Exothermic and Endothermic Reactions

Lecture Notes: Exothermic and Endothermic Reactions

1. Introduction to Chemical Reactions

• Chemical Reaction: A process where substances (reactants) change to form new substances (products).
• Energy in Reactions: Every chemical reaction involves a change in energy, often in the form of heat.

• This energy change is key to understanding exothermic and endothermic reactions.

2. Exothermic Reactions

Definition: Reactions that release energy, usually in the form of heat, light, or sound. The energy released comes from the chemical bonds of the reactants.

• Example: Combustion of wood, where heat and light are produced.

Characteristics:

• Surrounding temperature increases.
• Reactants have higher energy than products.
• Energy is released to the surroundings.

Key Examples:

• Combustion: Burning of fuels (e.g., natural gas, wood).
• Respiration: The metabolic process in cells that releases energy.
• Neutralization: Acid + base reactions (e.g., HCl + NaOH).

Energy Diagram:

• Reactants → (energy released) → Products

• The products are at a lower energy level than the reactants.

3. Endothermic Reactions

Definition: Reactions that absorb energy from the surroundings. These reactions require energy to proceed, usually in the form of heat.

• Example: Photosynthesis, where plants absorb light energy to convert CO₂ and H₂O into glucose and oxygen.

Characteristics:

• Surrounding temperature decreases.
• Products have higher energy than reactants.
• Energy is taken in from the surroundings.

Key Examples:

• Photosynthesis: Plants absorbing sunlight to produce glucose.
• Dissolving of ammonium chloride in water: The solution cools down as the reaction absorbs heat.
• Thermal decomposition: Breaking down calcium carbonate (CaCO₃) to form calcium oxide (CaO) and CO₂ requires heat.

Energy Diagram:

• Reactants → (energy absorbed) → Products

• The products are at a higher energy level than the reactants.

4. Comparison of Exothermic and Endothermic Reactions