The equilibrium constant "K" gives the value at which both the products and reactants in a reaction have reached a certain rate of reaction -- when they are no longer changing. This is when the reaction has reached an equilibria, and the rates of the forward reaction (into products) and the reverse reaction (into reactants) is equal, though the concentrations of the products and reactants will most likely not be equal(just constant).
Calculating the Equilibrium Constant
The value of K has no units, and can be calculated when the concentrations of the reactants and products is known. Avogadro gives us the equation for which the equilibrium constant can be calculated from as: K= (The Concentration of the Products) ^ stoichiometric coefficients / (The Concentration of the Reactants) ^ stoichiometric coefficients.
With "^" meaning to the power of, and the stoichiometric coefficients being the numbers next to the concentrations in the balanced chemical equation( the stoichiometric equation) e.g., aA , where "a" is the number next to the reactant/product, and "A" is the reactant/product in the equation.
When all of the reactants are gaseous, partial pressure might be used in place of concentrations, but concentrations are much easier to work with. For the reaction involving partial pressures, the equation for finding out the equilibrium constant is the same, except you substitute the partial pressure in place of concentration.
When calculating the equilibrium constant, pure solids, solids and pure liquids are given a value of 1 (but not aqueous solutions!). This is because they are in large amounts, and so on equilibrium the changes in concentration are so small that they can be taken as being constants. (Avogadro)
Concentration, Pressure and the Equilibrium Constant
Cartage states Le Chatelier's Principle as being, "when a system in chemical equilibrium is disturbed by a change of temperature, pressure, or a concentration, the system shifts in equilibrium composition in a way that tends to counteract this change of variable."
When a change happens in temperature, concentration or pressure, the reaction would of course be affected. Le Chatelier's Principle explains how the equilibrium shifts in such a way to minimise the change, so to remain stable.
Although the equilibria itself is affected and changed by a change in concentration or pressure, the equilibria shifts in such a way so to keep the equilibrium constant, K the same as before. For a change in concentration or pressure, the equilibrium will change to restore the values of concentration( or pressure of the reactants) to the same ratio as was present before the change, so that K is unaffected. This makes sense, as both the concentration and pressure can be used in the equilibrium constant equation.
So, a change in concentration or pressure will have no affect overall on the value of K, the equilibrium constant.
Temperature and the Equilibrium Constant
Equilibrium Constant is said to be "constant at a constant temperature".(Avogadro) What this means is that the value of K depends on the temperature - in other words if temperature changes, the equilibrium constant will also change.
The general association between reactions and temperature is endothermic reactions and exothermic reactions ; that is a reaction can be classed as endothermic if it takes in heat, and so has a positive change in temperature over the course of a reaction ( the reactants are colder than the products), and exothermic if it gives out heat, so over the course of the reaction the change in temperature is negative as the heat energy is lost (the reactants are warmer than the products).
In an endothermic reaction, for the reaction to go ahead heat (energy) is needed from the surroundings. An increase in temperature will favour an endothermic reaction, and increase the product yield because of this. From this, it can be concluded that K, the equilibrium constant will increase if the temperature increases for an endothermic reaction, as the concentration of products / concentration of reactants ratio from the equation will increase.
In an exothermic reaction, where heat energy is given out to the surroundings, an increase in temperature will favour the reactants ( the reverse reaction), and so the product yield will decrease. From this, we can see that the equilibrium constant will decrease if the temperature increases for an exothermic reaction, as the concentration of products / concentration of reactants ratio from the equation will decrease.
Do Catalysts Affect the Equilibrium Constant?
Catalysts have no affect on the equilibrium constant, as although ithey increase the speed of a reaction and the rate at which equilibrium is reached, they have no affect on the position of equilibrium, so do not effect the ratio of products to reactants, and thus have no effect on the value of K.
Sources
- Avogadro, 02/05/2011.
- Le Chatelier's Principle, Cartage, 02/05/2011.
- Archie Gibb, David Hawley, BrightRed Advanced Higher Chemistry, pg 32, 33.
Hayleigh Hunter - Hayleigh describes herself as the eternal student - she loves to learn. Her articles aim to provide the best quality information ...
Join the Conversation