BR Equilibrium Passage IV

[deleted388502]

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During an experiment, the closed glass cylinder is filled with 1.00 atm. of hydrogen (H2) according to the gauge in the cylinder. Flask I is filled with 1.0 atm carbon disulfide.The reaction starts when the stopcock is opened allowing the two gases to mix. The temperature of the glass column is maintained at 25"C using an external heat sink.

The internal pressure is monitored until it stays constant Figure 2 shows the internal pressure over time, where t = 0 represents the time at which the two gasses were mixed.

The pressure of each gas in the reaction mixture can be calculated from the change in internal pressure. The initial partial pressure of hydrogen gas is 0.8 atmospheres in the
l.25 L closed system. The decrease in partial pressure of hydrogen gas is double the decrease in the internal pressure based on the stoichiometry of the Reaction 1, which shows the reactivity of the compounds.
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So confused by the ice table for this question.

CS2 + 4H2 --> CH4 + 2H2S
They find the total pressure to be:
Ptotal = PCS2 + PH2 + PCH4 + PH2S
= 1-2x= (0.2-x) + (0.8-4x) + (x) + (2x)

Big confusions:
1) I don't understand why we're subtracting 4x from H2. I get why we're only doing one from Cs2 because it doesn't have more than 0.2 to give, but in general I'm so confused about how the ICE table is being set up
2) I don't get how we're determining that 0.8 H2 = 0.2 CS2?

Thanks!

 

[quicheduty]

I'd brush up on some review of ICE tables using other material because I don't think TBR covers it, but in general the "x's" come in during your "change" row. This is where you look at your stoich, so your mole proportions. Using your balanced equation, for every mole of CS2 you need 4 moles of H2 to react. So assign x for change of CS2 and 4x for change of H2. Does that make sense?

 

[deleted388502]

I'd brush up on some review of ICE tables using other material because I don't think TBR covers it, but in general the "x's" come in during your "change" row. This is where you look at your stoich, so your mole proportions. Using your balanced equation, for every mole of CS2 you need 4 moles of H2 to react. So assign x for change of CS2 and 4x for change of H2. Does that make sense?

Hey thanks! Maybe I'm missing something big here, but isn't the limiting reagent amount the amount that's considered 'x'? That's why I'm confused. Like how can you have a different amount of each being used? Sorry if that's super simplistic and I'm missing something ha

 

[quicheduty]

Hey thanks! Maybe I'm missing something big here, but isn't the limiting reagent amount the amount that's considered 'x'? That's why I'm confused. Like how can you have a different amount of each being used? Sorry if that's super simplistic and I'm missing something ha

Oh no. Limiting reagent is another beast. Limiting reagent implies your reaction is going to completion so you're using as much up as possible. Whereas here that might not be the case. "x" can stand for any amount up to 0.2, because if you need more than that then yes, CS2 would be your limiting reagent. But that's irrelevant, I think...is this an equilibrium passage?

 

[deleted388502]

Oh no. Limiting reagent is another beast. Limiting reagent implies your reaction is going to completion so you're using as much up as possible. Whereas here that might not be the case. "x" can stand for any amount up to 0.2, because if you need more than that then yes, CS2 would be your limiting reagent. But that's irrelevant, I think...is this an equilibrium passage?

Yeah is an equilibrium passage. Passage IV in BR chapter 3.

I have reviewed ICE passages and I guess I'm still confused in this case why we're subtracting different amounts from the reagents size. Per my understanding, your x value or how much is shifted from equilibrium is dependent on what's being produced, is it not?

 

[quicheduty]

Yeah is an equilibrium passage. Passage IV in BR chapter 3.

I have reviewed ICE passages and I guess I'm still confused in this case why we're subtracting different amounts from the reagents size. Per my understanding, your x value or how much is shifted from equilibrium is dependent on what's being produced, is it not?

Maybe you're overthinking a bit. It's super simple, promise:
Look at your balanced equation. Every stoichiometric mole of anything is one "x" equivalent
CS2 + 4H2 --> CH4 + 2H2S
You are going from reactants --> products, so if you start with all reactants, you will be subtracting certain amounts from initial concs of reactants and adding molar equivalent amounts of products. In this case, 1 mole CS2 reacts with 4 moles H2 to produce 1 mole CH4 and 2 moles H2S. If x = 1 molar equivalent, however many moles of CS2 you use, you must use 4x times that number of moles of H2 in order to be able to use up all that CS2.

So, let's flip that logic around: For every mole of H2 you use, you only need 1/4 that mole of CS2. Again, this is based purely on the stoichiometric ratios in your balanced equation. This is why when you start off with 0.8 atm H2, you only need 1/4 (0.8 atm) = 0.2 atm of CS2.

When it comes to the ICE chart, just start assigning "x" based on your stoichiometric ratios.
1 CS2 + 4H2 --> 1 CH4 + 2H2S
-1x -4x --> +1x +2x

How about now?

 

[deleted388502]

Maybe you're overthinking a bit. It's super simple, promise:
Look at your balanced equation. Every stoichiometric mole of anything is one "x" equivalent
CS2 + 4H2 --> CH4 + 2H2S
You are going from reactants --> products, so if you start with all reactants, you will be subtracting certain amounts from initial concs of reactants and adding molar equivalent amounts of products. In this case, 1 mole CS2 reacts with 4 moles H2 to produce 1 mole CH4 and 2 moles H2S. If x = 1 molar equivalent, however many moles of CS2 you use, you must use 4x times that number of moles of H2 in order to be able to use up all that CS2.

So, let's flip that logic around: For every mole of H2 you use, you only need 1/4 that mole of CS2. Again, this is based purely on the stoichiometric ratios in your balanced equation. This is why when you start off with 0.8 atm H2, you only need 1/4 (0.8 atm) = 0.2 atm of CS2.

When it comes to the ICE chart, just start assigning "x" based on your stoichiometric ratios.
1 CS2 + 4H2 --> 1 CH4 + 2H2S
-1x -4x --> +1x +2x

How about now?

AH! That makes PERFECT sense!
Now I'm just gonna push a little further to understand this, in questions where a certain "fixed" amount i.e.:

H2 + I2 --> 2 HI
-x -x +2x

What's different about this kind of an ICE table?

 

[deleted388502]

Also, just to make sure I understand: the 'x' is the molar amount/concentration that any of these compounds are present in during a state of equilibrium, and the difference is just the balanced numbers?

Thank you so much for your help, I feel like I had the idea down but now I feel like I can nail the passages.

 

[quicheduty]

AH! That makes PERFECT sense!
Now I'm just gonna push a little further to understand this, in questions where a certain "fixed" amount i.e.:

H2 + I2 --> 2 HI
-x -x +2x

What's different about this kind of an ICE table?

Hm, what do you mean? Keep in mind I didn't construct the full ice table and that your "initial" row should list the initial concentrations or pressures given. For a forward reaction you'll usually be given the initial concs/pressures (like in your original post), and you will subtract the x's and add the x's accordingly, and your "equilibrium" row will add the previous rows up. So what you have with the HI is an incomplete chart that doesn't say much unless you have more info and a question to answer!

Also, just to make sure I understand: the 'x' is the molar amount/concentration that any of these compounds are present in during a state of equilibrium, and the difference is just the balanced numbers?

Thank you so much for your help, I feel like I had the idea down but now I feel like I can nail the passages.

Not exactly--"x" is just kind of a placeholder for the ratio of things to help you calculate the equilibrium concentrations. So for CS2, 0.2 - x literally just means "some amount less than 0.2 is used up." It's not how much of something is present at equilibrium (you need to do the math to find that out).

 

[deleted388502]

Hm, what do you mean? Keep in mind I didn't construct the full ice table and that your "initial" row should list the initial concentrations or pressures given. For a forward reaction you'll usually be given the initial concs/pressures (like in your original post), and you will subtract the x's and add the x's accordingly, and your "equilibrium" row will add the previous rows up. So what you have with the HI is an incomplete chart that doesn't say much unless you have more info and a question to answer!


Not exactly--"x" is just kind of a placeholder for the ratio of things to help you calculate the equilibrium concentrations. So for CS2, 0.2 - x literally just means "some amount less than 0.2 is used up." It's not how much of something is present at equilibrium (you need to do the math to find that out).

So even though the x is a placeholder, we are still using the mole concentrations because it has to react in that ratio, correct?

 

[quicheduty]

So even though the x is a placeholder, we are still using the mole concentrations because it has to react in that ratio, correct?

Correct!