The number of endothermic process/es from the following is ______________.
A. I₂( g) → 2 I(g)
B. HCl(g) → H(g)+Cl(g)
C. H₂ O(l) → H₂ O(g)
D. C(s)+O₂( g) → CO₂( g)
E. Dissolution of ammonium chloride in water

Solution

Chemistry ma’am · 3 steps

For each process, ask whether something is being pulled apart; if it is, heat has to go in.

1. Pulling apart. Breaking a bond, or separating molecules from a liquid, always needs energy. Such processes have Δ H > 0, so they are endothermic. Forming strong new bonds releases energy, so Δ H < 0.

   Process	What happens	Sign of Δ H
   A. I₂(g) → 2I(g)	I–I bond is broken (atomisation)	+, endothermic
   B. HCl(g) → H(g) + Cl(g)	H–Cl bond is broken (bond dissociation)	+, endothermic
   C. H₂O(l) → H₂O(g)	Hydrogen bonds between molecules are overcome (vaporisation)	+, endothermic
   D. C(s) + O₂(g) → CO₂(g)	Combustion, strong C=O bonds form	-, exothermic
   E. NH₄Cl in water	Breaking the lattice costs more energy than hydration gives back	+, endothermic

2. Dissolving NH4Cl. When NH₄Cl dissolves, the beaker feels cold. The solution absorbs heat from its surroundings, which is the sign of an endothermic process.
3. So. A, B, C and E are endothermic and only D is exothermic. The count is 4.

The common slip: 3. Taking the dissolution of NH₄Cl as exothermic, the way dissolving many salts is, gives 3. It actually absorbs heat, and the solution turns cold.

The idea to keep. Atomisation, bond dissociation, melting and vaporisation are always endothermic, and combustion is always exothermic. For dissolution, remember examples: NH₄Cl and KNO₃ cool the water down.

Answer from the NTA final answer key. Solution by PrepWiser.