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  • The standard heat of formation  of ethane (in kJ/mol) , if the heat of combustion

The standard heat of formation (\Delta_{f}H^{^{\circ}}_{298} ) of ethane (in kJ/mol) , if the heat of combustion of ethane, hydrogen and graphite are -1560, -393.5 and -286 kJ/mol, respectively is __________
 

Answers (1)

best_answer

The reactions we have:

 

\mathrm{C_{2}H_{6}\: +\: \frac{7}{2}O_{2}\: \rightarrow \: 2CO_{2}\: +\: 3H_{2}O\quad \quad \quad \quad\quad\quad[\Delta H_{c}\: =\: -1560kJ/mol]}   ...................(i)

\mathrm{2C\: +\: O_{2}\: \rightarrow \: CO_{2}\quad \quad \quad \quad\quad\quad[\Delta H_{c}\: =\: --286 kJ/mol]}       ..........................(ii)

\mathrm{3H_{2}\: +\: \frac{1}{2}O_{2}\: \rightarrow \: H_{2}O\quad \quad \quad \quad\quad\quad[\Delta H_{c}\: =\: -393.5kJ/mol]}      ........................(iii)

Multiply eq(ii) by 2 and eq(iii) by 3.

Now, \DeltaHof = 2 x eq(ii) + 3eq(iii) - 1

Thus, the equaton is given as:

\mathrm{2C\: +\: 3H_{2}\: \rightarrow \: C_{2}H_{6}}

\mathrm{\Delta H^{o}_{f}\: =\: 2\, x\, (-286)\: +\: 3\, x\, (-393.5)\: -\: (-1560)}

\mathrm{\Delta H^{o}_{f}\: =\: -\: 572\: -\: 1180.5\: +\: 1560}

\mathrm{\Delta H^{o}_{f}\: =\: -\: 1752.5\: +\: 1560\: =\: -192.5\ kJ/mol}

Hence, the option number (1) is correct.

Posted by

Kuldeep Maurya

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