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2.3.4 Le Chatelier's principle

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Reactivity 2.3.4 - Le Châtelier’s principle enables the prediction of the qualitative effects of changes in concentration, temperature and pressure to a system at equilibrium

a system at equilibrium when subjected to a change will respond in such a way as to minimise the effect of the change

effect ofchange in position of equilibriumchange in value of
concentrationchangesno change
pressurechanges if gases are involvedno change
temperaturechangeschanges
catalystno changeno change
catalysts
  • catalysts increase the rate of chemical reaction
  • both rates (forward and reverse) of reaction are increased equally
  • have no effect on the position of the equilibrium
concentration
changeequilibrium shift
reactant increaseright, less reactant
reactant decreaseleft, more reactant
product increaseleft, less product
product decreaseright, more product

if all other conditions stay the same, the equilibrium constant is not affected by any changes in concentration of the reactants or products

pressure
changeequilibrium shift
increaseless gas (direction with smaller number of molecules of gas)
decreasemore gas (direction with larger number of molecules of gas)

if all other conditions stay the same, the equilibrium constant is not affected by any changes in the pressure of the reactants and products

if there are the same number of gas molecules on either side of the reaction, changes in pressure will not change the position of the equilibrium

temperature
changeequilibrium shift
increaseto endothermic direction
decreaseto exothermic direction

TEMPERATURE CAUSES THE VALUE OF THE EQUILIBRIUM CONSTANT TO CHANGE

changes in temperature have a different effect on the rates of the forward and backward reactions due to their different activation energies.

industrial processes
  • when conditions of industrial processes are chosen, le chatelier’s principle can be used to predict the conditions that would cause the equilibrium to lie towards the products, giving a high equilibrium yield
  • the kinematics of the reaction must also be considered as the rate of reaction needs to be sufficiently fast

for example: a reversible reaction whose forward reaction is exothermic

  • according to le chatelier’s principle, lower temperatures would produce a higher equilibrium yield
  • however, higher temperatures would give a faster rate of reaction
  • a compromise temperature is used which gives a lower yield, but is made more quickly
  • a catalyst can be added to speed up rate of production

recycling of unconverted reactants is commonly used in industrial processes, and allows processes with low equilibrium yield to be made commercially viable

heterogeneous equilibria

le chatelier’s principle can also be applied to heterogeneous equilibria

for example, in a carbonated soft drink, an equilibrium exists between the dissolved and the gaseous

when the bottle is opened, some escapes, so the equilibrium shifts to the left to reduce the effect of the change. thus, bubbles of are observed.

challenge questions
  1. use information from this section to explain why there is very little in the atmosphere under ordinary conditions, and why severe air pollution is often characterised by a brownish haze

INFO

the reaction takes place in motor vehicles where the heat released by combustion of the fuel is sufficient to cause the nitrogen and oxygen gases from the air to combine. unfortunately, the product, , is toxic, and, worse still, quickly becomes converted into other toxins that form the components of acid rain and smog. it is therefore of great interest to vehicle manufacturers to find ways of lowering the temperature during combustion in order to reduce the production of in the reaction above.

the value for at for the reaction is extremely low, so the equilibrium mixture lies to the left with almost no production of . however, at higher temperatures, such as in vehicle exhaust fumes, the reaction shifts to the right and a higher concentration of is produced. this gas is easily oxidised in the air, producing the brown gas which is responsible for the brownish haze

  1. consider the atom economy of the Haber process described here using the formula:
explain how the atom economy is different from the reaction yield

the atom economy is 100%, since there is only 1 product. the reaction yield refers to the amount of product obtained for the amount of reactant used. since this is a reversible reaction, the reaction yield will be lower than 100%

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