What are the sign of the entropy change (+ or -) in the following:
I : A liquid crystallises into a solid
II: Temperature of a crystalline solid is raised from 0K to 115K
III: `2NaHCO_(3(g)) rarr Na_(2)CO_(3(g)) + CO_(2(g)) + H_(2)O_((g))`
IV : `H_(2(g)) rarr 2H_((g))`

Explanation

i) After freezing, the molecules attain an ordered state and therefore, entropy decreases.
ii) At 0 K, the consituent particles are static and entropy is minimum. If temperature is raised to 115K, 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 are 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.