Question

Does the following reaction show an increase or decrease in entropy? N2(g)  +  3 H2(g)  --->  2 NH3(g)  

Answer 1

A decrease in entropy.

Entropy is associated with the degree of disorder, or number of degrees of freedom. In the reaction `N_2 + 3H_2 --> 2NH_3` we start with four moles of gaseous reactants! and end with two moles of gaseous products, which is less disordered.

In general, when a reaction has fewer moles of gaseous products than the moles of gaseous reactants, this will be reflected in a decrease in entropy, and vice versa. It's a good job the enthalpy change for this reaction is exothermic, or it would be impossible to make ammonia from nitrogen and hydrogen.