Section 1 : Physical Chemistry
Chemical equilibria, Le Chatelier’s principle and Kc
Chemical Equilibria and Le Chatelier’s Principle
Many chemical reactions are reversible, + can be represented by ⇌
In a reversible reaction at dynamic equilibrium:
• forward + reverse reactions proceed at equal rates and…
• the concentrations of reactants + products remain constant.
A dynamic equilibrium can only happen in a closed system which is at constant temp.
Le Chatelier’s Principle
✴ If a factor is changed which affects a system in equilibrium, the position of equilibrium will
shift in a direction to oppose the change.
Changes in Pressure
• An ↑ in pressure at constant temp. shifts the position of equilibrium to the side w/ fewer moles of gas
to oppose change + ∴ ↓ pressure.
• A ↓ in pressure at constant temp. shifts the position of equilibrium to the side w/ more moles of gas to
oppose change + ∴ ↑ pressure.
E.g. for the following reaction: 2SO2(g) + O2(g) ⇌ 2SO3(g)
• An ↑ in pressure will shift the position of equilibrium from left to right (as there are fewer moles of
gas molecules on the right) in order to oppose change, resulting in an ↑ in SO3 production + a ↓ in
pressure.
• A ↓ in pressure will shift the position of equilibrium from right to left (as there are more moles of gas
molecules on the left) in order to oppose change, resulting in an ↑ in SO2 and O2 production and an ↑
in pressure.
Changes in Temperature
For a reaction in equilibrium at constant pressure where the forward reaction is exothermic…
• an ↑ in temp. would shift the position of equilibrium in the direction of the reverse endothermic
reaction + the position of equilibrium would move from right to left in order to oppose change + ∴ ↓
temp. by absorbing heat.
• a ↓ in temp. would shift the position of equilibrium in the direction of the forward exothermic
reaction + the position of equilibrium would move from left to right in order to oppose change + ∴ ↑
temp. by giving out heat.
For a reaction in equilibrium at constant pressure where the forward reaction is endothermic…
• an ↑ in temp. would shift the position of equilibrium in the direction of the forward endothermic
reaction + the position of equilibrium would move from left to right in order to oppose change + ∴ ↓
temp. by absorbing heat.
• a ↓ in temp. would shift the position of equilibrium in the direction of the reverse exothermic
reaction + the position of equilibrium would move from right to left in order to oppose change + ∴ ↑
temp. by giving out heat.
E.g. for the following reaction: 2SO2(g) + O2(g) ⇌ 2SO3(g) ∆H = -197kJmol-1
• An ↑ in temp.will shift the position of equilibrium from right to left (the endothermic direction) to
absorb the extra heat in order to oppose change.
• A ↓ in temp. will shift the position of equilibrium from left to right (the exothermic direction) to give
out more heat in order to oppose change.
Chemical equilibria, Le Chatelier’s principle and Kc
Chemical Equilibria and Le Chatelier’s Principle
Many chemical reactions are reversible, + can be represented by ⇌
In a reversible reaction at dynamic equilibrium:
• forward + reverse reactions proceed at equal rates and…
• the concentrations of reactants + products remain constant.
A dynamic equilibrium can only happen in a closed system which is at constant temp.
Le Chatelier’s Principle
✴ If a factor is changed which affects a system in equilibrium, the position of equilibrium will
shift in a direction to oppose the change.
Changes in Pressure
• An ↑ in pressure at constant temp. shifts the position of equilibrium to the side w/ fewer moles of gas
to oppose change + ∴ ↓ pressure.
• A ↓ in pressure at constant temp. shifts the position of equilibrium to the side w/ more moles of gas to
oppose change + ∴ ↑ pressure.
E.g. for the following reaction: 2SO2(g) + O2(g) ⇌ 2SO3(g)
• An ↑ in pressure will shift the position of equilibrium from left to right (as there are fewer moles of
gas molecules on the right) in order to oppose change, resulting in an ↑ in SO3 production + a ↓ in
pressure.
• A ↓ in pressure will shift the position of equilibrium from right to left (as there are more moles of gas
molecules on the left) in order to oppose change, resulting in an ↑ in SO2 and O2 production and an ↑
in pressure.
Changes in Temperature
For a reaction in equilibrium at constant pressure where the forward reaction is exothermic…
• an ↑ in temp. would shift the position of equilibrium in the direction of the reverse endothermic
reaction + the position of equilibrium would move from right to left in order to oppose change + ∴ ↓
temp. by absorbing heat.
• a ↓ in temp. would shift the position of equilibrium in the direction of the forward exothermic
reaction + the position of equilibrium would move from left to right in order to oppose change + ∴ ↑
temp. by giving out heat.
For a reaction in equilibrium at constant pressure where the forward reaction is endothermic…
• an ↑ in temp. would shift the position of equilibrium in the direction of the forward endothermic
reaction + the position of equilibrium would move from left to right in order to oppose change + ∴ ↓
temp. by absorbing heat.
• a ↓ in temp. would shift the position of equilibrium in the direction of the reverse exothermic
reaction + the position of equilibrium would move from right to left in order to oppose change + ∴ ↑
temp. by giving out heat.
E.g. for the following reaction: 2SO2(g) + O2(g) ⇌ 2SO3(g) ∆H = -197kJmol-1
• An ↑ in temp.will shift the position of equilibrium from right to left (the endothermic direction) to
absorb the extra heat in order to oppose change.
• A ↓ in temp. will shift the position of equilibrium from left to right (the exothermic direction) to give
out more heat in order to oppose change.
