If the forward reaction is exothermic, the reverse is endothermic
Definition
Reversible reaction
A reaction in which the products can react to form the original reactants.
- In a reversible reaction, the forward and reverse changes transfer energy in opposite directions.
- If the forward reaction is exothermic, then the reverse reaction is endothermic.
- If the forward reaction is endothermic, then the reverse reaction is exothermic.
The same amount of energy is transferred each way
Definition
Surroundings
Everything outside the reacting chemicals that can transfer energy to or from them.
- The amount of energy given out in one direction equals the amount taken in the other direction.
- This is because the same bonds are broken and made, just the other way round.
- So the energy transferred to or from the surroundings is equal in size but opposite in direction.
Key Idea
- The two directions of a reversible reaction transfer equal amounts of energy.
- One direction releases energy while the other takes in the same amount.
Hydrated copper sulfate shows the energy swap
- Heating blue hydrated copper sulfate is endothermic, because energy is taken in to drive off the water.
- Adding water to white anhydrous copper sulfate is exothermic, so the solid warms up as it turns blue.
- The energy taken in on heating equals the energy released when the water is added back.
Exam technique
- If told one direction is exothermic, state that the reverse direction is endothermic.
- Say the energy transferred is equal in size in both directions.
- For copper sulfate, link the colour change to whether energy is taken in or given out.
Self review
- If the forward reaction is exothermic, what type of reaction is the reverse?
- How do the amounts of energy transferred in the two directions compare?
- Is heating hydrated copper sulfate exothermic or endothermic?
- Why is the same amount of energy transferred each way?
