EQUILIBRIUM

EQUILIBRIUM

1. 1. CHARACTERISTICS

   • Equilibrium is dynamic
     • Reactions don't stop but forward and backward reactions are still happening
   • It is achieved in a closed system
     • Where both reactants and products can react and recombine with each other
   • The concentrations of reactants and products stay constant in equilibrium
     • Produced and destroyed at an equal rate
   • No change at macroscopic properties
     • Those properties such as color or density do not change as they depend on the concentrations of the components of the mixture
   • It can be reached from either direction
     • The same equilibrium mixture will result under the same conditions, no matter wether the reaction is started with all reactants, products or a mixture of both of them.

2. The proportion of reactant and product in the equilibrium mixture is referred to as its EQUILIBRIUM POSITION. At equilibrium the rate of the forward reaction is equal to the rate of the backward reaction.

   • The equilibrium constant expression has the concentrations of products in the numerator and the concentrations of reactants in the denominator.
   • Each concentration is raised to the power of its coefficient in the balanced equation. (Where it is equal to one it does not have to be given).
   • Where there is more than one reactant or product the terms are multiplied together.

3. The magnitude of Kc gives information on the extent of the reaction.

   • Equilibrium mixture lies to the right and the reaction goes almost to completion.
   • A low value of Kc means that there are proportionally less products with respect to reactants, so the equilibrium mixture lies to the left and the reaction has barely taken place.
   • The magnitude of the equilibrium constant Kc gives information about how far a reaction goes at a particular temperature, but not about how fast it will achieve the equilibrium state.

4. The reaction quotient, Q, enables us to predict the direction of reaction.

   • 1. if Q=Kc, reaction is at equilibrium, no net reaction occurs. 2. if Q<Kc, reaction proceeds to the right in favour of products. 3. if Q>Kc, reaction proceeds to the left in favour of reactants.
     • When a system at equilibrium is subjected to a change, it will respond in such a way as to minimise the effect of the change.

5. When equilibrium is disrupted...

   • CHANGES IN CONCENTRATION
     • The equilibrium could be disrupted by a decrease in the concentration of product by removing it from the equilibrium mixture.
     • Suppose an equilibrium is disrupted by an increase in the concentration of one of the reactants. This will cause the rate of the forward reaction to increase while the backward reaction will not be affected, so the reaction rates will no longer be equal. When equilibrium re-establishes itself, the mixture will have new concentrations of all reactants and products, and the equilibrium will have shifted in favour of products. The value of Kc will be unchanged. This is in keeping with the prediction from Le Chatelier's principle: addition of reactant causes the system to respond by removing reactant — this favours the forward reaction and so shifts the equilibrium to the right.
   • CHANGES IN PRESSURE
     • Equilibria involving gases will be affected by a change in pressure if the reaction involves a change in the number of molecules. This is because there is a direct relationship between the number of gas molecules and the pressure exerted by a gas in a fixed volume. So if such a reaction at equilibrium is subject to an increase in pressure, the system responds to decrease this pressure by favouring the side with the smaller number of molecules. Conversely, a decrease in pressure will cause a shift in the equilibrium position to the side with the larger number of molecules of gas. A different equilibrium position will be achieved but the value of Kc, will be unchanged, so long as the temperature remains the same.
   • CHANGES IN TEMPERATURE
     • Kc is temperature dependent, so changing the temperature will change Kc. However, in order to predict how it will change we must examine the enthalpy changes of the forward and backward reactions. The enthalpy changes of the forward and backward reactions are equal and opposite to each other.
     • If we apply Le Chatelier's principle, including the energy change in the chemical reaction, we can predict how the reaction will respond to a change in temperature.

6. Equilibrium theory is applied in many industrial processes.

   • In reactions involving the manufacture of a chemical it is obviously a goal to obtain as high a conversion of reactant to product as possible. Application of Le Chatelier's principle enables chemists to choose conditions that will cause the equilibrium to lie to the right, and so help to achieve this. But the yield of a reaction is only part of the consideration. The rate is also clearly of great significance as it would be of limited value ifa process were able to claim a high equilibium yield of product, but took several years to achieve this.
   • The economics of the process will depend on considerations of both the equilibrium and the kinetics of the reaction —in other words on how far and how fast the reaction will proceed. Sometimes these two criteria work against each other, and so the best compromise must be reached. A few case studies of industrial processes are discussed here.

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