Study Notes
Energy changes in reactions involve the transfer of energy as bonds are broken and formed, resulting in either exothermic or endothermic processes.
- Chemical Reaction — a process where substances change to form new substances.
Example: Combustion of wood. - Reactants — substances that start a chemical reaction.
Example: Hydrogen and oxygen in water formation. - Products — substances formed from a chemical reaction.
Example: Water from hydrogen and oxygen. - Exothermic Reaction — a reaction that releases energy to the surroundings.
Example: Combustion of fuels. - Endothermic Reaction — a reaction that absorbs energy from the surroundings.
Example: Photosynthesis in plants. - Bond Breaking — an endothermic process where energy is absorbed.
Example: Breaking of H-H bonds in hydrogen gas. - Bond Forming — an exothermic process where energy is released.
Example: Formation of H2O from H2 and O2.
Exam Tips
Key Definitions to Remember
- Chemical Reaction: A process where reactants change to form products.
- Exothermic Reaction: A reaction that releases energy to the surroundings.
- Endothermic Reaction: A reaction that absorbs energy from the surroundings.
Common Confusions
- Confusing exothermic with endothermic reactions.
- Misunderstanding that bond breaking is endothermic and bond forming is exothermic.
Typical Exam Questions
- Name two types of reaction that are usually exothermic? Combustion and neutralisation.
- Give an example of a change which is endothermic? Photosynthesis.
- Name one type of change that can be exothermic or endothermic? Displacement reactions.
- Explain why the temperature increases during an exothermic reaction? Because energy is released to the surroundings, increasing their temperature.
What Examiners Usually Test
- Understanding of energy transfer in reactions.
- Ability to identify exothermic and endothermic reactions.
- Explanation of bond breaking and forming in terms of energy changes.