Prac: investigating heat of combustions of alcohols (1 Viewer)

[d00d]

yeah i did this chem prac where we investigated the heat of combustion of 1-propanol, 1-butanol and 1-pentanol. after doing the calculations, we have to draw a line graph of ∆H vs molar weight and then extrapolate the heat of combustion of ethanol.

now my question is, when calculating the heat of combustion, which formula should u use?

∆H = -{mC∆T (water) + mC∆T (copper container)}

or just ∆H = mC∆T (water) ?

i thought the 1st one is right coz u gotta take into account the heat absorbed by the container. however, most of the form is using the 2nd one.

 

[BlackJack]

Good scientific thinking, you are right to include the copper. However, copper takes little energy to heat (some decimal places compared to water) and there would be a much higher heat loss to the surrounding atmosphere, etc. which is harder to calculate (which you'd minimise if the experiment is done thoroughly scientifically, i.e. not at school, don't have the equipment).

So, to simplify things, the 2nd formula is used. When you compare it to the formula books, you'll see that the value obtained is often less than half, so adding copper doesn't change much.

I think that's the rationale behind it. btw, don't forget the minus, it's heat released from the molecules.

 

[d00d]

yeah that makes sense. conquering chem even says the error from not using copper's specific heat capacity is negligible. any other opinions?

 

[kini mini]

Originally posted by d00d
yeah that makes sense. conquering chem even says the error from not using copper's specific heat capacity is negligible. any other opinions?

Ideally, we'd account for the heating of the copper as BJ said. But yes, it is a relatively small error and it is also quite difficult to establish with any sort of precision. How would you be justified in taking the full mass of the can when not all of it is being heated for instance? Ultimately, the error in the experiment should be systematic and constant over all the runs.

 

[d00d]

so what ur saying is that i should use the formula that does not include copper's specific heat capacity?

 

[Twintip]

I think you should include it, if only to show correct scientific procedure. You know the ∆H of the copper calorimeter so you should include it in your calculations. It is a known factor. You can't quantitatively work out the heat loss to the atmosphere or the heat aborbed by any carbon build up on the calorimeter so you can just mention those things in your discussion as reasons for the experimental value being a fraction of the theoretical value.

That's just what I think anyway. I'm generally pretty cautious with my chem pracs and cover all bases that I can.

 

[d00d]

yeah after much consideration, i have included the copper. those who oppose my decision, do u think i'll lose marks for it?

 

[Twintip]

I wouldn't think so, no.

 

[kini mini]

Originally posted by d00d
yeah after much consideration, i have included the copper. those who oppose my decision, do u think i'll lose marks for it?

Hard to say, depends on your teacher. I wouldn't take marks off because it shows that you're thought about the problem more than others who just blindly followed the recommended procedure, even if I disagree with your conclusion.

 

[d00d]

ok i've got another question to do with this experiment. now the trend of the results is that an increase in molar wt results in higher heats of combustion, explain why this is? (due to bonds?)

 

[kini mini]

Originally posted by d00d
ok i've got another question to do with this experiment. now the trend of the results is that an increase in molar wt results in higher heats of combustion, explain why this is? (due to bonds?)

Basically, yes.

 

[d00d]

err i'm asking for an explanation of the trend, not a confirmation.

 

[d00d]

bump

 

[d00d]

bounce

 

[d00d]

kick