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Standard enthalpy of combustion of CH4 is -890 kJ mol^-1 and standard enthalpy of vaporisation of water is 40.5 kJ mol^-1. The enthalpy change of the reaction

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Standard enthalpy of combustion of CH4 is -890 kJ mol-1 and standard enthalpy of vaporisation of water is 40.5 kJ mol-1. The enthalpy change of the reaction

CH4(g) + 2O2(g) → CO2(g) + H2O(g)

(a) -809.5 kJ mol-1

(b) -890 kJ mol-1

(c) 809 kJ mol-1

(d) -971 kJ mol-1

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Correct answer: (a) -809.5 kJ mol-1

Explanation:

CH4(g) + 2O2(g) → CO2(g) + 2H2O(l)

ΔH = -890 kJ   .....(i)

2H2O(l) → 2H2O(g);

ΔH = 2 x 40.5 = 81 kJ   .....(ii)

From (i) + (ii), we get

CH4(g) + 2O2(g) → CO2(g) + 2H2O(l)

ΔH = -890 + 81 = -809 kJ

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