n which of the following gas-phase equilibria is the yield of products increased by increasing the total pres?

Question

In which of the following gas-phase equilibria is the yield of products increased by increasing the total pressure on the reaction mixture?

a) CO(g) + H2O(g) CO2(g) + H2(g)
b) 2NO(g) + Cl2(g) 2NOCl(g)
c) 2SO3(g) 2SO2(g) + O2(g)
d) PCl5(g) PCl3(g) + Cl2(g)

Answer 1

johnny m gave the correct answer and reason.
To expand on what he is saying, you want to find the equation in which there are fewer (gas) molecules on the right side of the (balanced) equation than on the left. This is true of only b, where 3 molecules of reactant produce 2 molecules of product.

The rule johnny m gives is a very important one in understanding equilibria. A more general way to state it is, "When a stress is placed on a system that is in equilibrium, the equilibrium will shift in the direction that tends to relieve that stress." In this particular problem, the stress is increased pressure, and b reduces the pressure by going toward more product. In another case, the stress could be a reduction in pressure, or an increase in temperature, or the introduction of additional reactant (or additional product). In each case, the equilibrium will shift to partially offset that stress (or that change in conditions).
Hope that helps you.

Answer 2

b. increase in pressure causes equilibrium to favor lower number of molecules.