Equilibrium questions and what to assume

[ChodeNode]

---

Members don't see this ad.

So I did EK1001 Chem and hit the following question:

Consier the rxn:
2 H2(g) + O2(g) --> 2 H2O(g)

Which would decrease the proportion of water vapor at equilibrium?
A. Increasing partial pressure of oxygen gas
B. Decreasing the volume of the container
C. Reducing the amount of catalyst present
D. Raising the temperature

I remembered thinking at the time there was no evidence either way for D since the question doesn't state if it's exothermic or endothermic. In a moment of doubt and rush, I chose another choice because I wasn't satisfied with any of them. The question said "Which would..." not "which could." If the question said it was endothermic, D would have been wrong.

What should I assume on questions where it could go one way or another, but the question gives no evidence for either? Or am I just looking at this wrong?

 

[Schenker]

You don't need to know whether the reaction is endothermic or exothermic in order to answer the question. We can look at it in terms of free energy:

deltaG = deltaH - (T)(deltaS)

While we know nothing about deltaH (enthalpy), we can make assumptions about deltaS (entropy). Because there are more moles of gas on the left side of the equation than on the right side, you can infer that deltaS is negative. Therefore, increasing T would increase deltaG, i.e. the reaction becomes less spontaneous. (By the same reasoning, the reverse reaction has a positive deltaS and becomes more spontaneous.) This means that there will be less water vapor at equilibrium.

I'm afraid of over-generalizing, but when the MCAT gives you a gaseous chemical reaction without any thermodynamic values, often times they are testing you on entropy.

 

[Blueprint MCAT Tutor]

While the explanation above is a good way to look at it, and you certainly should always be aware of the Gibbs Free Energy equation, I think the more important point here is that you made a classic test-taking mistake.

You're not looking for the "right" answer. You're looking for the credited one. It'd be nice if we could always have perfect, flawless logic, and the questions and answer choices were always written with absolute clarity such that we could find our way to THE. RIGHT. ANSWER.

Instead, we're taking the test in a murky rush of approximation, imperfect knowledge, and judgment clouded by anxiety.

So we've gotta be okay with the notion of selecting answer choices we don't like.

Here, if you know that the catalyst has no effect on equilibrium (eliminating C), that adding reactant would push a reaction forward (eliminating A), and that crunching down the volume, thereby increasing pressure, would push a reaction towards fewer moles of gas (eliminating B), you could say to yourself, "Well, D is the last man standing."

Sure, you'd have that moment of "wait WTF there's no info here about temperature" but then you've gotta say to yourself, "well whatever it's the only one left" and keep moving.

As a general matter with process of elimination: only eliminate answer choices you have a clear, explainable reason for eliminating. Choices that confuse you or you just don't like should be left in. Then, once you've eliminated everything you KNOW is wrong, consider your remaining choices. Sometimes that means picking a choice you don't like just b/c it's the only one left.

 

[ChodeNode]

Thanks for the responses.

As a general matter with process of elimination: only eliminate answer choices you have a clear, explainable reason for eliminating. Choices that confuse you or you just don't like should be left in. Then, once you've eliminated everything you KNOW is wrong, consider your remaining choices. Sometimes that means picking a choice you don't like just b/c it's the only one left.

On a related note, I sometimes find myself spending too much time vacillating between two choices. How much time should I allow myself before picking one and moving on? Do you have general rules for that?

 

[1TB4RKSB4CK]

I just remember that decrease in vol is associated with a shift to the side with less moles and increasing partial pressure is the same as that too, which means it would be increasing water vapor.
Catalyst has no effect and that should be rudimentary to the concept. If anyone picked this, they need to reread the chapter.
PoE would be best used on a question like this.
So for learning sakes, lets look at choice D and why it is right.
Raising the temp would have to shift it in the direction to the decrease proportion of water vapor right? Then one could assume that for it to be an endothermic reaction, the CHANGE of enthalpy would be increased (+) value right? An exothermic is one with a (-) CHANGE in enthalpy.
I just like to see it in a "bond-breaking/bond-forming" perspective that shows that energy (such as in the reaction you posted) indicates a bond forming reaction, or an exothermic reaction that is giving off energy (heat).
Raising the temperature so that the reaction moves left to decrease water vapor proportion is an exothermic reaction.
One can therefore say also that the K is being decreased.

 

[Blueprint MCAT Tutor]

How much time should I allow myself before picking one and moving on? Do you have general rules for that?

None at all.

Keep moving.

MOVE.

JUST GO. DUDE LEAVE IT! THEY'RE COMING! JUST GOooOoooOoOoOoooooo

No but seriously once you've eliminated what you know is wrong if you're still not sure just pick something immediately, mark the question, and keep going. Staring at it waiting for the choir of angles to start singing, the clouds to part, and a beam of light to hit you in the face with sudden insight is a sucker's bet. If you're stuck then either you flat-out don't know it, or your brain's gears have stripped a tooth and they're not meshing.

As long as you're hitting a good pace and willing to just guess-and-go you may find that you're able to finish with anywhere from 2 or 3 to as much as 10 minutes left. Use *that* time to go back and check and re-think the problem.

__________________

Bryan, Next Step MCAT Tutor
Check out my Office Hours thread and feel free to ask any questions here: Link