Chemistry, Third edition

DETERMINATION OF ΔHIN THE LABORATORY 223

● The chemical reaction should be rapid, so that the energy change is complete in a

short period of time. This is achieved by rapidly mixing the reactants.

● An exothermic chemical reaction increases the amount of heat energy contained in

the reaction mixture. This raises the temperature of the final mixture by TC, de-

fined as TfinalTinitial. An endothermic reaction decreases the amount of heat energy

contained in the reaction mixture, so lowering the temperature of the final mixture.

● The change in the amount of heat energy due to chemical reaction, qjoules, is cal-

culated using the equation

qmCT

(note the negative sign) where mis the mass and Cthe average specific heat

capacity of the final mixture (mainly water) whose temperature is being mea-

sured. The value of qis negative if the chemical reaction is exothermic (when T

is positive); the value of qis positive if the chemical reaction is endothermic

(whenTis negative).

● At constant pressure, the heat change undergone produced by the chemical reac-

tion is equal to the enthalpy change of the reaction, i.e. qH. Note that qand

Hhave the same sign.

● The calculations assume that the reaction ‘goes to completion’.

Experimental details

1.The reactants are contained in an insulated container vented to the atmosphere so

as to keep the contents at atmospheric pressure. Such a container is known as a con-

stant-pressure calorimeter, and at its simplest the container would be a polystyrene

coffee cup (Fig. 13.4). Temperature changes are measured using a thermometer.

2.A known quantity of each of the reactants is placed in separate containers (in the fol-

lowing example the reactants are powdered zinc and lead(II) nitrate solution) and

allowed 10 min to achieve the constant temperature, Tinitial, of the laboratory. This ‘equi-

libration time’ corresponds to the points between A and B in Fig. 13.5. If we want to mea-

sureH^ —(298 K), Tinitialshould be 298 K and the air pressure should be exactly 1 atm.

3.The reactants are mixed at time tseconds and rapidly stirred so that reaction is

complete within 30 s of mixing. As the reaction mixture can never be truly insulated,

the container and its contents immediately start to lose heat (points C to D in an

exothermic reaction, Fig. 13.5(a)) or gain heat (points E to F in an endothermic

reaction, Fig. 13.5(b)). The temperature of the mixture changes too rapidly for a sin-

gle definitive measurement to be taken immediately after mixing. Instead, tempera-

Plastic lid

Polystyrene beaker

Reactants

Thermometer

Stirrer

Fig. 13.4A simple constant-

pressure calorimeter.

Fig. 13.5Temperature–time profiles for (a) an exothermic reaction and (b) an endothermic reaction.