3.1.6 Le Chatelier's Principle

Le Chatelier’s Principle :

  • The position of equilibrium can be changed - If any factor is changed which affects equilibrium, the position of equilibrium will shift to oppose the change

Changes in concentration or amounts :

If the concentration of one of the substances at dynamic equilibrium is changed, the equilibrium will shift to oppose that change.

If Concentration is Increased :

  • The equilibrium will shift to the right

  • to oppose the addition of the [reactant]

  • so yield of [products] increases

If Concentration is Reduced :

  • The equilibrium will shift to the left

  • to oppose the reduction of the [reactant]

  • so yield of [reactants] increase

Changes in Pressure :

Pressure changes will only affect reactions involving gases.

If the pressure of one of the substances at dynamic equilibrium is changed, the equilibrium will shift to oppose that change.

Pressure Increased :

  • Fewer moles on the [Left/Right]

  • the equilibrium shifts to the [side with less moles] to oppose increase in pressure

  • so yield of [products/reactants] increases

Pressure Decreased :

  • More moles on the [Left/Right]

  • The equilibrium shifts to the [side with more moles] to oppose decrease in pressure

  • so yield of [Products/Reactants] increases

If there is an equal number of moles of products and reactants, then changing the pressure would not affect the position of equilibrium.

Volume :

  • Volume Increase = Pressure Decrease

  • Volume Decrease = Pressure Increase

Changes in Temperature :

If a reaction releases heat energy = exothermic and its enthalpy change is negative (-)

If a reaction absorbs heat energy = endothermic and its enthalpy change is positive (+)

Temperature Decrease :

  • Forward reaction is [Endothermic/Exothermic]

  • The equilibrium will shift to the [right/left] (whatever is opposite forward reaction/endo or exo) to oppose decrease in temperature

  • so yield of [substances on the side it shifts to]

Temperature Increase:

  • Backwards reaction is [Endo/Exo]

  • The equilibrium shifts to the [side with the hotter reaction] to oppose increase in temperature

  • So yield of [opposite side to hotter side increases]

Catalysts :

A catalyst is a substance that speeds up the rate of reaction by providing an alternative reaction pathway with a lower activation energy.

When a catalyst is added to a reversible reaction:

  • The rate of both forward and reverse reactions increases equally

  • there is no change to the position of equilibrium

  • dynamic equilibrium is reached faster

Haber Process :

Ammonia Yield requires for highest yield :

  • pressure of ~40,000 kPa

  • Temperature of 350 Celsius

Ammonia Yield of most industrial plants :

  • 200 atmospheres = pressure

  • 450 degrees Celsius = Temperature

Lower pressure and higher temperature = compromise conditions

  • higher pressure = too expensive

  • 450 Celsius = gives a good yield in a short amount of time

A catalyst is added in the Haber Process - doesn’t affect position of equilibrium but reduces the cost of the reaction as less fossil fuel is burned to generate heat energy.