Calculating Equilibrium Concentrations

Fundamentals of Equilibrium

Chemical equilibrium refers to state of reaction where the rate of forward reaction equals rate of the reverse reaction.
This point is usually reached when the concentration of reactants and products are constant over time.
The equilibrium constant (Kc) is a number obtained from the values of concentrations of the products and reactants at equilibrium at a certain temperature.

Balancing the Reaction

Before calculating equilibrium concentrations, it’s crucial to balance the chemical reaction.
The balanced reaction helps you know how each substance interacts with the others.
This is particularly necessary when calculating the Ratio of Concentration Changes.

The Ratio of Concentration Changes

Keep in mind that you’re often provided with the initial concentrations and the change in concentration.
You apply the change in concentration to each molecule based on its stoichiometric coefficients (the number before each element/molecule in a balanced equation).
Subtract the change from initial reactant concentrations and add it to initial product concentrations, considering the stoichiometric coefficients.

Setting up ICE (Initial, Change, Equilibrium) Table

Depending on the problem, setting up an ICE table can simplify your calculations.
In this table, I stands for Initial concentration, C stands for Change in concentration, and E stands for Equilibrium concentration.
Remember to take into account the direction of change noted in the step above.

Using the Equilibrium Constant Expression

The equilibrium constant expression is based on the law of mass action.
It involves the products of the concentrations of the products raised to their stoichiometric coefficients divided by the product of the concentrations of the reactants raised to their stoichiometric coefficients.
This expression is set equal to the equilibrium constant, and you’ll usually need to solve for one of the equilibrium concentrations.

Solving for Concentrations

The equation obtained from the equilibrium constant expression can be solved for the unknown concentration.
Keep in mind, sometimes the quadratic formula may be required to solve for concentrations.

Le Chatelier’s Principle

Remember that according to Le Chatelier’s Principle, equilibrium will shift to counteract any changes in conditions (like concentration, pressure, or temperature).
This principle can help predict the direction of concentration change in reaction adjustment, serving as a valuable tool in equilibrium understanding.