1. 5.4.6 Hess's Law of Heat Summation [1840]

2. The heat change for a reaction is the same whether the reaction takes place in one step or several steps.
Illustration: Shop takings for the day.
One can total the takings at morning tea, lunch, afternoon tea and closing - or simply at closing - the result will be the same.

3. S(s) + 11/2 O2(g) = SO3(s) H = - 395 kJ mol-1
Form SO3 in one step
S(s) + 11/2 O2(g) = SO3(s) H = kJ mol-1
If it is formed in two stages.
S(s) O2(g) = SO2(g) H = kJ mol-1
SO2(g) + 1/2 O2(g) = SO3(s) H = kJ mol-1
Total S(s) + 11/2 O2(g) = SO3(s) kJ mol-1
This can be used to calculate the heat of reaction for reactions which are difficult or even impossible to measure directly.

4. Find the heat of formation of CO C(s) + 1/2 O2(g) = CO(g)
One cannot do this directly because some CO2 is always going to be formed.
The following two equations can be worked out directly by experiments.
C(s) O2(g) = CO2(g) H = kJ mol-1
CO(g) + 1/2 O2(g) = CO2(g) H = kJ mol-1

5. If we arrange as in the previous example I. e
If we arrange as in the previous example I.e. as a two stage process – making CO first and then converting the CO to CO2 we get :-
C(s) + 1/2 O2(g) = CO(g) H = - ??? kJ mol-1
CO(g) + 1/2 O2(g) = CO2(g) H = kJ mol-1
C(s) + O2(g) = CO2(g) H = kJ mol-1
The unknown must be kJ mol-1
C(s) + 1/2 O2(g) = CO(g) H = kJ mol-1

6. Calculate the heat of formation of Fe2O3 given the following information.
2 Fe + O2 = 2 FeO ΔH = kJ
4 FeO + O2 = 2 Fe2O3 ΔH = -300 kJ
2 Fe + 1½ O2 = Fe2O3

7. * ½ Calculate the heat change for the reaction 2 Fe + 1½ O2 = Fe2O3
Equations given
2 Fe + O2 = 2 FeO ΔH = -280 kJ
4 FeO + O2 = 2 Fe2O3 ΔH = -300 kJ OK
* ½
Write modified equations
2 Fe + O2 = 2 FeO ΔH = kJ
2 FeO + ½ O2 = Fe2O3 ΔH = kJ
2 Fe + 1½ O2 = Fe2O3
ΔH = kJ

8. If the heat of formation of Al2O3 is – 1596 kJ mol-1
If the heat of formation of Al2O3 is – 1596 kJ mol-1. and the heat of formation of Cr2O3 is – 1134 kJ mol-1, calculate the heat of reaction for
2 Al(s) + Cr2O3(s) = Al2O3(s) + 2 Cr(s)

9. 2 Al(s) + Cr2O3(s) = Al2O3(s) + 2 Cr(s)
2 Al(s) + 11/2 O2(g) = Al2O3(s)
2 Cr(s) + 11/2 O2(g) = Cr2O3(s)

10. Calculate the heat change of the following reaction
C2H4(g) + H2(g) = C2H6(g)
Given that the heat of combustion of ethene [C2H4], hydrogen [H2] and ethane [C2H6] are -1393, and – 1561 kJmol-1 respectively

11. Calculate the heat of reaction for the for
HCOOH(l) = CO(g) + H2O(l)
Given the following heats of formation
C(s) + ½ O2(g) = CO(g) ΔH = -111 kJmol-1
H2(g) + ½ O2(g) = H2O(l) ΔH =
H2(g) + O2(g) + C(s) = HCOOH(l) ΔH =
Answer
kJ mol-1

12. Calculate the heat of reaction for the following equation
2H2S + SO2 = 3S + 2 H2O
H2(g) + S(s) = H2S(g) ΔH =
S(g) + O2(g) = SO2(g) ΔH = - 297
H2(g) + ½ O2(g) = H2O(g) ΔH = - 286
Answer =

13. 2H2S + SO2 = 3S + 2 H2O
(rev *2)H2(g) + S(s) = H2S(g) ΔH =
(Rev) S(g) + O2(g) = SO2(g) ΔH = - 297
(*2) H2(g) + ½ O2(g) = H2O(g) ΔH = - 286
2 H2S(g) = 2 H2(g) + 2 S(s) ΔH = + 42
SO2(g) = S(g) + O2(g) ΔH = +297
2 H2(g) + O2(g) = 2 H2O(g) ΔH = - 572
Answer = -233