[ideaessence]

Consider the reaction:

2A(g) + D(g)  <->  C(g) + E(g)

If only A and B are initially present, and we want to calculate Q_c, I don't know what the formula is. My suspicion is:

1 / [A]²*[D]

but then that seems strange. Why does it seem strange to me? Because it is seems like I'd be treating the numerator as if it comes out to be 1.0 M, but in reality, there are no products initially present.

I thought about trying to put a zero at the top, but it would just make Q_C = 0 no matter what concentrations are initially present.

PS - This forum having subscripts and superscripts is great.

[Borek]

Reaction quotient is always the same and contains concentrations of all substances. It may happen that initial value is absurd (0/x, x/0). Don't worry about that.

[ideaessence]

Thanks.

This question said that only the reactants are present. There were 2 separate problems like that and both required comparing Q_c to K_c. I got a decimal value for Kc but I'm still left without any value of Q to compare it to.

I guess it would make sense that it's zero? Because K_c is equilibrium position or how much reacted and zero would mean nothing reacted?

[Borek]

If there is no one of substances Q equals either zero or infinity. Just don't say that to any math nitpicker.

[ideaessence]

It's not that there are no substances present. In this example, there are only reactants (initially) present but no (initially) products.

[ideaessence]

Here is what I came up with so far.

Apparently, this web page is saying there's a case where Q can equal zero.

I found this in a thread:

If Q = reaction quotient, then when Q is equal to 0 (zero meaning there is no product and only reactant), ...

http://www.chemicalforums.com/index.php?topic=37315.0

Logically, the smaller Q, the higher the ratio of reactants, so zero would seem to be sensible even if not mathematically semantic.

[Borek]

Sorry, I meant "one of the substances is not present".

And I was referring to the other case - infinity. Math nitpickers would not like the idea that 1/0 = infinity. They will talk about limits, values approaching and so on

[ideaessence]

Oh. I see what you mean. I tried 1/0 here and got complex infinity:
http://www.wolframalpha.com/input/?i=1%2F0
That's really interesting. I want to learn more about that.

For this particular problem, I just didn't know if was supposed to put a 0 at the top, put a 1 at the top, or perform some other operation. I get it now. In this case of Q_c, there are reactants, but there are no products, so the 0 would be the numerator rather than the denominator. I would simply get 0 as an answer. Now I'm especially glad I asked because I not only had my question answered, I have something new to learn about (I consider myself a nitpicker for semantics too lol). Thank you.