Energetics

Exothermic and endothermic reactions, energy level diagrams, calorimetry and bond energies.

8 min read

Exothermic: products lower, energy released (ΔH negative). Endothermic: products higher, energy absorbed (ΔH positive).

An exothermic reaction releases energy, so the products are lower than the reactants and $Δ H$ is negative. An endothermic reaction absorbs energy ( $Δ H$ positive). If a reversible reaction is exothermic one way, it is endothermic the other.

Energy change from a temperature change: $q = m c Δ θ$ , where $m$ is the mass of solution, $c$ its specific heat capacity and $Δ θ$ the temperature change.

Worked example

Burning a fuel raises $100 g$ of water by $20° C$ ( $c = 4.2$ ). Find the energy released.

$q = m c Δ θ = 100 \times 4.2 \times 20 = 8400 J .$

Breaking bonds is endothermic; making bonds is exothermic. The overall change is $Δ H = (energy to break bonds) - (energy released making bonds)$ ; a negative result means exothermic.

Getting the sign of $Δ H$ backwards (exothermic is negative).
Thinking bond breaking releases energy (it requires energy).
Using the wrong mass in $q = m c Δ θ$ (use the solution being heated).

Energetics

Exothermic & endothermic

Calorimetry

Bond energies

Common mistakes