Equilibria

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Equilibria

All reversible reactions reach a dynamic equilibrium state in which the forward and backward reactions are occurring at equal rates and the concentrations of reactants and products stays constant.

The expression "Position of Equilibria" is often used to describe the composition of the equilibrium mixture. If the equilibrium "lies to the left" it means that the equilibrium mixture will contain mostly reactants.

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La Chatelier's Principle

Le Chatelier’s principle states that if an external

condition is changed the equilibrium will shift to

oppose the change.

Effect of Temperature

If temperature is increased the equilibrium will

shift to oppose this and move in the

endothermic direction to try and reduce the

temperature by absorbing heat. (and vice versa)

Low temperatures may give a higher yield of product but will also result in slow

rates of reaction. Often a compromise temperature is used that gives a

reasonable yield and rate

Effect of Pressure

Increasing pressure will cause the equilibrium to shift

towards the side with fewer moles of gas to oppose the change and therefore reduce the pressure. (and vice versa - if the number of gas moles are equal, pressure will not affect equilibrium)

Increasing pressure may not always give a higher yield of product but they will produce a faster

rate. Industrially high pressures are expensive to produce ( high electrical

energy costs for pumping the gases to make a high pressure) and the

equipment is expensive (to contain the high pressures)

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Effect of a Catalyst

A catalyst has no effect on the position of equilibrium, but it will speed up the rate at which the equilibrium is achieved. It does not effect the position of equilibrium because it speeds up the rates of the forward and

backward reactions by the same amount.

In Industry

In all cases catalysts speeds up the rate allowing lower temp to be used (and hence lower energy costs)

but have no effect on equilibrium. Also, in

all cases high pressure leads to too high energy costs for pumps to produce the pressure and

too high equipment costs to have equipment that can withstand high pressures. Recycling unreacted reactants back into the reactor can improve the overall yields of all these processes

Both methanol and ethanol can be used as fuels

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Equilibrium Constant Kc

To work out the equilibrium constant from the equation mA + nB <--> pC + qD, where [ ] = concentration.

To work out the units, replace A, B, C and D with mol dm-3 and follow indices rules.

Kc only changes with temperature.

It does not change if pressure or

concentration is altered. A catalyst also has no effect on Kc.

Both the position of equilibrium

and the value of Kc will change it

temperature is altered. If the amount of products increase, Kc will increase and vice versa.

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Moles at Equilibrium

To work out the number of moles at equilibrium, look at the moles you started out with and how many have been added/removed. If moles of products have increased, then moles of reactants have decreased and vice versa. The amount by which they increase/decrease is proportional to the molar ratio given by the equation.