[lokifenrir96]

Hi, I was fretting over this concept of Kc...

Kc is the ratio of concentration of products/concentration of reactants, raised to the appropriate power according to the coefficients of the species in the reaction equation.

It is said that Kc is a constant for a particular temperature, but changes with temperature. The explanation given was that at an equilibrium (what Kc is about), the rate of the forward and backward reaction are the same, so equating the two gives Kf[a][e]=Kb[c][d], where Kf is rate constant of forward reaction, Kb is rate constant of backward reaction, and [a] and [e] are reactant concentrations and [c] and [d] are product concentrations. Since Kf and Kb are rate constants which are given by Aexp(-Ea/RT), then when you increase temperature it would affect the rate constants of the forward and backward reactions differently depending on whether they are exothermic or endothermic, hence it would change the value of Kc as well.

But I don't really get questions about change in concentration or pressure. Assuming there is an increase in the concentration of a reactant, the model answer we should give is "the equilibrium shifts to favour the forward reaction", but "Kc remains the same, and the rate of reactions remain the same". So what exactly does this "shifting to favour" mean? I thought it meant the forward reaction increases in rate temporarily, but then stops when Kc reaches the original again, but it also contradicts with the concept of rate of reactions remaining the same at the same temperature.

Thank you for clarifying!

[Borek]

So what exactly does this "shifting to favour" mean?

It means reaction coordinate changes. Reaction coordinate is a number that is zero when you have only reactants and 1 when you have only products.

I thought it meant the forward reaction increases in rate temporarily, but then stops when Kc reaches the original again, but it also contradicts with the concept of rate of reactions remaining the same at the same temperature.

There is no contradiction here - reaction rate is not constant, it is reaction rate coefficient that is constant at a given temperature. As you wrote

v = K_f [a][e]

so v will change with concentrations.

[lokifenrir96]

Umm sorry I still don't really get it... could you please elaborate more on reaction rate coefficient? Sorry!

[fledarmus]

I thought it meant the forward reaction increases in rate temporarily, but then stops when Kc reaches the original again...

No, the reactions don't stop. The forward and reverse reactions in an equilibrium are both occurring. At equilibrium, they are both occurring at the same rate. However, the forward reaction rate depends on the concentrations of the starting materials, and the reverse reaction rate depends on the concentration of the products. If you do something that will increase the forward reaction rate, like increasing the concentration of one of the starting materials, then the forward reaction rate increases. In consequence, the concentration of the starting materials starts to fall as they get used in the reaction, slowing down the rate of the forward reaction, and the concentration of products starts to rise, speeding up the reverse reaction, and raising the concentration of the starting material. Eventually the forward and reverse reaction rates will match again, with new equilibrium concentrations. Both reactions are still occurring, and once again the rate of the forward reaction matches the rate of the reverse reaction.

[lokifenrir96]

Ah okay that makes sense. Sorry... by "stop" I meant "stop increasing"