Exothermic and endothermic reactions

Every chemical reaction transfers energy to or from the surroundings. An exothermic reaction gives out energy, so the surroundings get warmer: burning, respiration and neutralisation are exothermic. An endothermic reaction takes in energy, so the surroundings get colder (or energy must be supplied): thermal decomposition and photosynthesis are endothermic. A reaction profile shows the energy of the reactants and products and the activation energy needed to start the reaction.

Key facts

  • Exothermic: energy is given out to the surroundings and the temperature rises. Examples: combustion, respiration, neutralisation, most oxidation reactions.
  • Endothermic: energy is taken in from the surroundings and the temperature falls. Examples: thermal decomposition, photosynthesis, citric acid with sodium hydrogencarbonate.
  • Energy is conserved: whatever the reaction gives out, the surroundings gain.
  • The activation energy is the minimum energy particles need to collide and react. A catalyst lowers it.
  • On a reaction profile the products are lower than the reactants for an exothermic reaction and higher for an endothermic one.
  • The energy change is ΔH = energy of products − energy of reactants: negative for exothermic, positive for endothermic.
  • Uses: hand warmers and self-heating cans (exothermic); instant cold packs for sports injuries (endothermic).

Worked examples

Example 1

A student mixes 25 cm³ of hydrochloric acid and 25 cm³ of sodium hydroxide solution in a polystyrene cup. The temperature goes from 19.5 °C to 26.0 °C. Is the reaction exothermic or endothermic?

  1. Find the change in temperature. 26.0 − 19.5 = 6.5 °C
  2. The temperature rose, so the reaction gave out energy to the surroundings (the solution and the cup).
  3. So the neutralisation is exothermic. The polystyrene cup and lid reduce energy lost to the air.

Answer: exothermic: the temperature rose by 6.5 °C

Example 2

Use the reaction profile to find the activation energy and the energy change for this reaction. Is it exothermic or endothermic?

  1. The reactants are at 100 kJ/mol, the peak at 230 kJ/mol and the products at 40 kJ/mol.
  2. Activation energy: from the reactants up to the peak. 230 − 100 = 130 kJ/mol
  3. Energy change: products minus reactants. ΔH = 40 − 100 = −60 kJ/mol
  4. The products are lower than the reactants and ΔH is negative, so the reaction is exothermic.

Answer: activation energy 130 kJ/mol; ΔH = −60 kJ/mol; exothermic

Example 3

Sketch the reaction profile for an endothermic reaction and show the effect of a catalyst.

  1. Draw the products higher than the reactants: energy is taken in.
  2. Draw a hump between them: the activation energy is from the reactants to the top of the hump.
  3. A catalyst gives a different route with a lower hump. The reactants and products, and so ΔH, do not change.

Answer: products above reactants; the catalyst gives a lower peak