In chemical kinetics, it is stated that a forward reaction’s rate is dependent on the given concentration of the reactants. In other words, the relationship of the rate of a reaction is directly proportional to the concentration. While the reaction is taking place, the concentration of the reactants are decreased, as these reactants are formed into products. During the decrease of the reactants’ concentration, the rate of the forward reaction also decreases. As more and more products are being formed, they start to reform to their constituent reactants again. This causes the reverse reaction rate to increase this time.
This process continues to happen until the rate of the forward reaction and the backward reaction becomes equal. When this happens, chemical equilibrium is achieved by the reaction system. Whenever equilibrium is achieved, at any given time, both reactants and products will be present at any given point in time since their concentrations remain constant. Given a reaction, aA + bB ? cC + dD kf[A]a[B]b = kr[C]c[D]d kfkr = Keq = [C]c[D]d (1) [A]a[B]b A factor in determining whether to which direction a reaction will go to that has not yet reached equilibrium, is the reaction quotient Q.
Q is just the same as the Keq expression, but the main difference is that the concentrations of the reactants and the products used in the equation are still not yet at equilibrium. Whenever Q<Keq, the reaction will favor the forward reaction, but when Q>Keq, the reaction will favor the reverse reaction. Go or Gibb’s Free Energy that indicates a spontaneity of a reaction, and Keq are related through the general thermodynamic equation where both gaseous and solution forms appear in the chemical equation: Go = -RTlnK (2)
Whenever K is greater than 1, the forward reaction is spontaneous, meaning, the amount of products is greater than the amount of reactants at equilibrium. For this, Go<0. On the other hand, whenever Keq is less than 1, the reverse reaction is spontaneous, and the amount of reactants is great than the products at equilibrium. For this, Go>0. While the relationship of Go and Keq tells us about the spontaneity of a system and to which direction it will go to, Le Chatelier’s Principle tells us what would happen to a system at equilibrium whenever a stress is applied onto it.
The kinds of stresses that could be applied are: change in temperature, change in volume or pressure (applicable in gases only), and change in concentration of either the reactants or products. As said above, one of the stresses that could be applied to a system is changing the concentration of the reactants or the products. From the equilibrium expression, whenever concentration of the product is increased, it will favor the reverse reaction. The converse holds when the concentrations of the reactants are increased.