Once again, it is easy to suppose that, because the position of equilibrium will move to the right if you increase the pressure, K p will increase as well. An increase in pressure would move the position of equilibrium to the right.īecause this is an all-gas equilibriium, it is much easier to use K p: This one would be affected by pressure because there are 3 molecules on the left but only 2 on the right. Let's look at the same equilibrium we've used before. Where there are different numbers of molecules on each side of the equation If there are the same number of molecules on each side of the equation, then a change of pressure makes no difference to the position of equilibrium. It can do this by favouring the reaction which produces the fewer molecules. ![]() That means that if you increase the pressure, the position of equilibrium will move in such a way as to decrease the pressure again - if that is possible. According to Le Chatelier's Principle, the position of equilibrium moves in such a way as to tend to undo the change that you have made. The position of equilibrium may be changed if you change the pressure. The only thing that changes an equilibrium constant is a change of temperature. This only applies to systems involving at least one gas.Įquilibrium constants aren't changed if you change the pressure of the system. The position of equilibrium moves - not because Le Chatelier says it must - but because of the need to keep a constant value for the equilibrium constant. That happens until a new balance is reached when the value of the equilibrium constant expression reverts to what it was before. In order for that not to happen, the concentrations of C and D will have to increase again, and those of A and B must decrease. If you decrease the concentration of C, the top of the K c expression gets smaller. Why does the position of equilibrium move as it does? Let's assume that the equilibrium constant mustn't change if you decrease the concentration of C - because equilibrium constants are constant at constant temperature. This is actually the wrong question to ask! We need to look at it the other way round. If you have moved the position of the equilibrium to the right (and so increased the amount of C and D), why hasn't the equilibrium constant increased? The equilibrium constant, K c for this reaction looks like this: Note: The reason for choosing an equation with "2B" will become clearer when I deal with the effect of pressure further down the page.Įxplanation in terms of the constancy of the equilibrium constant Suppose you have an equilibrium established between four substances A, B, C and D.Īccording to Le Chatelier's Principle, if you decrease the concentration of C, for example, the position of equilibrium will move to the right to increase the concentration again. The position of equilibrium is changed if you change the concentration of something present in the mixture. ![]() This page should only be read when you are confident about everything else to do with equilibria.Įquilibrium constants aren't changed if you change the concentrations of things present in the equilibrium. Important: If you aren't happy about the basics of equilibrium, explore the equilibrium menu before you waste your time on this page. ![]() Students often get confused about how it is possible for the position of equilibrium to change as you change the conditions of a reaction, although the equilibrium constant may remain the same.īe warned that this page assumes a good understanding of Le Chatelier's Principle and how to write expressions for equilibrium constants. This page looks at the relationship between equilibrium constants and Le Chatelier's Principle. Equilibrium constants and changing conditionsĮQUILIBRIUM CONSTANTS and LE CHATELIER'S PRINCIPLE
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