Dynamic equilibrium is when the rate of the forward and reverse reactions are equal. The reaction will appear to have stopped, as the concentrations of each species won’t change, but the reaction continues to proceed in both directions.
Equilibrium constant expressions are the ratio of the concentrations of products to reactants using their coefficients from the balanced equation as exponents. Pure solids and liquids are not included in the equilibrium constant expression. Equilibrium concentrations are plugged into the expression to solve for the equilibrium constant. A large equilibrium constant means that the reaction “lies to the right” with more products than reactants at equilibrium. A small constant means that the reaction “lies to the left” with more reactants the products present at equilibrium.
The reaction quotient is the value when concentrations at any time are plugged into the equilibrium constant expression. If the reaction quotient equals the equilibrium constant, then the system is at equilibrium. If the reaction quotient is too large, the reaction will proceed to the left to produce more reactants (and reduce products). The opposite will happen if the reaction quotient is too small.
Solving equilibrium problems
A simple technique for solving equilibrium problems is the ICE chart method:
- Make a table with the reactants and products across the top
- Place “ICE” down the left hand side, for Initial, Change and Equilibrium.
- Fill in any given information from the problem.
- Use the balanced equation’s stoichiometric ratio to determine the “change” row.
To solve the problem using the ICE chart:
- If an equilibrium concentration is known, you can determine the “change” and find the other equilibrium concentrations to plug in and solve for K
- If you don’t know any equilibrium concentrations, write expressions for them and plug in the expressions into your “K” equation to solve for them.
- If the K is very tiny (10-5 or smaller), you may approximate that the change is insignificant compared to the original value (if the original value is > 0) e.g.: 0.25 M – x » 0.25 M
The solubility equilibrium constant is written for a dissolution reaction (a solid compound dissociates when dissolved in water). The pure solid reactant is not included in the expression. A saturated solution is one that is at equilibrium.
Le Chatelier’s Principle
Le Chatelier’s principle says that if a system at equilibrium is stressed or changed, the system will shift to reach equilibrium again. Whatever you do to the system, it will un-do it. For example, if you add more reactants, the reaction will proceed to the right to get rid of the extra reactants and make more products.