Question 9.4.1: Calculation of a Heat of Reaction from Heats of Combustion C......
Calculation of a Heat of Reaction from Heats of Combustion
Calculate the standard heat of reaction for the dehydrogenation of ethane:
C_{2}H_{6} → C_{2}H_{4} + H_{2}
From Table B.1 or using APEx function DeltaHcg,
(\Delta \hat{H}^{°}_{c})_{ C_{2}H_{6}} = – 1559.9 kJ/mol\\ (\Delta \hat{H}^{°}_{c})_{ C_{2}H_{4}} = – 1411.0 kJ/mol\\ (\Delta \hat{H}^{°}_{c})_{ H_{2}} = – 285.84 kJ/mol
From Equation 9.4-1, therefore,
As an illustration, let us demonstrate the validity of this formula using Hess’s law. The combustion reactions are
1. C_{2}H_{6} + \frac{7}{2}O_{2} → 2CO_{2} + 3H_{2}O
2. C_{2}H_{4} + 3O_{2} → 2CO_{2} + 2H_{2}O
3. H_{2} + \frac{1}{2} O_{2} → H_{2}O
It is easy to show that
4. C_{2}H_{6} → C_{2}H_{4} + H_{2}
is obtained as (1) – (2) – (3). (Show it.) The desired result follows from Hess’s law.