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.

 

[Borek]

You are right in your first paragraph, but I feel like you are misunderstanding some basics here.

Stoichiometry (described by the reaction reaction equation) and equilibrium are two separate (although in practice closely related) concepts.

Reaction equation describes the stoichiometry, and can be used to calculate amount of products/amount of required reactants assuming reaction went to completion (see below). It doesn't say anything about molarities or amounts of substances involved, it just describes their stoichiometric proportions when they react. That helps calculate amounts of substances reacting and helps determine if there are limiting reagents, or if something is in excess.

Technically no reaction goes to completion, technically every reaction goes to the equilibrium point - which is sometimes so close to completion we don't care about details. If we are very close to completion we can use the reaction equation to calculate yield, if we know reaction would go to some equilibrium point that is far from completion we can't - but we still can do some rough estimates of amounts of reactants required, or what would be the maximum stoichiometric yield.

Reaction quotient is a number that describes the mixture at any given point in time - could be the system is at equilibrium, could be it is not there yet, so Q can take (almost) any positive value.

Equilibrium constant is the value reaction quotient will have once the reaction reaches the equilibrium and stops - which actually doesn't mean "nothing is reacting any longer", more like "reaction still goes both ways, but the same amount of products is converted to reactants and the same amount of reactants is converted to products in any given period of time, so their amounts remain constant".

 

[njh]

Thankyou Borek, very helpful. There was a gap in my understanding, which is now more clear.

 

[Chestermiller]

Here is an example. If you initially have 1 mole of H2 and 1 mole of I2, and you let the reaction to to equilibrium, forming x moles of HI, then the final amounts of H2 and I2 will be 1-x/2 and the final amount of HI will be x.

 

[Mayhem]

Careful with describing a "reduction" in reactants. Say decrease/increase instead.

Between reactants and products, the only thing that is conserved is mass and charge. Every other quantity is potentially subject to change.