Reaction Profiles

Section 1 of 7Activation energy

Collisions are needed

• Collisions are needed Particles must collide with the correct orientation and enough energy to react.

Minimum energy

• Minimum energy The smallest energy colliding particles need to react is the activation energy, E_a.
• Bonds in the reactants must be partly broken before new bonds form.

Different reactions, different E_a

• Different reactions, different E_a A high activation energy means few collisions are successful, so the reaction is slow at room temperature.

Section 2 of 7Features of a reaction profile

Feature	What it shows
y-axis	Relative energy of the chemicals
x-axis	Progress of reaction
Start level	Energy of the reactants
Peak	Energy needed for the transition state
Height from reactants to peak	Activation energy
Difference between reactants and products	Overall energy change, ΔH

Section 3 of 7Exothermic profile

1Reactants start high

• Reactants start high They begin at a higher energy level than the products.

2Climb to the peak

• Climb to the peak Energy is needed to break bonds, the activation energy.

3Fall to products

• Fall to products Energy is released as bonds form, ending lower than the start.

4Overall change

• Overall change The downward arrow shows a negative energy change; heat goes to the surroundings.

Section 4 of 7Endothermic profile

1Reactants start low

• Reactants start low They begin at a lower energy than the products.

2Climb to the peak

• Climb to the peak Activation energy is again supplied to start the reaction.

3Settle higher

• Settle higher Products finish at a higher energy than the reactants.

4Overall change

• Overall change The upward arrow shows a positive energy change; heat is taken from the surroundings.

Section 5 of 7Drawing a reaction profile

1

Draw axes: energy on the vertical, progress of reaction on the horizontal.

2

Mark the reactants on the left at the correct height and the products on the right.

3

Draw a smooth curve over a peak between them.

4

Add a vertical arrow from the reactants to the peak and label it activation energy.

5

Add a vertical arrow between the reactant and product levels and label it overall energy change, pointing down for exothermic and up for endothermic.

Section 6 of 7Key terms

Term	Meaning
Activation energy	The minimum energy colliding particles need for a reaction to occur.
Reaction profile	A graph showing energy of reactants, transition state and products during a reaction.
Overall energy change	The difference between the energy of products and reactants.
Exothermic	Products have less energy than the reactants.
Endothermic	Products have more energy than the reactants.

Section 7 of 7Exam technique

1

Drawing the activation energy arrow from the products rather than the reactants.

2

Forgetting that exothermic reactions also have an activation energy.

3

Showing the overall energy change arrow from the base of the axes instead of between reactant and product levels.

4

Confusing the energy change with the activation energy; they are different measurements.