Hi Guys,
Why doesn't a change in pressure affect the equilibrium constant (Kc)?
The equilibrium constant is defined as the value of the concentration fraction when equilibrium is reached. It indicates the extent of reaction (i.e. how far a reaction will proceed towards the products).
Let's say a decrease in pressure causes a net forward reaction due to Le Chatelier's principle (as there are more particles of products than there are reactants). Wouldn't this increase the yield of products AND increase the value of the equilibrium constant (Kc)? Because an increase in yield = increase in products = reaction proceeds more towards products (i.e. increased extent of reaction)?
I might sound confusing here, however remember that equilibrium constant is different from equilibrium position. Changes in pressure affect the equilibrium position but not the equilibrium constant (Kc). And why they do not affect both, please help me out haha. The book states that the equilibrium constant (Kc) is altered ONLY by changes in temperature and no other factor(s).
I know Bri MT beat me to it, but I thought I would add my inferior and probably convoluted 2 cents
I also had trouble wrapping my head around this and this reasoning here helped me understand a bit.
As we know, If the pressure of the system is increased, the system will partially oppose this change favouring the reaction that produces less moles of gas, such to decrease the pressure.
If we regard the equilibrium equation:
Initially, the Kc of this reaction would be given as
Making up random concentration values for an established equilibrium where
Thus
Now, if we were to double the pressure, the reaction would no longer be at equilibrium, and the reaction quotient would no longer equal the equilibrium constant as shown:
As we also know, the system will partially oppose this increase in pressure by favouring the forward reaction, thus increasing the concentration of N2 O4(g) and decreasing the concentration of NO2(g). This would occur to the extent that equilibrium is re-attained as the reaction quotient becomes equal to the initial equilibrium constant.
Thus, a change in pressure can be said to change the equilibrium position, however, does not change the equilibrium constant since, as it partially opposes the change in pressure, the reaction quotient becomes equal to the initial equilibrium constant.
It is important to now, however, that any of this only occurs when there is a difference in the molarity of gasses on each side of the reaction.
However, if we regard an increase in temperature for the reaction:
The system is no longer at equilibrium as it will favour the reverse reaction to decrease the temperature. However initially, the reaction quotient is unchanged as the change in temperature does not initially affect the concentrations of the products and reactants. Thus, as the reverse reaction is favoured and equilibrium is reattained, the equilibrium concentrations are changed thus changing equilibrium constant.
Hopefully, this logic is sound, although, please do point out anything nonsensical.