Question

In which of the following reaction $K_p = K_c$ ?

• A. $PCl_{3}+Cl_{2}\to PCl_{5}$
• B. $H_{2}+I_{2}\to2HI$
• C. $H _{2( g )}+ Cl _{2( g )} \rightleftharpoons 2 HCl _{( g )}$
• D. $N_{2}+3H_{2}\to2NH_{3}$

Answer 1

Answer: (C)

$
K _{ p }= K _{ c }( RT )^{ \Delta n }
$
If $\Delta n =0, K _{ p }= K _{ c }$
If $\Delta n >0$, then $K _{ p }> K _{ c }$
If $\Delta n <0$, then $K _{ p }< K _{ c }$
For the reaction: $H _{2}( g )+ Cl _{2}( g ) \rightleftharpoons 2 HCl _{( g )}$
$\Delta n$ is the number of moles of gaseous products - number of moles of gaseous reactants in a balanced equation.
$
\begin{array}{l}
\Delta n =2-(1+1)=0 \\
\therefore K _{ p }= K _{ c }
\end{array}
$