Concise Physical Chemistry

c04 JWBS043-Rogers September 13, 2010 11:24 Printer Name: Yet to Come

68 THERMOCHEMISTRY

The heat capacity curve of the simple salt solution (the baseline) is subtracted from

the experimental result. There may be multiple peaks if there is more than one protein

in the test solution or if the protein is capable of unfolding in sequential steps.

PROBLEMS AND EXAMPLE

Example 4.1 Oxygen Bomb Calorimetry

Exactly 0.5000 g of benzoic acid C 6 H 5 COOH were burned under oxygen. The

combustion produced a temperature rise of 1.236 K. The same calorimetric setup was

used to burn 0.3000 g of naphthalene (C 10 H 8 ) and the resulting temperature rise was

1.128 K. The heat of combustion of benzoic acid isqV=
cU^298 =–3227 kJ mol−^1

(exothermic). What is the heat of combustionqV=
cU^298 of naphthalene?

Solution 4.1 The corresponding molar masses are: benzoic acid, 122.12 g mol−^1 ;

and naphthalene, 128.19 g mol−^1. The temperature rise for each combustion was: ben-

zoic acid, 1.236/0.5000=2.472 K g−^1 ; and naphthalene, 1.128/0.3000=3.760 K g−^1.

Multiplying by the molar masses in each case, one obtains 301.9 K mol−^1 for benzoic

acid and 482.0 K mol−^1 for naphthalene. This gives us the ratio

301. 9

482. 0

=

− 3227

x

x=− 3227

482. 0

301. 9

=−5152 kJ mol−^1

qV=
cU^298 =−5152 kJ mol−^1

Notice that the units cancel on the left; thusxhas the units of kJ mol−^1 , not K mol−^1.

The handbook value is
cU=−5156 kJ mol−^1.

Problem 4.1

A resistor of precisely 1 ohm is immersed in a liter (1 dm^3 ) of water in a perfectly

insulated container. Suppose that precisely 1 ampere flows through the resistor for

precisely 1 second. What is the temperature rise of the water?

Problem 4.2

Exactly one gram of a solid substance is burned in a bomb calorimeter. The bomb

absorbs as much heat as 300 g of water would absorb. (Its water equivalent is 300 g.)

The bomb was immersed in 1700 g of water in an insulated can. During combustion

of the sample, the temperature went from 24.0◦C to 26.35◦C. What is the heat of

combustion per gram of the sample? What is the molar enthalpy of combustion if the

molar mass of the substance is 60.0 g mol−^1 and 2 mol of gas are formed in excess

of the O 2 burned?