# Does the way a reaction with gas moles shifts depend on Q or number of moles on either side of the reaction?

Given a container with the reaction $\ce{C2H4{(g)} + HCl{(g)} -> C2H5Cl{(g)}}$, if we inject $\ce{C2H5Cl{(g)}}$ to the container, what will happen?

I thought of this in two different ways and got two different answers:

1. If we inject $\ce{C2H5Cl{(g)}}$, $Q>K$ so reaction will shift to the left, leading to the formation of more $\ce{C2H4{(g)} and HCl{(g)}}$.
2. Injecting $\ce{C2H5Cl{(g)}}$ also increases the pressure I suppose, and if you increase pressure, the reaction will shift to the side with the least moles of gas. In this case, it will shift right, forming more $\ce{C2H5Cl{(g)}}$

Which answer is correct?

• Injecting chloroethane will increase the pressure of it. Once the pressure of the product is greater than the pressures of the reactants, the equilibrium will shift to the left, and so more reactant will react to oppose the change and restore Kp. Your first statement seems correct. I'm not sure though; the reaction isn't in equilibrium, so there isn't an equilibrium to be shifted. – user60221 Apr 21 '18 at 14:10