Lesson focus: exothermicGives out heat energy to the surroundings, so the surroundings get warmer. and endothermicTakes in heat energy from the surroundings, so the surroundings get colder. reactions differ in the direction of net energy transfer between systemThe substances you are studying in a reaction, such as the chemicals inside a flask. and surroundingsEverything outside the system you are studying, such as the air around a beaker..
Classify energy changes from temperature evidence and interpret simple reaction-energy profiles.
Build on earlier learning
Burning fuelsCombustion is a familiar exothermic reaction; this lesson generalises energy transfer.
Practical
Temperature changes in reactions
- Measure the starting temperature of 25 cm³ of a solution in an insulated cup.
- Add the second reactant and stir.
- Record the highest or lowest temperature reached.
Record: The temperature change for each reaction, classified as exothermic or endothermic.
Safety: Wear eye protection.
You will be successful when you can…
- Define exothermic and endothermic.
- Interpret temperature evidence.
- Identify activation energy.
- Compare reactant and product energy levels.
Words that carry the science
- exothermic
- A process with net energy transfer from system to surroundings.
- endothermic
- A process with net energy transfer from surroundings to system.
- system
- The reacting substances being studied.
- surroundings
- Everything outside the chosen system.
- activation energy
- Minimum energy needed for particles to react successfully.
Direction defines the energy change
01Direction defines the energy change
An exothermicGives out heat energy to the surroundings, so the surroundings get warmer. reaction transfers energy to surroundingsEverything outside the system you are studying, such as the air around a beaker., often making the measured mixture and container warmer. An endothermicTakes in heat energy from the surroundings, so the surroundings get colder. reaction takes in energy, often making them cooler.
Temperature is evidence about energy transfer, but a fair comparison needs the same quantities, insulation and starting conditions.
02Reactions still need a start
Even exothermicGives out heat energy to the surroundings, so the surroundings get warmer. reactions can need activation energyThe smallest amount of energy particles need when they collide to make a reaction start. to break initial bonds or reach a reactive arrangement. A spark can start combustion without being its main energy source.
A catalyst lowers activation energy but does not change the overall energy difference between reactants and products.
03Profiles show chemical energy change
In an exothermic profile, products lie lower than reactants; the difference transfers to surroundings. In an endothermic profile, products lie higher.
Bond breaking requires energy and bond making releases energy. The net balance determines the overall classification.
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 from temperature data
A mixture changes from 21.0 °C to 15.5 °C in an insulated cup.
- Temperature change = 15.5 − 21.0 = −5.5 °C.
- The surroundings represented by solution and cup cool.
- Energy transfers from surroundings into the reacting systemThe substances you are studying in a reaction, such as the chemicals inside a flask..
- Classify the reaction as endothermic under these conditions.
How to make the reasoning visible: The negative temperature change supports an endothermicTakes in heat energy from the surroundings, so the surroundings get colder. process.
Reaction-energy
Compare illustrative measurements made with equal volumes and insulation.
| Reaction | Start / °C | Peak or minimum / °C | Classification |
|---|---|---|---|
| A | 20.0 | 31.5 | ? |
| B | 20.0 | 14.0 | ? |
| C | 20.0 | 20.8 | ? |
| D | 20.0 | 18.9 | ? |
Classify each process.
Why is C less secure than A?
Check your understanding
Answer from memory, then check against the suggested answer.
Define exothermic.
Define endothermic.
What is activation energy?
Why does a spark not make combustion endothermic?
Explain why bond formation can make a reaction exothermic.
Exam Question Practice
Exam Question Practice
Mark points shown after marking
The temperature of a reaction mixture rises from 19 °C to 34 °C after the reaction starts.
Explain what the temperature change shows.
(3)Explain the energy changes in terms of bonds.
(2)Total for this question: 5 marks
Command word: Separate activation energy from the net energy transfer.