3.1.6 Chemical equilibria & Le Chatelier's principle - Chemistry Alevel

Symbol for reversible reactions

⇌

What conditions must there be for a reaction to reach dynamic equilibrium?

The reaction must be done in a closed container for dynamic equilibrium to be reached.

Position of equilibrium - reactant > product

Equilibrium lies to the left. Higher concentration of reactant.

Position of equilibrium - reactant < product

Equilibrium lies to the right. Higher concentration of product.

Concentration - ↑ reactants - equilibrium

Shift to the right to react away the extra _______ (reactant) added.

Concentration - ↓ reactant - equilibrium

Shift to the left to make more _______ (reactant).

Concentration - ↑ product - equilibrium

Shift to the left to react away the extra _______ (product) added.

Concentration - ↓ product - equilibrium

Shift to the right to make more _______ (product).

↑ pressure - equilibrium

Shift to the side with fewer gas molecules to decrease the pressure.

↓ pressure - equilibrium

Shift to the side with more gas molecules to increase the pressure.

-ve forwards reaction - ↑ temperature - equilibrium

Shift to the left in favour of the endothermic backwards reaction to decrease the temperature/absorb heat.

-ve forwards reaction - ↓ temperature - equilibrium

Shift to the right in favour of the forwards exothermic reaction to increase temperature/release heat.

+ve forwards reaction - ↑ temperature - equilibrium

Shift to the right in favour of the endothermic forwards reaction to decrease the temperature/absorb heat.

+ve forwards reaction - ↓ temperature - equilibrium

Shift to the left in favour of the exothermic backwards reaction to increase temperature/release heat.

Choosing conditions - factors

How much the desired product is formed (equilibrium position), the speed at which the product is formed (rate of reaction), safety and cost considerations (high pressure is explosive...).

Catalysts effect of equilibrium position

Catalysts do not affect equilibrium position but speed up the rate at which equilibrium is reached, as they speed up the rate of the forward reaction and backward reaction (equally).

Why do catalysts work

They provide an alternative reaction pathway with a lower activation energy so more particles will have sufficient energy to overcome the energy barrier & react.

Uses to liquid fuels

Easier to transport (pump/pour - less viscous), low boiling point (less energy needed to vaporise shorter chained alcohols), burn cleanly (relatively).

Manufacture methanol - temperature

Compromise, 500K/250°C approx. (FR is exothermic)

Manufacture methanol - pressure

High, 50 - 100 atmospheres (5-10 MPa)

Manufacture methanol - catalyst

Aluminium oxide/alumina

Manufacture methanol - increasing the yield

Continuously fractionally distilling the methanol to cause the equilibrium to shift → while the reactants are recycled after removing the product.

Manufacture ethanol - temperature

Compromise, 300°C

Manufacture ethanol - pressure

High, 60 - 70 atmospheres (6 - 7 MPa)

Manufacture ethanol - catalyst

Concentrated phosphoric acid.

Haber process - raw materials

The hydrogen is from hydrocarbons in crude oil or natural gas. The nitrogen is from fractional distillation of the air (80%)

Haber process - uses

Haber process is used to produce ammonia which is needed for fertilisers an explosives.

Haber process - why needed

Nitrogen and hydrogen are hard to combine because nitrogen is very unreactive.

Haber process - word equation

Nitrogen + Hydrogen ⇌ Ammonia

Haber process - symbol equation

N₂ + 3H₂ ⇌ 2NH₃ (FR is exothermic)

Haber process - temperature

450°C - the temperature chosen reduces the yield of ammonia but allows it to be produced in a shorter time. The forward reaction is exothermic.

Haber process - pressure

200 atm - the pressure chosen is not extremely high because the engineering costs of constructing the reaction vessel and the running costs of maintaining the high pressure outweighs the economic benefit of producing ammonia more quickly.

Haber process - catalyst

Iron catalyst.

Haber process - ratio

1:3 N:H

Haber process - how is waste reduced

By cooling the reaction mixture and liquefying the ammonia produced (higher bp as hydrogen bonds) but allows the unreacted nitrogen and hydrogen to be recirculated.

Equilibrium constant - higher concentration of reactant

Small Kc < 1

Equilibrium constant - higher concentration of product

Large Kc > 1

What does Kc refer to?

Kc refers to the concentration in moldm⁻³ and the concentration of a solid does not change, therefore any solid chemicals are not included in the Kc expression.

What effects Kc?

Temperature