[ajax0604]

A question on a practice exam paper asks "How is it possible to tell from pressure measurements, when the system has reached equilibrium?" The solutions say that equilibrium has been reached when the pressure no longer changes.  I don't agree with this because the reaction in question has the same number of particles on both sides of the equation which means that the pressure will stay constant at all times. Is my reasoning correct?

[Borek]

You may be right, but to be sure - show the reaction equation.

[ajax0604]

SO₂ + NO₂  NO + SO₃

[Borek]

In this case definitely observation of pressure is not a good way of controlling the reaction progress.

That is - pressure should not change assuming ideal gas behavior. Real gases behave a little bit different. Whether the changes are possible to see depends on how precisely the pressure is measured, and how far from the ideal gas we are.

[curiouscat]

In this case definitely observation of pressure is not a good way of controlling the reaction progress.

That is - pressure should not change assuming ideal gas behavior. Real gases behave a little bit different. Whether the changes are possible to see depends on how precisely the pressure is measured, and how far from the ideal gas we are.

@Borek

I agree with you.

But here's a modification I was thinking about. Say you took equimolar SO2 & NO2 at 30°C & atmospheric pressure. And mixed them inside a constant volume closed vessel kept isothermal also at 30°C.

As the reaction proceeds NO & SO3 would be formed of which SO3 should be a liquid at 30°C. It might be interesting to predict the resultant change in pressure as the reaction proceeds.

Do you have any idea how to do this? Would mutual solubilities etc. make it too messy?

[Borek]

If one gets a liquid between products change in the volume should be huge. Huge enough to be equivalent of just removing the reactant from the mixture.

But you are definitely right SO₃ is not necessarily a gas at STP, I missed that part. It is volatile enough I always think about it being a gas, but I just checked - its boiling point is 45°C and melting point 17°C. I thought they were at least 30 degrees lower.

[curiouscat]

If one gets a liquid between products change in the volume should be huge. Huge enough to be equivalent of just removing the reactant from the mixture.

But you are definitely right SO₃ is not necessarily a gas at STP, I missed that part. It is volatile enough I always think about it being a gas, but I just checked - its boiling point is 45°C and melting point 17°C. I thought they were at least 30 degrees lower.

Correct, but I was wondering about complexities such as:

(a) Say the initial mix is at 1 atm. The BP of SO₃is 45°C. But as soon as you start converting to SO3 & that liquifies, the Pressure drops (since gas phase moles reduce) but that means the BP of SO3 also reduces.

(b) Since this is a multi-component situation would we have to worry about dew points & bubble points instead of a sharp 45°C BP?

In any case, there must be one unambiguous P=fn(conversion) function (assume isothermal at 30°C & fixed volume container) but I'd love to work on trying to analytically / numerically evaluate what it is.

Might be a challenging problem.