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Heat of combustion of methane at constan...

Heat of combustion of methane at constant volume and 298 K has been found to be `-885.0 kJ mol^(-1).` Calculate the enthalpy of combustion under constant pressure. (`R=8.314 Jmol-K^(-1)`).

Text Solution

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The combustion of methane is
`CH _(4) (g) + 2 O _(2) (g) to CO _(2) (g) + 2 H _(2) O (l)`
Here `Delta n _(g) = 1 ` mol ( gasesous product) -3 mole (gaseous reactants) = - 2 mols.
Change in enthalpy `Delta H = Delta U + Delta n _(g) RT`
`Delta H =- 885000 J mol ^(-1) + (-2 mol) (8.314 J K ^(-1) mol ^(-1)) ( 298 K)`
`Delta H =- 885000 J mol ^(-1) - 4955 J mol ^(-1) =- 889955 J mol ^(-1) =- 889.955 kJ mol ^(-1)`
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