CHEM1312H - Le Chatelier's Principle

le chatelier’s principle

when a chemical system at equilibrium is disturbed, the system tends to shift in a direction that minimizes the disturbance

effect of addition or removal of a reactant or product

-look at example on slides

-the disturbance incurred in adding more product, the reaction is no longer at equilibrium because there is extra product

-which means that when the new equilibrium position is established, the concentration of the reactants will increase

-please look at example on slides

effect of pressure and volume changes on gas-phase equilibria

-gas-phase reactions are affected by changes by volume and pressure changes (ideal gas law: PV = nRT; boyle’s law:P ∝ 1/V)

-equilibria of gas-phase reactions can be disturbed in three ways

• add or remove a gaseous reactant or product (changes in the partial pressure of the gas)

• changing the volume of the container

• adding an inert gas (one that is not involved in the reaction)

effect of pressure and volume changes on gas-phase equilibria (boyle’s law)

-from the ideal gas law PV = nRT

-P = n/V (R and T are constant)

-increasing the pressure caused by a decrease in volume for a gas-phase reaction shifts the equilibrium position to the side with fewer moles of gas

change in volume

-a change in volume (like concentration) will change the value of Q

-if Q < K reaction shifts to the direction with fewer gas moles

effect of addition of an inert gas on the equilibrium position

-addition of an inert gas at constant volume increases the total pressure (P_tot) in the reaction vessel but has no effect on the concentrations or partial pressures of the reactants or products (the equilibrium position does not change)

-the number of moles per unit volume of reactants and products will decrease and the equilibrium will shift in the direction that will increase the number of moles of gases

-if the number of moles of gas particles on the reactant and product side are equal the, equilibrium position does not shift in either direction

effect of temperature changes on equilibrium composition

-a change in temperature changes the actual value of the equilibrium constant (K)

-when the temperature of a system is increased at equilibrium, the system reacts as if we added a reactant (endothermic reaction) or a product (exothermic reaction)

-the equilibrium will shift in the direction that consumed excess heat

effect of temperature changes on K

-increasing the temperature of an endothermic reaction (heat as a reactant) increases the value of K and vice versa

-increasing the temperature of an exothermic reaction (heat as a product) decreases the value of K and vice versa

-look at example on slides

effect of temperature (kinetic energy)

-consider K = k_f/k_r

-when an endothermic reaction is heated, the kinetic energy of the reactant particles becomes greater than E_a (forward) and the magnitude of the rate constant for the forward reaction increases more than the rate constant for the reverse reaction

-when an exothermic reaction is heated, the kinetic energy of the product particles becomes greater than E_a (reverse) and the magnitude of the rate constant for the reverse reaction increases compared to the rate constant for the forward reaction (look at images on slides)

effect of catalyst on equilibrium

-a catalyst lowers the activation energy for a process by taking an alternate mechanism increasing the rate of the reaction

-since equilibrium is a reversible process, a catalyst increases both the rate of the forward and the rate of the reverse and has no effect on the value of K

-a catalyst increases the rate at which equilibrium is achieved but does not change the composition of the equilibrium mixture