Calculate the standard enthalpy of combustion of methane, if the standard enthalpy of formation of methane, carbon dioxide and water are –74·8, –393·5 and –285·8 kJmol–1 respectively.

Q.17 Calculate the standard enthalpy of combustion of methane, if the standard enthalpy of formation of methane, carbon dioxide and water are –74·8, –393·5 and –285·8 kJmol^–1 respectively.

Doubtly Asked 29 Oct 2026 Board exam · March 2024 hsc 12th Save WhatsApp Report

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Answer: \(\Delta_cH^\circ = -890.3\) kJ mol\(^{-1}\)

Combustion: \(\mathrm{CH_4(g) + 2O_2(g) \rightarrow CO_2(g) + 2H_2O(l)}\)

\(\Delta_cH^\circ = \sum \Delta_fH^\circ(\text{products}) - \sum \Delta_fH^\circ(\text{reactants})\)

\(= [\Delta_fH^\circ(\mathrm{CO_2}) + 2\Delta_fH^\circ(\mathrm{H_2O})] - [\Delta_fH^\circ(\mathrm{CH_4}) + 2\Delta_fH^\circ(\mathrm{O_2})]\)

\(= [-393.5 + 2(-285.8)] - [-74.8 + 0]\)

\(= -965.1 + 74.8 = -890.3\) kJ mol\(^{-1}\)

Doubtly AI · 29 Oct 2026 AI-generated · not yet verified Report

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Calculate the standard enthalpy of combustion of methane, if the standard enthalpy of formation of methane, carbon dioxide and water are –74·8, –393·5 and –285·8 kJmol–1 respectively.

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