Reactions That Release or Absorb Heat

Activities to try

1. 1

   Measure, graph and classify: a temperature-change bench

   Why it works: Every chemical reaction — and many physical changes such as dissolving — involves an energy transfer with its surroundings, and the thermometer reading is the everyday signature of which way energy flowed. 'Mixing chemicals' can make things colder as well as hotter, because energy can be absorbed from the surroundings.

   1. Choose a reaction or dissolving process (e.g. acid + alkali neutralisation, a combustion example, dissolving ammonium nitrate, or citric acid + sodium bicarbonate) and record the starting temperature. Note that dissolving is a physical change, but it still involves an energy transfer with the surroundings.
   2. Begin the reaction or dissolving process (e.g. pour the reactants together, or stir the salt into water; a combustion example would be run as a demonstration rather than mixed in a cup) and watch the thermometer, recording temperature over time to produce a temperature–time trace.
   3. Decide from the direction of change whether energy flowed OUT to the surroundings (temperature rose, exothermic) or IN from the surroundings (temperature fell, endothermic).
   4. Run at least one case you expect to warm and one that unexpectedly cools (dissolving ammonium nitrate, or the sherbet reaction), and compare.
   5. Sort each result into an 'exothermic' or 'endothermic' bin and state the rule you discovered: temperature up = energy out; temperature down = energy in.
2. 2

   Design a hand-warmer or cold pack to a target temperature

   Why it works: Exothermic and endothermic processes are the basis of real devices; choosing the right direction of energy transfer lets you engineer a warming or cooling effect.

   1. Pick a goal: a device that gets warm (hand-warmer) or one that gets cold (instant cold pack).
   2. Decide whether you need an exothermic or endothermic process to reach that goal, explaining the direction of energy transfer.
   3. Assemble, test and measure the temperature your device reaches, then modify it to get closer to a target temperature.
   4. Explain in terms of energy transfer with the surroundings why your device feels hot or cold.
3. 3

   Energy changes in changes of state, and conservation

   Why it works: Energy changes accompany physical changes (melting, freezing, dissolving) as well as dramatic reactions, and because energy is conserved, cooling means energy was transferred from the water to the dissolving/reacting system, not that 'cold' arrived.

   1. Classify melting ice and freezing water as endothermic or exothermic, justifying each by whether energy is absorbed from or released to the surroundings.
   2. Explain how a cold pack that cools works even though 'nothing cold' was added.
   3. State what gained the energy when the water in a cold pack cooled, using the idea that total energy is conserved.