Changes in Pressure

[Alondra Cortes 1F]

Textbook Question 5J.5
State whether reactants of products will be favored by an increase in the total pressure (resulting from compression) on each of the following equilibria. If there is no change, explain why that is so.

(b) H20(g) + C(s) <--> H2(g) + CO(g)

According to the answer key, the equilibrium would shift towards the reactants. But how do we know this? I know that increasing total pressure shifts the equilibrium to the side with less moles of gas. But a decrease in pressure will cause the equilibrium to shift towards the side with more moles of gas. However, I'm not really understanding why or how that works. What would be a better way to visualize or think about this?

Thanks!

[Shannon Shams Disc 3I]

I think the best way to visualize this is using the ideal gas law (PV = nRT). Le Chatlier's principle states that a system will go in the direction that reduces the effect of any change on that system. By decreasing the volume, the pressure has gone up, and now that volume is constant, we need to find a different term in the equation to change. The only option that can change with the direction of the reaction is moles, and since we know increasing the moles will increase the pressure, the reaction will go in the direction that has more moles of gas.

Conversely and also more correctly, you can think of how decreasing the volume will increase the concentration of all species in the container. If you recalculate Q, it should be greater than K, indicating that the reaction will shift towards the reactants.