The following enthalpy changes are known (kJ, per mole of P₄O₁₀ formed):
$P_{4} (white) + 5 O_{2} \to P_{4} O_{10}; Δ H = - 2984$
$4P (red) + 5 O_{2} \to P_{4} O_{10}; Δ H = - 2967$ How much heat is evolved when 12.4 g of white phosphorus changes to red phosphorus? (P = 31 g/mol)

Question

The following enthalpy changes are known (kJ, per mole of P₄O₁₀ formed):

$P_{4} (white) + 5 O_{2} \to P_{4} O_{10}; Δ H = - 2984$

$4P (red) + 5 O_{2} \to P_{4} O_{10}; Δ H = - 2967$ How much heat is evolved when 12.4 g of white phosphorus changes to red phosphorus? (P = 31 g/mol)

TestHub · Accepted Answer

Subtract the second equation from the first: P₄(white) → 4P(red).

ΔH = -2984 - (-2967) = -17 kJ per mole of P₄.

12.4 g of P₄ (124 g/mol) = 0.1 mol.

Heat evolved = 0.1 x 17 = 1.7 kJ.

Answer: (C).

Chemistry - Thermochemistry Question with Solution | TestHub

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