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At 25°C and 1 atm pressure, the enthalpies of combustion are given below: Substance: H2, C(graphite), C2H6(g) D_cH° (kJ mol^-1): -286.0, -394.0, -1560.0 The enthalpy of formation of ethane is
- +54.0 kJ mol^-1
- -68.0 kJ mol^-1
- -86.0 kJ mol^-1
- +97.0 kJ mol^-1
Correct answer: +97.0 kJ mol^-1
Solution
The enthalpy of formation of a compound can be calculated using the enthalpies of combustion of its elements and the compound itself. For ethane (C2H6), the enthalpy of formation is derived from the combustion values, leading to a positive value of +97.0 kJ mol^-1, indicating that forming ethane from its elements requires energy.
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