Related conceptsEnthalpy of neutralisation, calorimetry, heat capacity.

PrincipleWhen a strong acid is neutralised with a strong base in dilutesolution, the same amount of heat is always released. If the reac-tion takes place under isobaric conditions, this heat is known asthe enthalpy of neutralisation. The chemical reaction which gen-erates this heat is the reaction of protons and hydroxyl ions toform undissociated water. It therefore correlates to the enthalpyof formation of water from these ions.

Tasks1. Measure the temperature change during the neutralisation of

a dilute potassium hydroxide solution with dilute hydrochlo-ric acid.

2. Calculate the enthalpy of neutralisation.

EquipmentCobra3 Basic-Unit 12150.00 1Power supply 12 V/2 A 12151.99 1Data cable, RS232 14602.00 1Temperature measuring module Pt 100 12102.00 1Software Cobra 3 Temperature 14503.61 1Temperature probe Pt 100 11759.01 1Calorimeter, transparent 04402.00 1Delivery pipette, 50 ml 04402.10 1

Pipettor 36592.00 1Rubber bulb, double 39287.00 1Pinchcock, w = 15 mm 43631.15 1Heating coil with sockets 04450.00 1Work and power meter 13715.93 1Universal power supply 13500.93 1Connection cable, l = 500 mm, black 07361.05 4Magnetic heating stirrer 35720.93 1Magnetic stirrer bar, l = 30 mm, oval 35680.04 1Separator for magnetic bars 35680.03 1Support rod, l = 500 mm, M10 thread 02022.20 1Right angle clamp 37697.00 2Universal clamp 37715.00 2Laboratory balance

with data output, 800/1600/3200 g 48803.93 1Volumetric flask, 500 ml 36551.00 2Glass beaker, 100 ml, tall 36002.00 1Glass beaker, 600 ml, tall 36006.00 1Pasteur pipettes 36590.00 1Rubber bulbs 39275.03 1Stop watch, digital, 1/100 s 03071.01 1Wash bottle, 500 ml 33931.00 1Potassium hydroxide for 1 l

of 1 M solution, ampoule 31425.00 1Hydrochloric acid for 1 l

of 1 M solution, ampoule 30271.00 1Water, distilled, 5 l 31246.81 1PC, Windows® 95 or higher

PHYWE series of publications • Laboratory Experiments • Chemistry • © PHYWE SYSTEME GMBH & Co. KG • D-37070 Göttingen P3020811 1

LEC02.08

Determination of the enthalpy of neutralisation

Fig. 1. Experimental set-up.

Set-up and procedureSet up the experiment as shown in Fig. 1 but for the time beingdo 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 for1 l of 1 M solution in a 500 ml volumetric flask and topping offwith water to the calibration mark. Proceed in a similar fashionwith a second 500 ml volumetric flask using 1 ampoule ofhydrochloric acid for 1 l of 1 M solution to produce hydrochloricacid 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 measuringparameters as shown in Fig. 2. Under <Diagram 1> selectTemperature T0a, the appropriate range for the temperature andthe X bounds and ‘auto range‘. Now calibrate your sensor under<Calibrate> by entering a temperature value measured with athermometer and pressing <Calibrate>. After having made thesesettings, press <Continue> to reach the field for the recording ofmeasured values. Arrange the displays as you want them.Pour approximately 750 g water and 60 g of the 2 M potassiumhydroxide solution (both weighed to an accuracy of 0.1 g) intothe calorimeter. Using a delivery pipette and a pipettor, drawaround 50 ml of the 2 M hydrochloric acid from a small glassbeaker. The exact mass of the hydrochloric acid contained in thedelivery pipette is calculated from the difference between themasses of the filled and the empty delivery pipette (accuracy0.1 g). The 600 ml beaker is used as a pipette stand.Place the filled calorimeter on the magnetic stirrer, put in the ovalmagnetic stirrer bar and switch on the stirrer (Caution: Do notswitch on the heating unit by mistake!). Push the delivery pipettethrough the cap of the calorimeter from below and mount the lidon the calorimeter vessel. Now attach the pipette to the supportrod using a clamp in such a manner that the opening is abovethe level of the liquid and that the stirrer bar can rotate unhin-dered. Insert the heating coil and the temperature probe into thelid of the calorimeter and fix them in position.

When the temperature equilibrium has been reached (afterapproximately 10 min) start the measurement by pushing <Startmeasurement>. Wait 3 to 4 minutes, then blow the hydrochloric

acid out of the delivery pipette into the potassium hydroxidesolution in the calorimeter. To do this, first clamp a pinchcockonto the tube of the rubber bulb, blow up the air reservoir of therubber bulb and quickly release the pinchcock. Continue tomeasure until a new equilibrium has been reached. Subse-quently perform electrical calibration to determine the total heat capacity of the calorimeter. Supply 10 V AC to thework 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 con-tinuously heated and the supplied quantity of energy is mea-sured. The temperature increase in the system should beapproximately 2 K. When this value has been reached, switch offthe heating and read the exact quantity of electrical energy sup-plied. After a further three minutes, stop the recording of tem-perature.Fig. 3 shows the graph as it is presented by the programmewhen the measurement is stopped. If you use <survey> from thetoolbar you can read the temperature difference data.

Theory and evaluationThe value of the enthalpy of neutralisation ∆RH for the reactionof strong acids with strong bases is independent of which strongacid or base is used, because the heat of reaction is generatedby the reaction of hydrogen and hydroxyl ions to form water.

H+ + OH- S H2O ∆RH = -57.3 kJ · mol-1

In the case of the neutralisation of weak acids and bases, addi-tional heat effects arise from dissociation, hydration and associ-ation of molecules, so that the value of the enthalpy of neutrali-sation will differ to that given above.The heat capacity of the system must be determined in order tobe able to determine the system change in enthalpy ∆h. This isundertaken, after completion of the neutralisation reaction, byintroducing a specific amount of heat into the filled calorimeterusing electrical heating. The electrical energy Wel = I · U · twhich is converted into heat Q causes an increase in tempera-ture ∆Tcal. From this the heat capacity of the system Csys can becalculated using equation (1).

Q = I · U · t = Csys . ∆Tcal = Wel (1)

PHYWE series of publications • Laboratory Experiments • Chemistry • © PHYWE SYSTEME GMBH & Co. KG • D-37070 GöttingenP30208112

LEC02.08

Determination of the enthalpy of neutralisation

Fig. 2: Measurement parameters

Fig. 3: Temperature-time curve of neutralisation and deter-mining the heat capacity of the system

Using the heat capacity of the system, the enthalpy of neutrali-sation ∆RH can be calculated from the temperature increase ∆Tof the neutralisation reaction for a known amount n of convertedhydrochloric acid.

(2)

n Amount of hydrochloric acid introducedcHCl Concentration of hydrochloric acid (= 2 mol/l)mHCl Mass of hydrochloric acid introducedrHCl Density of hydrochloric acid (= 1.0344 g/ml for 2 M HCl

at 20°C)∆RH Enthalpy of neutralisationCsys Heat capacity of system

For reasons of simplification it is assumed that the heat capaci-ty of the dilute salt solution differs only negligibly from that ofwater.

Data and resultsEnthalpy of neutralisation:∆RH = -57.3 kJ · mol-1

∆RH � �Csys · ∆T

n� �

rHCl

cHCl · mHCl� Csys · ∆T

PHYWE series of publications • Laboratory Experiments • Chemistry • © PHYWE SYSTEME GMBH & Co. KG • D-37070 Göttingen P3020811 3

LEC02.08

Determination of the enthalpy of neutralisation

PHYWE series of publications • Laboratory Experiments • Chemistry • © PHYWE SYSTEME GMBH & Co. KG • D-37070 GöttingenP30208114

LEC02.08

Determination of the enthalpy of neutralisation