Effect of inert gas on Q
Wouldn't the partial pressures of all the reactants change by the same factor, so Q wouldn't change?


If the number of gas particles is the same for reactants and products, then Q would not change. However, if you have a reaction like 

$$\ce{2N2O(g) &lt;=&gt; N2O4(g)}$$

even though the reactant and product partial pressure changes by the same factor, the changes don't cancel out because the coefficients are different.

$$ Q = \frac{[\ce{N2O4}]}{[\ce{N2O}]^2}$$