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Please enter city name. Please enter category. Male Female Please select your gender. Please enter either mobile no. An increase in pressure would move the position of equilibrium to the right.
Not so! Remember the relationship between partial pressure, mole fraction and total pressure? How can that happen if you increase P? Increasing the terms on the top means that you have increased the mole fractions of the molecules on the right-hand side. Decreasing the terms on the bottom means that you have decreased the mole fractions of the molecules on the left. That is another way of saying that the position of equilibrium has moved to the right - exactly what Le Chatelier's Principle predicts.
There are the same numbers of molecules on each side of the equation. In this case, the position of equilibrium is not affected by a change of pressure.
Why not? Equilibrium constants are changed if you change the temperature of the system. Look at the equilibrium involving hydrogen, iodine and hydrogen iodide:.
This is typical of what happens with any equilibrium where the forward reaction is exothermic. Increasing the temperature decreases the value of the equilibrium constant. Where the forward reaction is endothermic, increasing the temperature increases the value of the equilibrium constant.
The position of equilibrium also changes if you change the temperature. If you increase the temperature, the position of equilibrium will move in such a way as to reduce the temperature again. It will do that by favoring the reaction which absorbs heat. 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?
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. Why does the position of equilibrium move as it does? If you decrease the concentration of C, the top of the K c expression gets smaller.
That would change the value of K c. 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. That happens until a new balance is reached when the value of the equilibrium constant expression reverts to what it was before. 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.
Equilibrium constants aren't changed if you change the pressure of the system. The position of equilibrium may be changed if you change the pressure. 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.
It can do this by favouring the reaction which produces the fewer molecules. 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.
Let's look at the same equilibrium we've used before. This one would be affected by pressure because there are 3 molecules on the left but only 2 on the right. An increase in pressure would move the position of equilibrium to the right.
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. Not so! Note: If you aren't happy with this, read the beginning of the page about K p before you go on.
If you sort this out, most of the "P"s cancel out - but one is left at the bottom of the expression. Now, remember that K p has got to stay constant because the temperature is unchanged.
How can that happen if you increase P? To compensate, you would have to increase the terms on the top, x C and x D , and decrease the terms on the bottom, x A and x B. Increasing the terms on the top means that you have increased the mole fractions of the molecules on the right-hand side.
Decreasing the terms on the bottom means that you have decreased the mole fractions of the molecules on the left. That is another way of saying that the position of equilibrium has moved to the right - exactly what Le Chatelier's Principle predicts. The position of equilibrium moves so that the value of K p is kept constant. There isn't a single "P" left in the expression. Changing the pressure can't make any difference to the K p expression.
The position of equilibrium doesn't need to move to keep K p constant.
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