Final answer:
Adding a noble gas does not change concentrations or Keq, while decreasing volume increases concentrations and causes the equilibrium to shift in the direction that produces fewer moles of gas.
Step-by-step explanation:
When the pressure of a reaction is increased by adding a noble gas, the concentrations of all things involved in the reaction do not change. This is because the noble gas does not participate in the reaction. The equilibrium constant (Keq) also remains unchanged.
On the other hand, if the pressure of a reaction is increased by decreasing the volume, the concentrations of the reactants and products that are gases will increase. This is because decreasing the volume increases the pressure, which forces the gas particles closer together, thus increasing their concentration. The equilibrium will shift in the direction that produces fewer moles of gas in order to reduce the pressure. The equilibrium constant, however, will remain the same.