chemical equation - Waterford Schools

The evaporation of a puddle of water
 Rust forming on a metal fence


Recall that chemical changes occur
when one substance rearranges itself to
form a new substance
› A burning candle
› Toasting a marshmallow
› Iron rusts
› Silver tarnishes

Chemical changes are also called
CHEMICAL REACTIONS!
What evidence
of a chemical
reaction did we
observe with
the
decomposition
of glucose?

During a chemical reaction,
matter (mass) cannot be
created or destroyed!
› Law of Conservation of Matter
 Defined by Antoine Lavoisier in
early 1700s

Even though the matter may
change from one form to
another, the same number of
atoms exists before and after
the change takes place!
› All atoms present going in MUST
COME OUT!

No matter is created or destroyed by
such changes!

The total amount of matter remains
constant

The mass of the reactants – the
substances there before the reaction
occurs – must equal the mass of the
product – the substances there after the
reaction takes place
Mass of
Reactants
Mass of
Products
Decomposition of HgO
using heat

Chemical reactions are
represented in chemistry
by writing a chemical
equation

Reactants are shown on
the left

Products are shown on
the right

Arrow indicates direction
of the chemical change
and reads as “yields” or
“produces”

Identify the
product(s) and
reactant(s) in the
following reaction:
𝑤𝑎𝑡𝑒𝑟 → ℎ𝑦𝑑𝑟𝑜𝑔𝑒𝑛 + 𝑜𝑥𝑦𝑔𝑒𝑛

Identify the
product(s) and
reactant(s) in the
following reaction:
𝑁𝑖𝐶𝑙2 + 2 𝑁𝑎𝑂𝐻 → 𝑁𝑖(𝑂𝐻)2 +2 𝑁𝑎𝐶𝑙

Iron
› Hard, gray-colored
metal

Rust
› Flaky, orange-red solid
 This
is written as:
Iron + Oxygen
Iron (III) Oxide
TAKE A
LOOK AT
THE
NUMBERS
Fe
Fe
Fe
Fe
How many
Iron
molecules?
O2
O2
O2
How many
Oxygen
molecules?
Fe2O3
Fe2O3
What about
the
PRODUCT?

With iron (III) oxide, the MASS STAYS THE
SAME!
› The total number of iron atoms and oxygen
atoms in the reactants is the same as that in
the product.
› The atoms were just rearranged!
 No new atoms were created and none were
destroyed!

All atoms present in
the reactants must
be accounted for
among the products!
› Same number of
each type of atom
on the product side
as on the reactant
side of the arrow

Referred to as
balancing the
chemical equation

Subscripts
› Shows number of
atoms in a
compound

Coefficients
› Number written in
front of the
chemical formulas
(element symbols) in
a balanced
chemical equation
› Indicates relative
numbers of
reactants and
products
3 H2SO4
H ____ S ____ O ____
Fe(NO3)3 + 3LiOH
How many atoms of
each type are
represented by the
following formulas?
Fe ____ N ____ O ____ Li _____ H ____
2 NH4NO3 + H2S
N ____ H ____ O ____ S ____
2 K + 2 H2O
K ____ H ____ O ____
So, let’s look at the steps we need to take to
BALANCE chemical equations:
Let’s work with the following equation:
P +
O2

P2O5
19
Step 1:
Create a RAP table
• What’s a RAP table ??
• It’s a table that shows us:
• What atoms are present in the
reaction
• How many atoms are present
• Are the atoms reactants or
products?
P +
O2

P2O5
#Reactant Atom #Product
1
2
P
O
2
5
Step 2:
• Go to the first atom that’s not balanced and
balance it!
• Since P atoms are not balanced, what do
we need to do to balance it?
• Multiply it by 2!
#Reactant Atom #Product
2x
1
2
P
O
2
5
• In step 2, we balanced the number of P atoms
by multiplying the reactant side by 2
• This now becomes the new coefficient in
the chemical equation
• Modify the equation to reflect the change:
2P +
O2

P2O5
#Reactant Atom #Product
2
2
P
O
2
5
Step 3:
• Move to the next unbalanced atom
• What is it?
#Reactant Atom #Product
2
2
P
O
2
5
• How can we balance the oxygen?
• Multiply reactants by 5 and products by 2
#Reactant Atom #Product
2
5x 2
P
O
2
5
X2
#Reactant Atom #Product
2
10
P
O
2
10
• Adjust the chemical equation to reflect your changes:
2P +
5O2  2 P2O5
• But notice that by changing oxygen, we
also changed phosphorus!
• We need to go back and fix this!
Step 4:
• Write out the updated RAP table
• How can we balance the phosphorus?
• Multiply the # of reactant P atoms by 2 !
#Reactant Atom #Product
2x
2
10
P
O
2
10
#Reactant Atom #Product
4
10
P
O
4
10
• Adjust the chemical equation to reflect your changes:
4P +
5O2  2 P2O5
Do we have a balanced chemical equation now?
Yes we do!
• Look at this balanced chemical equation:
2AgNO3 + MgCl2  2AgCl + Mg(NO3)2
• When an equation has polyatomics in it and the
polyatomic appears on BOTH the reactant and
product side of the equation, COUNT THE ENTIRE
POLYATOMIC as an “ATOM”!
• So, the RAP table for the above
equation would be:
• If the same polyatomic does
not appear on both sides, break
the polyatomic down into
individual atoms!
#R atom #P
2
2
1
2
Ag
NO3
Mg
Cl
28
2
2
1
2

Look at your chemical equation of the
decomposition of glucose
› Write the chemical equation using formulas
of each reactant and product
› Is the reaction balanced? How do you
know?
 If not, BALANCE IT using coefficients!


Learning to read a sentence or paragraph to extract out a
chemical equation can be tough
Follow these hints to be successful:
› Learn the diatomic gases – HIGHLIGHT THEM ON YOUR PT!
 H2, N2, O2, F2, Cl2, Br2, I2
› Know the states of matter symbols




Solid – (s)
Liquid – (l)
Gas – (g)
If you read the word solution, use the matter symbol (aq)
› Use your periodic table and pink ion sheet to correctly write the
formulas of the compounds and elements from the names given
› Balance using coefficients AFTER ALL SUBSCRIPTS ARE CORRECT
 DO NOT ALTER THE CHEMICAL FORMULAS ONCE WRITTEN WITH
CORRECT SUBSCRIPTS!

Read description of chemical reaction
› What are reactants, products, and their
states (solid, liquid, gas, aqueous)?
› Write appropriate formulas using Periodic
Table and Pink Sheet!

Write the unbalanced equation that
summarizes the above step

Make a RAP table
› Proceed element by element to determine
what coefficients are necessary so that
same number of each type of atom appears
on both the reactant side and product side
 DO NOT CHANGE THE FORMULA OF ANY
REACTANT OR PRODUCT!
YOU CAN ONLY
CHANGE COEFFICIENTS!

Check to see that the coefficients are
the smallest whole number that give the
balanced equation

A solid piece of zinc reacts with
hydrochloric acid to produce a solution
of zinc chloride and hydrogen gas as the
products