Question

Arrange the following reaction in order of increasing S value.

1. H2(g) + F2(g) 2HF(g)

2. NH4NO3(s) N2O(g) + 2H2O(I)

3. (NH4)2 Cr2O7(s) Cr2O3(s) + 4H2O(I) +N2(g)

Answer 1

CRITERIA USED FOR ARRANGING :

Entropy (S) tells us about the degree of randomness or disorder in a system. A system which is more disordered possesses higher entropy.

In a chemical reaction, if more molecules are present, then there are more collisions amongst them, which means greater randomness or disorder. Thus we simply calculate the change in the number of molecules in the system. If the number of moleclues increase, then there is more entropy change.

Reaction 1 : H₂(g)+ F₂(g)---> 2 HF (g)

Number of molecules of reactant, n_r = 1+1 = 2

Number of molecules of product, n_p = 2

So, change in number of molecules = n_p-n_r = 0

Similarly,

Reaction 2 : NH₄NO₃ (s) ----> N₂O(g) + 2H₂O (l)

n_p-n_r = 3-1 = 2

Reaction 3 : (NH₄)₂Cr₂O₇ (s) -----> Cr₂O₃ (s) + 4 H₂O (l) + N₂ (g)

n_p-n_r = 6-1 = 5

Thus, from the np-nr values calculated above, the correct order for increasing S values is :

Reaction 1 < Reaction 2 < Reaction 3