General Chemistry/Chemical Equilibria/Le Chatelier's Principle

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Contents 1 Le Chatelier's Principle 1.1 Concentration 1.2 Pressure/Volume 1.3 Temperature

[edit] Le Chatelier's Principle

Le Chatelier's Principle is a very important concept in chemical equilibrium. It states that a system will shift its equilibrium to oppose a change in the system.

There are several changes that can effect the equilibrium position of a system:

• Concentration
• Pressure/Volume
• Temperature

[edit] Concentration

If the concentrations in a system are changed, Le Chatelier's Principle predicts that the equilibrium position will shift to minimize the change.

For example, a system is at equilibrium, but then you add more reactants, thereby increasing their concentrations. They will react to form products, resulting in a decrease in the reactant concentrations. The system is now at a new equilibrium position, and the change you created has been minimized.

[edit] Pressure/Volume

If some the the substances in a system are gas, changing their partial pressure is the same as changing their concentrations.

If the volume is increased, the overall pressure decreases (and vice versa). Consider the reaction A + B → 3C. There are three moles of products for every two moles of reactants. The product side has higher pressure than the reactant side. So, if pressure is increased (or volume decreased), equilibrium will shift towards the lower pressure side. Products will be consumed (by the reverse reaction) and reactants will form.

If pressure is decreased (or volume increased), equilibrium will shift towards the higher pressure side.

If an inert gas (such as neon) is added to a system, it will raise the overall pressure. However, the partial pressures of all products and reactants are unchanged, so no shift in equilibrium occurs.

[edit] Temperature

Changes in concentration, pressure, and volume affect the equilibrium position, but the equilibrium constant K_eq is unchanged. These changes can be calculated using the equilibrium expression and known values of K_eq and concentrations.

Temperature, however, does change the value of K_eq. When given a value of K, you will also be given a temperature because K is dependent on the temperature.

An increase in temperature will favor the endothermic (heat-absorbing) side of a reaction. A decrease in temperature will favor the exothermic (heat-releasing) side of a reaction.

For example, the following reaction is very exothermic:

Because it is exothermic, you can think of it like this:

At room temperature, K_eq for this reaction is 3.2 x 10⁸¹. If this reaction happens at a very high temperature, what will happen? Less product will form (or maybe the reverse reaction will occur if the temperature is high enough) and K_eq will be smaller. What will happen if this reaction occurs at a very low temperature? More product will form and K_eq will be larger.