AS Chemistry (Edexcel)

Can anyone please help me with the following question?

"The following system was allowed to reach equilibrium at 300 Celsius

2HI(g) --> H2(g) + I2(g) delta H= -53 kj/mol
(colorless) (colorless) (purple)

The equilibrium mixture at 300 Celsius was compressed in a gas syringe to occupy a smaller voluem. What would be seen immediately?

a) no visible change
b) the color gets lighter
c) the mixture turns colorless
d) the mixture goes darker purple

The correct answer is D, I don't understand why can anyone please explain?

For starters, what do you think happens to the equilibrium?

well, what i thought was as it said "smaller" volume occupied so that means the pressure would increase. and pressure affects gases in equilibrium, but since there are equal number of moles of gases on the side of the equation, there would be no effect of the pressure on equilibrium mixture.
But they have mentioned temperature in the question as well, and given forward reaction is exothermic, the increase temperature should shift the equilibrium to the left..in turn making the mixture paler ( option B)

I'm not sure that a temperature change is what they're getting at.

Using a solution as an analogy, what would you expect to observe if you had a coloured solution and decreased its volume by removing solvent?

okay yeah, i get it that in a solution we could expect the mixture to get darker if we decrease the amount of volume of solvent

But how can we apply such an analogy in this question, as they have stated all the substances taking part in the reaction are in gaseous state? I dont really understand how volume affects the rate of reaction of the mixture..

It's nothing to do with rates. When you decrease the volume there are more molecules of the coloured species per unit volume (i.e. a higher 'concentration'.

why only more molecules of colored species per unit volume? why not more volume of colorless species as well?

Well you have that too, but you can't see it.

oh, so the concentration of all three species increase? but since we can only observe purple so we only account that?

But if two species of the mixture are colorless, then don't they like outweigh the purple color? and the mixture should be like lighter purple color (im sorry if im annoying you)

Yep.



The vast majority of a gases volume is empty, so the probability of the colourless species 'blocking' the coloured one is incredibly small. The colourless species likely can't absorb much of the purple light anyway.