lesson
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Snap an instant cold pack, and it turns icy cold within seconds. Click a reusable hand warmer, and it instantly glows with heat.
To understand where that heat goes, we divide every experiment into two parts: the system (the reacting chemicals) and the surroundings (everything else, including the water, beaker, thermometer, and your hands).
๐Interactive diagram
So what happens to the thermometer when the system dumps heat into its surroundings?
Exothermic Reactions
An exothermic reaction transfers thermal energy (heat) from the reacting system to the surroundings. Because the surroundings absorb this energy, their temperature rises.
๐Interactive diagram
For example, when you mix 25ย mL of hydrochloric acid with 25ย mL of sodium hydroxide, the temperature rises from 21.0โC to 28.5โC. Since ฮT=+7.5โC, heat left the chemicals and warmed the water, confirming it is exothermic.
What happens if the chemicals need energy from the outside world just to react?
Endothermic Reactions
An endothermic reaction takes in thermal energy from the surroundings into the system. As the surroundings lose heat, their temperature drops.
๐Interactive diagram
When solid ammonium nitrate dissolves in water, the temperature plunges from 22.0โC down to 14.0โC. Because ฮT=โ8.0โC, the reaction absorbed energy from the solvent, marking it as endothermic.