question about equilibrium constant?

Question

while calculating the equilibrium constant for the reaction between hydrogen and iodine, how can we just assume that the concentrations of hydrogen and iodine at equilibrium are equal? does the initial concentration of hydrogen and iodine have to be equal in order for this to be true or are their eqm, concentrations equal in every case?

Answer 1

Lots of people get muddled over this.

The ONLY thing you can assume is that

{products}/{reactants} = equilibrium constant.

In this case, the reaction is

H2 + I2 = 2 HI

So if you are using equilibrium constant in pressure units (this is called Kp),

p(HI) squared /(p(H2) x p(I2) ) = Kp

and if you are using concentrations,

[HI]squared/([H2] x [I2]) = Kc

Squared because thee are two HI in the products.

I have assumed that the temperature is high enough for all the I2 to be vaporised.

If this is true, then if you are at equilibrium the rest that I have written above is true whatever concentrations you started with. That's why it's called the equilibrium CONSTANT.

You don't need to assume that the initial concentrations of H2 and I2 are the same, unless you are told that they are. You should notice that the CHANGE in H2 and I2 are the same, and that the CHANGE in HI is twice as big,and in the opposite direction. That is because of what the reaction equation tells you.

Answer 2

H2 + I2 = 2HI

It doesn't matter whether or not either the hydrogen or the iodine is initially in excess. There are literally an infinite number of combinations of [H2], [I2), and [HI] that will mathematically yield the same value of the equilibrium constant "K." You can also have an initial excess of hydrogen iodide present, and the same ratio or products to reactants will be reached at equilibrium. Remember, equilibrium can be approached from either the forward or the reverse directions.