equilibrium constant

So i understand that Kc is a number that expresses the relationship between the amounts of products and reactants etc, but why does a greater amount of products lead to a larger value for Kc?

and it states that when ' temperature changes, an increase in concentration of products will increase the value of Kc', but if it is this increase in concentration of products that affects Kc then why doesn't the value of Kc change with other factors like concentration and only temperature?

Reply 1

charco

Study Forum Helper

Original post by s362

As you say, kc is a function of the products concentrations divided by the reactants concentrations at equilibrium (each raised to the power of their stoichiometric coefficients, but this can be glossed over here). This function can also be called the reaction quotient.

As a constant, kc is ONLY affected by temperature.

Changing any other factor simply disturbs the equilibrium, making the reaction quotient not equal to kc. The system then responds to restore the equality: reaction quotient = kc.

For example, if you add more reactant to a system at equilibrium, then the reaction quotient is reduced, as the denominator of the function is increased. The equilibrium is disturbed and the system responds by making more of the product until the reaction quotient = kc once again.

Reply 2

dela.1

Hi, as you mentioned yourself it is a fact that Kc or any other equilibrium constant is affected by temperature only, and this can be proven by mathematical models which are beyond A-level complexity (assuming that this is your level of education) and I currently don't have the knowledge of. However, you can make sense of this without needing to get into any of that. Kc is a ratio of [products]/ [reactants], thus increasing the concentration of products will increase the numerator of this fraction and so the value of Kc.

Now moving onto the second part of your question try thinking of Kc as this constant that only gets changed by temperature. Say you have the following equilibrium 2A + B > C + D The Kc expression will look like this [C] [D] / [A]^2 . Increasing the concentration of the species in this reaction will disproportionally increase the concentration of reactants more than that of the products because there is a greater molar ratio of reactants in this particular equilibrium. This will alter the Kc ratio from the original Kc which must be restored in order to keep Kc value constant. So according to Le Chatelliers's principle to minimise the change on the equilibrium mixture the equilibrium position will shift towards the side with the fewest moles in this case toward the right (products). By doing so the rate of the forward reaction is increased and so the yield of the products also increases. This increases the numerator of the ratio counteracting the initial rise in the denominator which temporarily decreased the value of Kc, and as a result, restoring the original Kc value. Hope this highlights to you that any change to the equilibrium mixture caused by changes in concentration is temporary as the equilibrium position shifts to restore the original balance of products to reactants. This is not the case with temperature changes which lead to permanent changes in Kc value as the equilibrium position shifts according to Le Chatelier's principle and alter the ratio of products to reactants.
I know this is a bit wordy, but hopefully you can take something from it. :smile:

equilibrium constant

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