Understanding Reaction Quotient and Molarity

[njh]

Homework Statement

I understand that a Reaction Quotient states that Products/ Reactants balance at the Equilibrium Constant. Therefore, if one of the Reactants in the numerator rises in concentration, then you expect to see a REDUCTION in the other Reactant(s) and an INCREASE in the Product.

Firstly, may I confirm that my understanding is correct please?

However, I think that I am getting confused with a part of this and wondered if I could ask for clarification:

I have given an example equation below. A balanced equation, such as the example given, shows two Reactants of 1 mole each (H_2 and I_2), with one Product of 2 moles (2HI). I can not have (1.5 moles of H) and (0.5 moles of I) to create (2 moles of HI).

So, where is my understanding going wrong?

Relevant Equations

$K_c = \frac{Products} {Reactants}$

$H_2 + I_2 \leftrightarrow 2HI$

$Q_c = \frac {[HI]^2}{[H_2] [I_2]}$

I think the answer is that in a balanced equation you do have the fixed molarity. If there is an excess concentration of one of the Reactants, then this remains unutilised.