Question

Given $S_{C_2{H}_6}^{\circ }=225Jmo{l}^{-1}{K}^{-1},S_{C_2{H}_4}^{circ}=220Jmo{l}^{-1}{K}^{-1},S_{H_2}^{\circ }=130Jmo{l}^{-1}{K}^{-1}$
Then $△S^{\circ }$ for the process $C_2{H}_4+H_2\to C_2{H}_6$ is:

• A. $+25J$
• B. $-125J$
• C. $135J$
• D. $315J$

Answer 1

The correct option is B $-125J$

For the reaction:

$C_2{H}_4+H_2\to C_2{H}_6\Delta S^{\circ }=\sum S_{product}-\sum S_{reactant}$

Given:

$S_{C_2{H}_4}^{\circ }=220J/K/mol$, $S_{H_2}^{\circ }=130J/K/mol$, $S_{C_2{H}_6}^{\circ }=225J/K/mol$

Therefore on substitution we get

$\Delta S^{\circ }=225-(130+220)\Delta S^{\circ }=-125J{K}^{-1}mo{l}^{-1}$

Hence option B is the correct answer.