Predict in which of the following, entropy increases/decreases:
(i) A liquid crystallizes into a solid.
(ii) Temperature of a crystalline solid is raised from 0 K to 115 K.
(iii) `2NaHCO_(3(s)) to Na_(2) CO_(3(s)) + CO_(2(g)) + H_(2) O_((g))`
(iv) `H_(2(g)) to 2H_((g))`

Explanation

(i) After freezing, the molecules attain an ordered state and therefore, entropy decreases.
(ii) At 0 K, the constituent particles are static and entropy is minimum. If temperature is· raised to 115 K, these begin to move and oscillate about their equilibrium positions in the lattice and system becomes more disordered, therefore entropy increases.
(iii) Reactant, `NaHCO_(3)` is a solid and it has low entropy. Among products there is one solid and two gases. Therefore, the products represent a condition of higher entropy.
(iv) Here one molecule gives two atoms i.e., number of particles increases leading to more disordered state. Two moles of H atoms have higher entropy than one mole of dihydrogen molecule.