Chemical Equilibria

• Reversible reactions involve reactants forming products, which can then react to reform the original reactants, represented by a double arrow
• In reversible reactions, one reaction is endothermic while the opposite reaction is exothermic
• Dynamic equilibrium is reached when the forward and backward rate of reaction is the same and remains constant, with constant concentrations of reactants and products
• Dynamic equilibrium can only occur in closed systems where no substances can enter or exit
• Le Chatelier’s Principle predicts how an equilibrium mixture will change if reaction conditions are altered
• When a system is subject to change, it will alter to lessen the effect of that change
• Increasing temperature favors the endothermic reaction, while decreasing temperature favors the exothermic reaction
• In exam questions, the ∆H of the forward reaction will be given to indicate the direction of endothermic and exothermic reactions
• Increasing pressure favors the side of the reaction with fewer moles, while decreasing pressure favors the side with more moles
• Increasing the concentration of reactants favors the production of products, while decreasing the concentration of products favors the reformation of reactants
• Catalysts affect the forward and backward reactions equally, allowing equilibrium to be reached faster
• Equilibrium constant (Kc) indicates the position of equilibrium for a reaction at a certain temperature
• Kc for a reaction has the same value unless the reaction conditions and position of equilibrium are changed
• To find the value of Kc, the concentrations of substances at equilibrium need to be known, calculated as the concentration of products divided by the concentration of reactants
• The equilibrium constant has varying units depending on the chemical reaction
• Kc is not affected by concentration change or the use of a catalyst, but it is affected by changing the reaction temperature