Determine the standard enthalpy of reaction: , using the given enthalpies under standard conditions: — Thermodynamics and Thermochemistry Chemistry Question
Question
Determine the standard enthalpy of reaction: $\text{C}_3\text{H}_8\text{(g)} + \text{H}_2\text{(g)} \to \text{C}_2\text{H}_6\text{(g)} + \text{CH}_4\text{(g)}$, using the given enthalpies under standard conditions:<br>Compound: $\text{H}_2\text{(g)}$, $\text{CH}_4\text{(g)}$, $\text{C}_2\text{H}_6\text{(g)}$, C (graphite)<br>$\Delta H^\circ$ (kJ/mol): –285.8, –890.0, –1560.0, –393.5<br>The standard enthalpy of formation of $\text{C}_3\text{H}_8\text{(g)}$ is –103.8 kJ/mol
💡 Solution & Explanation
The table provides standard enthalpies of combustion. Calculate $\Delta H_f$: $\Delta H_f(\text{CH}_4) = -393.5 + 2(-285.8) - (-890) = -75.1\text{ kJ/mol}$. $\Delta H_f(\text{C}_2\text{H}_6) = 2(-393.5) + 3(-285.8) - (-1560) = -84.4\text{ kJ/mol}$. For the main reaction, $\Delta H_{rxn} = [-84.4 + (-75.1)] - [-103.8 + 0] = -159.5 + 103.8 = -55.7\text{ kJ/mol}$.