S-Cool Revision Summary

S-Cool Revision Summary

A reversible reaction is one where there is a forward and backwards reaction occurring:

aA + bB cC + dD

The double arrow signifies a reversible reaction.

If in the above reactions the concentrations of the reactants and products does not change, although the reaction is still in progress, then the forward rate must equal the backward rate.

A situation known as dynamic equilibrium is achieved.

Any dynamic equilibrium can be described in terms of its equilibrium constant, Kc.

The equilibrium constant is the product of the molar concentrations of the products raised to the power of the coefficient in the stoichiometric equation, divided by the product of molar concentrations of the reactants, each raised to the power of its coefficient in the stoichiometric equation.

For the above reaction,

[ ] represents concentration of species.

Le Chatelier's principle states that the position of the equilibrium of a system changes to minimise the effect of any imposed change in conditions.

Changing concentration, does not change the numerical value of the equilibrium, but does change its position. In general equilibrium shifts to the right if concentration of reactant is increased.

Changing pressure, for reactions involving gases may cause a change in the position of the equilibrium, but the constant remains the same. For a reaction where there is an increase in the number of moles from reactants to products, increasing the pressure moves the equilibrium to the left.

Changing temperature, does shift the equilibrium. For an exothermic reaction, the increase in temperature causes the equilibrium to shift to the left, since the opposing endothermic reaction will tend to reduce the temperature.

A catalyst has no effect on the position of the equilibrium, but does increase the rate of the forward and backward reactions.