# hess law

Watch
#1
https://www.youtube.com/watch?v=xkDkYX6eRbE&t=173s 2.58seconds into the question it asks for formation but then the equations are in combustion im confused? someone explain
0
3 months ago
#2
(Original post by Jshek)
https://www.youtube.com/watch?v=xkDkYX6eRbE&t=173s 2.58seconds into the question it asks for formation but then the equations are in combustion im confused? someone explain
If they are enthalpy change of combustion values the arrows go downwards, whereas enthalpy change of formation arrows go upwards. This is because in combustion you are going from the species in the equation and combusting to the combustion products (CO2 and H2O) so they go downwards to those products. For formation, you are forming the species in the equation from their constituent elements so the arrows must go upwards to the species.
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#3
(Original post by gs--xa)
If they are enthalpy change of combustion values the arrows go downwards, whereas enthalpy change of formation arrows go upwards. This is because in combustion you are going from the species in the equation and combusting to the combustion products (CO2 and H2O) so they go downwards to those products. For formation, you are forming the species in the equation from their constituent elements so the arrows must go upwards to the species.
i meant like the question ask for enthalpy change of Formation but the answer he is doing is enthalpy change of combustion
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3 months ago
#4
(Original post by Jshek)
i meant like the question ask for enthalpy change of Formation but the answer he is doing is enthalpy change of combustion
In the question the enthalpy changes you are given are for combustion (hence the oxygen in the reactants). So in order to solve the enthalpy cycle you must use the combustion enthalpies. You must go the opposite direction to his arrows as you are for looking enthalpy of formation values which go up the arrow. The arrows he has drawn are correct.
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