Answer. The reactions which have very small value of Kc do not go to completion. These reactions attain equilibrium state on conversion of very less amount of reactants to products. At equilibrium state, almost all reactants are present but the amount of products formed is negligible. For example, decomposition of ammonia to nitrogen and hydrogen never goes to completion.
2NH 3(g) <====> N 2(g) + 3H2(g)Kc = 3.0 x 10-9