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Energy changes are not just a topic in the chemistry lab — they are part of our daily lives. This lesson explores how exothermic and endothermic reactions are used in everyday products and situations, linking the theoretical concepts from AQA GCSE Chemistry to real-world applications. Understanding these applications will help you answer context-based questions in the exam.
Exothermic reactions release energy to the surroundings. Many products and processes rely on this energy release.
Disposable hand warmers contain iron powder, salt, water, activated charcoal, and vermiculite. When exposed to air (by opening the packaging), the iron undergoes a slow oxidation reaction:
iron + oxygen --> iron oxide
This reaction is exothermic and releases heat steadily over several hours.
| Feature | Detail |
|---|---|
| Reaction type | Oxidation (exothermic) |
| Reactants | Iron powder and oxygen from the air |
| Product | Iron oxide (rust) |
| Temperature reached | Approximately 40-50 degrees C |
| Duration | Up to 10 hours |
| Reusable? | No — the iron is fully oxidised |
Reusable hand warmers use a supersaturated solution of sodium acetate. Clicking a metal disc triggers crystallisation, which is exothermic. These can be recharged by boiling the pack to redissolve the crystals.
Exam Tip: If asked about hand warmers, be specific about the type. Disposable hand warmers use an irreversible oxidation reaction. Reusable hand warmers use a reversible physical process (crystallisation). Both are exothermic.
Self-heating food products use an exothermic reaction to heat the contents without an external heat source. A common reaction used is:
calcium oxide + water --> calcium hydroxide
This reaction is highly exothermic and can raise the temperature of a drink or meal by 40-60 degrees C within minutes.
| Component | Purpose |
|---|---|
| Outer container | Holds the food or drink |
| Inner compartment | Contains calcium oxide (quickite) and water, separated by a barrier |
| Activation | Pressing a button breaks the barrier, allowing water to mix with CaO |
| Heat transfer | The exothermic reaction heats the surrounding food/drink |
The most common exothermic reactions in daily life are combustion reactions:
graph TD
A["Exothermic Reactions in Daily Life"] --> B["Hand Warmers"]
A --> C["Self-Heating Cans"]
A --> D["Combustion - Cooking"]
A --> E["Combustion - Vehicle Engines"]
A --> F["Respiration in Living Cells"]
B --> G["Iron oxidation"]
C --> H["CaO + Water"]
D --> I["Burning natural gas"]
E --> J["Burning petrol/diesel"]
F --> K["Glucose + Oxygen -> CO2 + Water"]
Endothermic reactions absorb energy from the surroundings. They have practical applications where cooling is needed.
Instant cold packs are used to treat sports injuries, reduce swelling, and relieve pain. They contain:
When the pack is squeezed, the inner sachet breaks and the ammonium nitrate dissolves in the water. This dissolving process is endothermic — it absorbs heat from the surroundings, making the pack cold.
| Feature | Detail |
|---|---|
| Reaction type | Dissolving (endothermic) |
| Chemical | Ammonium nitrate (NH4NO3) dissolving in water |
| Temperature drop | Can drop to approximately 0-5 degrees C |
| Duration | 15-30 minutes |
| Reusable? | No — cannot be reversed easily |
Exam Tip: Strictly speaking, dissolving ammonium nitrate is not a chemical reaction — it is a physical change. However, it is an endothermic PROCESS, and AQA treats it as an example of an endothermic change. Be prepared for questions about this distinction.
Sodium hydrogencarbonate (baking soda, NaHCO3) undergoes thermal decomposition when heated in an oven:
2NaHCO3 --> Na2CO3 + H2O + CO2
This endothermic reaction absorbs energy and produces carbon dioxide gas, which causes bread, cakes, and biscuits to rise. The gas creates bubbles in the dough, giving baked goods their light, airy texture.
Photosynthesis is an endothermic reaction that takes place in the chloroplasts of plant cells:
carbon dioxide + water --> glucose + oxygen (in the presence of light)
The energy for this reaction comes from sunlight, which is absorbed by chlorophyll. Without this endothermic process, life on Earth could not exist.
Respiration is an exothermic reaction that occurs in every living cell:
glucose + oxygen --> carbon dioxide + water (energy released)
This reaction releases energy that cells use for:
| Reaction | Type | Energy Transfer |
|---|---|---|
| Respiration | Exothermic | Releases energy for life processes |
| Photosynthesis | Endothermic | Absorbs light energy to make glucose |
| Digestion (some steps) | Endothermic | Energy needed to break down large molecules |
Exam Tip: Respiration and photosynthesis are often linked in exam questions. Remember that respiration is exothermic (releases energy) and photosynthesis is endothermic (absorbs energy). They are essentially reverse processes.
Many industrial processes involve significant energy changes:
| Process | Type | Details |
|---|---|---|
| Haber process (making ammonia) | Exothermic | N2 + 3H2 --> 2NH3, releases energy |
| Thermal decomposition of limestone | Endothermic | CaCO3 --> CaO + CO2, requires continuous heating in a kiln |
| Cracking of hydrocarbons | Endothermic | Large molecules broken into smaller ones using heat and/or catalyst |
| Contact process (making sulfuric acid) | Exothermic | 2SO2 + O2 --> 2SO3, releases energy |
Understanding energy changes helps us make informed decisions:
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