Lesson focus: Energy-level diagrams compare the relative chemical energy stores of reactants and products and show the activation-energy barrier.
Draw and interpret energy-level diagrams for exothermicDescribes a reaction that transfers heat energy to the surroundings, so the temperature of the surroundings increases. and endothermicDescribes a reaction that takes in heat energy from the surroundings, so the temperature of the surroundings decreases. reactions without treating energy as a substance.
Build on earlier learning
Exothermic and endothermic reactionsYear 8 classified exothermic and endothermic reactions. This lesson represents those energy changes and the activation barrier graphically.
You will be successful when you can…
- Identify reactant and product levels.
- Classify exothermic and endothermic reactions.
- Define activation energy.
- Distinguish energy transfer from material flow.
Words that carry the science
- activation energy
- The minimum energy required for particles to react during a collision.
- exothermic
- Describes a process that transfers energy to the surroundings.
- endothermic
- Describes a process that takes in energy from the surroundings.
- reaction pathway
- A representation of progress from reactants to products, not elapsed time.
- energy change
- The difference between the energy stores of products and reactants for the stated system.
- catalyst
- A substance that increases reaction rate by providing an alternative pathway with lower activation energy and is regenerated overall.
Read energy changes from relative levels and pathways
01Relative height represents energy, not amount
On an energy-level diagram, the vertical axis represents energy and the horizontal axis represents reaction progress, not time. Reactants and products are drawn as levels connected by a pathway.
If products are lower than reactants, energy has been transferred to the surroundings overall: the reaction is exothermicDescribes a reaction that transfers heat energy to the surroundings, so the temperature of the surroundings increases.. If products are higher, the reaction is endothermicDescribes a reaction that takes in heat energy from the surroundings, so the temperature of the surroundings decreases..
02The peak represents an activation barrier
Particles must reach a higher-energy arrangement before bonds can reorganise. The energy difference from the reactant level to the peak is the activation energyThe smallest amount of energy particles need when they collide to make a reaction start..
An exothermic reaction can still need heating to start because activation energyThe smallest amount of energy particles need when they collide to make a reaction start. and overall energy changeThe overall amount of energy given out or taken in during a reaction. answer different questions. Once started, released energy may help further particles cross the barrier.
03Catalysts alter the pathway, not the endpoints
A catalystA substance that speeds up a reaction without being used up, by providing a route with a lower activation energy. provides an alternative route with lower activation energy. The reactant and product energy levels remain the same, so the overall energy change is unchanged.
Catalysts can reduce energy use and increase industrial rate. They are regenerated overall rather than consumed as a reactant.
Exothermic and endothermic reactions
Exothermic reactions transfer energy to the surroundings, so the temperature goes up. Endothermic reactions take in energy from the surroundings, so the temperature goes down. An energy-level diagram shows the energy of the reactants and products.
Dashed curve: the reaction path. The hump is the energy needed to start the reaction. Orange dot: progress of the reaction.
Controls
Other exothermic reactions: burning, respiration. Other endothermic reactions: thermal decomposition, photosynthesis.
Classify a reaction from levels
Reactants are at 180 kJ and products at 95 kJ on the same scale.
- Compare product and reactant levels.
- Products are 85 kJ lower: 95 − 180 = −85 kJ.
- A decrease in the reacting system corresponds to transfer to the surroundings.
- Classify the reaction as exothermic.
How to make the reasoning visible: The diagram represents an overall energy transfer of 85 kJ to the surroundings for the stated reacting amounts.
Interpret energy diagrams
Use relative levels and peak heights; the values are illustrative.
| Diagram | Reactants / kJ | Peak / kJ | Products / kJ | Activation energy / kJ | Type |
|---|---|---|---|---|---|
| A | 120 | 190 | 70 | 70 | Exothermic |
| B | 80 | 165 | 145 | 85 | Endothermic |
| A with catalyst | 120 | 155 | 70 | 35 | Exothermic |
| B with catalyst | 80 | 125 | 145 | 45 | Endothermic |
Why is the energy change identical for A with and without a catalyst?
Why can reaction B occur even though its products are higher?
Check your understanding
Answer from memory, then check against the suggested answer.
What does the horizontal axis show?
Where are products for an exothermic reaction?
What does a catalyst lower?
Describe the effect of a catalyst on an endothermic profile.
Explain why a hand-warmer can be exothermic yet remain unused until a trigger is activated.
Exam Question Practice
Exam Question Practice
Mark points shown after marking
An energy-level diagram is drawn for an exothermic reaction.
Describe what the diagram shows.
(3)Explain how a catalyst changes the diagram.
(2)Total for this question: 5 marks
Command word: For explainA command word: say how or why something happens, giving reasons with words like "because" or "so"., distinguish the endpoint difference from the barrier height.