Chemistry

Chemistry

Problem

The following enthalpy changes were measured at 298 K298\,\text{K}: (i) C(s) + O2_2(g) → CO2_2(g) ΔH=−394 kJ\Delta H = -394\,\text{kJ} (ii) CO2_2(g) + H2_2(g) → CO(g) + H2_2O(l) ΔH=+41 kJ\Delta H = +41\,\text{kJ} (iii) 22 H2_2(g) + O2_2(g) → 22 H2_2O(l) ΔH=−572 kJ\Delta H = -572\,\text{kJ} Using these data, what is ΔH\Delta H for the reaction C(s)+12O2(g)+H2(g)→CO(g)+H2O(l)C(s) + \frac{1}{2}O_2(g) + H_2(g) → CO(g) + H_2O(l)? a) −639 kJ-639\,\text{kJ} b) +353 kJ+353\,\text{kJ} c) −353 kJ-353\,\text{kJ} d) −156 kJ-156\,\text{kJ}
Intuitive explanation

This is a classic Hess's Law problem—you're combining given reactions like building blocks to construct a target reaction. The key insight is that enthalpy is a state function, so you can add, subtract, and scale the given equations to match your goal, then apply the same operations to their ΔH\Delta H values. Work backwards from your target: identify which reactants and products you need, then figure out which given reactions to keep, reverse, or halve to make them cancel correctly.

Intuitive explanation

This is a classic Hess's Law problem—you're combining given reactions like building blocks to construct a target reaction. The key insight is that enthalpy is a state function, so you can add, subtract, and scale the given equations to match your goal, then apply the same operations to their ΔH\Delta H values. Work backwards from your target: identify which reactants and products you need, then figure out which given reactions to keep, reverse, or halve to make them cancel correctly.

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