Exothermic reactions transfer energy to the surroundings, so the temperature rises; combustion, neutralisation and hand warmers are examples. Endothermic reactions take in energy from the surroundings, so the temperature falls; thermal decomposition and sports ice packs are examples.
A reaction profile shows the energy of reactants and products. In an exothermic reaction the products are lower in energy than the reactants. The activation energy is the minimum energy particles need to react: the hump on the profile. Breaking bonds takes in energy and making bonds releases energy. If more energy is released making new bonds than is taken in breaking old ones, the reaction is exothermic overall: ΔH = energy in (breaking) − energy out (making), and is negative.
Worked example
Breaking bonds takes in 1370 kJ and making bonds releases 1530 kJ. Is the reaction exothermic?
- ΔH = 1370 − 1530 = −160 kJ.
- A negative ΔH means more energy is released than taken in.
Answer:Yes, the reaction is exothermic.
Key words
- exothermic
- releases energy; temperature rises
- endothermic
- takes in energy; temperature falls
- activation energy
- minimum energy needed to react
- reaction profile
- a diagram of energy during a reaction
New questions every time.
NGSS-referenced: HS-PS1-4