The heat evolved on combustion of 1 g of starch, , into and is 4.6 kcal. Heat of formation of and ar — Thermodynamics and Thermochemistry Chemistry Question
Question
The heat evolved on combustion of 1 g of starch, $(\text{C}_6\text{H}_{10}\text{O}_5)_x$, into $\text{CO}_2\text{(g)}$ and $\text{H}_2\text{O(l)}$ is 4.6 kcal. Heat of formation of $\text{CO}_2\text{(g)}$ and $\text{H}_2\text{O(l)}$ are –94.2 and –68.4 kcal/mol, respectively. The magnitude of standard enthalpy of formation of 1 g of starch (in kcal) is
💡 Solution & Explanation
Molar mass of one starch unit $(\text{C}_6\text{H}_{10}\text{O}_5) = 162\text{ g/mol}$. Heat of combustion per unit = $162 \times (-4.6) = -745.2\text{ kcal/mol}$. Reaction: $\text{C}_6\text{H}_{10}\text{O}_5 + 6\text{O}_2 \to 6\text{CO}_2 + 5\text{H}_2\text{O}$. $\Delta H_c = 6(-94.2) + 5(-68.4) - \Delta H_f(\text{starch}) \implies -745.2 = -565.2 - 342.0 - \Delta H_f \implies \Delta H_f = -907.2 + 745.2 = -162.0\text{ kcal/mol}$ of unit. Per gram, $\Delta H_f = -162.0 / 162 = -1.0\text{ kcal/g}$. Magnitude is 1.