Question 5.4.2: Use reaction stoichiometry to calculate enthalpy change. The......
Use reaction stoichiometry to calculate enthalpy change.
The reaction of HCl with O_{2} is exothermic.
4 HCl(g) + O_{2}(g) → 2 H_{2}O(ℓ) + 2 Cl_{2}(g) ∆H(1) = −202.4 kJ
Calculate the enthalpy change for the reaction of water with elemental chlorine to produce HCl and O_{2}.
H_{2}O(ℓ) + Cl_{2}(g) → 2 HCl(g) + ½ O_{2}(g) ∆H(2) = ?
You are asked to calculate the enthalpy change for a given reaction.
You are given the enthalpy change for the reaction under a different set of stoichiometric conditions.
The second reaction is related to the first reaction by (1) reversing reactants and products and (2) multiplication by a constant (× ½). Thus, the enthalpy change for the second reaction is equal to −∆H(1) × ½.
∆H(2) = −∆H(1) × ½ = −(−202.4 kJ) × ½ = 101.2 kJ
Is your answer reasonable? The second reaction is the reverse of the exothermic first reaction, so it is endothermic (+ΔH). It also involves only half as many reactants and products, so the amount of energy absorbed is half as much as the amount of energy released in the first reaction.