Enthalpy Question

[dearabbey3]

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Hey guys,
On one of my million practice tests, there is the following enthalpy question and I'm not sure if I'm overlooking the obvious or if my brain is just completely fried. 😱

CH4 + 2 o2 ------> CO2 + 2 H2O

For the above reaction ∆H = -890 kJ. If 10 g of CH4 is reacted with 64 g of O2, how much heat will be generated?



The answer is shown to be 555 kJ but no explanation. 😕

I have looked in old textbooks and my study materials, I'm not sure what to do. Any help would be greatly appreciated.

 

[Phlame217]

This is a limiting reactant problem

Convert to moles first:
take the grams of each and convert to moles, so 10g(CH4)/16.04 and 64g(O2)/32g which equals .625 and 2 respectivly
[note: for reasons of speed on the PCAT, i'd leave 10/16 as a fraction and reduce to 5/8. Also i rounded because we only need approx answer to find the one of four correct]

Next, figure the limiting reactant:
Since its a 1CH4 to 2O2 ratio (1/2) then devide the O2 by 2 to get to 1/1 ratio. It comes out to be 5/8's CH4 to 1 O2. CH4 is limiting.
[Note: try to get to where you can do this step of determining the reactant in your head, it will increase speed greatly.]

Now to determine heat generated:
Since the Full reations would be "CH4 + 2O2 > CO2 + 2H2O + Heat(∆H=-890)"
you would switch the sign of ∆H to make positive and then multiply by the limiting reactant (5/8CH4 x -1(-890)) = 556.

Now this is a little high, but for sake of speed, remember you cut a decimal, but a small decimal. So you're going to look for an answer that is close to that, but also lower.

 

[dearabbey3]

Is there any way of calculating or guesstimating without the MW's? All of the questions in this series of practice tests gives the MW's of the elements is they are needed in the problem but this one did not. So I am wondering if there is another method or if this was a misprint?

Thanks for your help 😀

 

[Phlame217]

Im not sure, CH4 is methane aka an alkane and so in organic chemistry you may have known some shortcut... But i havent taken that yet.

I know its also common that if tests give you the reaction formula or the compouneds with their molecular formula, itll be something you have to decipher.

 

[Knickerbocker]

Is there any way of calculating or guesstimating without the MW's? All of the questions in this series of practice tests gives the MW's of the elements is they are needed in the problem but this one did not. So I am wondering if there is another method or if this was a misprint?

Thanks for your help 😀

You should know approximate values for common used elements:

O=16, C=12, H=1
CH4 = 12+4 = 16
O2 = 16*2 = 32

For methane and oxygen, it should take a couple seconds.

I imagine that for anything complicated that you'd be given the MW. Worse case, just make sure you remember your multiples of 12.

 

[RobertZimmerman]

It all comes down to did you learn from your pre-reqs. Common "MWs" should be permanantly printed on your brain by the time you take a hs chem class, and 2-3 semesters of general/organic chem.

 

[dearabbey3]

I agree 100% unfortunately I haven't been in class for almost 4 years and it shows. 🙄