Exothermic Reactions:
1. Combustion of Methane:
- Balanced Reaction: CH4 + 2O2 → CO2 + 2H2O
- Explanation: In this reaction, methane (CH4) reacts with oxygen (O2) to produce carbon dioxide (CO2) and water (H2O) along with the release of heat and light. This is a classic example of an exothermic reaction as it releases energy in the form of heat.
2. Neutralization of Hydrochloric Acid and Sodium Hydroxide:
- Balanced Reaction: HCl + NaOH → NaCl + H2O
- Explanation: When hydrochloric acid (HCl) reacts with sodium hydroxide (NaOH), salt (NaCl) and water (H2O) are formed with the release of heat. This reaction is exothermic as it releases energy in the form of heat.

Endothermic Reactions:
3. Photosynthesis:
- Balanced Reaction: 6CO2 + 6H2O + light energy → C6H12O6 + 6O2
- Explanation: In the process of photosynthesis, carbon dioxide (CO2) and water (H2O) react in the presence of sunlight to produce glucose (C6H12O6) and oxygen (O2). This reaction requires energy in the form of light, making it an endothermic reaction.
4. Thermal Decomposition of Calcium Carbonate:
- Balanced Reaction: CaCO3 → CaO + CO2
- Explanation: When calcium carbonate (CaCO3) is heated, it decomposes into calcium oxide (CaO) and carbon dioxide (CO2). This reaction absorbs heat from the surroundings, making it endothermic.
5. Evaporation of Water:
- Balanced Reaction: H2O(l) → H2O(g)
- Explanation: When liquid water (H2O) evaporates to form water vapor (H2O(g)), it absorbs heat from the surroundings to overcome the intermolecular forces holding the liquid together. This process is endothermic as it requires energy input to occur.