestatthatpoint.Asthe
reactioncontinuestheratedecreasesuntilthereactionfinallystops.
Thesteepnessof thelineisameasureof therateof reaction.
a plot of the volume of hydrogen
formed against time for a reaction of
magnesium with hydrochloric acid.
Key word
D rate of reaction
238
Question
1 For each of these reactions, pick a method from the opposite page
that could be used to measure the rate of reaction: (hint: look at
the states of the reactants and products.)
CaCl2(aq) + CO2(g) + h2O(l)
a CaCO3(s) + 2hCl(aq)
ZnSO4(aq) + h2(g)
b Zn(s) + h2SO4(aq)
2naCl(aq) + SO2(g) + S(s) + h2O(l)
c na2S2O3(aq) + 2hCl(aq)
C6: Chemical Synthesis
Methods of measuring rates of reaction
Collecting and measuring a gas product
Record the volume at regular intervals, such as every 30 or
60 seconds. A gas syringe could be used to collect the gas,
instead of a measuring cylinder – see the next page.
Timing how long it takes for a small
amount of solid reactant to disappear
Mix the solid and solution in the flask and start
the timer. Stop it when you can no longer see
any solid.
Measuring the loss of mass as a gas forms
Record the mass at regular intervals such as every
30 or 60 seconds.
Timing how long it takes for a solution to turn cloudy
This is for reactions that produce an insoluble
solid. Mix the solutions in the flask and start the
timer. Stop it when you can no longer see the
cross on the paper through the solution.
Manual timing of reactions is only suitable for reactions that are over in a
few minutes (or more). Modern methods using lasers and dataloggers allow
chemists to measure the rates of reactions that are over in less than one
thousand billion billionths of a second (one femtosecond).
F: Rates of reac tion
239
Key words
DD concentration
DD surface area
DD catalyst
Factors affecting reaction rates
Powdered Alka-Seltzer reacts faster in water than a tablet. Milk
standing in a warm kitchen goes sour more quickly than milk kept in
a refrigerator. Changing the conditions alters the rate of these processes
and many others.
Factors that affect the rate of chemical reactions are:
• the concentration of reactants in solution – the higher the
concentration, the faster the reaction
• the surface area of solids – powdering a solid increases the surface
area in contact with a liquid, solution, or gas, and so speeds up the
reaction.
• the temperature – typically a 10 °C rise in temperature can roughly
double the rate of many reactions
• catalysts – these are chemicals that speed up a chemical reaction
without being used up in the process.
The factors in action
The apparatus in the diagram was used in an investigation into the effect
of changing the conditions on the reaction of zinc metal with sulfuric
acid. The graph below shows the results.
Apparatus used to investigate
the factors affecting the rate of
reaction of zinc with sulfuric acid.
The volume of hydrogen formed over time during an investigation of the factors
affecting the rate of reaction of zinc with sulfuric acid. The investigator used the same
mass of zinc each time. There was more than enough metal to react with all the acid.
Zn(s) 1 H2SO4(aq)
ZnSO4(aq) 1 H2(g)
The red line on the graph plots the volume of hydrogen gas against
time using zinc granules and 50 cm3 of dilute sulfuric acid at 20 °C.
The reaction gradually slows down and stops because the acid
concentration falls to zero. There is more than enough metal to react
with all the acid. The zinc is in excess.
240
C6: Chemical Synthesis
The effect of concentration
Line A on the graph shows the result of using acid that was half as
concentrated while leaving all the other conditions the same as in the
original set up.
The investigator added 50 cm3 of this more dilute acid. Halving the acid
concentration lowers the rate at the start. The final volume of gas is
cut by half because there was only half as much acid in the 50 cm3 of
solution to start with.
The effect of surface area
Line B on the graph shows the result of using the same excess of zinc
metal in fewer larger pieces. All other conditions were the same as in the
original setup. Fewer larger lumps of metal have a smaller total surface
area so the reaction starts more slowly. The amount of acid is unchanged
and the metal is still in excess so that the final volume of hydrogen is
the same.
Questions
2 When a lump of calcium
carbonate is placed into a
beaker of acid it reacts and
carbon dioxide is given off.
Suggest three ways to
speed up the reaction.
3 How is it possible to
control conditions to speed
up these changes:
athe setting of an epoxy
glue
b the cooking of an egg
cthe conversion of oxides
of nitrogen in car
exhausts to nitrogen.
4 When investigating the
effect of temperature on a
chemical reaction, why is it
important to keep all other
conditions the same?
Breaking up a solid into smaller pieces increases the total surface area. This
increases the amount of contact between the solid and the solution, making it
possible for the reaction to go faster.
5 The effect of concentration
on the rate of reaction
between zinc and sulfuric
acid was investigated. The
results were plotted on
a graph.
The effect of temperature
Line C on the graph shows the result of carrying out the reaction at
30 °C while leaving all the other conditions the same as in the original
set up. This more or less doubles the rate at the start. The quantities of
chemicals are the same so the final volume of gas collected is the same
as it was originally.
The effect of adding a catalyst
Line D on the graph shows what happens when the investigation is
repeated with everything the same as in the original set up but with a
catalyst added. The reaction starts more quickly. Catalysts do not
change the final amount of product, so the volume of gas at the end is
the same as before.
aIs there a correlation?
If so, describe it.
bWhich result is an
outlier? Suggest a
reason why this result is
different from the
expected value.
F: Rates of reac tion
241
Catalysts in industry
What is a catalyst?
Acatalystisachemicalthatspeedsupachemicalreaction.
Ittakespartinthereaction,butisnotusedup.
Moderncatalystscanbehighlyselective.Thisisimportant
whenreactantscanundergomorethanonechemical
reactiontogiveamixtureof products.Withasuitable
catalystitcanbepossibletospeedupthereactionthat
givestherequiredproduct,butnotspeedupotherpossible
reactionsthatcreateunwantedby-products.
Better catalysts
Catalystsareessentialinmanyindustrialprocesses.They
makemanyprocesseseconomicallyviable.Thismeans
thatchemicalproductscanbemadeatareasonablecost
andsoldataffordableprices.
Researchintonewcatalystsisanimportantareaof scientificwork.Thisisshownbytheindustrial
manufactureof ethanoicacid(seeSectionB)from
methanolandcarbonmonoxide.Thisprocesswasfirst
developedbythecompanyBASFin1960usingacobalt
compoundasthecatalystat300ºCandatapressure700
timesatmosphericpressure.
AboutsixyearslaterthecompanyMonsantodevelopeda
processusing thesamereaction,butanewcatalystsystem
basedonrhodiumcompounds.Thisranundermuch
milderconditions:200ºCand30–60timesatmospheric
pressure.
the manufacture of ethanoic acid from methanol
and carbon monoxide uses a catalyst to speed up
the reaction.
242
methanol 1 carbon monoxide
ethanoic acid
Ch3Oh(g) 1
Ch3COOh(g)
CO(g)
In1986,thepetrochemicalcompanyBPboughtthe
technologyformakingethanoicacidfromMonsanto.It
hassincedevisedanewcatalystbasedoncompoundsof iridium.Thisprocessisfasterandmoreefficient.Iridium
ischeaper,andlessof thecatalystisneeded.Iridiumis
evenmoreselectivesotheamountof ethanoicacid
produced(yield)isgreaterandtherearefewer
by-products.Thismakesiteasierandcheapertomake
pureethanoicacidandthereislesswaste.
C6: Chemical Synthesis
Collision theory
Chemists have a theory to explain how the various factors affect
reaction rates.
Key word
DD collision theory
The basic idea is that particles, such as molecules, atoms, and ions can
only react if they bump into each other. Imagining these particles
colliding with each other leads to a theory that can account for the
effects of concentration, temperature, and catalysts on reaction rates.
According to collision theory, when molecules collide some bonds
between atoms can break while new bonds form. This creates new
molecules.
Molecules are in constant motion in gases, liquids, and solutions. There
are millions upon millions of collisions every second. Most reactions
would be explosive if every collision led to a reaction. It turns out that
only a very small proportion of all the collisions are successful and
actually lead to a reaction. These are the collisions in which the molecules
are moving with enough energy to break bonds between atoms.
Any change that increases the number of successful collisions per
second has the effect of increasing the rate of reaction. Increasing the
concentration of solutions of dissolved chemicals increases the
frequency of collisions. The same small proportion of these collisions
will be successful, but now there are more of them. This means that
there will also be more successful collisions.
Questions
Molecules have a greater chance of colliding in a more concentrated solution.
More frequent collisions means – faster reactions. Reactions get faster if the
reactants are more concentrated.
Breaking up a solid into smaller pieces increases its surface area.
Increased surface area means that there are more atoms, molecules, or
ions of the solid available to react. This speeds up the reaction by
increasing the frequency of successful collisions with particles in liquid,
solution, or gas.
6aWhere do cobalt,
rhodium and iridium
appear in the periodic
table?
bWhy is not surprising
that these three metals
can be used to make
catalysts for the same
process?
7 Suggest reasons why it is
important to develop
industrial processes that:
arun at lower
temperatures and
pressures
b produce less waste.
F: Rates of reac tion
243
G
Stages in chemical synthesis
Find out about
D steps in synthesis
D making a soluble salt
Making calcium chloride for dialysis
Chemicalsynthesisisawayof makingnewcompounds.Synthesisputs
thingstogethertomakesomethingnew.Itistheoppositeof analysis,
whichtakesthingsaparttoseewhattheyaremadeof.
Thekidneysremovetoxicchemicalsfromtheblood.Incasesof kidney
failure,patientsareputondialysismachinesthatdothejoboutsidethe
bodywhiletheyawaitatransplant.Bloodpassesoutof thebodythrough
atubeintothedialysismachine.
Insidethemachine,thebloodflowspastaspecialmembrane.Onthe
othersideof themembraneisasolutioncontainingamixtureof saltsat
thesameconcentrationsasthesamesaltsintheblood.Thetoxic
chemicalspassfromthebloodthroughthemembraneintothesolution
andarecarriedaway.Itisalsopossibleforusefulsaltstopassbackinto
theblood.
Kidney dialysis.
Oneof thesaltsinthedialysissolutioniscalciumchloride.Itisa
particularlyimportantsaltasthelevelof calciuminthebloodhastobe
maintainedataparticularlevel.Justalittlebittoomuchortoolittleand
thepatientcouldbecomeveryillindeed.Thecalciumchloridetherefore
hastobeverypure,andthequantityaddedtothesolutionhastobe
measuredaccurately.
Theprocessformakingcalciumchlorideisshownintheflowchart.
the process for making very pure calcium chloride for dialysis.
244
C6: ChemiCal SyntheSiS
Making a sample of magnesium sulfate
Magnesiumsulfateisanothersaltproducedbythechemicalindustry.It
hasmanyusesincludingasanutrientof plants.
Theprocessof makingmagnesiumsulfate(oranyothersolublesalt)on
alaboratoryscaleillustratesthestagesinachemicalsynthesis.
Inthefollowingmethodanexcessof solidisaddedtomakesurethatall
theacidisusedup.Thismethodisonlysuitableif thesolidaddedtothe
acidiseitherinsolubleinwaterordoesnotreactwithwater.
Choosing the reaction
Anyof thecharacteristicreactionsof acidscanallbeusedto
make salts:
• acid1metal
salt1hydrogen
• acid1metaloxideorhydroxide
salt1water
• acid1metalcarbonate
salt1carbondioxide1water
Magnesiummetalisrelativelyexpensivebecauseithastobe
extractedfromoneof itscompounds.Soitmakessensetouseeither
magnesiumoxideorcarbonateasthestartingpointformaking
magnesiumsulfatefromsulfuricacid.
Carrying out a risk assessment
Itisalwaysimportanttominimiseexposurestorisk.Youshouldtake
caretoidentifyhazardouschemicals.Youshouldalsolookforhazards
arisingfromequipmentorprocedures.Thisisarisk assessment.
an operator emptying magnesium
sulfate into the tank of a sprayer on
a farm. he is wearing protective
clothing because magnesium
sulfate is harmful. as well as being
a micronutrient needed for healthy
plant growth, the salt is a:
• raw material in soaps and
detergents
• laxative in medicine
• refreshing additive in bath water
• raw material in the manufacture
of other magnesium compounds
• supplement in feed for poultry
and cattle
• coagulant in the manufacture of
some plastics.
Inthispreparationthemagnesiumcompoundsarenothazardous.
Thedilutesulfuricacidisanirritant,whichmeansthatyoushould
keepitoff yourskinandespeciallyprotectyoureyes.Youshould
alwaysweareyeprotectionwhenhandlingchemicals,forexample.
Questions
1 refer to the flow diagram for the process of
making calcium chloride.
a Write the word and balanced symbol
equations for the reaction used to make
the salt.
b identify steps taken to make the yield of the
pure salt as large as possible.
2 epsom salts consist of magnesium sulfate.
magnesium sulfate is soluble in water. Produce a
flow diagram to show how you could remove
impurities that are insoluble in water from a
sample of epsom salts. Processes you might use
include: crystallisation, dissolving, drying,
evaporation, filtration.
G: StaGeS in ChemiCal SyntheSiS
245
Questions
3 Write the balanced equation
for the reaction of magnesium
carbonate with sulfuric acid.
4 Why does the mixture of
magnesium carbonate and
sulfuric acid froth up?
5 What is the advantage of:
a using powdered
magnesium carbonate?
b warming when most
of the acid has been
used up?
c adding a slight excess of
the solid to the acid?
6 Why is it impossible to use
this method to make a pure
metal sulfate by the reaction
of dilute sulfuric acid with:
a lithium metal?
b sodium hydroxide?
c potassium carbonate?
7 look at the procedure on
this and the following page
for making magnesium
sulfate, and identify the step
when risks might arise from:
a chemicals that react
vigorously and spill over
b chemicals that might
spit or splash on heating
c hot apparatus that
might cause burns
d apparatus that might
crack and form sharp
edges.
Key word
D risk assessment
246
Working out the quantities to use
Reactingmassescanbeusedtoworkouttheamountof reactants
neededtoproduceaparticularamountof product(seeSectionH).
Inthisprocedurethesolidisaddedinexcess.Thismeansthatthe
amountof productisdeterminedbythevolumeandconcentration
of thesulfuricacid.Theconcentrationof dilutesulfuricacidis
98 g/litre,andavolumeof 50 cm3dilutesulfuricacidisused.This
contains4.9 gof theacid.
Carrying out the reaction in suitable apparatus under
the right conditions
Thereactionisfastenoughatroomtemperature,especiallyif the
magnesiumcarbonateissuppliedasafinepowder.
Thisreactioncanbesafelycarriedoutinabeaker.Stirringwitha
glassrodmakessurethatthemagnesiumoxideorcarbonateand
acidmixwell.Stirringalsohelpstopreventthemixturefrothingup
andoutof thebeaker.
1
measure the required volume of acid
into a beaker. add the metal oxide
or carbonate bit by bit until no more
dissolves in the acid. Warm gently
until all of the acid has been used up.
make sure that there is a slight excess
of solid before moving on to the
next stage.
Separating the product from the reaction mixture
Filteringisaquickandeasywayof separatingthesolutionof the
productfromtheexcesssolid.Themixturefiltersmorequicklyif themixtureiswarm.
2
Filter off the excess solid, collecting
the solution of the salt in an
evaporating basin. the residue on
the filter paper is the excess solid.
C6: ChemiCal SyntheSiS
Purifying the product
Afterthemixturehasbeenfiltered,thefiltratecontainsthepure
saltdissolvedinwater.Evaporatingmuchof thewaterspeedsup
crystallisation.Thisisconvenientlycarriedoutinanevaporating
basin.Theconcentratedsolutioncanthenbelefttocooland
crystallise.Thedryingcanbecompletedinawarmoven.The
crystalscanthenbetransferredtoadesiccator.Thisisaclosed
containerthatcontainsasolidthatabsorbswaterstrongly.
3
4
Pour the concentrated solution
into a labelled glass Petri dish
and set it aside to cool slowly.
heat gently to evaporate some of the
water. evaporate until crystals form
when a droplet of solution picked up
on a glass rod crystallises on cooling.
5
Crystals of pure magnesium sulfate
seen through a Polaroid filter (360).
Complete the drying in an
oven and then store in a
dessicator.
Measuring the yield and checking the purity of the product
Thefinalstepistotransferthedrycrystalstoaweighedsample
tubeandre-weighittofindtheactualyieldof crystals.Oftenitis
importanttocarryoutteststocheckthattheproductispure.
Theappearanceofthecrystalscangiveacluetothe
purityoftheproduct.Amicroscopecanhelpifthe
crystalsaresmall.Thecrystalsofapureproductare
oftenwell-formedandeveninshape.
6
the weighed sample of product
showing the name and formula of
the chemical, the mass of product,
and the date it was made.
Questions
8 identify the impurities
removed during the
purification stages of the
magnesium sulfate
synthesis.
9 Why is it important that the
magnesium carbonate is
added in excess to the
sulfuric acid?
G: StaGeS in ChemiCal SyntheSiS
247
H Chemical quantities
Find out about
DD reacting masses
DD yields from chemical
reactions
Key words
DD relative formula mass
DD reacting mass
DD actual yield
DD theoretical yield
DD percentage yield
Chemists wanting to make a certain quantity of product need to work
out how much of the starting materials to order. Getting the sums right
matters – especially in industry, where a higher yield for a lower price
can mean better profits.
The trick is to turn the symbols in the balanced chemical equation into
masses in grams or tonnes. This is possible given the relative masses of
the atoms in the periodic table.
Reacting masses
Adding up the relative atomic masses for all the atoms in the formula
of a compound gives the relative formula mass of chemicals . Given
the relative formula masses, it is then possible to work out the masses
of reactants and products in a balanced equation. These are the
reacting masses.
Question
1 What mass of:
aHCl in hydrochloric acid
reacts with 100 g
calcium carbonate?
b
HNO3 in dilute nitric
acid neutralises 56 g of
potassium hydroxide?
cc opper(II) oxide (CuO)
would you need to react
with sulfuric acid (H2SO4)
to make 319 g of copper
sulfate (CuSO4)?
Worked example
What are the masses of reactants and products when sulfuric acid reacts with sodium hydroxide?
248
Step 1
2NaOH 1 H2SO4
Step 2
relative formula mass of NaOH 5 23 1 16 1 1 5 40
relative formula mass of H2SO4 5 (2 3 1) 1 32 1 (4 3 16) 5 98
relative formula mass of Na2SO4 5 (2 3 23) 1 32 1 (4 3 16) 5 142
relative formula mass of H2O 5 (2 3 1) 1 16 5 18
Steps 3 & 4
2NaOH 1 H2SO4 Na2SO4 1 2H2O
Na2SO4 1 2H2O
2 3 40 5 80 98
142
142 g
80 g
98 g
2 3 18 5 36
36 g
C6: Chemical Synthesis
Yields
The yield of any synthesis is the quantity of product obtained from
known amounts of starting materials. The actual yield is the mass of
product after it is separated from the mixture, purified, and dried.
Theoretical yield
The theoretical yield is the mass of product expected if the reaction
goes exactly as shown in the balanced equation. This is what could be
obtained in theory if there are no by-products and no losses while
chemicals are transferred from one container to another. The actual
yield is always less than the theoretical yield.
Worked example
What is the theoretical yield of ethanoic acid made from 8 tonnes of
methanol?
Step 1 Write down the balanced equation
methanol
1 carbon monoxide
CH3OH(g) 1CO(g)
CH3COOH(g)
ethanoic acid
32 60
Step 2 Work out the relative formula masses
Questions
2 What is the mass of salt
that forms in solution
when:
a
hydrochloric acid
neutralises 4 g sodium
hydroxide?
b 12.5 g zinc carbonate,
ZnCO3, reacts with
excess sulfuric acid?
3 A preparation of sodium
sulfate began with 8.0 g of
sodium hydroxide.
aCalculate the theoretical
yield of sodium sulfate
from 8.0 g sodium
hydroxide.
bCalculate the percentage
yield, given that the
actual yield was 12.0 g.
methanol:12 1 4 1 16 5 32
ethanoic acid:
Steps 3 & 4
24 1 4 1 32 5 60
Write down the relative reacting masses
and convert to reacting masses by adding the units
Theoretically, 32 tonnes of methanol should give 60 tonnes of ethanoic
acid.
Step 5 Scale to the quantities actually used
If the theoretical yield of ethanoic acid 5 x tonnes, then
mass of ethanoic acid 60 tonnes x tonnes
mass of methanol 5 32 tonnes 5 8 tonnes
So, the yield of ethanoic acid from 8 tonnes of methanol should be
8 tonnes 3 60 tonnes
5 15 tonnes
32 tonnes
Percentage yield
Worked example
What is the percentage yield if
8 tonnes of methanol produces
14.7 tonnes of ethanoic acid?
From the previous example:
theoretical yield 5 15 tonnes
actual yield 5 14.7 tonnes
percentage
actual yield
yield 5 theoretical yield 3 100
14.7 tonnes
3 100
15 tonnes
5
5 98%
The percentage yield is the percentage of the theoretical yield that is
actually obtained. It is always less than 100%.
H: Chemical quantities
249
Science
Explanations
Chemists use their knowledge of chemical reactions to plan and carry out
the synthesis of new compounds.
ow:
You should kn
 thatthechemicalindustryprovidesusefulproductssuchasfood
additives,fertilisers,dyestuffs,paints,pigments,and
pharmaceuticals
 howchemistsuseindicatorsandpHmeterstodetectacidsand
alkalisandtomeasurepH
 thatcommonalkalisincludethehydroxidesof sodium,
potassium,andcalcium
 thattherearecharacteristicreactionsof acidswithmetals,
metaloxides,metalhydroxides,andmetalcarbonatesthat
producesalts
 howchemistsuseionictheorytoexplainwhyacidshavesimilar
properties,andhowalkalisneutraliseacidstoformsalts
 thatsafetyprecautionsareimportantwhenworkingwith
hazardouschemicalssuchascorrosiveacidsandalkalis
 howtofollowtherateof achangebymeasuringthe
disappearanceof areactantortheformationof aproductand
thentoanalysetheresultsgraphically
 thattheconcentrationsof reactants,theparticlesizeof solid
reactants,thetemperature,andthepresenceof catalystsare
factorsthataffecttheratesof reaction
 howcollisiontheorycanexplainwhychangingthe
concentrationof reactants,ortheparticlesizeof solids,affects
therateof areaction
 thatreactionsareexothermicif theygiveoutenergy,and
endothermicif theytakeinenergy
 thatachemicalsynthesisinvolvesanumberof stagesanda
rangeof practicaltechniques,whichareimportanttoachieving
agoodyieldof apureproductinasafeway
 thatatitrationisaprocedurethatcanbeusedtocheckthe
purityof chemicalsusedinsynthesis
 howtousethebalancedequationforareactiontoworkoutthe
quantitiesof chemicalstouseinasynthesis,andtocalculatethe
theoreticalyield.
250
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3 cal
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CHEMICAL
SYNTHESIS
r
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in
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6
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of na, K, and Ca
is
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C6: ChemiCal SyntheSiS
C6: ChemiCal SyntheSiS
251
Ideas about Science
Scientistscanneverbesurethatameasurement
tellsthemthetruevalueof thequantity
being measured.Dataismorereliableif itcan
be repeated.If youtakeseveralmeasurements
of the samequantity,forexample,inatitration,
the resultsarelikelytovary.Thismaybebecause:
• thequantityyouaremeasuringisvarying,for
example,thepurityof separatesolidsamplesof aproductmaynotbeuniform
• therearevariationsinthejudgementof when
theindicatorcolourcorrespondstotheendpoint,orof thepositionof ameniscus
• therearelimitationsinthemeasuring
equipment,suchasburettes.
Usuallythebestestimateof thetruevalueof a
quantityisthemean(oraverage)of severalrepeat
measurements.Youshould:
• beabletocalculatethemeanfromasetof repeatmeasurements
• knowthatameasurementmaybeanoutlierif itlieswelloutsidetherangeof theothervalues
inasetof repeatmeasurements
• beabletogivereasonstoexplainwhyanoutlier
shouldberetainedaspartof thedataor
rejectedwhencalculatingthemean.
Whencomparingsetsof titrationresultsyou
shouldknowthat:
• adifferencebetweentheirmeansisrealif their
rangesdonotoverlap.
Toinvestigatetherelationshipbetweenafactor
andanoutcome,itisimportanttocontrolallthe
otherfactorsthatmightaffecttheoutcome,such
astemperatureandtheconcentrationsof other
reactants.Wheninvestigatingtheratesof chemicalreactionsyoushouldbeableto:
• identifytheoutcomeandfactorsthatmay
affect it
• suggesthowanoutcomemightalterwhena
factorischanged.
252
Inaplanforaninvestigationof theeffectof a
factoronanoutcome,youshould:
• beabletoexplainwhyitisnecessarytocontrol
allthefactorsthatmightaffecttheoutcome
• recognizethatthefactthatotherfactorsare
controlledisapositivedesignfeature,andthe
factthattheyarenotisadesignflaw.
If anoutcomeoccurswhenaspecificfactoris
presentbutdoesnotwhenitisabsent,orif an
outcomevariableincreases(ordecreases)steadily
asaninputvariableincreases,wesaythatthereis
acorrelationbetweenthetwo.Inthecontextof studyingreactionratesyoushould:
• understandthatacorrelationshowsalink
betweenafactorandanoutcome
• beabletoidentifywhereacorrelationexists
whendataispresentedastext,asagraph,orin
atable
• understandthatacorrelationdoesnotalways
meanthatthefactorcausestheoutcome.
• identifythatwherethereisamachanismto
explainacorrelation,scientistsarelikelyto
acceptthatthefactorcausestheoutcome.
C6: ChemiCal SyntheSiS
Review
Questions
1
Astudentisdevelopinganewsportspack.
Sheneedstofindtwochemicalsthatwillcool
aninjurywhenmixedinsidethepack.
a Shouldthereactionbeendothermicor
exothermic?
b Whichof theenergy-leveldiagramsbelow
representsthereactionthatislikelyto
coolinjuriesmoreeffectively?
A
3
B
products
energy
energy
products
reactants
a Explainwhythechemistrepeatedthe
titrationfourtimes.
b Calculatethevolumeof alkaliaddedin
run4.
c Thechemistcalculatesthemeanof his
titrationresults.Whydoeshedothis?
d Suggesttherangewithinwhichthetrue
valueprobablylies.
Ateacheradds3gof zincgranulestodilute
hydrochloricacidinaflask.Sheusesagas
syringetomeasurethevolumeof hydrogen
gasgivenoff.
reactants
Time (min)
progress of reaction
D
reactants
energy
energy
C
progress of reaction
products
reactants
products
progress of reaction
2
Run 1 Run 2 Run 3
Run 4
0.1
0.1
25.0
21.9
25.2
25.0
50.0
46.9
25.1
24.9
25.0
reading(cm3)
Finalburette
reading(cm3)
Volumeof acid
added(cm3)
1
syringe (cm3)
32
2
56
3
74
4
87
5
95
6
95
progress of reaction
Achemisttestedthepurityof asampleof tartaricacidbytitratingwithdilutesodium
hydroxide.Thetableshowshisresults.
Initialburette
Volume of gas in
a Drawalinegraphof theresults.
b Calculatetheaveragerateof reaction
betweenthreeandfourminutes,incm3
perminute.
c Theteacherdecidestorepeatthe
experimentwith3gof zincpowder.She
keepsallotherfactorsthesame.Drawand
labelalineonthegraphtoshowthe
expectedresults.Usecollisiontheoryto
explaintheeffectof thischange.
d Explainwhyitwasimportantforthe
teachertokeepallotherfactorsthesame.
C6: ChemiCal SyntheSiS
253