[the_zom]

I'm really struggling to get through this correspondence course and was hoping to bounce something off of you fine folk.

The question:

Chlorine and carbon monoxide gas are mixed in a 1.00L container and the following equilibrium is established:

CO_(g) + Cl_{2 (g)}  COCl_{2 (g)}

Initially 1.5 mol of chlorine was present with excess carbon monoxide.  At equilibrium, 0.8 mol of COCl_{2 (g)} was found.

Calculate the percent reaction.

My thoughts:

If I understand what is being asked here, I need to write a balanced equation and determine what the expected yield of COCl₂ was.  Then actual yield/expected yield x 100% = percent yield.

In this particular instance, given the information I'm provided I'm inclined to say that the % yield is simply 0.8/1.5 x 100 = 53%?

I'm starting to think I shouldn't have skipped Chem 11.  Any input would be much appreciated.

[AWK]

OK

[Denu]

We sorta just started this stuff in my AS Levels, and I`m not that good with this equilibrium stuff, but I think I know what you`re doing wrong. It says, "Initially" 1.5 mol of chlorine was present with excess of CO. That means that at equilibrium, there would be a different amount of Chlorine, which would mean a different amount of COCl2. You`re also given the actual equilibrium amount of COCl2. In other words, I think you need to find the theoretical yield of COCl2 at equilibrium, instead of simply assuming that there`s 1.5 mol of it present because you were given the initial amount of Cl2, and because the molar ratio of Cl2:COCl2 = 1:1

Well, sorry about the confusion. I think that`s the problem. And you shouldn`t skip Chem. 11. It can be really frustrating sometimes, but it gets interesting too, and I really think it`ll help you later on.

Correct me if I`m wrong with the method, it`s sorta just like an assumption.

[Lord Hades]

Yeah, it says at EQUILIBRIUM. That is when both reactants are completely used up but in your case your left with an excess product.
Take this into consideration and attempt again.

Hope that helps!

[rabolisk]

I'm not quite sure what Denu and Lord Hades are trying to say, but I think 53% is the correct answer.

[skbuncks]

I'm not quite sure what Denu and Lord Hades are trying to say, but I think 53% is the correct answer.

Whatever it is that they are wittering on about it is wrong and can be safely ignored. The answer given in the OP is correct for the following reasons.

As CO is in xs Cl2 is the limiting reagent. Therefore the number of moles of COCl2 that can be formed is limited by the number of moles of Cl2.
1 mole of Cl2 will form 1 mole of COCl2.
0.8 moles of COCl2 have been formed from 1.5 moles of Cl2.
By telling us the reaction has reached equilibrium we know that no further COCl2 will be formed (unless the system is perturbed is some way, which it hasn't).

Percentage molar yield is therefore = 0.8/1.5 x 100% = 53.3%

skb

[AWK]

_{As CO is in xs Cl2 is the limiting reagent. Therefore the number of moles of COCl2 that can be formed is limited by the number of moles of Cl2.
1 mole of Cl2 will form 1 mole of COCl2.
0.8 moles of COCl2 have been formed from 1.5 moles of Cl2.}
This is not a limiting reagent problem. This is also an equilibrium reaction though we are not compelled to calculate any equlibrium. We should calculate yield of reaction knowing moles of resulting phosgene and knowing only moles of one of the two reagent entering in the reaction. But if  moles of CO would be known  (eg 0.9) then we could calculate two yields based on both reagents. Even then we usually calculate one yield based on the most important reagent or the most expensive one or the base of calculations should be done at the beginning of the problem.
If only entering reagent is known we can calculate the yield in the way analogous to the limiting reagent problem (but only analogously).