Chapter 13, Problem 68APP

According to Le Châtelier's principle, does the equilibrium shift in the direction of products or reactants when ${\text{N}}_2$ is added to the equilibrium mixture of each of the following reactions? (13.5)
a. $2{\text{NH}}_3(g)\rightleftarrows 3{\text{H}}_2(g)+{\text{N}}_2(g)$
b. ${\text{N}}_2(g)+{\text{O}}_2(g)\rightleftarrows 2\text{NO}(g)$
c. $2{\text{NO}}_2(g)\rightleftarrows {\text{N}}_2(g)+2{\text{O}}_2(g)$
d. $4{\text{NH}}_3(g)+3{\text{O}}_2(g)\rightleftarrows 2{\text{N}}_2(g)+6{\text{H}}_2\text{O}(g)$