# Question about the enthalpy change of combustion of propane.

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#1
I can't figure out how to do this question I've been given as homework for AS Chemistry. The question says:

The enthalpy change of combustion of propane can be measured by burning a known mass of propane below a container of water and measuring the temperature rise of the water.
The heat capacity of the apparatus is found by calibrating it with a a fuel with known enthalpy of combustion.
The results of an experiment are shown below.
Mass of propane burned: 0.500g
Temperature of water at start: 21.0 degrees C
Temperature of water at end: 39.0 degrees C
Heat capacity of apparatus: 1.35kJoC^-1
Calculate the number of kilojoules of energy transferred when the 0.500g sample of propane burns in this experiment.

I'm really stuck on this one so any help is hugely appreciated 0
4 years ago
#2
Sorry you've not had any responses about this. Why not try posting in a specific subject forum- you might have more luck there.

Here's a link to our subject forum which should help get you more responses.  0
4 years ago
#3
(Original post by French Zebra)
I can't figure out how to do this question I've been given as homework for AS Chemistry. The question says:

The enthalpy change of combustion of propane can be measured by burning a known mass of propane below a container of water and measuring the temperature rise of the water.
The heat capacity of the apparatus is found by calibrating it with a a fuel with known enthalpy of combustion.
The results of an experiment are shown below.
Mass of propane burned: 0.500g
Temperature of water at start: 21.0 degrees C
Temperature of water at end: 39.0 degrees C
Heat capacity of apparatus: 1.35kJoC^-1
Calculate the number of kilojoules of energy transferred when the 0.500g sample of propane burns in this experiment.

I'm really stuck on this one so any help is hugely appreciated lol i've been stuck on these and gotten help with it. Now it's my time to help you so q=mc delta t
(in this example)
m= mass of propane
c= specific heat capacity of apparatus
delta t= change in temperature

so the mass of propane is 0.5g
c=1.35kJc¹ <-- divide by 1000 because it's supposed to be in J so 1350JKC¹
delta t is 39-21 so = 18

q=0.5*1350*18 = 12150J

so it's supposed to be in kJ so divide by 1000

12150/1000= 12.15kJ

this should be right

edit: this is not right
0
4 years ago
#4
B_9710
help did i do it right?
0
4 years ago
#5
(Original post by thefatone)
B_9710
help did i do it right?
Isn't, m, the mass normally the mass of the water?
0
4 years ago
#6
(Original post by B_9710)
Isn't, m, the mass normally the mass of the water?
but they didn't give mass of water?
0
4 years ago
#7
You use the equation Q=mc delta t
M=mass of water
C= 1.35 ( but it's normally 4.18)
Delta t= temperature change
You multiply all these figures together and then divide by a thousand to give the answer in kilojoules
You then want to find the number of moles of propane to find out the energy output per mole of propane.
Moles=mass/rfm
You divide the mass of the propane by its relative formula mass
You then divide the energy in kilojoules by the moles of propane and put a negative sign in front of it to give energy produced per mole of propane.
I'd do it with numbers but I haven't got paper right now :/
Hope it helps!
0
4 years ago
#8
(Original post by thefatone)
but they didn't give mass of water?
But the temperature change is of the water so to find the energy being transferred to the water you use q=mcΔT where c is the shc of water and ΔT is the change in temperature of the water. For q the mass of the fuel is never considered so I'm not too sure here.
What I'm saying is, for q=mcΔT c, m and T must be all measuring the same thing - you can't mix it up and use c for the apparatus, m for the mass of the fuel and ΔT for the temperature change of the water.
I'm sure you need the mass of water to do this question. But I honestly cannot be 100% sure - chemistry is not my 'speciality'.
0
4 years ago
#9
(Original post by B_9710)
But the temperature change is of the water so to find the energy being transferred to the water you use q=mcΔT where c is the shc of water and ΔT is the change in temperature of the water. For q the mass of the fuel is never considered so I'm not too sure here.
What I'm saying is, for q=mcΔT c, m and T must be all measuring the same thing - you can't mix it up and use c for the apparatus, m for the mass of the fuel and ΔT for the temperature change of the water.
I'm sure you need the mass of water to do this question. But I honestly cannot be 100% sure - chemistry is not my 'speciality'.
oh yea i see....
0
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