# Confusion on enthalpy changes

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#1
I got a question saying: calculate the standard enthalpy change of combustion of glucose using the standard enthalpy changes below:
C6H12O6 -1250
CO2 -394
H2O -286
C6H12O6 + 6O2 gives 6CO2+6H20
But I’m confused I thought to work out the enthalpy change using enthalpy change of combustion u use combustion data
And not formation data
How would I go about drawing a Hess’ cycle for this
And would I put CO2 and H20 in the element box to show that they combust or just the elements Carbon hydrogen and oxygen?
0
9 months ago
#2
first state what is the equation for combustion of glucose, which is written on the upper side of the cycle, the bottom side is the elements needed to make glucose, and this points to both the product of combustion and the glucose+o2,then use delta Hc=deltaH2-delta H1
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#3
But isn’t that for formation
I thought with enthalpy change of combustion
CO2 and h20 go at the bottom
I can’t seem to find an explanation of it online
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#4
It gave me formation data but wanted me to calculate the enthalpy of combustion
So I just use Hess’ law in the way of calculating enthalpy change using enthalpy change of formation
And calculated it using products-reactants ?
And I got -2830 kJ mol -1
Last edited by Blue_skies124; 9 months ago
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9 months ago
#5
(Original post by Blue_skies124)
But isn’t that for formation
I thought with enthalpy change of combustion
CO2 and h20 go at the bottom
I can’t seem to find an explanation of it online
u shouldn't use the enthalpy change of combustion, this is what they want u to find.using the data of formation to calculate the H change of conbustion.check my attachment.
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9 months ago
#6
(Original post by Blue_skies124)
But isn’t that for formation
I thought with enthalpy change of combustion
CO2 and h20 go at the bottom
I can’t seem to find an explanation of it
if you use the enthalpy change of combustion, that means everything is formed by the addition of oxygen, but in this equation is it not like that, just use formation is ok, it does not mean if u want to find enthalpy change of combustion, then you have to use combustion enthalpy for everything, in Hess's law we often use formation enthalpy for calculation.
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