LEC 02.08 Determination of the enthalpy of neutralisation
Transcription
LEC 02.08 Determination of the enthalpy of neutralisation
LEC 02.08 Determination of the enthalpy of neutralisation Related concepts Enthalpy of neutralisation, calorimetry, heat capacity. Principle When a strong acid is neutralised with a strong base in dilute solution, the same amount of heat is always released. If the reaction takes place under isobaric conditions, this heat is known as the enthalpy of neutralisation. The chemical reaction which generates this heat is the reaction of protons and hydroxyl ions to form undissociated water. It therefore correlates to the enthalpy of formation of water from these ions. Tasks 1. Measure the temperature change during the neutralisation of a dilute potassium hydroxide solution with dilute hydrochloric acid. 2. Calculate the enthalpy of neutralisation. Equipment Cobra3 Basic-Unit Power supply 12 V/2 A Data cable, RS232 Temperature measuring module Pt 100 Software Cobra 3 Temperature Temperature probe Pt 100 Calorimeter, transparent Delivery pipette, 50 ml 12150.00 12151.99 14602.00 12102.00 14503.61 11759.01 04402.00 04402.10 1 1 1 1 1 1 1 1 Pipettor Rubber bulb, double Pinchcock, w = 15 mm Heating coil with sockets Work and power meter Universal power supply Connection cable, l = 500 mm, black Magnetic heating stirrer Magnetic stirrer bar, l = 30 mm, oval Separator for magnetic bars Support rod, l = 500 mm, M10 thread Right angle clamp Universal clamp Laboratory balance with data output, 800/1600/3200 g Volumetric flask, 500 ml Glass beaker, 100 ml, tall Glass beaker, 600 ml, tall Pasteur pipettes Rubber bulbs Stop watch, digital, 1/100 s Wash bottle, 500 ml Potassium hydroxide for 1 l of 1 M solution, ampoule Hydrochloric acid for 1 l of 1 M solution, ampoule Water, distilled, 5 l PC, Windows® 95 or higher 36592.00 39287.00 43631.15 04450.00 13715.93 13500.93 07361.05 35720.93 35680.04 35680.03 02022.20 37697.00 37715.00 1 1 1 1 1 1 4 1 1 1 1 2 2 48803.93 36551.00 36002.00 36006.00 36590.00 39275.03 03071.01 33931.00 1 2 1 1 1 1 1 1 31425.00 1 30271.00 1 31246.81 1 Fig. 1. Experimental set-up. PHYWE series of publications • Laboratory Experiments • Chemistry • © PHYWE SYSTEME GMBH & Co. KG • D-37070 Göttingen P3020811 1 LEC 02.08 Determination of the enthalpy of neutralisation Set-up and procedure Set up the experiment as shown in Fig. 1 but for the time being do not connect the heating coil to the work and power meter. Prepare the potassium hydroxide solution required (cKOH = 2 mol/l) by dissolving one ampoule of potassium hydroxide for 1 l of 1 M solution in a 500 ml volumetric flask and topping off with water to the calibration mark. Proceed in a similar fashion with a second 500 ml volumetric flask using 1 ampoule of hydrochloric acid for 1 l of 1 M solution to produce hydrochloric acid of the same concentration (cHCl = 2 mol/l). Connect the temperature probe to T1 of the measuring module. Call up the Measure programme in Windows and enter <Temperature> as measuring instrument. Set the measuring parameters as shown in Fig. 2. Under <Diagram 1> select Temperature T0a, the appropriate range for the temperature and the X bounds and ‘auto range‘. Now calibrate your sensor under <Calibrate> by entering a temperature value measured with a thermometer and pressing <Calibrate>. After having made these settings, press <Continue> to reach the field for the recording of measured values. Arrange the displays as you want them. Pour approximately 750 g water and 60 g of the 2 M potassium hydroxide solution (both weighed to an accuracy of 0.1 g) into the calorimeter. Using a delivery pipette and a pipettor, draw around 50 ml of the 2 M hydrochloric acid from a small glass beaker. The exact mass of the hydrochloric acid contained in the delivery pipette is calculated from the difference between the masses of the filled and the empty delivery pipette (accuracy 0.1 g). The 600 ml beaker is used as a pipette stand. Place the filled calorimeter on the magnetic stirrer, put in the oval magnetic stirrer bar and switch on the stirrer (Caution: Do not switch on the heating unit by mistake!). Push the delivery pipette through the cap of the calorimeter from below and mount the lid on the calorimeter vessel. Now attach the pipette to the support rod using a clamp in such a manner that the opening is above the level of the liquid and that the stirrer bar can rotate unhindered. Insert the heating coil and the temperature probe into the lid of the calorimeter and fix them in position. When the temperature equilibrium has been reached (after approximately 10 min) start the measurement by pushing <Start measurement>. Wait 3 to 4 minutes, then blow the hydrochloric Fig. 2: acid out of the delivery pipette into the potassium hydroxide solution in the calorimeter. To do this, first clamp a pinchcock onto the tube of the rubber bulb, blow up the air reservoir of the rubber bulb and quickly release the pinchcock. Continue to measure until a new equilibrium has been reached. Subsequently perform electrical calibration to determine the total heat capacity of the calorimeter. Supply 10 V AC to the work and power meter for the electric heating. Push the <Reset> button and then put the free ends of the heating coil connection cables into the output jacks. The system is now continuously heated and the supplied quantity of energy is measured. The temperature increase in the system should be approximately 2 K. When this value has been reached, switch off the heating and read the exact quantity of electrical energy supplied. After a further three minutes, stop the recording of temperature. Fig. 3 shows the graph as it is presented by the programme when the measurement is stopped. If you use <survey> from the toolbar you can read the temperature difference data. Theory and evaluation The value of the enthalpy of neutralisation ∆RH for the reaction of strong acids with strong bases is independent of which strong acid or base is used, because the heat of reaction is generated by the reaction of hydrogen and hydroxyl ions to form water. H+ + OH- S H2O In the case of the neutralisation of weak acids and bases, additional heat effects arise from dissociation, hydration and association of molecules, so that the value of the enthalpy of neutralisation will differ to that given above. The heat capacity of the system must be determined in order to be able to determine the system change in enthalpy ∆h. This is undertaken, after completion of the neutralisation reaction, by introducing a specific amount of heat into the filled calorimeter using electrical heating. The electrical energy Wel = I · U · t which is converted into heat Q causes an increase in temperature ∆Tcal. From this the heat capacity of the system Csys can be calculated using equation (1). Q = I · U · t = Csys . ∆Tcal = Wel (1) Measurement parameters Fig. 3: 2 ∆RH = -57.3 kJ · mol-1 P3020811 Temperature-time curve of neutralisation and determining the heat capacity of the system PHYWE series of publications • Laboratory Experiments • Chemistry • © PHYWE SYSTEME GMBH & Co. KG • D-37070 Göttingen LEC 02.08 Determination of the enthalpy of neutralisation Using the heat capacity of the system, the enthalpy of neutralisation ∆RH can be calculated from the temperature increase ∆T of the neutralisation reaction for a known amount n of converted hydrochloric acid. ∆RH n cHCl mHCl rHCl ∆RH Csys Csys · ∆T n rHCl Csys · ∆T cHCl · mHCl (2) Amount of hydrochloric acid introduced Concentration of hydrochloric acid (= 2 mol/l) Mass of hydrochloric acid introduced Density of hydrochloric acid (= 1.0344 g/ml for 2 M HCl at 20°C) Enthalpy of neutralisation Heat capacity of system For reasons of simplification it is assumed that the heat capacity of the dilute salt solution differs only negligibly from that of water. Data and results Enthalpy of neutralisation: ∆RH = -57.3 kJ · mol-1 PHYWE series of publications • Laboratory Experiments • Chemistry • © PHYWE SYSTEME GMBH & Co. KG • D-37070 Göttingen P3020811 3 LEC 02.08 4 P3020811 Determination of the enthalpy of neutralisation PHYWE series of publications • Laboratory Experiments • Chemistry • © PHYWE SYSTEME GMBH & Co. KG • D-37070 Göttingen