Equilibrium confusion.. any help appreciated

[ilovemedi]

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How do you solve these types of problems:

TBR Passage I asks:
If a flask were filled with pure CO2 (g) to a total pressure of 1.00 atm., then once equilibrium is reached, the total pressure of the system is which of the following?

2CO (g) + O2 (g) <--> 2CO2 (g)

A. Less than 1.00 atm.
B. Exactly 1.00 atm.
C. Between 1.00 atm. and 1.50 atm.
D. Exactly 1.50 atm.

Answer: C
I did it this way: 3/x=2/1 (proportion of n/P=n/P) and got the correct answer (basically x=1.5 atm total, but since it's equilibrium, not all are reactants, so it's smaller than that).

But for another similar passage III, Q17:
If a flask were filled with PCl3 and Cl2 to a total pressure of 1.5atm such that the mole fraction of PCl3 is twice that of Cl2, then what is the total pressure of the system at equilibrium?

PCl3 + Cl2 =double arrow= PCl5
A) Less than 1.00atm
B)between 1 and 1.25 atm
c) between 1.25 and 1.5 atm
d) greater than 1.5 atm.

Answer: B

I did the same: set up proportions 2/1.5=1/x and got .75.. which is wrong. What am I doing wrong? Should I set up ICE boxes though they take up more time?! Any help appreciated, please!

 

[Gauss44]

How do you solve these types of problems:

TBR Passage I asks:
If a flask were filled with pure CO2 (g) to a total pressure of 1.00 atm., then once equilibrium is reached, the total pressure of the system is which of the following?

2CO (g) + O2 (g) <--> 2CO2 (g)

A. Less than 1.00 atm.
B. Exactly 1.00 atm.
C. Between 1.00 atm. and 1.50 atm.
D. Exactly 1.50 atm.

Answer: C
I did it this way: 3/x=2/1 (proportion of n/P=n/P) and got the correct answer (basically x=1.5 atm total, but since it's equilibrium, not all are reactants, so it's smaller than that).

But for another similar passage III, Q17:
If a flask were filled with PCl3 and Cl2 to a total pressure of 1.5atm such that the mole fraction of PCl3 is twice that of Cl2, then what is the total pressure of the system at equilibrium?

PCl3 + Cl2 =double arrow= PCl5
A) Less than 1.00atm
B)between 1 and 1.25 atm
c) between 1.25 and 1.5 atm
d) greater than 1.5 atm.

Answer: B

I did the same: set up proportions 2/1.5=1/x and got .75.. which is wrong. What am I doing wrong? Should I set up ICE boxes though they take up more time?! Any help appreciated, please!

Not sure the following is how you are suppose to do it, but I got to the right answer like this:

For your second example, I would have noted the coefficients or 2 moles on the left side and 1 mole on the right side, so double the pressure on the left than the right. Say all .75 went to the left side, it would be 1.5. So, I took the average .75 + 1.5 = 1.125

 

[ilovemedi]

I did it by PV=nRT, thus P/n=P/n, and did proportions.

But I'm confused as to what you mean by "Say all .75 went to the left side, it would be 1.5". I understand how you got .75, but what does it represent? Total pressure of products at equilibrium ? And how did you get 1.5?

 

[sazerac]

In the first example, you have two units of product in your equation, and one atm of gas. Thus each unit in the equation represents 0.5 atm. What if the equation shifted all the way to the left? You would now have three units of gas, or 1.5 atm. You know the truth must lie between these two extremes.

In the second example, you have one atm of pcl3, and 0.5 atm of cl2. But the equation says one unit of pcl3 combines with one unit of cl2. So this is a limiting reagent problem. What if the equation tried to shift all the way to the right? Half an atm of pcl3 will combine with half an atm of cl2 and make half an atm of pcl5. There is still going to be half an atm of pcl3 leftover, with no cl2 to react with. This makes a total of 1.0 atm. We know the answer lies between 1.5 and 1.0 atm.

And now we are kinda stuck between answers B and C. I assume the passage gave some additional info like a K value or something that hinted this reaction heavily favors products, or something.

 

[FeinMS]

In the first example, you have two units of product in your equation, and one atm of gas. Thus each unit in the equation represents 0.5 atm. What if the equation shifted all the way to the left? You would now have three units of gas, or 1.5 atm. You know the truth must lie between these two extremes.

In the second example, you have one atm of pcl3, and 0.5 atm of cl2. But the equation says one unit of pcl3 combines with one unit of cl2. So this is a limiting reagent problem. What if the equation tried to shift all the way to the right? Half an atm of pcl3 will combine with half an atm of cl2 and make half an atm of pcl5. There is still going to be half an atm of pcl3 leftover, with no cl2 to react with. This makes a total of 1.0 atm. We know the answer lies between 1.5 and 1.0 atm.

And now we are kinda stuck between answers B and C. I assume the passage gave some additional info like a K value or something that hinted this reaction heavily favors products, or something.

Yup, the K value is given 1.3 something which corresponds to what you say. Since the equilibrium favors the product side (with less moles of gas) the total pressure will be less than 1.25.

 

[DrBTS]

Hey both of these really good/elegant questions/explanations. While I get that the two extremes are pressures of 1 to 1.5, I think this explanation of there being slightly more product gas molecules which are at a lower pressure and therefore total pressure is on the lower half of the range as being too vague. I, on the other hand, can't really wrap my head around a definitive way of finding out the total pressure at this K value.

This much has made sense to me thus far: as K value increases, more products and less pressure for PCl3 and Cl2 bringing total pressure closer and closer to 1 which makes sense with the equilibrium equation. But I don't get how you would definitively find the value as being less or more than half the range at 1.25. Any thoughts?

 

[sazerac]

At the "half way" point you would have 0.75 atm of one reactant, 0.25 of the second, and 0.25 of the product. The instantaneous K value here would be products over reactants, or 0.25/0.75/0.25. Which is 4/3 or 1.333333. So if the equilibrium K of this reaction is more than 4/3 then the reaction will go more than "half way".

I don't have the problem in front of me, and I don't know what the given K value was, beyond "1.3something".

 

[ilovemedi]

In the first example, you have two units of product in your equation, and one atm of gas. Thus each unit in the equation represents 0.5 atm. What if the equation shifted all the way to the left? You would now have three units of gas, or 1.5 atm. You know the truth must lie between these two extremes.

In the second example, you have one atm of pcl3, and 0.5 atm of cl2. But the equation says one unit of pcl3 combines with one unit of cl2. So this is a limiting reagent problem. What if the equation tried to shift all the way to the right? Half an atm of pcl3 will combine with half an atm of cl2 and make half an atm of pcl5. There is still going to be half an atm of pcl3 leftover, with no cl2 to react with. This makes a total of 1.0 atm. We know the answer lies between 1.5 and 1.0 atm.

And now we are kinda stuck between answers B and C. I assume the passage gave some additional info like a K value or something that hinted this reaction heavily favors products, or something.

Is it a Limit Reg problem because the reactants are at unequal concentrations, thus you can't do proportions of moles/pressure=moles/pressure?! So the first example wasn't a LR reaction because each had equal pressures,? Sorry, I don't know why this chapter is so confusing for me.

 

[NeverBackDown]

How do you solve these types of problems:

TBR Passage I asks:
If a flask were filled with pure CO2 (g) to a total pressure of 1.00 atm., then once equilibrium is reached, the total pressure of the system is which of the following?

2CO (g) + O2 (g) <--> 2CO2 (g)

A. Less than 1.00 atm.
B. Exactly 1.00 atm.
C. Between 1.00 atm. and 1.50 atm.
D. Exactly 1.50 atm.

Answer: C
I did it this way: 3/x=2/1 (proportion of n/P=n/P) and got the correct answer (basically x=1.5 atm total, but since it's equilibrium, not all are reactants, so it's smaller than that).

But for another similar passage III, Q17:
If a flask were filled with PCl3 and Cl2 to a total pressure of 1.5atm such that the mole fraction of PCl3 is twice that of Cl2, then what is the total pressure of the system at equilibrium?

PCl3 + Cl2 =double arrow= PCl5
A) Less than 1.00atm
B)between 1 and 1.25 atm
c) between 1.25 and 1.5 atm
d) greater than 1.5 atm.

Answer: B

I did the same: set up proportions 2/1.5=1/x and got .75.. which is wrong. What am I doing wrong? Should I set up ICE boxes though they take up more time?! Any help appreciated, please!

I question the relevance of the first question, because it is asked as a system in which mass enters but you don't know how the equation behaves under STP. I would think you at least you have to know the pressure of the CO2 before it enters the system. Using entropy and mole ratio you can assume CO2 is favored....but that would lead you to the wrong answer. Using the gas law you have to assume T is constant at 273K...that means the flask is either a vacuum(it is not doing PV work, cuz it's isothermal) or being heated as it enters for the reactants to be favored (because it is at a higher energy). The key to this question is the K constant at STP which is not given and assuming STP at the initial conditions and at equilibrium is a stretch as i just explained. Sorry for being a dork, but you said any help would be appreciated.