Question

The enthalpies of combustion of methane and ethane are –890.3 and –1559.7 kJ mol^–1 respectively. Which of the two has a greater efficiency of the fuel per gram?

Solution

The fuel efficiency can be predicted from the amount of heat evolved for every gram of fuel consumed.
(i) Thermochemical equation for the combustion of methane is,
$CH subscript 4 left parenthesis straight g right parenthesis space plus space 2 straight O subscript 2 left parenthesis straight g right parenthesis space rightwards arrow space space CO subscript 2 left parenthesis straight g right parenthesis space plus space 2 straight H subscript 2 straight O left parenthesis straight l right parenthesis semicolon space increment subscript straight r straight H to the power of 0 space equals space 890.3 space kJ space mol to the power of negative 1 end exponent$
Gram molecular mass of methane $open parentheses CH subscript 4 close parentheses$
= 12 + 4 X 1 = 16 g
$therefore space space Heat space produced space per space gram space equals space fraction numerator 890.3 over denominator 16 end fraction equals space 55.64 space kJ$
(ii) Thermochemical equation for combustion of ethane is
$straight C subscript 2 straight H subscript 6 left parenthesis straight g right parenthesis space plus space 7 over 2 straight O subscript 2 left parenthesis straight g right parenthesis space rightwards arrow space space 2 CO subscript 2 left parenthesis straight g right parenthesis space plus space 3 straight H subscript 2 straight O left parenthesis straight l right parenthesis semicolon space increment subscript straight r straight H to the power of 0 space equals space minus 1559.7 space kJ space mol to the power of negative 1 end exponent$
Gram molecular mass of ethane $open parentheses straight C subscript 2 straight H subscript 6 close parentheses$
= 2 x 12 + 6 x 1 = 30 g
$therefore space space Heat space produced space per space gram space equals space fraction numerator 1559.7 over denominator 30 end fraction space equals space 51.99 space kJ$
Thus, methane has greater fuel efficiency than ethane.

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Thermodynamics

The enthalpies of combustion of methane and ethane are –890.3 and –1559.7 kJ mol^–1 respectively. Which of the two has a greater efficiency of the fuel per gram?

Some More Questions From Thermodynamics Chapter

Define intensive properties.

Define extensive properties.

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Define state variables of a system.

From thermodynamic point of view, to which system the animals and plants belong?

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Define irreversible change.

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For Daily Free Study Material Join wiredfaculty

Thermodynamics

The enthalpies of combustion of methane and ethane are –890.3 and –1559.7 kJ mol–1 respectively. Which of the two has a greater efficiency of the fuel per gram?

Some More Questions From Thermodynamics Chapter

Define intensive properties.

Define extensive properties.

Why all living systems need to be 'open systems'?

Define state variables of a system.

From thermodynamic point of view, to which system the animals and plants belong?

What is a state function?

A thermodynamic state function is a quantity

What is reversible process in thermodynamics?

Define irreversible change.

Name the variable that is kept constant in isochoric process and isobaric process.

Mock Test Series

NCERT Sample Papers

Entrance Exams Preparation

The enthalpies of combustion of methane and ethane are –890.3 and –1559.7 kJ mol^–1 respectively. Which of the two has a greater efficiency of the fuel per gram?