Enthalpy

[Gundam00]

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What is the standard enthalpy of formation of white phosphorus from red phosphorus?

ΔH-formation of oxide: -2900 kJ mol-1(red), -3020 kJ mol-1(white)

This is from GS2 test.

 

[justadream]

Isn't it just products - reactants?

 

[tdod]

What is the standard enthalpy of formation of white phosphorus from red phosphorus?

ΔH-formation of oxide: -2900 kJ mol-1(red), -3020 kJ mol-1(white)

This is from GS2 test.

∆H(white) – ∆H(reactants) = -3020 – -2900 = -120

∆ is always [final - initial], or in this case [products - reactants]. This question is about Hess's law, which is easy to understand if thought of in terms of height. ∆H(formation) is the absolute height from the ground, but ∆H(rxn) is just the distance between two points.Think of white phosphorous as the top of the empire state building; it is 3020 feet tall; it's formation releases 3020 kJ of heat. Think of red phosphorous as the third-to-last story of the empire state building; it is 2900 feet tall its formation releases 3020 kJ of heat. The distance (∆H) between the two points is 120; when going from red to white, 120 kJ must be released.


i hope that helps.

 

[Gundam00]

∆H(white) – ∆H(reactants) = -3020 – -2900 = -120

∆ is always [final - initial], or in this case [products - reactants]. This question is about Hess's law, which is easy to understand if thought of in terms of height. ∆H(formation) is the absolute height from the ground, but ∆H(rxn) is just the distance between two points.Think of white phosphorous as the top of the empire state building; it is 3020 feet tall; it's formation releases 3020 kJ of heat. Think of red phosphorous as the third-to-last story of the empire state building; it is 2900 feet tall its formation releases 3020 kJ of heat. The distance (∆H) between the two points is 120; when going from red to white, 120 kJ must be released.


i hope that helps.

Hey thanks thats what i figured. But the answer choices do not list -120 as a possible answer but the explanation does have -120 that is why i was a little confused