Energy Changes in Reactions
Some reactions release heat so intensely they can melt metal. Others absorb heat so effectively they can freeze water in seconds. Understanding exothermic and endothermic reactions unlocks the chemistry behind hand warmers, cold packs, bushfires and the very process that powers life on Earth.
Before You Begin
Think about these two products:
- A hand warmer that feels hot when you snap the metal disc inside.
- An instant cold pack that feels ice-cold when you break the inner pouch.
Write down your answers before reading on:
- Where does the heat in the hand warmer come from?
- Where does the cold in the cold pack go?
- Are these physical changes or chemical reactions? How do you know?
Know
- The definitions of exothermic and endothermic reactions
- Examples of each: combustion, neutralisation (exothermic); thermal decomposition, photosynthesis (endothermic)
- That energy can be thought of as a reactant or product conceptually
Understand
- Why exothermic reactions release heat to the surroundings
- Why endothermic reactions absorb heat from the surroundings
- How energy changes relate to the breaking and forming of bonds conceptually
Can Do
- Classify a reaction as exothermic or endothermic from observations
- Explain everyday and industrial examples in terms of energy transfer
- Describe the direction of energy flow between reaction and surroundings
Exothermic Reactions
Reactions that release heat
Conservation Of Mass
An exothermic reaction releases heat energy to the surroundings. The word comes from exo- (out) + therm (heat). If you touch the container during an exothermic reaction, it feels warm or hot.
Conceptually, we can think of energy as a product in an exothermic reaction:
reactants → products + energy
Common exothermic reactions
| Reaction | What you observe |
|---|---|
| Combustion (burning) | Heat and light released; surroundings get hot |
| Neutralisation | Test tube feels warm; temperature increases |
| Hand warmers | Iron oxidising slowly releases sustained heat |
| Concrete setting | Hydration of cement releases heat over hours |
Endothermic Reactions
Reactions that absorb heat
An endothermic reaction absorbs heat energy from the surroundings. The word comes from endon- (in) + therm (heat). If you touch the container during an endothermic reaction, it feels cold.
Conceptually, we can think of energy as a reactant in an endothermic reaction:
reactants + energy → products
Common endothermic reactions
| Reaction | What you observe |
|---|---|
| Thermal decomposition | Heat must be continuously supplied; container cools if heating stops |
| Photosynthesis | Light energy absorbed; no temperature rise |
| Cold packs | Dissolving ammonium nitrate in water absorbs heat; pack feels cold |
| Dissolving some salts | Solution temperature drops as salt dissolves |
Energy as Reactant or Product
A conceptual model for Stage 5
At Stage 5, we do not calculate energy changes using bond energies or enthalpy. Instead, we use a simple but powerful conceptual model:
- In exothermic reactions, more energy is released when new bonds form in the products than is needed to break bonds in the reactants. The excess energy escapes as heat.
- In endothermic reactions, more energy is needed to break bonds in the reactants than is released when new bonds form in the products. The missing energy must be supplied from outside.
This is why combustion (exothermic) gives out so much heat — strong bonds form in CO2 and H2O, releasing lots of energy. And why thermal decomposition (endothermic) needs continuous heating — strong bonds in the original compound must be broken before weaker products can form.
Common Misconceptions
"All chemical reactions produce heat." No — many reactions absorb heat. Photosynthesis, thermal decomposition and cold packs are all endothermic.
"Endothermic reactions are rare." No — endothermic reactions are common in nature. Photosynthesis, which powers almost all life on Earth, is endothermic.
Bushfires, Sport and Industry
Australian bushfires are among the most dramatic exothermic events on Earth. When eucalyptus leaves burn, the combustion reaction releases enormous heat — enough to create firestorms that generate their own weather. CSIRO researchers study these reactions to predict fire behaviour and protect communities.
At the other end of the temperature scale, Australian sports teams and medical services use instant cold packs for injuries. These packs contain ammonium nitrate and water separated by a barrier. When the barrier is broken, the ammonium nitrate dissolves endothermically, drawing heat from the injured area and reducing swelling. From the Outback to the operating theatre, exothermic and endothermic reactions are part of Australian life.
✍ Copy Into Your Books
▾Exothermic Reactions
- Release heat to surroundings
- Energy is a product conceptually
- Examples: combustion, neutralisation, hand warmers
- Surroundings get warmer
Endothermic Reactions
- Absorb heat from surroundings
- Energy is a reactant conceptually
- Examples: thermal decomposition, photosynthesis, cold packs
- Surroundings get cooler
Energy Flow
- Exothermic: system → surroundings (heat out)
- Endothermic: surroundings → system (heat in)
- Conservation of energy applies
Classify by Energy Change
Energy in Everyday Life
Test Your Understanding
1. An exothermic reaction is one that:
2. Which of the following is an endothermic reaction?
3. A reaction feels cold to the touch. This means:
4. Which pair of reactions are both exothermic?
5. In an endothermic reaction, energy can be thought of conceptually as a:
Short Answer Questions
1. Define exothermic and endothermic reactions. Give one example of each and explain how you could tell which is which by observation. 4 MARKS
2. Combustion and photosynthesis can be described as opposite reactions in terms of energy. Explain this statement with reference to whether energy is a reactant or product in each case. 4 MARKS
3. A sports trainer uses an instant cold pack to treat an ankle injury. When the inner pouch is broken, two chemicals mix and the pack becomes cold. Explain whether this is an exothermic or endothermic reaction, and describe the energy transfer between the reaction and the surroundings. 4 MARKS
Revisit Your Thinking
Go back to your Think First answer. Has your understanding changed?
- Can you now explain where the heat in a hand warmer comes from?
- Can you explain where the 'cold' in a cold pack goes?
Answers
▾MCQ 1
B — An exothermic reaction releases heat energy to the surroundings.
MCQ 2
C — Photosynthesis is endothermic because it absorbs light energy from the sun to convert carbon dioxide and water into glucose.
MCQ 3
B — A reaction that feels cold is absorbing heat energy from the surroundings (including your skin). This is characteristic of an endothermic reaction.
MCQ 4
A — Both combustion and neutralisation are exothermic reactions. They release heat to the surroundings.
MCQ 5
C — In an endothermic reaction, energy is absorbed from the surroundings. Conceptually, energy acts as a reactant that must be supplied for the reaction to proceed.
Short Answer 1
Model answer: An exothermic reaction is a chemical reaction that releases heat energy to the surroundings. For example, combustion of wood is exothermic because the surroundings get hot and light is produced. An endothermic reaction is a chemical reaction that absorbs heat energy from the surroundings. For example, photosynthesis is endothermic because it requires light energy from the sun. You can tell them apart by touch: exothermic reactions make the container feel warm, while endothermic reactions make it feel cold.
Short Answer 2
Model answer: Combustion and photosynthesis are opposite reactions in terms of energy. Combustion is exothermic, meaning energy is released as a product — the reaction gives out heat and light. Conceptually: fuel + oxygen → carbon dioxide + water + energy. Photosynthesis is endothermic, meaning energy is required as a reactant — the reaction absorbs light energy. Conceptually: carbon dioxide + water + energy → glucose + oxygen. Together, they form a cycle: photosynthesis stores solar energy in glucose, and respiration or combustion releases that stored energy.
Short Answer 3
Model answer: The instant cold pack involves an endothermic reaction. When the chemicals mix, the reaction absorbs heat energy from the surroundings. This energy transfer goes from the surroundings (the injured ankle, the air, the pack itself) into the chemical system. As heat is removed from the surroundings, the temperature drops and the pack feels cold. This reduces blood flow and swelling at the injury site. The reaction is endothermic because energy must be supplied from outside for the reaction to proceed.
Energy Blaster
Test your knowledge of exothermic and endothermic reactions. Classify reactions, predict energy transfers and blast your way to a high score!
Mark lesson as complete
Tick when you have finished all activities and checked your answers.