Heat of Combustion Question (1 Viewer)

boxhunter91

Member
Joined
Nov 16, 2007
Messages
737
Gender
Male
HSC
2009
LOL Aerath you raped me.
+ Rep.
Would we still get 2/3?
EDIT: You must spread reputation yadadada
 

Gibbatron

Member
Joined
Mar 20, 2009
Messages
339
Gender
Male
HSC
N/A
I don't think so. It means you have to find out how many moles of ethanol have been combusted using:

1367=mCAT/n

you can find mCAT so you solve for the moles of ethanol and then multiply this value by 2 because if half the heat is lost then you need twice as much to give the same total heat. Using this you can find the mass, etc, etc
 

gurmies

Drover
Joined
Mar 20, 2008
Messages
1,223
Location
North Bondi
Gender
Male
HSC
2009
I started racking my brain over this bullshit, do I divide or multiply by 2...but then I realised, fuck it, I'm multiplying final mass by 2.
 

captainvagina

Member
Joined
Jun 25, 2009
Messages
68
Gender
Male
HSC
2010
this is the solution to the combustion question:

Heat = -0.21 x 4.18 x 10^3 x 65
=57057J
=57.057kJ

n=57.057/1367
=0.0417mol
then you times this number by 2 as twice as many moles of ethanol is needed to achieve the heat energy of 57.057
=0.0834mol

then using n=m/Mm

m= 0.0834mol x ((12 x 2) + (1 x 6) + 16)
= 3.84g
 

flyace

New Member
Joined
Oct 29, 2009
Messages
11
Gender
Male
HSC
2009
yeh i doubled the value since the mass of ethanol burtn has to make up for the heat lost in order to still raise the temp. of the water to 65 degrees.

The question dint say nethin about the delta T changing altho heat was lost

i got 3.84 g
 

humdiddle

New Member
Joined
Feb 16, 2009
Messages
4
Gender
Male
HSC
2009
Heat 210g of water by 65ºC using ethanol

specific heat capacity= 4.18

heat of combustion of ethanol = 1367KJ/mol

∆H=mc∆t

= 210 x 65 x 4.18

= 57057 J (57.057 KJ)

Since half heat is lost to surroundings, only 683.5 KJ/mol of heat was utilized from the ethanol.

so 57.057/moles of ethanol = 683.5

...since n=mass/molecular mass

57.057/(mass/molecular mass) = 683.5

therefore mass/molecular mass = 57.057/683.5

= 0.083477688....

so mass of ethanol = 0.083477688 x molecular mass (which for ethanol is 46)

therefore, mass of ethanol = 0.083477688 x 46

= 3.8 g (2 s.f) --> which is the corrent amount of significant figures based on the question.
 
Last edited:

hermand

je t'aime.
Joined
Aug 28, 2008
Messages
1,433
Gender
Female
HSC
2009
so could you just divide the initial energy output by two instead of multiplying by two in the end? because only half of the energy of the ethanol is getting to the water?

pretty sure i got around 3.8. don't remember but yeah.
 

Aerath

Retired
Joined
May 10, 2007
Messages
10,174
Gender
Undisclosed
HSC
N/A
so could you just divide the initial energy output by two instead of multiplying by two in the end? because only half of the energy of the ethanol is getting to the water?

pretty sure i got around 3.8. don't remember but yeah.
Yeah, that's correct. :)
 

Users Who Are Viewing This Thread (Users: 0, Guests: 1)

Top