Question

The enthalpy of formation of ethane at $25^{\circ} C ,$ if the enthalpies of combustion of carbon, hydrogen and ethane are $94.14,68.47$ and $373.3\, kcal$ respectively is

• A. $20.39\, kcal$
• B. $210.69\, kcal$
• C. $-210.69\, kcal$
• D. $-20.39\, kcal$

Answer 1

Answer: (D)

(Heat of combustion is always negative.)

The reaction shows the formation of ethane from its elemental constituent.

$\Delta H_{\text {reaction }}=2(-94.14)+3(-68.47)-(-373.3)=-20.39\, kcal$