Energy Profiles

Section 1 of 7Reading an energy profile

The axes

• The axes The vertical axis shows the relative energy of the chemicals.
• The horizontal axis shows the progress of the reaction from reactants on the left to products on the right.

The overall energy change

• The overall energy change The vertical gap between the level of the reactants and the level of the products is the overall energy change of the reaction.
• An arrow drawn between the two levels shows its direction: down for energy released, up for energy absorbed.

The curve

• The curve A curve joins reactants to products.
  • The rise to the peak of the curve represents the energy barrier that must be crossed first.

Section 2 of 7Exothermic and endothermic reactions

Feature	Exothermic	Endothermic
Energy transfer	Energy is given out to the surroundings	Energy is taken in from the surroundings
Products compared with reactants	Lower in energy	Higher in energy
Sign of the energy change	Negative	Positive
Arrow on the profile	Points downwards	Points upwards
Temperature of surroundings	Rises	Falls

Section 3 of 7Activation energy

1Particles collide

• Particles collide For a reaction to happen, particles must first collide with each other.

2Enough energy needed

• Enough energy needed Collisions must have at least a minimum energy, and the particles must meet in the right orientation.

3Bonds are broken

• Bonds are broken This minimum energy, called the activation energy (E_a), is needed to break bonds in the reactants.

4Reaction proceeds

• Reaction proceeds Collisions with at least E_a are successful, so new bonds form and products appear.

Section 4 of 7Drawing a full profile

1

Draw labelled axes: energy on the vertical axis and progress of reaction on the horizontal axis.

2

Draw a horizontal line at the left for the reactants and label it. For an exothermic reaction, draw the products line lower on the right; for an endothermic reaction, draw it higher.

3

Join the two levels with a smooth curve that rises to a peak above the higher of the two levels.

4

Mark the activation energy with an upward arrow from the reactants' level to the top of the peak, and label it E_a.

5

Mark the overall energy change with an arrow from the reactants' level to the products' level, pointing down for exothermic and up for endothermic.

Section 5 of 7Interpreting profiles

Energy profile

Taller peak

• Taller peak A bigger rise means a higher activation energy, so more energy (for example more heating) is needed to start the reaction.

Every reaction has E_a

• Every reaction has E_a Even exothermic reactions need an initial input of energy to break bonds, so their curves also have a peak.

Identifying the type

• Identifying the type Compare the end level with the start level.
  • Lower means exothermic;
    • higher means endothermic.

Section 6 of 7Key terms

Term	Meaning
Energy profile	A graph showing the relative energy of reactants and products as a reaction progresses.
Exothermic	A reaction that transfers energy to the surroundings, so the products have less energy than the reactants.
Endothermic	A reaction that takes in energy from the surroundings, so the products have more energy than the reactants.
Activation energy	The minimum energy that colliding particles need for a reaction to take place.
Overall energy change	The difference in energy between the products and the reactants.

Section 7 of 7Exam technique

1

Drawing the arrow for the overall energy change from the baseline to the peak: that arrow is the activation energy, not the overall change.

2

Leaving out the activation energy peak on an exothermic profile because the reaction gives out energy.

3

Labelling the axes incorrectly, for example writing 'time' instead of 'progress of reaction'.

4

Saying exothermic reactions have a positive energy change: it is negative because the products have less energy.

5

Starting the activation energy arrow at the bottom of the graph instead of at the level of the reactants.