EquilibriaRevision Slides

The Idea of an Equilibrium• In a reversible reaction, there comes a point where the rate of

the forward and reverse reactions, and thus the concentration of reactants and products become constant. This point is known as the equilibrium. The mixture of the reactants and products is known as the equilibrium mixture.• Remember, at equilibrium, the reactions do not stop, they just

occur at a constant rate.• The conditions for an equilibrium are as follows:1. The system must be closed (thus, nothing is gained or lost)2. The equilibrium will remain the same with constant

concentrations, pressure and temperature3. Equilibrium is a dynamic process- it is reached when the rates

of two opposing reactions become constant

Le Châtelier’s Principle• When changing the conditions of an equilibrium, it is

important to remember:-If a system at equilibrium is disturbed, the equilibrium moves in the direction that tends to reduce the disturbance• So, in other words, the position of the equilibrium will shift to

oppose the change and return the conditions of the equilibrium back to its previous, normal state.• However, as the principle does not tell us how far the

equilibrium will be shifted, we cannot predict any quantifiable changes in the reactions.• Remember, when an equilibrium shifts, it shifts to the side

that has the products of the desired reaction.

If Temperature is Increased:

• If the system’s temperature is increased, the equilibrium will shift to favour the reaction that is endothermic. Let’s say, in this case, the forward reaction is endothermic. The equilibrium of the system will shift to oppose the change- the increased temperature- and thus will shift right. This is because the forward reaction’s rate will increase, so the heat energy that is added to the system will be used up, cooling the system down again, reducing the disturbance to the system.

If Temperature is Decreased

• If the system’s temperature is decreased, the equilibrium will shift to favour the reaction that is exothermic. Let’s say, in this case, the reverse reaction is exothermic. The equilibrium of the system will shift to oppose the change- the decreased temperature- and thus will shift left. This is because the reverse reaction’s rate will increase, so the heat energy that is taken from the system will be added back, heating the system up again, reducing the disturbance to the system.

If Pressure is Increased

• If the system’s pressure is increased, the equilibrium will shift to favour the reaction that produces the least moles of gas. Let’s say, in this case, the forward reaction produces the least moles of gas. The equilibrium of the system will shift to oppose the change- the increased pressure- and thus will shift right. This is because the forward reaction’s rate will increase, so the increased pressure will be reduced again by the fact there are less moles of gas in the system, reducing the disturbance to the system.

If Pressure is Decreased

• If the system’s pressure is decreased, the equilibrium will shift to favour the reaction that produces the most moles of gas. Let’s say, in this case, the reverse reaction produces the most moles of gas. The equilibrium of the system will shift to oppose the change- the decreased pressure- and thus will shift left. This is because the reverse reaction’s rate will increase, so the decreased pressure will be increased again by the fact there are more moles of gas in the system, reducing the disturbance to the system.

If the Concentration of Reactants is Increased• If the reactants’ concentration is increased, the equilibrium

will shift to favour the reaction that uses up the additional reactants, in this case it will always be the forward reaction. The equilibrium of the system will shift to oppose the change- the increased concentration- and thus will shift right. This is because the forward reaction’s rate will increase, so the additional reactants that are added to the system will be used up, reducing their concentration again, reducing the disturbance to the system.

If the Concentration of Products is Increased• If the products’ concentration is increased, the equilibrium

will shift to favour the reaction that uses up the additional products, in this case it will always be the reverse reaction. The equilibrium of the system will shift to oppose the change- the increased concentration- and thus will shift left. This is because the reverse reaction’s rate will increase, so the additional products that are added to the system will be used up, reducing their concentration again, reducing the disturbance to the system.

Catalysts

• Catalysts have no effect on the equilibrium, they just increase the rate of both reactions, decreasing the amount of time taken for the equilibrium to be reached.

Equilibria in Industry

• A number of industrial processes involve reversible reactions, such as the Haber Process’ use in the production of fertilisers and explosives. In these cases, the yield of the reaction is important and Le Châtelier’s Principle can be used to find the best conditions for a high yield. However, the yield is not the only consideration- other factors affecting the conditions used include cost of running a system with these conditions, the value of the desired product (and thus its cost-effectiveness) and also perhaps the setup costs. E.g. to produce a pressure in the Haber Process that would give a high ammonia yield would require an expensive pump, which would also require large amounts of money to run, reducing profits, so a lesser pressure is used.

The Equilibrium Constant, Kc

• The equilibrium constant gives the ratio of reactants’ concentration to products’ concentration. If Kc<1, then the concentration of reactants is greater. If Kc>1, then the concentration of products is greater. • Kc is given by this formula:

([Product 1]n of product 1) ([Product 2]n of product 2)([Reactant 1]n of reactant 1) ([Reactant 2]n of reactant 2)

• Square brackets represent concentration

• n is the amount of moles of that substance

• If all moles are equal, then the powers are not used

Kc=

The Equilibrium Constant-Calculating Kc

https://www.youtube.com/watch?v=JjvAD9xwz0Ihttps://www.youtube.com/watch?v=WA8dfmMwrrM

The Effect of Condition Changes on Kc

If the equilibrium shifts left, Kc decreases.

If the equilibrium shifts right, Kc increases.Therefore, you can determine whether Kc will

increase or decrease based on the direction in which the equilibrium has shifted.

Catalysts and Kc

• Catalysts, as they do not effect the position of an equilibrium, do not affect Kc’s value. This is because they have an equal effect on the reaction rates of both reactions, forward and reverse.