Which equation represents an exothermic reaction at 298 K? **Hint: Use Ref Tbl I**

N2(g) + O2 (g) → 2 NO (g)
4 Al (s) + 3 O2 (g) → 2 Al2O3 (s)
H2 (g) + I2 (g) → 2 HI (g)
2 C (s) + 2 H2 (g) → C2H4 (g)

2C + 2H₂ →C₂H₄ is the exothermic reaction.

Option D

Explanation:

The reactions which will be giving energy out to the surroundings are termed as exothermic reactions. In the first equation, the reactants as well as the products are in the same phase of gas, so no energy will be released.

Then for the second equation, one of the reactants is a solid while another is a gas and the product formed is also a solid, so more energy will be required to form a solid.

This is because, solid has tight bond formation, so the energy required to form a solid as a product should also be more. Thus, the first two reactions cannot be exothermic.

In the third reaction also, the reactants and products are gases, so there is very less chance of release in energy. But in the fourth reaction, one of the reactant is a solid while the other is a gas and the product formed is also in gas phase.

So the excess of energy which should have been present during the breakage of bonds of solid reactant will be released after utilizing the necessary amount of energy for the compound formation in gas phase. Thus, the fourth reaction will be exothermic in nature.