Which one of the following is not an exothermic reaction?

Exothermic reactions release heat into the surroundings. The dissolution of calcium oxide in water (hydration of quicklime) is a highly exothermic combination reaction that produces slaked lime and significant heat [3]. Similarly, the decomposition of vegetable matter into compost is classified as an exothermic process [1]. The dissolution or dilution of sodium hydroxide in water is also known to evolve heat, making it exothermic [1]. In contrast, the decomposition of lead nitrate is a thermal decomposition reaction. Thermal decomposition reactions are endothermic because they require the absorption of energy, typically in the form of heat, to break the chemical bonds of the reactant into simpler substances. Therefore, the decomposition of lead nitrate is not an exothermic reaction.

Sources

1. [2] Science , class X (NCERT 2025 ed.) > Chapter 1: Chemical Reactions and Equations > C6H12O6 (aq) + 6O2 (aq) → 6CO2 (aq) + 6H2O(l) + energy (1.18) (Glucose) > p. 7
2. [3] Science , class X (NCERT 2025 ed.) > Chapter 1: Chemical Reactions and Equations > Figure 1.4 > p. 8
3. [1] Science , class X (NCERT 2025 ed.) > Chapter 1: Chemical Reactions and Equations > Figure 1.3 > p. 6