One part of determining the spontaneity of a reaction, is to determine whether or not the entropy of the system is increasing or decreasing. When we look at these reactions, what we are going to be looking at primarily are the moles of gas. When I look at my reactant side, from my first reaction I see I have 2 moles of gas compared to just 1 mole of gas on the product side. Because I'm decreasing the moles of gas, this is seen as a decrease, In entropy. For the second reaction, I have no moles of gas, going to 1 mole of gas, and so this, it would be an increase in the entropy of the system. For the last one, I go from 2 moles of gas to 1 mole of gas, and so again, I see this as a decrease in the entropy of the system. Understanding the entropy of the system and the entropy of the surroundings will allow me to predict whether or not a reaction is spontaneous or not.