The thermochemical equation for the dissociation of hydrogen gas into atoms may be written as: . Wha — Thermodynamics and Thermochemistry Chemistry Question
Question
The thermochemical equation for the dissociation of hydrogen gas into atoms may be written as: $\text{H}_2 \to 2\text{H}; \Delta H = 432.0\text{ kJ}$. What is the ratio of the energy yield on combustion of hydrogen atoms to steam to the yield on combustion of an equal mass of hydrogen molecules to steam? Heat of formation of steam is –240.0 kJ/mol.
💡 Solution & Explanation
Combustion of $\text{H}_2$ molecules: $\text{H}_2 + 0.5\text{O}_2 \to \text{H}_2\text{O}$ yields 240.0 kJ per mole. Combustion of equal mass of H atoms (2 moles of H): Since forming $2\text{H}$ from $\text{H}_2$ takes 432.0 kJ, the combustion $2\text{H} + 0.5\text{O}_2 \to \text{H}_2\text{O}$ yields the molecule's heat plus the atomization energy $= 240.0 + 432.0 = 672.0\text{ kJ}$. Ratio $= 672.0 / 240.0 = 2.80$.