Question

Calculate the enthalpy change for the oxidation of pyruvic acid to aceticacid under standard conditions (298 K, 1 atm):

2 CH₃COCOOH(l) + O₂(g) -> 2 CH₃COOH(l) + 2 CO₂(g)

The enthalpies of combustion for pyruvic acid and acetic acid understandard conditions are −279 kcal/mol and −207 kcal/mol, respectively. Heats of combustion are determined by reacting pyruvic acid or acetic acid with O₂(g) to give H₂O(l) and CO₂(g). As a first step, write balanced chemical equations for the combustion reactions for pyruvic acid and acetic acid, then combine them to make the overall reaction given above for the oxidation of pyruvic acid to acetic acid.

Answer 1

The enthalpy change for the oxidation of pyruvic acid to acetic acid is calculated using Hess's law by combining the combustion equations of both acids. The result is a change of -288 kcal for the reaction.

Step-by-step explanation:

The balanced chemical equations for the combustion reactions of pyruvic acid and acetic acid, which are given by: Pyruvic acid combustion: CH₃COCOOH(l) + 3 O₂(g) → 3 CO₂(g) + 3 H₂O(l). Acetic acid combustion: CH₃COOH(l) + 2 O₂(g) → 2 CO₂(g) + 2 H₂O(l).

We reverse the combustion of acetic acid (since it appears as a product in the target reaction) and combine it with the combustion of pyruvic acid (since it's a reactant in the target reaction). The reversed combustion reaction for acetic acid is: 2 CO₂(g) + 2 H₂O(l) → CH₃COOH(l) + 2 O₂(g), ΔH = +414 kcal.

Combining this with the combustion of pyruvic acid gives: CH₃COCOOH(l) + O₂(g) → CH₃COOH(l) + CO₂(g), ΔH = (-558 kcal) + (414 kcal) = -144 kcal/mol per mole of pyruvic acid or -288 kcal for the reaction as given. Hence, the enthalpy change for the oxidation of pyruvic acid to acetic acid under standard conditions is -288 kcal.