# Predict the sign of Delta S and then calculate Delta S for each of the fololwoing reactions?

a) H2(g) + 1/2O2(g) = H2O(l)

b) N2(g) + 3H2(g) = 2NH3(g)

c) HCl(g) = H^+(aq) + Cl^-(aq)

can anyone explain this to me ?

• Entropy (ΔS) is a measure of molecular randomness or disorder of a system. If in a process disorder increases, the sign of ΔS is positive, if disorder decreases the sign is negative.

(a) two freely move gaseous reactants produce a liquid, a condensed form of matter. Disorder decreases. ΔS < 0.

(b) gaseous reactants form a gaseous product. But, 3 moles of reactant form 2 mole of product. Number of freely move particles decreases, dis order decreases, ΔS < 0.

(c) gaseous substance dissolves in water and produce 2 ions. Although it seems that disorder increases, it is not. Because you confine a freely move substance into a restricted volume. Its disorder decreases, ΔS < 0.

In order to calculate the ΔS values you will need a thermodynamic data.

ΔS = S final - S initial

(a) ΔS = S(H2O) - [S(H2) + 1/2S(O2)]

ΔS = 70 - [131 + 1/2(205)] = - 163.5 J/K.mol

(b) ΔS = 2S(NH3) - [S(N2) + 3S(H2)]

ΔS = 2x193 - [192 + 3(131)] = - 199 J/K.mol

(c) ΔS = [S(H+) + S(Cl-)] - S(HCl)

ΔS = [(0) + (57)] - (187) = - 130 J/K.mol