TC4 : Reversible reactions and equilibria

What is a reversible reaction?

Where the products can react with each other to produce the original reactants - reaction can go both ways

What is an example of a reversible reaction?

The Haber process

What happens during the Haber process?

Nitrogen and hydrogen react to form ammonia

What is the equation of what happens during the Haber process?

N₂ + 3H₂ ⇌ 2NH₃

Where is the nitrogen (N₂) obtained from for the Haber process?

Obtained easily from the air, which is about 78% nitrogen

Where is the hydrogen (H₂) obtained from for the Haber process?

Can be extracted from hydrocarbons from sources such as natural gas & crude oil

What are the conditions for the Haber process?

Haber process is carried out at 450°C, with a pressure of 200 atmospheres and an iron catalyst

What is dynamic equilibrium?

The forwards and backward reactions are both happening at the same time and at the same rate, and the concentrations of reactants and products have reached a balance, and won’t change

What will reversible reactions reach?

An equilibrium

As the reactants react, what happens to their concentrations, and what happens to the speed of the reaction?

Their concentrations fall, so the forward reaction will slow down

As more and more of the products are made, what will happen to their concentrations, and what happens to the speed of the reaction?

Their concentrations rise, and the backward reaction will speed up

After a while, what will happen?

The forward reaction will be going at exactly the same rate as the backward one

After a while, the forward reaction will be going at exactly the same rate as the backward one. What is this?

Equilibrium

What happens at equilibrium?

Both reactions are still happening, but there’s no overall effect - this is a dynamic equilibrium

Equilibrium can only be reached if the reversible reaction takes place in a …

Closed system

What is a closed system?

None of the reactants or products can escape

When a reaction is at equilibrium, does it mean that the amounts of reactants and products are equal?

No

What does it mean if the equilibrium lies to the right?

Lots of products, not much of the reactants

(The concentration of products is greater than the concentration of reactants)

What does it mean if the equilibrium will lie to the left?

Lots of the reactants, not must of the products

(The concentration of reactants is greater than the concentration of the products)

What does the exact position of equilibrium depend on?

The conditions (as well as the reaction itself)

What 3 things can change the position of equilibrium?

• Temperature

• Pressure (for equilibria involving gases)

• Concentrations (of reactions or products)

What changes if the position of equilibrium changes?

The amounts of products and reactants present at equilibrium

Ammonium chloride ⇌ ammonia + hydrogen chloride

What happens if you heat this reaction?

The equilibrium shifts to the right (more ammonia and hydrogen chloride)

Ammonium chloride ⇌ ammonia + hydrogen chloride

What happens if you cool this reaction?

Equilibrium moves to the left (more ammonium chloride)

What does La Chatelier’s Principle state?

If there’s a change in concentration, pressure or temperature in a reversible reaction, the equilibrium position will move to help counteract that change

All reactions are … in one direction and … in the other

Exothermic, endothermic

What happens if you increase the temperature?

The equilibrium will move in the endothermic direction to absorb the extra heat

What happens if you decrease the temperature?

The equilibrium will move in the exothermic direction to produce more heat

Changing the pressure only affects equilibria involving …

Gases

What happens if you increase the pressure?

The equilibrium will move towards the side that has fewer moles of gas to reduce pressure

What happens if you decrease the pressure?

The equilibrium will move towards the side that has more moles of gas to increase pressure

What happens if you increase the concentration of the reactants?

The equilibrium will move to the right to use up the reactants (making more products)

What happens if you increase the concentration of the products?

The equilibrium will move to the left to use up the products (making more eactants)

What happens if you decrease the concentration of the products?

The equilibrium will move to the right, favouring the forward reaction to produce more products

What happens if you decrease the concentration of the reactants?

The equilibrium will shift to the left, favouring the formation of more reactants