Question 9.3.1: Determination of a Heat of Reaction from Heats of Formation ......
Determination of a Heat of Reaction from Heats of Formation
Determine the standard heat of reaction for the combustion of liquid n-pentane, assuming H_{2}O(l) is a combustion product.
C_{5}H_{12}(l) + 8O_{2}(g) → 5CO_{2}(g) + 6H_{2}O(l)
From Equation 9.3-1
To verify the formula for ΔH°_{r}, we may write the stoichiometric equations for the formation reactions of the reactants and products:
1. 5C(s)+6H_{2}(g) \rightarrow C_{5}H_{12}(l): ΔH^{°}_{r1} = 1 mol C_{5}H_{12}(\Delta \hat{H}^{°}_{f})_{C_{5}H_{12}(l)}
2. C(s)+ O_{2}(g) \rightarrow CO_{2}(g): ΔH^{°}_{r2} = 1 mol CO_{2} (\Delta \hat{H}^{°}_{f})_{ CO_{2}(g)}
3. H_{2}(g)+ \frac{1}{2} O_{2}(g) \rightarrow H_{2}O (l): ΔH^{°}_{r3} = 1 mol H_{2}O (\Delta \hat{H}^{°}_{f})_{ H_{2}O(l)}
The desired reaction,
4. C_{5}H_{12}(l) + 8O_{2}(g) → 5CO_{2}(g) + 6H_{2}O(l): ΔH^{°}_{r} = ?
may be obtained as 5 × (2) + 6 × (3) – (1) (verify), and the given formula for ΔH°_{r}then follows from Hess’s law.