Chemistry: Reversible reactions

What are reversible reactions?

Chemical reactions that can go either forward or backwards

What happens in the different directions?

The forward reaction forming the products

The reverse reaction forming the reactants

What do you use when writing the equation?

Use ⇌

Where can reversible reactions be seen?

In some hydrated salts, those that contain water of crystallisation

What is an example?

hydrated copper(II) sulfate ⇌ anhydrous copper(II) sulfate + water

CuSO₄•5H₂O ⇌ CuSO₄ + 5H₂O

What happens in forward reaction and what reaction is it?

Blue crystals

The hydrated salt can be heated / dehydrated to form anhydrous copper(II) sulfate, CuSO₄ 

This reaction is endothermic as energy is taken in to remove the water

What happens in backward reaction and what reaction is it?

This is usually seen as white crystals / powder

Adding water to the anhydrous salt forms the hydrated copper(II) sulfate pentahydrate, CuSO₄•5H₂O

This reaction is highly exothermic

How should reversible reactions be done?

in CLOSED SYSTEM to prevent products from escaping/reactants from outside reacting to allow for reverse reaction for equilibrium

What is equilibrium?

When the rate of the forward reaction equals the rate of the reverse reaction.

What is the haber process?

manufacturing ammonia by combining nitrogen from the air and hydrogen

Describe this process in relation to reaching equilibrium

• At the start of the reaction, only nitrogen and hydrogen are present

  • This means that the rate of the forward reaction is at its highest, since the concentrations of hydrogen and nitrogen are at their highest

• As the reaction proceeds, the concentrations of hydrogen and nitrogen gradually decrease

  • So, the rate of the forward reaction will decrease

• However, the concentration of ammonia is gradually increasing and so the rate of the backward reaction will increase

  • Ammonia will decompose to reform hydrogen and nitrogen

• In a closed system, the two reactions are interlinked and none of the gases can escape

• So, the rate of the forward reaction and the rate of the backward reaction will eventually become equal and equilibrium is reached

Changes in what can be used to predict the changes to the position of equilibrium?

Temperature

Pressure

When does the position of the equilibrium shifts right?

when the forward reaction is favoured

This means that there is an increase in the amount of products formed

When does the position of the equilibrium shifts left?

when the reverse reaction is favoured

So, there is an increase in the amount of reactants formed

If the temperature of the reaction increases, where will the equilibrium shift?

The equilibrium will shift in the direction of the endothermic reaction

If the temperature of the reaction decreases, where will the equilibrium shift?

The equilibrium will shift in the direction of the exothermic reaction

What is the effect of a catalyst on the equilibrium?

not affect the position of equilibrium but it does increase the rate at which equilibrium is reached

As catalyst increases the rate of both the forward and backward reactions by the same amount (by providing an alternative pathway requiring lower activation energy)

As a result, the concentration of reactants and products is nevertheless the same at equilibrium as it would be without the catalyst