The Foundations of Chemistry

EXAMPLE 15-2 Heat Measurements Using a Calorimeter

A 50.0-mL sample of 0.400 Mcopper(II) sulfate solution at 23.35°C is mixed with 50.0 mL of

0.600 Msodium hydroxide solution, also at 23.35°C, in the coffee-cup calorimeter of Example

15-1. After the reaction occurs, the temperature of the resulting mixture is measured to be

25.23°C. The density of the final solution is 1.02 g/mL. Calculate the amount of heat evolved.

Assume that the specific heat of the solution is the same as that of pure water, 4.184 J/g°C.

CuSO 4 (aq)2NaOH(aq)88nCu(OH) 2 (s)Na 2 SO 4 (aq)

Plan

The amount of heat released by the reaction is absorbed by the calorimeter andby the solu-

tion. To find the amount of heat absorbed by the solution, we must know the mass of solution;

to find that, we assume that the volume of the reaction mixture is the sum of volumes of the

original solutions.

Solution

The mass of solution is

__? g soln(50.050.0) mL

1.02

m

g

L

soln

102 g soln

The amount of heat absorbed by the calorimeter plusthe amount absorbed by the solution is

amount of heat amount of heat

absorbed by calorimeter absorbed by solution

__?J

24

°

.

C

0J

(25.2323.35)°C102 g

4

g

.



1

°

8

C

J

(25.2323.35)°C

45 J801 J846 J absorbed by solution plus calorimeter

Thus, the reaction must have liberated 846 J, or 0.846 kJ, of heat.

You should now work Exercise 62(a).

THERMOCHEMICAL EQUATIONS

A balanced chemical equation, together with its value of 
H, is called a thermochemical

equation.For example,

C 2 H 5 OH(
)3O 2 (g)88n2CO 2 (g)3H 2 O(
)1367 kJ

1 mol 3 mol 2 mol 3 mol

is a thermochemical equation that describes the combustion (burning) of one mole of

liquid ethanol at a particular temperature and pressure. The coefficients in such a descrip-

tion mustbe interpreted as numbers of moles.Thus, 1367 kJ of heat is released when one

mole of C 2 H 5 OH(
) reacts with threemoles of O 2 (g) to give twomoles of CO 2 (g) and

threemoles of H 2 O(
). We can refer to this amount of reaction as one mole of reaction,

which we abbreviate “mol rxn.” This interpretation allows us to write various unit factors

as desired.

, , and so on

2 mol CO 2 (g)



1 mol rxn

1 mol C 2 H 5 OH(
)



1 mol rxn

15-5

When dilute aqueous solutionsare mixed,
their volumes are very nearly additive.

598 CHAPTER 15: Chemical Thermodynamics

 

See the Saunders Interactive
General Chemistry CD-ROM,
Screen 6.14, Enthalpy Change and
H.

The heat released by the reaction of
HCl(aq) with NaOH(aq) causes the
temperature of the solution to rise.