# Change in equilibrium

## Homework Statement

2NOCl(g) <--> 2NO(g) +Cl2(g)[/B]
What will be the effect on equilibrium concentration of NOCL when equal moles of NOCL and NO are introduced in the mixture at constant temperature?

## Homework Equations

Kconcentration = [NO]2[Cl2] / [NOCl]2

## The Attempt at a Solution

In my opinion, the concentration of NOCl will decrease because the number of moles of chlorine is less than the new equilibrium concentration and so NOCl will decompose to give more chlorine.

## Answers and Replies

Borek
Mentor
What if - before adding NOCl and NO - you leave them in a separate vessel till they reach their own equilibrium, and you combine two equilibrated mixtures?

But even if in the equilibrium of NOCl and NO, if NOCl is trying to reach equilibrium each attempt would result in much more production of Cl2

Borek
Mentor
Question doesn't ask about what happens to the concentration of Cl2.

Assuming you combine two mixtures at equilibrium - does the reaction proceed?

In general the question is poorly worded, as it is not entirely clear what it means "introduced into the mixture". Keeping pressure constant? Keeping volume constant? Each case is slightly different and should be treated separately, but in general I have yet to see a case where adding a substance to the mixture results in lowering its amount. Sure, final amount can be lower than the sum of initial+added, but I have never seen a case where final is lower than the initial without added.

epenguin
Homework Helper
Gold Member

## The Attempt at a Solution

In my opinion, the concentration of NOCl will decrease because the number of moles of chlorine is less than the new equilibrium concentration and so NOCl will decompose to give more chlorine.

If there is something, as was said, misleading about the question, it is that it maybe suggests that there is an answer like this.

Anyway you are not asked about the concentration of just one thing.

I presume the answer would have to be on the lines of "it depends on..." And then the answer is rather generic and almost trivial.

The temperature has remained constant so the pressure is also constant.
and in my opinion the concentration of NOCl should decrease to compensate for the less moles of Cl2

epenguin
Homework Helper
Gold Member
The temperature has remained constant so the pressure is also constant.
and in my opinion the concentration of NOCl should decrease to compensate for the less moles of Cl2

Where does the bolded part come from? I am not sure what it even means.

I don't think there can be a unique answer to this question In the form either increases or decreases. We are not told an equilibrium constant or conditions, pressures or anything. Still if the equilibrium constant was such as to strongly favour NOCl, then introducing equimolar NO and NOCl will result in a net reaction of NO with any chlorine about (about which we are not told either) increasing NOCl.

Then when you say the pressure is constant, if this is a closed vessel (about which again we are not told) then during the reaction it cannot stay constant!

So one of two things: If the question is as you have related it, then in your answer you have to reframe, restate, the question so that it is clear what you are answering. Or else if, as very frequently happens here, a question has not been accurately relayed, then you should give it to us exactly.verbatim.

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Borek
Mentor
The temperature has remained constant so the pressure is also constant.

Say, you have 1 mole of gas in 22.4 L tank, at 0°C. The pressure is - as it is easy to calculate - 1 atm. Now you add another mole of gas to the same tank, keeping the temperature at 0°C. Temperature has not changed - is the pressure still 1 atm?

and in my opinion the concentration of NOCl should decrease to compensate for the less moles of Cl2

You were told several times what is wrong with this line of thinking.