Endothermic and Exothermic Processes

The process in which heat is absorbed from the outside, is called endothermic process.

The processes in which heat is given out are called exothermic processes.

A chemical reaction in which heat is absorbed from the surroundings or must be continuously supplied is called an endothermic reaction.

A chemical reaction in which heat is released when reactants are converted to products is called an exothermic reaction.

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.