Energy transfers: explain warming and cooling
| English | 中文 | Pinyin |
|---|---|---|
| exothermic/eɡzəˈðɜːmɪk/ | 放热的 | fàng rè de |
| endothermic/ˌendəʊˈθɜːmɪk/ | 吸热的 | xī rè de |
What would explain this observation?
- A hand warmer releases energy to your hands. An instant cold pack takes energy from its surroundings. The direction of energy transfer explains their different uses.
- Start with a prediction. State the quantities or features you would compare, then decide what evidence could distinguish two explanations.
Build the model
- Energy is conserved in chemical reactions. An exothermic 放热的 reaction transfers energy to the surroundings, which become warmer; products have less energy than reactants by the amount transferred. Combustion, many oxidation reactions and neutralisation are exothermic examples. An endothermic 吸热的 reaction takes energy from the surroundings, which cool; products have more energy than reactants. Thermal decomposition and citric acid reacting with sodium hydrogencarbonate are endothermic examples.
- exothermic: Transferring energy from reacting chemicals to the surroundings; endothermic: Taking energy from the surroundings into reacting chemicals.
A solution cools during a reaction with no external cooling. Which description fits?
Distinguish the reacting chemicals from their surroundings. A thermometer in a reacting solution measures the temperature of that solution, which receives or supplies energy during the chemical change. A temperature rise supports an exothermic interpretation under the stated conditions; a fall supports endothermic behaviour. Heating a vessel externally can obscure this evidence. Not every process involving cooling is a chemical reaction: new substances must also be formed.
Match each technical term to its precise meaning.
Use the definitions to distinguish related quantities and processes.
Choose evidence that can test it
- Distinguish the reacting chemicals from their surroundings. A thermometer in a reacting solution measures the temperature of that solution, which receives or supplies energy during the chemical change. A temperature rise supports an exothermic interpretation under the stated conditions; a fall supports endothermic behaviour. Heating a vessel externally can obscure this evidence. Not every process involving cooling is a chemical reaction: new substances must also be formed.
- Evaluate a supplied warmer or cold-pack design against its purpose: suitable temperature, duration, risk of leakage, storage and single-use waste. Use information supplied for the particular product. Many self-heating cans exploit exothermic changes; some sports injury packs use endothermic changes. These examples explain energy transfers and do not recommend applying an untested chemical mixture to skin.
Which two habits make the investigation or model in this case more defensible?
Evaluate a supplied warmer or cold-pack design against its purpose: suitable temperature, duration, risk of leakage, storage and single-use waste. Use information supplied for the particular product. Many self-heating cans exploit exothermic changes; some sports injury packs use endothermic changes. These examples explain energy transfers and do not recommend applying an untested chemical mixture to skin.
Work from known quantities
- State the known values and their units. Choose the relation because its assumptions fit this case, then rearrange before substitution.
- Known: a supervised reaction starts at 21.0 °C and reaches 28.5 °C. The temperature rise is 28.5−21.0=7.5 °C, supporting an exothermic interpretation with no external heater. A second starts at 22.0 °C and falls to 17.0 °C: the change is −5.0 °C and its cooling magnitude is 5.0 °C. Energy has been transferred, not destroyed.
A reaction starts at 19.5 °C and reaches 27.0 °C. Find the temperature rise. Use the same sequence: known quantities → model → relation → substitution → unit and interpretation.
A reaction starts at 19.5 °C and reaches 27.0 °C. Find the temperature rise.
The result is 7.5 °C. Known: a supervised reaction starts at 21.0 °C and reaches 28.5 °C. The temperature rise is 28.5−21.0=7.5 °C, supporting an exothermic interpretation with no external heater. A second starts at 22.0 °C and falls to 17.0 °C: the change is −5.0 °C and its cooling magnitude is 5.0 °C. Energy has been transferred, not destroyed.
Check the conclusion and its limits
- AQA 4.5.1.1 requires measuring temperature changes, not calculating transferred energy using heat capacity or calculating molar enthalpy. Those quantities are not substitutes for the required temperature evidence. A warmer that becomes hotter is not necessarily more suitable if it can burn skin or runs out too quickly.
- Return to the original observation. Explain what the result supports, which conditions it assumes, and one way to test a competing explanation.
An exothermic reaction destroys energy as its surroundings warm. This claim is false: AQA 4.5.1.1 requires measuring temperature changes, not calculating transferred energy using heat capacity or calculating molar enthalpy. Those quantities are not substitutes for the required temperature evidence. A warmer that becomes hotter is not necessarily more suitable if it can burn skin or runs out too quickly.
Energy transfers: explain warming and cooling: Distinguish the reacting chemicals from their surroundings. A thermometer in a reacting solution measures the temperature of that solution, which receives or supplies energy during the chemical change. A temperature rise supports an exothermic interpretation under the stated conditions; a fall supports endothermic behaviour. Heating a vessel externally can obscure this evidence. Not every process involving cooling is a chemical reaction: new substances must also be formed.
An exothermic reaction destroys energy as its surroundings warm.
AQA 4.5.1.1 requires measuring temperature changes, not calculating transferred energy using heat capacity or calculating molar enthalpy. Those quantities are not substitutes for the required temperature evidence. A warmer that becomes hotter is not necessarily more suitable if it can burn skin or runs out too quickly.
Transferring energy from reacting chemicals to the surroundings: write the technical term.
exothermic means Transferring energy from reacting chemicals to the surroundings.