Question

The ∆ f H° of O₃, CO₂, NH₃ and HI are 142.2, –393.3, –46.2
and + 25.9 kJ per mol respectively. The order of their
increasing stabilities will be
(a) O₃, CO₂, NH₃, HI (b) CO₂, NH3, HI, O₃
(c) O₃, HI, NH₃, CO₂ (d) NH₃, HI, CO₂, O₃

Answer 1

$\huge\mathcal\red{Answer:-}$

• (c) O₃, HI, NH₃, CO₂

Answer 2

The Heat of formation is defined as the amount of heat required to form 1 mole of a substance from its most stable forms of constituents.

• Greater the heat of formation, tells us that the compound requires greater amount of heat in its formation.
• If large quantity of heat is consumed by it to form its 1 mole, then it shows that the compound has higher energy in it.
• The thermodynamically stable compound always prefers to be in lower energy state.
• As higher energy compound can liberate higher energy, by reacting with the other compound.
• The relation is therefore, Higher the energy, lower the stability.
• Now, by seeing the options given we can say, Ozone has higher energy of formation ,i.e lower stability and CO₂ has lower energy and greater stability.
• Therefore the increasing order of stability is: c) O₃, HI, NH₃, CO₂