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Enthalpy vs. Bond Enthalpy equations?
For a chemical reaction, the total change in enthalpy = sum of enthalpies of products - sum of enthalpies of reactants.
If the reaction is a combustion reaction, this is reversed.
Reactants are always "bonds broken" and products are always "bonds formed."
However, we also learned in Chemistry class that for BOND enthalpies, the equation is bonds broken - bonds formed.
This is weird, because bonds broken are reactants, and bonds formed are products... so in a sense it is saying that for the total change in bond enthalpy, it is reactants - products. Before, it was products - reactants.
Does anyone know why there is a difference between "enthalpy" and "bond enthalpy"; an exact opposite difference? Does "bond enthalpy" only refer to combustion? Or are some of these equations incorrect?
Much thanks.
1 Answer
- DannyLv 48 years agoFavorite Answer
Given a simple reaction:
H2O (l) --> H2O (g)
This equation is the vaporization of water. The enthalpy of water vapor is, of course, higher than that of liquid water. Liquid water forms hydrogen bonds, which are broken by adding heat. Since there are no bonds being formed, so the change in enthalpy is simply the bond enthalpy of H-bonds.
I see what you mean by the reactants - products, but it doesn't really work like that. When you break bonds, the system because more random. When you form bonds, the system becomes less random. That's the principle of the bond enthalpy equation.
