Mistake in Practice Test? Help with these two questions.

[stester77s]

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A researcher investigated the equilibrium between CO2, C, and CO as a function of temperature. The equation is given below:

CO2(g) + C(s) 2 CO(g)

The furnace was then sealed so that pressure would increase as temperature rose.

1. When the system is stabilized at 1200 K, a sample of helium was injected into the furnace. What should happen to the amount of carbon dioxide in the system?
a) It should increase.
b) It should decrease.
c) It should be completely converted to CO.
d) It should remain the same.

The answer is D, and they argue that although the total pressure increases, the partial pressures of the reaction don't change, so they stay the same. Fine, that's fair, makes sense.

Here's the problem.

2. Which of the following is not necessarily true about the equilibrium reaction between CO2, C, and CO.
a) The standard entropy change is positive.
b) A decrease in pressure at constant temperature would shift the equilibrium to the right.
c) Addition of CO will shift the equilibrium to the left.
d) The standard Gibbs' free energy change is negative.

Answer: D.

But that means that B is necessarily true for question 2. But what if the pressure was uniformly decreased by taking equal percentages of all reactants and products? Or what is there was helium in the reaction chamber and you only removed the helium to reduce the pressure. The equilibrium would then not shift according to question 1. So how can you say B is true? Please help here, thanks all.

 

[dudewheresmymd]

What exam is this?

1) Helium is a inert gas. Does not appear in the equilibrium reaction, therefore has no impact on amount of CO2.


See wiki on le chatliers no idea what's going on with their mnemonic

2) Not sure but by process of elimination D is the best answer (A, B, C, are necessarily true) according to the equation you wrote:

 

[stester77s]

Thank you for the reply, but you did not cover the only part of the two problems that I had a problem with.

For #2, B, why does decreasing the pressure necessarily shift the equilibrium, when you just said that adding helium (and thus increasing the pressure) does not shift the equilibrium.

I mean for #2 B, we could be removing some helium to "decrease the pressure" - and you and Kaplan just said that that won't shift equilibrium.

For anyone who can see my issue, please help me and clarify this.

 

[Chrisz]

Delta g=Delta H-T(Delta S)

See Delta S is scaled by Temp. what if temp is extremely low. I dont know if the passage mentions whether the reaction is endothermic or exothermic. If endo, delta H is positive. Therefore, whether delta G is pos or neg depends on T. If T is small enough, Delta G will be positive