Question

What is the difference between

ΔH^oreaction=∑ΔH^of(Products)−∑ΔH^of(Reactants)

and

ΔH reaction=∑ΔH^of(Reactants)−∑ΔH^of(Products)

I'm not sure when you would use one or the other.

Answer 1

Delta H of reaction

For calculating enthalpy of formation of reaction always the 1st one is used

When you are given with the bond energies and you are asked to calculate the enthalpy of reaction then you use 2nd equation with bond energies

2nd equation interpreted in other form as Delta H= sum of bond energies of bond broken - sum of bond energies of bond formed

Defination for 1st one

The Heat of Reaction (also known and Enthalpy of Reaction) is the change in the enthalpy of a chemical reaction that occurs at a constant pressure. It is a thermodynamic unit of measurement useful for calculating the amount of energy per mole either released or produced in a reaction. Since enthalpy is derived from pressure, volume, and internal energy, all of which are state functions, enthalpy is also a state function.

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Consider as reference state all your reactants and products in their atomic states, that is, with all bonds broken. Forming a bond releases energy, so the reactants and products all have a negative enthalpy

ΔH=−∑ΔHbonds

with respect to this reference state. The enthalpy of reaction remains

ΔHrxn=ΔHprod−ΔHreac=−∑ΔHbonds,prod−(−∑ΔHbonds,reac)=∑ΔHbonds,reac−∑ΔHbonds,prod,

so the inconsistency you've pointed out is just the neglect of a minus sign.