Summary
In chemical reactions, heat energy changes occur as energy is either absorbed or released, affecting the temperature of the surroundings.
- Exothermic Reaction — a reaction that releases energy into the surroundings, increasing the temperature. Example: Combustion reactions.
- Endothermic Reaction — a reaction that absorbs energy from the surroundings, decreasing the temperature. Example: Thermal decomposition.
- Activation Energy — the minimum energy required to start a chemical reaction. Example: Energy needed to initiate combustion.
- Bond Energy — the energy required to break a bond or released when a bond is formed. Example: Energy calculations in bond formation and breaking.
Exam Tips
Key Definitions to Remember
- Exothermic Reaction: Releases energy, feels hot.
- Endothermic Reaction: Absorbs energy, feels cold.
- Activation Energy: Minimum energy needed to start a reaction.
Common Confusions
- Exothermic reactions feel hot, not cold.
- Endothermic reactions feel cold, not hot.
Typical Exam Questions
- Which is a correct definition of rate of reaction? Increase in amount of products against time
- In which of the chemical reactions can the rate be determined by measuring the change in the gas volume? Calcium carbonate with dilute hydrochloric acid
- In an endothermic reaction the products have ........ ? More energy than the reactants
- If the energy required to break the bonds is less than the energy given out by making new bonds the reaction is ...........? Exothermic
What Examiners Usually Test
- Understanding of exothermic and endothermic reactions.
- Ability to calculate energy changes using bond energies.