Energy Change in Chemical Reactions

In an endothermic reaction, heat is absorbed from the surroundings, causing the temperature of the surroundings to decrease. This means energy acts as a reactant, and the enthalpy change (ΔH) is positive.

Examples of Endothermic Reactions:

1. Decomposition of Calcium Carbonate

• When calcium carbonate (CaCO₃) is heated, it decomposes into calcium oxide (CaO) and carbon dioxide (CO₂) while absorbing heat.
• Equation: CaCO₃(s) + heat → CaO(s) + CO₂(g), ΔH = +177.8 kJ

2. Physical Changes Involving Heat Absorption

• Melting of ice: Ice absorbs heat to become water.
• Dissolution of potassium nitrate (KNO₃) in water: Absorbs heat during dissolution.

In an exothermic reaction, heat is released into the surroundings, causing the temperature of the surroundings to increase. Here, energy is a product, and the enthalpy change (ΔH) is negative.

Examples of Exothermic Reactions:

1. Combustion of Methane Gas

• When calcium oxide (CaO) reacts with water (H₂O), it forms calcium hydroxide (Ca(OH)₂) and releases heat.
• Equation: CaO(s) + H₂O(l​) → Ca(OH)₂(aq​) + heat, ΔH = −890.4 kJ

2. Physical Changes Involving Heat Release

• Freezing of water (formation of ice): Heat is released as water turns into ice.
• Dissolution of sodium hydroxide (NaOH) in water: Produces heat when dissolved.

Precaution in Exothermic Reactions:

• Some exothermic reactions release a large amount of heat rapidly, which can be dangerous. For example, while diluting concentrated sulphuric acid (H₂SO₄) with water, heat is released so quickly that water evaporates instantly, leading to splashes and burns.
• Safety Tip: Always add acid to water slowly with constant stirring to control the heat release and prevent splashing.

1. Aim: To observe and compare endothermic and exothermic processes by dissolving potassium nitrate (KNO₃) and sodium hydroxide (NaOH) in water.

2. Requirements

• Apparatus: two plastic bottles, a measuring cylinder, and a thermometer.
• Chemicals: Potassium nitrate (KNO₃), sodium hydroxide (NaOH), water.

(Note: NaOH is corrosive; handle it carefully under supervision.)

3. Procedure

• Take 100 mL of water in each of the two plastic bottles. (Plastic prevents heat dissipation.)
• Measure and record the initial temperature of the water in both bottles.
• Dissolve 5 g of potassium nitrate (KNO₃) in one bottle and shake well.
• Dissolve 5 g of sodium hydroxide (NaOH) in the other bottle and shake well.
• Measure and record the final temperature of both solutions.

4. Observations

• The temperature decreases when KNO₃ dissolves, indicating heat absorption from surroundings (endothermic process).
• The temperature increases when NaOH dissolves, indicating heat release (exothermic process).

5. Conclusion

• The dissolution of KNO₃ is an endothermic process, as heat is absorbed, lowering the solution temperature.
• The dissolution of NaOH is an exothermic process, as heat is released, increasing the solution temperature.