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# Difference between Enthalpy of Combustion and Formation?? [PLEASE EXPLAIN]

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1. I know that from the Enthalpy Change of Formation
ΔrH = ∑(Products) - ∑(Reactants)

So Why is it that for Enthalpy Change of Combustion
ΔrH = ∑(Reactants) - ∑(Products)

???

Thanks.
2. If you do a Hess law cycle, for ∆Hc , the arrows go down but for formation , it goes Up. So it's the route u take but just in a equation format
3. If you look at the formulae you posted there would be no difference. However, you must consider the definitions.
Enthalpy of formation is the enthalpy change when 1 mole of a compound is made from its components in their standard states.
Enthalpy of combustion is the enthalpy change when 1 mole of a compound is completely burned in oxygen under standard conditions.
From the definitions you should clearly see the difference

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4. (Original post by nwmyname)
I know that from the Enthalpy Change of Formation
ΔrH = ∑(Products) - ∑(Reactants)

So Why is it that for Enthalpy Change of Combustion
ΔrH = ∑(Reactants) - ∑(Products)

???

Thanks.
In Formation:

We need to break all reactants into their constituent elements and from these form the new reactants. Since breaking reactants is the opposite of formation we give the total enthalpy of reactants a negative. Hence your formulae.

In Combustion:

We need to burn all reactants in oxygen to form combustion products of reactants, from theses combustion products we want to go to the products. As burning reactants in o2 is same as combustion enthalpy we want to keep these positive. As we go from combustion products to products of reaction, we are going in the reverse direction. Therefore we give these a negative value. Hence your equation.

Hess law states a reactions enthalpy will be the same in whatever direction, however we assign a direction by givning a sign.

Draw the thermochemical cycles, to get a visual understanding
5. (Original post by nwmyname)
I know that from the Enthalpy Change of Formation
ΔrH = ∑(Products) - ∑(Reactants)

So Why is it that for Enthalpy Change of Combustion
ΔrH = ∑(Reactants) - ∑(Products)

???

Thanks.
Check out this interactive on that there Hess stuff
In Formation:

We need to break all reactants into their constituent elements and from these form the new reactants. Since breaking reactants is the opposite of formation we give the total enthalpy of reactants a negative. Hence your formulae.

In Combustion:

We need to burn all reactants in oxygen to form combustion products of reactants, from theses combustion products we want to go to the products. As burning reactants in o2 is same as combustion enthalpy we want to keep these positive. As we go from combustion products to products of reaction, we are going in the reverse direction. Therefore we give these a negative value. Hence your equation.

Hess law states a reactions enthalpy will be the same in whatever direction, however we assign a direction by givning a sign.

Draw the thermochemical cycles, to get a visual understanding
You literally just wrote an essay format of what I said meaning your complicating things.

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Updated: April 18, 2016
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