Le chatelier's principle regarding pressure

[Ella Figueroa 2H]

There was a problem on the recent midterm in which I was confused on. The problem gave an equation at equilibrium, with an equal amount of gaseous molecules on either side. The problem asked what would the outcome look like if 3x the concentration of helium was added (if it would shift towards products or reactants or remain the same)

[jadyndavidson]

Hi! Not sure if this was the question you were asking about, but when you are increasing the pressure by adding inert gas (in this case helium) the equilibrium composition is unaffected; however, when the system is compressed, the equilibrium will shift towards the side that decreases the number of gas molecules the most.

[Cara W]

I agree that there would be no change in the concentrations. This could be confusing because if the pressure was increased by COMPRESSION, not adding another unrelated gas, then the reaction would go towards the side with fewer molecules. I think Dr. Lavelle said in lecture that its very important to pay attention to how the pressure is changed to determine its affect.

[Andres Ibarra 2F]

I'm pretty sure when pressure is increased from an inert gas the equilibrium constant won't be affected. Equilibrium will only shift when compressed to the side that has fewer mols.

[Miles Dailey 1K]

Increasing pressure through the addition of an inert gas (such as helium) does not affect the equilibrium constant, but increasing pressure through a decrease in volume does. When it is increased by decreasing volume, the gas molecules on each side of the equation hit the walls more frequently, and thus the reaction shifts to attempt to curb this. When an inert gas is added, the gas molecules on each side of the equation hit the walls with the same frequency, so the equilibrium is unaffected.

[tristenaguinaldo]

Hi! Equilibrium would remain the same as the amount of moles on the reactant and product side are equal and helium is an inert gas. However, the reaction would lean towards the reactants or products based on the amount of moles for gases on each side.

[Ashlinn_Limjoco_1I]

Hi,
I believe the question you are referring to is the one that included adding helium gas. Helium is an inert gas so the total pressure would change but not the partial pressure (which is what we are looking at). Also, because it is inert it would not be reacting with the products or reactants or changing rates of anything.

[Jonathan_Yu_2C]

when you add helium, it does not react with any of the reactants or products, it just overall increases the pressure. Because both sides have the same number of gas molecules, the increased pressure does not make reactants or products increase relatively.

[Trinabh Khera Sahni]

When there are unequal moles of gas on the reactants and products side and pressure is increased, then the reaction will shift in the direction of either the reactants or products depending on which one has fewer moles of gas. The opposite is true if the pressure is decreased. Therefore, if you have equal moles of gas on both the reactants and products side then changing the pressure will not have an effect on whether or not the reaction moves to the left or right, and therefore, the concentrations of the reactants and products stay the same.

[Margreet Gawargious lecture 2 section 2H]

Hey to solve this problem, you need to understand that Helium is an inert gas, so changes in its concentration do not affect equilibrium. If helium was not an inert gas, you would just have to evaluate whether it was a reactant or product and see whether its concentration was increasing or decreasing, based on this information you would then have to apply Chatelier's principle to offset the stressor/change. the chart below can help you understand how how a system offsets a particular change in concentration, temperature, pressure, and volume!

STRESS: SHIFT:
Increase in concentration of inert gas no shift
decrease in concentration of inert gas no shift
Increase the concentration of a substance away from substance
decrease concentration of a substance towards substance
increase pressure of system towards fewer moles of gas
decrease pressure of system towards more moles of gas
increase temperature of system away from heat/energy
decrease temperature of system towards heat/energy
add a catalyst No shift

Remember that Le Chatelier's principle states that if a constraint (such as a change in pressure, temperature, or concentration of a reactant) is applied to a system in equilibrium, the equilibrium will shift so as to tend to counteract the effect of the constraint)
also treat Temperature as a reactant or product depending on whether the reaction is exothermic (releases energy) or endothermic (needs energy)

[Alex Ottersbach]

Adding gas to a system to change the pressure does not change partial pressures and therefore has no affect. Decreasing the volume however by shrinking the container (compression) would shift the equilibrium to the side with less moles.