8.2: Discussion

Chemical equilibrium

Some chemical reactions will, after forming product reverse and reform reactants. These are reversible reactions. For example, the generation of ammonia is a reversible reaction.

N₂ (g)   +   3H₂(g)                                   2NH₃ (g) 

Reactants                   equilibrium arrow                Products

Once ammonia is formed, the reaction will reverse, re-forming nitrogen and hydrogen. Eventually, the rate of the formation of ammonia (the forward reaction) and the rate of formation of nitrogen and hydrogen gases (the reverse reaction) becomes equal. The balance of the rates of reaction is called chemical equilibrium. A special type of arrow, called an equilibrium arrow (shown in the equation), is used in this chemical equation to indicate that both the forward and reverse reactions can take place simultaneously. This does not mean that the concentrations of ammonia and of nitrogen and hydrogen are equal. It means there is no net change in the concentrations. In other words, the concentrations of products and reactants stay the same.

The Equilibrium Constant K

If we measure the concentrations of ammonia, nitrogen and hydrogen present, the ratio of products to reactants would be constant. This value is called the equilibrium constant, K, and it is a characteristic of equilibrium reactions at a given temperature. K is defined as

The brackets, [ ], mean “molar concentration of.” So, the equation states that the equilibrium constant, K, is equal to the molar concentration of the products divided by that of the reactants. If there is more than one reactant or product, the concentrations are multiplied together. For the generation of ammonia, the expression for K is shown below. The superscripts in the expression come from the coefficients in the balanced chemical equation.

Only substances with concentrations that change appear in an equilibrium expression. Solids (s) and pure liquids (l) do not appear in the equilibrium expression.

Effect of Concentration on Equilibrium – Le Chatelier’s Principle

Suppose that we had our reaction for the generation of ammonia sitting in a reaction vessel and that it was at equilibrium. What would happen if we decided to inject more nitrogen into the vessel?

According to the Le Chatelier’s principle, if we disturb an equilibrium-chemists refer to this as applying stress (upset) to the equilibrium-the rate of the forward or reverse reaction will change to offset the stress and regain equilibrium. Applying this principle, if we add more N₂ to our system (it appears on the reactant side of our equation), the rate of the forward reaction will increase, shifting the equilibrium to produce more products. This rate increase occurs because adding more N₂ molecules increases the chance that N₂ will collide with H₂ molecules, forming ammonia more rapidly.

As a contrast, suppose that we must remove one of the reactants, H₂. To regain equilibrium, the reverse reaction must be faster than the forward reaction, allowing the H₂ to be replenished. The equilibrium shifts to the left, forming more reactants and reestablishing the equilibrium between the two rates.

In general, we can think of equilibrium as a balancing act between forward and reverse reaction. If one side of the reaction gains a substance, the reaction shifts to the other side to regain its equilibrium. If one side of the reaction loses a substance, the reaction will shift towards that side to regain its equilibrium.