Chemistry That Does Work
Energetics & Enthalpy
13 minStudent plan
Why some reactions release heat and others absorb it, and how bond energies let you predict which.
After this lesson you can
- Say whether a reaction gives out heat or takes it in
- Explain what activation energy is and why a catalyst helps
- Compare fuels and batteries by energy density
Breaking costs, making pays
Breaking a chemical bond always requires energy. Forming one always releases it. A reaction's overall energy change is simply the difference between the two.
If more energy is released in forming new bonds than was spent breaking old ones, the surplus leaves as heat and the reaction is exothermic. Combustion, neutralisation and the reaction in a battery are all exothermic.
If the opposite is true, the reaction absorbs heat from its surroundings and is endothermic, which is why an instant cold pack goes cold when you break the seal.
Enthalpy change, ΔH, is this quantity measured at constant pressure. Negative means exothermic, and the sign convention catches people out constantly: energy released is a negative number, because the system has lost it.
In the full lesson
- Breaking costs, making pays
- Activation energy
- Calorific value and energy density
This lesson comes with the Student plan. The first module of Chemistry is free.