2. Enthalpy change of formation from enthalpy
change of combustion
Enthalpy change of combustion when 1 mole of a
substance reacts completely with oxygen under
standard conditions
3. the enthalpy change of an element in its
normal state is zero
you must take into account the number of
moles of reactants and products in each part
of the energy cycle
Important
4. Example 1 Kjmol-1
C(S) H2 (g) C2H6
-394 -286 -1561
Calculate standard enthalpy change of formation of
ethene
3C(S) +3H2 (g) f C2H6
3CO2(g) +3H2O (I)
-394X3+-286X3 = -1182+-858
-2040
= -2040+1561= -479KJ mol-1
H
H
5. Example 2
Use HC
value to calculate H for
3C(s) + 4H2 C3H8
H
3CO2 +4H2O
H = 3x-394+4x-286=-1182+-1144=-2326
right side = +2219 =
H +2219+ -2326 = -107 Kjmol-1
Hc KJmol-
C = -394
H= -286
C3H8 = -2219
6. Example 3
Use HC
value to calculate H for
2C (s) +3H2 +
𝟏
𝟐
O2 CH3CH2OH
2CO2 +3H2
H
Hc KJmol-
C = -394
H= -286
C3H2OH = -1367
Solution
H+-1367=(-394x2)+( -286x3)
H ? = -394 x2+(-286x3) - -1367
=-279 Kjmol-1
7. The standard enthalpy of formation of pentane
relates to the equation
5C(s) + 6H2(g) C5H12(l)
The standard enthalpy changes of combustion
for the three substances in the equation are:
C(s) -394 kJ mol-1
H2(g) -286 kJ mol-1
C5H12(l) -3509 kJ mol-1
Homework
