Heat of combustion & formation-
Heat of combustion –
“It is defined as the change in internal energy or heat enthalpy when one mole of substance is completely oxidized in O2.”
For Ex-
2H2(g) + O2(g) —> 2H2O(g) ; ΔH = -115 Kcal.
2 mole H2 gives energy = 115 Kcal
1 mole H2 gives energy = 115/2 = 57.5 Kcal
Heat of combustion of H2 is -57.5 Kcal.
For Ex-
C(s) + O2(g) —> CO2(g) ; ΔH = -94.3 Kcal.
1 mole Carbon gives energy = 94.3 Kcal
Heat of combustion of Carbon is – 94.3 Kcal.
Heat of combustion is always exothermic . It means ΔH = negative
Exception –
N2(g) +1/2O2(g) —> N2O(g) ; ΔH = positive
N2(g)+O2 (g) —> 2NO(g) ; ΔH = positive
Heat of formation-
“It is defined as the change in internal energy or heat enthalpy when one mole of substance is formed from its initial components or elements”
For Ex –
C(s) + O2(g) —> CO2(g) ; ΔH = – 94.3 Kcal.
In this reaction 1 mole of CO2 is formed from its elements i-e Carbon & oxygen.
Therefore , heat of formation of CO2 ‘ ΔH’ = -94.3 Kcal.
For Ex-
2H2(g) + O2(g) —> 2H2O(g) ; ΔH = -115 Kcal.
In this reaction 2 mole of H2O is formed from its elements i-e Hydrogen & oxygen.
Therefore , heat of formation of H2O ‘ ΔH’ = -115 / 2 = -57.5 Kcal.
heat of formation of H2O ‘ ΔH’ = -57.5 Kcal.
Numerical-
Question – The molar heat of combustion of acetylene C2H2(g) ,C(graphite) & H2(g) are -310.62 Kcal, -94.05 Kcal,&
– 68.32 Kcal respectively. Find heat of formation of acetylene ?
Solution –
C2H2(g) + 2.5O2(g) —> 2Co2(g) + H2O(g) ; ΔH = -310.62 Kcal ———eq(1)
C(graphite) +O2(g) —> CO2(g) ; ΔH = -94.05 Kcal ————- eq (2)
H2 (g) + 1/2 O2(g) —> H2O(g) ; ΔH = -68.32 Kcal ————-eq (3)
2C(graphite) +H2(g) —> C2H2(g) ; ΔH = ? Kcal ——– eq (4)
Multiply Eq (2) by 2 & then add eq (3),
2C(graphite) +2O2(g) + H2 (g) + 1/2 O2(g)—> 2CO2(g) + H2O(g)
ΔH = (-94.05 x 2 ) + (-68.32) = -256.42 Kcal
2C(graphite) +2O2(g) + H2 (g) + 1/2 O2(g)—> 2CO2(g) + H2O(g) ; ΔH = -256.42 Kcal —– eq (5)
Now to get eq (4), substract eq ( 1 )from eq (5) ,
2C(graphite) +2O2(g) + H2 (g) + 1/2 O2(g) – C2H2(g) – 2.5O2(g)—> 2CO2(g)+H2O(g)- 2Co2(g) – H2O(g) ; ΔH = -256.42 – (-310.62 )
= + 54.2 Kcal
2C(graphite)+ H2 (g) —> C2H2(g) ; ΔH = +54.2 Kcal