When does the "x is small" approximation break down and lose its usefulness? view available hint(s) when does the "x is small" approximation break down and lose its usefulness? when the value of k is small and the initial concentration of reactants is much larger than the initial concentration of products, very little new products will form and the change in concentration of the reactants will be small. when the initial concentrations of reactants and products are such that qc is significantly different from kc, then x is likely to be large and the approximation will break down. when x is less than 5% of the starting values of each of the products and reactants, the approximation will break down. when only reactants are present at the start of the reaction, a lot of products will have been formed by the time equilibrium is reached, so the change in concentration will not be negligible.