# 13: Chemical Equilibrium

• 13.0: Prelude to Equilibria
The term chemical equilibrium is used to describe a chemical reaction in which the concentrations of the substances involved remain constant. Read on to learn how chemical equilibrium is defined.
• 13.1: The Equilibrium State
The term chemical equilibrium is used to describe a chemical reaction in which the concentrations of the substances involved remain constant. Read on to learn how chemical equilibrium is defined.
• 13.2: The Equilibrium Constant
The equilibrium constant represents the constant ratio between reactants and products when a reaction has reached equilibrium. Read on to find out more about how this ratio is calculated.
• 13.3: The Law of Chemical Equilibrium
Chemical equilibrium is attained when the concentration of reactants and products stops changing. Since reactants and products eventually reach a constant value (given constant temperature and pressure) a ratio called the rate constant can be used to describe the equilibrium. This section unpacks this ratio and how it is calculated.
• 13.4: The Equilibrium Constant in Terms of Pressure
In its most familiar form, the equilibrium constant is described in terms of the concentration of products and reactants. However, for gases it is often more convenient to relate the equilibrium constant in terms of pressure. Read on to find out more about expressing the equilibrium constant in terms of pressure.
• 13.5: Calculating the Extent of a Reaction
In chemistry, it's often convenient to predict what the outcome of a reaction will be in numerical terms. This section teaches you how to calculate the extent of a reaction - how much product will be formed.
• 13.6: Successive Approximation
An approximation is often useful even when it is not a very good one, because we can use the initial inaccurate approximation to calculate a better one. With practice, using this method of successive approximations is much faster than using the quadratic formula. It also has the advantage of being self-checking.
• 13.7: Predicting the Direction of a Reaction
Often you will know the concentrations of reactants and products for a particular reaction and want to know whether the system is at equilibrium. If it is not, it is useful to predict how those concentrations will change as the reaction approaches equilibrium. A useful tool for making such predictions is the reaction quotient, Q. Q has the same mathematical form as the equilibrium-constant expression, but Q is a ratio of the actual concentrations (not the equilibrium concentrations).
• 13.8: Le Chatelier’s Principle
Le Chatelier’s principle states that if a system is in equilibrium and some factor in the equilibrium conditions is altered, then the system will (if possible) adjust to a new equilibrium state so as to counteract this alteration to some degree.
• 13.9: The Effect of a Change in Pressure
In general, whenever a gaseous equilibrium involves a change in the number of molecules (Δn ≠ 0), increasing the pressure by reducing the volume will shift the equilibrium in the direction of fewer molecules. This applies even if pure liquids or solids are involved in the reaction.
• 13.10: The Effect of a Change in Temperature
Similar to a change in volume, a change in temperature forces a reaction to change in order to offset it's effect.
• 13.11: Effect of Adding a Reactant or Product
Just as varying temperature or volume can affect equilibrium, so can adding/subtracting a reaction/product. Read on to learn the specifics.
• 13.12: The Molecular View of Equilibrium
Chemical equilibrium can seem to be an unchanging phenomenon from a macroscopic perspective. Diving into the microscopic perspective, we find a different story. Read on to find out more.