kind of urgent- heat of combustion calculation (1 Viewer)

[estoriel]

A student carried out an experiment using 200ml of water to find the heat of combustion of ethanol

When 1.635g of ethanol was burnt, the temperature of the water increased by 47oC.
Assuming no heat loss occurred. What is the amount of energy released when 1.50g of ethanol was burnt?

How would you do this, any help is appreciated, thanks.

edit: the answer is 36.4kj according to the answers
applying mcdeltat
200x4.18x47=39292j
???
unless the answer is wrong...

 

[HeroicPandas]

Q
= mCdeltaT
= 200g x 4.186 J/g/C x 47 C
= 39348.4 J
= 39.3484 kJ (how much energy was produced by ethanol and was absorbed by water)

molar heat of combustion of ethanol
= deltaH_C = -deltaH/moles of ethanol burnt
= -39.3484/ (1.635/46.068)
= 1108.686.. kJ/mol (how much heat energy PER MOLE produced by ethanol, which was absorbed by water)

Now molar heat of combustion for 1.50g ethanol,

n = m/M
n = 1.5/46.068
n = 0.03256...mol (moles for particular ethanol quantity)

For 1 mol of ethanol burnt, 1108.686.. kJ of energy is produced (recall)

For 0.03256.. mol of ethanol burnt, 1108.686..x 0.03256... = 36.099.. kJ of energy is produced

Answer = 37 kJ????

Please tell me if wrong

EDIT: increased clarity

 

[HeroicPandas]

Hey estoriel, sorry I didn't see the edit to your OP

Applying Q = mCdeltaT like that will yield the energy produced by 1.635g of ethanol, which does not answer the question!!

The question is: What is the amount of energy released when 1.50g of ethanol was burnt?

Do you understand??

I think you got confused because your aim was unclear and you haven't devised a logical plan to reach the goal.
Maybe writing reasons next to your working out for why you are doing those steps and how does that link with the aim.

So what you did is apply Q = mCdeltaT, what was the reason for you doing this? If you thought of Q = mCdeltaT without reason and it came into mind when the question involved releasing energy and combustion, then you must introduce some reason to this or else you are rote learning ways to solve this question

 

[estoriel]

yeah i was thrown off by the 1.5g of ethanol and i wan't sure what to do after mcdeltat, but your answer cleared things up nicely. thanks for your help!

 

[HeroicPandas]

yay! i probably made some rounding off error somewhere

BUT just to make sure that you actually understand this, do a question similar to this and try to get the answer without looking at my working

 

[anomalousdecay]

yay! i probably made some rounding off error somewhere

Just in case you were wondering:

For 0.03256.. mol of ethanol burnt, 1108.686..x 0.03256... = 36.099.. kJ of energy is produced

Answer = 37 kJ????

That was the error.

But yeah I got 36.1 kJ as well but using a shorter method. Instead I took the heat of combustion (in units J/kg) released by the ethanol and then found the total energy released when 1.50 g of ethanol is used. Saves the time of having to find the molar mass, the molar heat of combustion (units of J/mol), etc.

 

[HeroicPandas]

Just in case you were wondering:



That was the error.

But yeah I got 36.1 kJ as well but using a shorter method. Instead I took the heat of combustion (in units J/kg) released by the ethanol and then found the total energy released when 1.50 g of ethanol is used. Saves the time of having to find the molar mass, the molar heat of combustion (units of J/mol), etc.

Nice, thanks