Sulfur trioxide gas, one of the causes of acid rain, is produced in the upper atmosphere when oxygen reacts with sulfur dioxide gas in the reaction shown below:

Question

2SO2(g) + O2(g) <--> 2SO3(g) deltaH0 = -197kJ

The gases are placed in a reaction vessel and allowed to come to equilibrium at temperature T, pressure P, and volume V. Predict and explain the effects that each of the following will have on the equilibrium composition of the reaction:

1. The partial pressure of SO3(g) is increased by the addition of SO3(g).
2. The pressure in the vessel is increased by the addition of He.
3. The total pressure in the vessel is increased by decreasing the volume of the vessel.
4. The temperature of the system is decreased.
5. The partial pressure of O2(g) is decreased.

Answer 1

This is all about Le Chatelier's Principle. In a nutshell, that says that a system will shift to undo what we've done to it.
Here is the same equation rewritten to include the heat.
2SO2(g) + O2(g) <--> 2SO3(g) + heat

1. If we add SO3 the reaction will try to get rid of the added SO3. How can it do that? By reacting to the left.
2. Addition of He has no effect since the partial pressures of the components remain the same.
3. Total pressure is increased. The system will try to decrease the pressure. How can it do that? By decreasing the number of moles (it's the molecules impinging on the walls that contributes to the pressure). You have 3 mol on the left; two on the right. Therefore, it will move to the right (to the side with fewer moles of gas)
I'll be glad to critique your thoughts on the others.