Energy in chemical reactions — Chemistry, 11–13 years
Chemical reactions can move energy between substances and their surroundings. Some warm things up, while others need energy from outside to happen.
Reactions can release or take in energy
Breaking and making chemical connections involves energy. In some reactions, more energy is released than taken in, so the surroundings warm up. In others, the reaction takes in more energy than it releases, so the surroundings cool down unless energy is supplied.
Why notice the temperature?
A reaction may look like a change of colour or bubbles, but those signs do not tell us where energy goes. Measuring temperature gives evidence about energy transfer. This helps explain why fuels can power engines and why some cold packs cool injuries without a freezer.
A warming reaction
Measure 20 ml of dilute hydrochloric acid at 20 °C, then add sodium hydroxide solution. Stir and measure again: the mixture reaches 27 °C. The temperature rose by 7 °C, so energy moved from the reacting chemicals into the mixture and its surroundings.
Hot does not mean more matter
It is reasonable to think that a reaction which gets hot has made extra energy appear, or that every reaction must give out heat. Temperature is only one way energy is transferred, and some reactions absorb energy instead. The chemicals have not become hotter because new matter was created.
Fuel, batteries and cold packs
Burning fuel releases energy that can move a car or heat a home. Batteries use chemical reactions to transfer energy as an electric current. Instant cold packs use a process that absorbs energy from nearby materials, making the pack and the skin feel colder.
Keep exploring
Other languages
Loading MyLeoNes™…