The heat evolved on combustion of 1 g of starch, (C 6 H 10 O 5 ) x , into CO 2 (g) and H 2 O(l) is 4.6 kcal. Heat of formation of CO 2 (g) and H 2 O(l) are -94.2 and -68.4 kcal/mol, respectively. The

Question

The heat evolved on combustion of 1 g of starch, (C 6 H 10 O 5 ) x , into CO 2 (g) and H 2 O(l) is 4.6 kcal. Heat of formation of CO 2 (g) and H 2 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

TestHub · Accepted Answer

Per C ₆ H ₁₀ O ₅ unit (162 g): Δ H comb = 162 × (-4.6) = -745.2 kcal The combustion reaction for C ₆ H ₁₀ O ₅ is: C 6 H 10 O 5 + 6O 2 tarrow 6CO 2 + 5H 2 O(l) The enthalpy of formation for the unit, Δ f H(unit), is calculated as: Δ f H(unit) = 6(-94.2) + 5(-68.4) - (-745.2) Δ f H(unit) = -565.2 - 342 + 745.2 Δ f H(unit) = -162 kcal/mol This corresponds to -162/162 = -1 kcal/g. The magnitude is 1 kcal/g.

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