Hi, I was fretting over this concept of Kc...
Kc is the ratio of concentration of products/concentration of reactants, raised to the appropriate power according to the coefficients of the species in the reaction equation.
It is said that Kc is a constant for a particular temperature, but changes with temperature. The explanation given was that at an equilibrium (what Kc is about), the rate of the forward and backward reaction are the same, so equating the two gives Kf[a][e]=Kb[c][d], where Kf is rate constant of forward reaction, Kb is rate constant of backward reaction, and [a] and [e] are reactant concentrations and [c] and [d] are product concentrations. Since Kf and Kb are rate constants which are given by Aexp(-Ea/RT), then when you increase temperature it would affect the rate constants of the forward and backward reactions differently depending on whether they are exothermic or endothermic, hence it would change the value of Kc as well.
But I don't really get questions about change in concentration or pressure. Assuming there is an increase in the concentration of a reactant, the model answer we should give is "the equilibrium shifts to favour the forward reaction", but "Kc remains the same, and the rate of reactions remain the same". So what exactly does this "shifting to favour" mean? I thought it meant the forward reaction increases in rate temporarily, but then stops when Kc reaches the original again, but it also contradicts with the concept of rate of reactions remaining the same at the same temperature.
Thank you for clarifying!