Chemistry

Chemistry

Problem

Consider the following thermochemical data: (1) C(graphite) + O₂(g) → CO₂(g)  ΔH = –393.5 kJ (2) 2CO(g) + O₂(g) → 2CO₂(g)  ΔH = –566.0 kJ Using these reactions, determine ΔH for CO(g) + 12\tfrac{1}{2}O₂(g) → CO₂(g). a) −566 kJ-566\,\text{kJ} b) + 283 kJ+\,283\,\text{kJ} c) −283 kJ-283\,\text{kJ} d) −393.5 kJ-393.5\,\text{kJ}
Intuitive explanation

This is a Hess's Law problem where you combine given thermochemical equations to construct a target equation. The key insight is that enthalpy is a state function, so you can add and manipulate reactions algebraically—multiply, divide, or reverse them—and their ΔH\Delta H values transform the same way. Figure out which combination of reactions (1) and (2), possibly scaled, gives you your target, then apply those same operations to their ΔH\Delta H values.

Intuitive explanation

This is a Hess's Law problem where you combine given thermochemical equations to construct a target equation. The key insight is that enthalpy is a state function, so you can add and manipulate reactions algebraically—multiply, divide, or reverse them—and their ΔH\Delta H values transform the same way. Figure out which combination of reactions (1) and (2), possibly scaled, gives you your target, then apply those same operations to their ΔH\Delta H values.

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