CHEMISTRY COMMUNITY

When determining the change in pressure, how do you know whether the "x" will be positive or negative? What determines this?

If the reaction is going to the right then the products will gain some partial pressure so it will be a positive x value for the product and a negative x value for the reactants. The same is true for the opposite in which a reaction going to the left should have a negative x value for the products and a positive x value for the reactants.

If the question gives you the pressures for all of the substances in the reaction (both the reactants and products), calculate Q and compare Q to K. If Q > K then X will be negative for the products and positive for the reactants as the reaction favors the formation of reactants. If Q < K then X will be positive for your products and negative for your reactants as the reaction favors the formation of products.

You have to calculate Q first.
If Q < 1, then you subtract from the reactants as products are forming. If Q > 1, then you add to the reactants!