Ethanol boils at a temp. of 78.29 degrees C. What amount of energy, in joules, is necessary to heat to boiling and then completely vaporize a 23 g sample of ethanol initially at 12 degrees C?
I know that the molar enthalpy heat of vaporization is 35.56 kJ/mol and the specific heat of ethanol (C2H5OH) is approximately constant at 2.44 J/gK. However, I am unsure of what to do with these numbers or even which equation to use. Any help at all will be appreciated!
q1 = heat necessary to raise T of ethanol from 12o to 78.29.
q1 = mass x specific heat x delta T.
mass = 23 g
specific heat = 2.44 J/g*K
delta T = 78.29 - 12.00
q2 = heat necessary to vaporize all of the ethanol.
q2 = mass x heat of vaporization
mass = 23 g
heat of vap = 35.56 kJ/mol
qtotal = q1 + q2
Don't forget to change q2 from kJ to J or q1 from J to kJ before adding q1 and q2.
Ok so got 3.72 kJ for q1 and 817.88 kJ for q2. I added them up and got 821.6 J. However, this answer is not right. What did I do wrong?
There is nothing wrong with what you have done and I obtained 821.6 kJ (NOT J) when I worked the problem. Does your prof stick to significant figures. Did the original problem quote 23 g and not 23.0 or 23.00? Did the original problem quote 12 degrees and not 12.0 or 12.00 degrees? If so, then you are allowed only two significant figures in the final answer (from the 12 AND from the 23) so the answer may have only two s.f. So 821.6 kJ would round to 822 kJ and that would round to 820 and that would be written as 8.2 x 10^2 kJ. Check it out. Check my thinking. (Also make sure you gave the answer in the correct units.)
Well the question said to answer in joules, so I've tried 8.2 x 10^5 multiple times with no luck. I don't think sig figs matter. I think the whole answer may be incorrect, but I'm just not sure what exactly I'm doing wrong...hmmm.
ok. I see the problem.
The specific heat is quoted as J/g*K BUT the heat of vaporization is quoted in kJ/mol. You need to convert that to kJ/g OR convert the 23 g to mols. let me know if that doesn't do it. I missed that when I first read through the problem.
I got approx. .5 mols when I converted it from grams. Do I then just plug this number in where it calls for mass?
23/18.015 (Check that) is not 0.5 mol, is it?
Oh, 18.015? I thought it was divided by the molar mass of ethanol.
ok. 23/46.069 = 0.499
Right. So do I plug in that number where it calls for mass?
it is divided by the molar mass of ethanol. I was working another problem with water and that slipped in.
yes.
Ok so I did that...got 1.9 x 10^4 J. STILL not right. This is very frustrating!
I don't have that.
(23/46.069)mols*35.56 kJ/mol = 17.75 kJ
17.75 kJ + 3.72 kJ = 21.47 kJ.
AH!! I see what I did! I kept putting the enthalpy for ethanol as 35.56, but it was actually 38.56. So I got 2.3E4, and it was right.
Thank you SO much for all of your help!
You're welcome. Look back at one of the first things I told you to check. ALWAYS check the board to make sure your actual post agrees with the problem. Typos are so easy to make. And it didn't help too much when I overlooked the difference in units. Have a good day. And come back anytime.