Exothermic & Endothermic Reactions Notes

Q: How is the energy change represented?

The symbol \Delta H is negative for exothermic reactions and positive for endothermic reactions. A negative \Delta H means the products have less energy than the reactants (energy was released). A positive \Delta H means the products have more energy (energy was absorbed).

Oxford AQA IGCSE Chemistry: Revision notes

Key facts

Every reaction transfers energy to or from the surroundings.
Exothermic: energy given out, surroundings get hotter, $\Delta H$ is negative.
Endothermic: energy taken in, surroundings get colder, $\Delta H$ is positive.
Exothermic examples: combustion, many oxidations, neutralisation.
Endothermic example: thermal decomposition.

\Delta H

	Exothermic	Endothermic
Energy	Energy given out	Energy taken in
Surroundings	Surroundings hotter	Surroundings colder
$\Delta H$	Negative	Positive

Exothermic

Energy:: Energy given out
Surroundings:: Surroundings hotter
$\Delta H$ :: Negative

Endothermic

Energy:: Energy taken in
Surroundings:: Surroundings colder
$\Delta H$ :: Positive

Energy and the surroundings

Energy is always transferred to or from the surroundings, and shows up as a temperature change.

The surroundings are the air, water or solution around the reaction. If they get hotter, energy has been given out. If they get colder, energy has been taken in.

1
Reaction happens
energy moves to or from the surroundings
2
Temperature changes
measured with a thermometer
3
Conclusion
hotter: given out. Colder: taken in

Detecting an energy transfer

The temperature of the solution falls during a reaction. What does this show?

Exothermic reactions

An exothermic reaction transfers energy to the surroundings, so their temperature increases.

Examples are combustion, many oxidation reactions and neutralisation. When methane burns in a Bunsen burner, the air and nearby objects heat up.

Exothermic: products lower than reactants, so

\Delta H

is negative

Name two types of exothermic reaction.

Endothermic reactions

An endothermic reaction takes in energy from the surroundings, so their temperature decreases.

A common example is thermal decomposition, such as heating a metal carbonate: $\ce{CaCO3 -> CaO + CO2}$ .

Endothermic: products higher than reactants, so

\Delta H

is positive

Is thermal decomposition exothermic or endothermic?

Representing energy change

The symbol $\Delta H$ is negative for exothermic reactions and positive for endothermic reactions.

A negative $\Delta H$ means the products have less energy than the reactants (energy was released). A positive $\Delta H$ means the products have more energy (energy was absorbed).

Illustrative energy profiles from the same reactants: products lower (ΔH negative, exothermic) or higher (ΔH positive, endothermic)

Exothermic $\Delta H$ negative (−)
Endothermic $\Delta H$ positive (+)

A reaction has a positive $\Delta H$ . What can you say about the surroundings?

Try an exam question

Explain what is meant by an exothermic reaction and state the sign of $\Delta H$ for it.

[3 marks]

That's the notes covered.

Carry on to the next subtopic.