Chemistry, Third edition

230 13 · ENERGY CHANGES IN CHEMICAL REACTIONS

say CH 4 (g) and SO 2 (g), since neither compound contains an element common to

both. However, a comparison of H—f^ values (kJ mol^1 ) for compounds in the series:

HF(g) HCl(g) HBr(g) HI(g)

 271  92  36  27

is useful since it suggests a gradual reduction in stability from left to right. This trend

fits in with experimental observations which show that HI(g) is the least stable of

the series, with HI(g) being the only hydride in the series that decomposes at room

temperature. However, H—f^ values cannot be used to predict how fastsuch a

decomposition will occur.

Standard enthalpy of combustion

The standard enthalpy of combustion H—c^ is the

standard enthalpy change when one mole of substance is burned in oxygen.

For example, methane (the main constitutent of natural gas) burns as follows:

CH 4 (g)2O 2 (g)CO 2 (g)2H 2 O(l) H—c^ (298 K)  890 kJ mol^1

(From now on we shall assume that all H^ —cvalues apply at 298 K.)

The heating power of fuels is often expressed as the energy value, defined as the

heat produced when one gram of fuel burns completely. Its units are kilojoules per

gram (kJ g^1 ).

We work out the energy value of methane as follows. The molar mass of methane

is 16.0 g mol^1. When 16.0 g of methane burns, 890 kJ of heat is produced. When 1 g

of methane is burned, 890/16.0 56 kJ of heat is produced, so that the energy value

of methane is 56 kJ g^1. The energy values of common fuels are shown in Table

13.3.

Determination of enthalpy changes of combustion in the

laboratory

The determination of Hcis, in principle, similar to that for other enthalpy changes.

In practice, the fuel (or food) is mixed with an oxidizing agent (such as solid sodium

peroxide (Na 2 O 2 ) or pure oxygen gas) and sealed in a steel-walled vessel called a

bomb calorimeter. The mixture is ignited by an electric current and the temperature

rise recorded. The combustion reaction in a bomb calorimeter is not taking place at

constant pressure, but this is easily allowed for in the final calculations.

13.6

Fig. 13.6A comparison of the standard

enthalpies of formation of six substances

at 25 ° C. (The value for diamond has been

multiplied by five; the value for graphite is

zero.).

Table 13.3Energy

values

Fuel Energy
value/
kJ g^1

Hydrogen (H 2 (g)) 142
Methane (CH 4 (g)) 56
Ethyne (acetylene, 50
C 2 H 2 (g))
Octane (C 8 H 18 (l)) 48
Coal (bituminous)
30
Pine wood 18
Methanol (CH 3 OH(l)) 23
Apples 2
White bread 11
Butter 30
Cheese 4–18
Hard boiled eggs 7
Full cream milk 3
Baked potatoes 4
Chips 11
Nitroglycerine 7
TNT 5
Ammonium nitrate 2