43. Exothermic & Endothermic Reactions

1. Key Definitions

• Exothermic Reaction: A reaction that transfers energy to the surroundings, usually in the form of heat. This results in a temperature increase in the surroundings.

• Endothermic Reaction: A reaction that takes in heat energy from the surroundings. This results in a temperature decrease in the surroundings.

• Activation Energy: The minimum amount of energy that reactant particles must have to collide and react with each other.

2. Exothermic Reactions

• Energy Change: The products have less energy than the reactants.

• Reaction Profile: The products are shown at a lower energy level than the reactants.

• Common Examples:

  • Combustion: Burning fuels.

  • Neutralisation: Reactions between acids and bases.

  • Oxidation: Most oxidation reactions.

3. Endothermic Reactions

• Energy Change: The products have more energy than the reactants because energy is absorbed.

• Reaction Profile: The products are shown at a higher energy level than the reactants.

• Common Examples:

  • Thermal Decomposition: Such as breaking down calcium carbonate into calcium oxide and carbon dioxide.

4. Reaction Profiles and Activation Energy

Reaction profiles use a y-axis for total energy and an x-axis for the progress of the reaction.

• The Curve: A curve is drawn from the reactants to the products to show energy changes.

• Activation Energy Location: This is the energy difference between the reactants' level and the highest point (peak) on the curve.

• Starting the Reaction: Even exothermic reactions require some initial energy (activation energy) to get started.

• Specific Labeling: When drawing a profile for a specific reaction, you should replace the generic terms "reactants" and "products" with the actual chemical formulas from the equation.