Page 96 - Chemical engineering design

FUNDAMENTALS OF ENERGY BALANCES
Taking the overall efficiency of the furnace as 70% the gas rate required
Heat input 79
D
calorific value ð efficiency
18,002.3
3
D D 768 m /h
33.5 ð 0.7
3.11. STANDARD HEATS OF FORMATION
Ž
The standard enthalpy of formation H of a compound is defined as the enthalpy change
f
when one mol of the compound is formed from its constituent elements in the standard
state. The enthalpy of formation of the elements is taken as zero. The standard heat of
Ž
any reaction can be calculated from the heats of formation H of the products and
f
reactants; if these are available or can be estimated.
Conversely, the heats of formation of a compound can be calculated from the heats of
reaction; for use in calculating the standard heat of reaction for other reactions.
The relationship between standard heats of reaction and formation is given by
equation 3.26 and illustrated by Examples 3.8 and 3.9
Ž Ž
Ž
H D H , products H , reactants 3.26
r f f
A comprehensive list of enthalpies of formation is given in Appendix D.
As with heats of reaction, the state of the materials must be specified when quoting
heats of formation.

Example 3.8
Calculate the standard heat of the following reaction, given the enthalpies of formation:
4NH 3 (g) C 5O 2 (g) ! 4NO(g) C 6H 2 O(g)

Standard enthalpies of formation kJ/mol
NH 3 (g) 46.2
NO(g) C90.3
H 2 O(g) 241.6

Solution
Note: the enthalpy of formation of O 2 is zero.
Ž Ž
Ž
H D H , products H , reactants
r f f
D 4 ð 90.3 C 6 ð 241.6 4 ð 46.2
D 903.6kJ/mol

Ž
Heat of reaction H D 904 kJ/mol
r