Heat of Combustion Help (1 Viewer)

samantha.liu

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Hey guys,

I'm really struggling with the heat of combustion prac. Not sure what I need to learn for it and then how to do some of the calculations. I'm finding using the formula quite challenging because I don't know what to plug in. Can anyone give me a hand?

Thanks!
 

jazz519

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Usually for practical exams you should know:
- Aim
- Hypothesis
- Safety issues and how to resolve them
- Method (and why you do things in the method)
- Discussion (discussing whether the experiment is reliable, accurate, valid. If any of those things isn't fully there, then you need to talk about the reasons why. E.g. most times students do the molar heat of combustion prac, they only do it once so the result isn't reliable (you would need to repeat it a few times to make sure there isn't any outlier). Also the prac is invalid as it can't answer your aim which is to actually calculate the true value for molar heat of combustion due to the heat loss. Inaccurate because your value won't be the theoretical. This is the kind of analysis you want)
- Improvements to make it more reliable, more valid and more accurate (can repeat experiment more times for reliability and accuracy you can use things like heat guards or reduce distance between the can and spirit burner to reduce heat loss)

You can probably find some detailed notes online for this practical since it's a practical that was also in the old syllabus

Calculations: for molar heat of combustion there is only two formulas. So you should memorise both and recognise that the questions can only ask you something that uses those two formulas

One thing I was always recommend to my tutoring students is if you can't figure out which formula to use in a calculation question, write down all the possible ones you could use for that question and select the one in which you only have one unknown variable. Then usually you can solve that formula for something and it then allows you to use another formula and you keep continuing until you get your answer

An example to show you how you can apply that thought process:
Question: The molar heat of combustion of pentan-1-ol is 2800 kJ/mol. A quantity of pentan-1-ol was combusted, generating 108 kJ of heat. What mass of pentan-1-ol was combusted?

Begin by summarising the info in the question and assigning variables to it:
delta H c = 2800 kJ/mol
delta H = 108 kJ

Write down your two formulas if you are stuck on which one to choose:
delta H = mC delta T
delta H c = delta H / n(fuel)

So now look for the formula in which you have everything known but 1 variable. If we look at the first equation; we know delta H, we don't know mass, we know C (from data sheet), we don't know delta T. So you can't use this equation because there is 2 unknowns

Look at your 2nd equation: we know delta H c and we know delta H. We don't know n(fuel), but we know all the variables except for 1 so you can calculate for n(fuel)

2800 = 108 / n(fuel)
n(fuel) = 108 / 2800 = 0.03857... moles

Read your question again: it says find mass. Just have to think about how to make moles --> mass (n=m/MM formula)

pentan-1-ol has formula C5H11OH (draw it to see this)

m(pentan-1-ol) = n x MM = (0.03857...)(88.146) = 3.39... g
 

cathy.l

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there is a pretty good video online explaining it here

 

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