5.2 a: Le Chatelier's Principle & Changes to Equilibrium

Le Chatelier's Principle & Changes to Equilibrium

Y12 Chem

Syllabus dot points

• investigate the effects of temperature, concentration, volume and/or pressure on a system at equilibrium and explain how Le Chatelier’s principle can be used to predict such effects, for example:

– heating cobalt(II) chloride hydrate

– interaction between nitrogen dioxide and dinitrogen tetroxide

– iron(III) thiocyanate and varying concentration of ions

Open Ended

Do you remember what Le Chatelier's Principle is? Give your best definition. 

​Le Chatelier's Principle

• if conditions in a system change, the system will no longer be at equilibrium

• ​equilibrium can be established if the forward or reverse reaction proceeds at a faster rate

• ​'if a system is subjected to a change in conditions, then the system will counteract this change'

• ​but which way will it 'shift'?

​Shifting Reactions

• ​as a change is being counteracted, the forward or reverse reaction will be favoured

• the reaction will shift to the left or the right (reactants or products) to demonstrate what is being favoured

• ​if the forward reaction is favoured then equilibrium has shifted right, if reverse is favoured then it has shifted left

• ​'shifting' is just describing what will be formed more

• ​whatever direction it is shifting, that's what will be formed more

Multiple Choice

Le Chaltelier's Principle states that if a chemical system at equilibrium is stressed,

• the system will adjust to increase the stress
• the system will adjust to reduce the stress
• the system will not adjust

Multiple Choice

A forward reaction is exothermic (warmer).

This means the reverse reaction is endothermic (cooler).

If you INCREASE temperature the reaction will want to do the opposite to resist the change.

Will equilibrium shift reverse (left) or forwards (right)?

• Forward/Right (its going to make more product)

• Reverse/Left (its going to make more reactant)

Multiple Choice

A forward reaction is endothermic (cooler).

This means the reverse reaction is exothermic (warmer).

If you INCREASE temperature the reaction will want to do the opposite to resist the change.

Will equilibrium shift reverse (left) or forwards (right)?

• Forward/Right (its going to make more product)

• Reverse/Left (its going to make more reactant)

​What changes equilibrium?

• ​temperature

• ​concentration

• ​partial pressure

• ​volume

• ​addition of a catalyst

​Concentration

• ​if concentration of a reactant is increased, the system will want to counteract this and will want to use it up - the forward reaction will be favoured and more product will be formed

  • ​equilibrium has shifted right

  • ​same as if a product concentration is decreased

• ​concentration will remain different, but equilibrium will be restored

• ​more iodine is added into the system

• ​rate of forward reaction increases - product concentration increases, whilst reactant concentration decreases

• ​rate of reverse reaction also then increases, until equilibrium is again established

​Partial pressure = part of the total pressure of a gas mixture exerted by a single gas molecule

​

​As partial pressure changes, so does the concentration of that particular particle

​Volume and Pressure

• ​changes to V and P will change the concentrations

• ​E.g. if the V is increased, the P is decreased, and the concentration is decreased

  • ​reaction that will be favoured is the one that contains more gas molecules (to increase the P)

• ​same applies with increasing or decreasing of water in a solution

  • ​increased water = decreased concentration of aq substances

  • ​reaction that will be favoured is the one with more aq molecules

​Temperature

• ​no sudden changes, but a gradual changes as equilibrium is being re-established

• ​e.g. an exothermic reaction releases heat with the products. If the temp of the system is increased the system will favour the reaction that absorbs the energy

  • ​the reverse reaction, so there will be a shift left

​Catalysts

• ​does not effect equilibrium

• ​affects how quickly equilibrium is achieved

• ​decreases the activation energy of forward and reverse reactions

Multiple Choice

What happens to the equilibrium if we add methane (CH₄)?

• It shifts left, towards the reactants

• It shifts right, towards the products

• It does not shift

Multiple Choice

For the reaction...
SO_2(g) + O_2(g) <−>  SO_3(g)
If the concentration of SO_2(g)  is increased, the equilibrium of the reaction will ___________.

• shift to the left
• shift to the right
• not shift

Multiple Choice

For the reaction...
N_{2 (g)} +  3 H_2 (g) <−> 2 NH_{3 (g)}

If the pressure in the system is increased, the reaction will __________________.

•  shift to the left
• shift to the right
• not shift

Multiple Choice

An exothermic reaction is allowed to reach equilibrium. If heat energy is then removed, the equilibrium will shift

• toward the middle
• toward the reactant side
• toward the product side