Energy changes in chemical reactions — Chemistry, 14–17 years
Chemical reactions can transfer energy to the surroundings or take energy from them. Chemistry, 14–17 years.
Reactions move energy
Breaking and forming chemical bonds involves energy. In a reaction, the energy needed to break old bonds may be greater or smaller than the energy released when new bonds form. If the surroundings warm up, the reaction is exothermic; if they cool down, it is endothermic. Energy has moved, even though the substances have changed.
Why measure the energy change?
Chemists need to know whether a process will heat a room, cool a pack or require a supply of energy. Simply saying that a reaction happens does not answer those practical questions. Measuring the temperature change gives evidence about the direction and size of the energy transfer, so a reaction can be chosen and controlled.
Finding energy transferred to water
A reaction heats 100 g of water from 20 °C to 25 °C. The temperature rise is 5 °C. Using q = mcΔT, with water’s specific heat capacity 4.18 J/(g °C), q = 100 × 4.18 × 5 = 2,090 J. The water gained 2,090 J, so the reaction released at least that much to it, ignoring heat lost elsewhere.
Temperature is not total energy
A common mistake is to say that the hotter sample contains more total energy. Temperature measures the average energy of particles, not the total amount. A large warm bath can contain more energy than a tiny cup of boiling water, because it has far more matter. This is why mass matters in q = mcΔT.
Energy changes in everyday technology
Fuels release chemical energy to move vehicles and generate electricity. Instant cold packs use a process that absorbs heat, while hand warmers use one that releases it. Engineers compare these energy changes when designing batteries, heating systems and industrial processes. The idea applies only when the chemical process is actually responsible for the transfer.
Keep exploring
Other languages
Loading MyLeoNes™…