Le Chatelier's princple

Question

In which of the following reactions will an increasing pressure at constant temperature favor the formation of products?

(= is the arrow that goes both ways since I can not type that on my laptop)

A. CaCO3(solid) = CaO(solid) + CO2(g)

B. C(solid) + O2(g)= CO2(g)

C. 2NO(gas) + O2(gas) = 2NO2(g)

D. H2(gas) + Br2(gas) = 2HBr(g)

The answer is C. Please explain to me in detail how to solve this. (preferably by typed explanation,

if handwriting please avoid cursive, thank you in advance.)

Answer 1

According to Le Chatelier's principle, the equilibrium will shift in a direction to counteract the stress placed on it. So, when you are dealing with gases, if you change the pressure, this is a stress, and the system will react to alleviate the stress. When you increase pressure, the side of the reaction that has the most moles of gas will be affected more than the side with the fewer number of moles of gas. So, the equilibrium will shift to the side with the fewer moles of gas.

A. CaCO3(solid) = CaO(solid) + CO2(g) - 1 mol gas on left. Increase pressure shifts equilib. to reactants

B. C(solid) + O2(g)= CO2(g) - equal mols gas on both side. Increase pressure has no effect

C. 2NO(gas) + O2(gas) = 2NO2(g) - 3 mols gas on left, 2 on right. Increase pressure shifts equil. to products

D. H2(gas) + Br2(gas) = 2HBr(g) - 2 mols gas on each side. Increase pressure has no effect