Question

What is the value of ${\Delta }_f{H}^{\circ }(C_2{H}_6,g)$ ?

Given:

${\Delta }_f{H}^{\circ }(C{H}_4,g)$ = $-66kJ/mol$; ${\Delta }_{sub}{H}^{\circ }(C,S)=718kJ/mol$

B.E.(C - C) = $345kJ/mol$, B.E.(H - H) = $436kJ/mol$

• A. -85 kJ/mol
• B. 633 kJ/mol
• C. -185 kJ/mol
• D. None of these

Answer 1

The correct option is A

-85 kJ/mol

Let B.E. of C - H is $x$

$C\left(s\right)+2H_2\left(g\right)\to C{H}_4\left(g\right)$

$-66=718+2\times 436-4\times x$,

B.E(C - H) = $414kJ/mol$

$2C\left(s\right)+3H_2\left(g\right)\to C_2{H}_6\left(g\right)$

${\Delta }_f{H}^{\circ }(C_2{H}_6,g)=2\times 718+3\times 436-345-6\times 414$

= $-85kJ/mol$