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Are Limiting Reactants always completely consumed?

  1. Nov 11, 2013 #1

    Qube

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    1. The problem statement, all variables and given/known data

    I was doing a gas law stoichiometry problem -

    https://scontent-b-mia.xx.fbcdn.net/hphotos-prn2/v/1395119_10201040722401960_650319510_n.jpg?oh=e9741ad3f8f73c8bda9bf689ae17e5a8&oe=52834E47

    2. Relevant equations

    Pressure is proportional to the number of moles. We can convert pressure to moles. 2.4 moles of CO and 4.5 moles of O2 react to form CO2.

    The balanced equation is 2CO + O2 --> 2CO2.

    3. The attempt at a solution

    The limiting reactant is oxygen. With 4.5 moles of oxygen only 2.25 moles of carbon dioxide can be formed.

    Even though oxygen is a limiting reactant, this does not imply that oxygen is completely consumed, correct? I somehow had the misconception that limiting reactants were completely consumed, so I incorrectly calculated the final pressure as the pressure of the carbon dioxide generated (2.25 atm) added to the pressure of whatever carbon monoxide remained (0.15 atm, since 2.25 moles of CO was used).

    I thought of an analogy - imagine a car. There must be a certain ratio of fuel and oxygen. Fuel is the limiting reactant in a flooded engine. THERE IS STILL FUEL. Although fuel is limiting, it doesn't mean there is 0 fuel. There simply way too much oxygen for the fuel to combust. (ETA: I googled flooded engines and I'm still not completely clear on which is in excess but either way, it's clear that one component - either fuel or oxygen - is preventing a car from starting up; that one component is limiting, but neither is 0. Not a car mechanic here!)

    So ... two things really. Is my work and reasoning correct? Also, where may this misconception that the limiting reactant is completely spent have come from?

    ----

    ETA: googling "limiting reactant completely consumed" (https://www.google.com/search?q=lim...limiting+reactant+completely+consumed&spell=1)

    seems to yield reinforcements of my misconception:

    So ... is the LR completely consumed?
     
    Last edited: Nov 11, 2013
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  3. Nov 11, 2013 #2

    mfb

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    I think you got a factor of 2 wrong here.

    I think in this reaction, you can safely assume that one reactant will get consumed completely (unless it gets really hot). In other reactions, this can be different.
     
  4. Nov 11, 2013 #3

    Qube

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    Ah. I see. 4.5 moles of oxygen results in 9 moles of CO2. Oopsie.
     
  5. Nov 11, 2013 #4

    Qube

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    But in general, about LR. Let's say we're making sandwiches with 1 slice of ham for every 2 slices of cheese. We have 10 slices of ham and 9 slices of cheese.

    With 10 slices of ham we can make 10 sandwiches, if cheese is unlimited.

    With 9 slices of cheese we can make 4 sandwiches, if ham is unlimited.

    Cheese is the LR.

    We can only make 4 sandwiches. However, the LR isn't completely used up. 4 sandwiches consumes 8 slices of cheese, and we have 1 slice left over.
     
  6. Nov 11, 2013 #5

    mfb

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    Well, 1 atom in the whole substance does not matter in chemical reactions.
     
  7. Nov 13, 2013 #6

    Borek

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    Limiting reagent is never completely consumed, it is all a matter of chemical equilibrium. What we typically mean by that is that amount left unreacted is pretty small compared with what have reacted. "Pretty small" doesn't have a well defined meaning, sometimes it will mean 1%, sometimes 10-10 of the initial amount.
     
  8. Nov 13, 2013 #7

    Qube

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    I'm doing this problem, and I keep getting 5.775 atm of pressure, which doesn't exactly equal 5.70 atm. Even if the problem wanted 3 sig. figs, then shouldn't the answer be instead 5.80 atm?

    I converted pressure to moles and I know that we get 2.25 moles of CO2 and that means we only use 1.125 moles of O2 and 2.25 moles of CO from the coefficients.

    That means total pressure is 2.25 + 3.375 + 0.15 = 5.775.
     
  9. Nov 13, 2013 #8

    Borek

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    You are still repeating the same error. 2.25 moles of CO2 is effect of your mistake, it has nothing to do with the real answer.

    In this case you can easily assume equilibrium is shifted so far to the right equilibrium amounts of oxygen and CO don't matter at all.
     
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