# Hess cycles and bond enthalpiesWatch

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#1
Hi, I'm really stuck on these questions and have no idea what to do so would be really grateful for any help about the answer and how to do it.

a. A spirit burner containing ethanol was weighed at 58.37g. It was then lit, and used to heat a can containing 200cm3 of water for three minutes, after which it was extinguished and was found to weigh 57.52g. The temperature of the water increased from 21.5C to 52.0C.
Calculate the enthalpy of combustion of ethanol.
b. Given your answer to part a, and that the data book value for the standard enthalpy of combustion of ethene is -1409kJmol-1, set up an appropriate Hess cycle and use it to calculate the enthalpy change for the reaction below.
C2H4(g) + H2O(l) ------> C2H5OH(l)
0
3 years ago
#2
(Original post by cassie05)
Hi, I'm really stuck on these questions and have no idea what to do so would be really grateful for any help about the answer and how to do it.

a. A spirit burner containing ethanol was weighed at 58.37g. It was then lit, and used to heat a can containing 200cm3 of water for three minutes, after which it was extinguished and was found to weigh 57.52g. The temperature of the water increased from 21.5C to 52.0C.
Calculate the enthalpy of combustion of ethanol.
b. Given your answer to part a, and that the data book value for the standard enthalpy of combustion of ethene is -1409kJmol-1, set up an appropriate Hess cycle and use it to calculate the enthalpy change for the reaction below.
C2H4(g) + H2O(l) ------> C2H5OH(l)
For a) you must have come across the formula q = mc T
You know what the mass of water is (For water, 1000cm3 = 1kg), you know what the specific heat capacity of water is (4.2) and you know what the temperature change is.
Plug those numbers in to get the energy change of this reaction.

However, that's the energy change when a certain amount of ethanol is burnt. Enthalpy of combustion is calculated when one mole of a compound is burnt. You know the mass of the ethanol burnt, so calculate the moles of ethanol burnt. If x moles of ethanol releases q energy, how much does 1 mole of ethanol release?

Part b) do part a) first
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#3
(Original post by RMNDK)
For a) you must have come across the formula q = mc T
You know what the mass of water is (For water, 1000cm3 = 1kg), you know what the specific heat capacity of water is (4.2) and you know what the temperature change is.
Plug those numbers in to get the energy change of this reaction.

However, that's the energy change when a certain amount of ethanol is burnt. Enthalpy of combustion is calculated when one mole of a compound is burnt. You know the mass of the ethanol burnt, so calculate the moles of ethanol burnt. If x moles of ethanol releases q energy, how much does 1 mole of ethanol release?

Part b) do part a) first
Thanks for the reply I did the equation and got -20.09kJmol-1.
For part b, I got -1388.91kJmol-1.
I was wondering if this seems about right?
Thanks.
0
3 years ago
#4
(Original post by cassie05)
Thanks for the reply I did the equation and got -20.09kJmol-1.
For part b, I got -1388.91kJmol-1.
I was wondering if this seems about right?
Thanks.
Could you post your working to part a)?
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#5
(Original post by RMNDK)
Could you post your working to part a)?
Q=mcΔT
Q=0.2 x 4.18 x 30.5
Q=25498J
Q=25.498kJ

n=m/Mr
n=58.37/46
n=1.2689

ΔH=energy/n
ΔH=25.498/1.2689
ΔH=-20.09kJmol-1
0
3 years ago
#6
(Original post by cassie05)
Q=mcΔT
Q=0.2 x 4.18 x 30.5
Q=25498J
Q=25.498kJ

n=m/Mr
n=58.37/46
n=1.2689

ΔH=energy/n
ΔH=25.498/1.2689
ΔH=-20.09kJmol-1
Ah. You see the question illustrates that the ethanol changed in mass. That was the actual mass of ethanol burnt.

Everything else is fine.
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#7
Oh okay. I think I understand now So I have to use the new mass of ethanol or the change in mass of ethanol in the equation?
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3 years ago
#8
(Original post by cassie05)
Oh okay. I think I understand now So I have to use the new mass of ethanol or the change in mass of ethanol in the equation?
The change in mass of ethanol. because that's the amount of ethanol that had burnt.
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