Question

Identify the equilibrium in which $K_c$ has no units.

• A. Decomposition of $HI$
• B. Decomposition of $P{Cl}_5$
• C. Decomposition of ${NH}_3$
• D. Decomposition of $Ca{CO}_3$

Answer 1

Answer: (A)

Decomposition of $HI$

The units of Equilibrium constant $K_c$ will depend on the number of moles of reactants and products. When the total number of moles of products is equal to the total number of moles of reactants, then $K_c$ has no units.

In the question the reactions are expressed as:

Decomposition of $HI$,

$2HI\rightleftharpoons H_2+I_2$

Decomposition of $P{Cl}_5$,

$P{Cl}_5\rightleftharpoons P{Cl}_3+{Cl}_2$

Decomposition of $N{H}_3$,

$2N{H}_3\rightleftharpoons N_2+3H_2$

Decomposition of $Ca{CO}_3$,

$Ca{CO}_3\rightleftharpoons CaO+C{O}_2$

In the above reactions, only in the reaction of decomposition of $HI$, moles of reactants and product are the same.

Hence Equilibrium constant $K$ has no units.