# Equilibrium - Pressure Vs. Concentration [closed]

Does increasing the pressure increase the concentration of reactants as well?

If: $$\ce{A(g) + B(g)<=>C(g) + D(g)}$$

Assuming all reactants and products are gases.

According to Le Chatelier's Principle, increasing the pressure will cause the system to shift wherever has the least amount of gaseous molecules. However, in this case, the molar ratios are 1:1; thus, increasing the pressure will not cause a shift. That being said, when the pressure is increased, does the concentration of reactants and products both go up? Or do they maintain the same.

Thanks!

## closed as off-topic by andselisk♦, Mithoron, Todd Minehardt, Tyberius, aventurinFeb 22 at 6:12

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• There are some cases where le chateliers fails if you follow typical wisdom. It is better to approach it quantitatively using differential sensitivity analysis. This is probably out of the scope of the question, but it works every time. – Charlie Crown Feb 22 at 3:17
• See the following for a prime example of why you can't just follow Le Chateliers principle blindly... researchgate.net/publication/… – Charlie Crown Feb 22 at 3:26

## 1 Answer

when the pressure is increased, does the concentration of reactants and products both go up? Or do they maintain the same?

Le Chatelier's principle in its most general form makes statements about what happens to a reaction that used to be at equilibrium when changes are made to concentrations, temperature or pressure.

To keep things simple, let's say the temperature stays constant, but we are changing the overall pressure of the reaction mix by decreasing the volume. As a result, all concentrations (or partial pressures) will increase by the same factor. If the sum of the stoichiometric factors for reactants in the gas phase is equal to that of the products, the reaction quotient Q will not change (all factors cancel out) and the system stays at equilibrium. If this is not the case, the reaction will shift to re-establish equilibrium.

On the other hand, if you change the pressure at constant volume by changing the temperature, the concentrations (partial pressures) will stay the same, but you are out of equilibrium anyway because the equilibrium constant is temperature dependent.

In both cases, you can use Le Chatelier's rules to predict in which direction the reaction has to shift to re-establish equilibrium.