2.4 Chemical reactions and energy
Cards (6)
- Exothermic reactions
- An exothermic reaction is one that transfers energy to the surroundings so the temperature of the surroundings increases
- Examples of exothermic reactions include combustion, many oxidation reactions and neutralisation
- Everyday examples of exothermic reactions include self-heating cans (e.g. for coffee) and hand warmers
- Endothermic reactions
- An endothermic reaction is one that takes energy in from the surroundings so the temperature from the surroundings decrease
- Examples of endothermic reactions are thermal decomposition and the reaction of citric acid and sodium hydrogen carbonate
- Some sports injury packs are based on endothermic reactions
- Chemical reactions can occur only when reacting particles collide with each other and with sufficient energy
- Activation energy is the minumum amount of energy that particles must have to react
- Endothermic reaction energy profile
- Exothermic reaction energy profile
- During a chemical reaction:
- Energy must be supplied to break bonds in the reactants
- Energy is released when bonds in the products are formed
- Energy needed to break bonds and energy released whebn bonds are formed can both be calculated from bond energies
- Overall energy change = Sum of energy to break bonds - sum of energy released when bonds form
- If the overall energy change is negative the reaction is exothermic
- If the overall energy change is positive the reaction is endothermic
- Energy needed to break > energy released ENDOTHERMIC
- Energy needed to break < energy released EXOTHERMIC