Chemistry, Third edition

224 13 · ENERGY CHANGES IN CHEMICAL REACTIONS

ture measurements are recorded at regular intervals (say every 5 s) after reaction

completion and extrapolation of the resulting graph (shown by the dotted lines in

Fig. 13.5) is used to give the temperature change Tat mixing.

Example 13.3

Find the enthalpy change for the exothermic reaction

Zn(s)Pb(NO 3 ) 2 (aq)Zn(NO 3 ) 2 (aq)Pb(s)

The ionic reaction involved is

Zn(s)Pb^2 (aq)Zn^2 (aq)Pb(s)

In an experiment, 0.500 g of finely powdered zinc was rapidly stirred into

100.0 cm^3 of 0.100 mol dm^3 lead(II) nitrate solution (this is an excess of

lead(II) nitrate solution) in a polystyrene cup. The recorded temperature of the

mixture rose from 20.0C (before mixing) to 22.6C so that

TTfinalTinitial22.620.0 2.6C.

We make the approximation that the heat capacity of the polystyrene cup and

of the reactants and products is relatively small and may be ignored. The

lead(II) nitrate solution is so dilute that its specific heat capacity may be

taken to be the same as that of pure water (4.18 J g^1 C^1 ). In effect, we

are assuming that the heat produced by the reaction is entirely absorbed by

the 100 g of water.

The change in heat energy due to the chemical reaction, q, is calculated as

follows:

qmCT100.04.182.61087 J 1.087 kJ

(units: g J g^1 °C^1 °CJ)

The experiment involves the conversion of 0.500 g of Zn to Zn^2 (aq):

0.500 g

amount of Zn 0.007 65 mol

65.39 g mol^1

Therefore, 1.087 kJ of heat is produced by the chemical reaction (and

subsequently absorbed by the water) when 0.007 65 mol of Zn is used up. If

one mol of Zn(s) were consumed in the reaction, the heat change would be

1

q(per mol) 1.087142 kJ mol^1

0.007 65

As we have only measured Tto two significant figures, qshould be rounded

down to 140 kJ mol^1. Finally, as qandHare equal,

H140 kJ mol^1

This will be approximately equal to H^ —(298 K). The accepted value of H^ —

(298 K) is 152 kJ mol^1 , the difference between the experimental and

literature values being mainly due to excessive heat losses by the mixture in

the polystyrene cup and by the approximations made in the calculation. More

accurate experimental determinations rely upon more efficient insulation and

a more rapid (electronic) means of recording temperature. In these cases,

the heat capacities of the container and of the reactants and products are

also taken into account.

Determining

enthalpy changes 1

In Example 13.3

(i)Why was the zinc

powdered?

(ii)Calculate the number of

moles of lead nitrate in

the solution, and hence

show that the lead

nitrate is in excess.

Exercise 13E