Why do we have these two equations:
ΔH° = ΣD (bond broken) – ΣD (bond formed)
and
ΔH = Σ ΔHf products - Σ ΔHf reactant
I don't understand when we use which one and why?
Please help. Thanks.
The second equation gives the delta H for the REACTION as it is written from reactants producing products. The numbers one uses comes from tables which have been prepared from experimentally measured HEATS OF FORMATION (or calculated from different heats of formation using Hess' Law). They are called standard heats of formation and labeled delta Hof and they are at 25 degrees C. They are quite accurate. The values from the first equation CAN be used for the same thing; however, whereas the delta H formation values have a number for each molecule, the bond values for the first equation are averages for C-C bonds, C-H bonds, C-F bonds, etc., from ALL kinds of atoms/molecules. Thus, the first equation gives a quick way of calculating if one doesn't have the formation values handy but they only give approximate values since they are formed from averages.
That helps. But I still don't understand why we subtract the reactants from the products in one case and when using bonds we subtract the products from the reactants.
The easiest answer is that "that's the nature of the beast."
The bottom line is that when reactions take place, we usually go from a higher energy state to a lower energy state. Subtracting as we do gives the correct sign for our "defined" delta H (negative for exothermic reactions and positive for endothermic reactions). We define exothermic and endothermic together with what we define a + delta H and a - delta H to mean.
Thanks so much!
The two equations you mentioned are used to calculate the change in enthalpy (ΔH) during a chemical reaction. They are applied in different situations, and I'll explain each one in detail:
1. ΔH° = ΣD (bond broken) – ΣD (bond formed):
This equation is known as the bond dissociation energy approach. It is commonly used when dealing with reactions involving covalent compounds or molecules. The equation calculates the change in enthalpy (ΔH°) by summing up the bond dissociation energies of the bonds broken (ΣD) and subtracting the bond dissociation energies of the bonds formed (ΣD).
To use this equation, you need to know the bond dissociation energies of the specific bonds involved in the reaction. These values can be found in reference tables or databases. By calculating the total energy associated with breaking the bonds and subtracting the total energy released when forming new bonds, you can determine the overall change in enthalpy for the reaction.
2. ΔH = Σ ΔHf products - Σ ΔHf reactants:
This equation is known as the standard enthalpy of formation approach. It is commonly used when dealing with reactions involving elements and their compounds. The equation calculates the change in enthalpy (ΔH) by summing up the standard enthalpies of formation (ΔHf) of the products and subtracting the sum of the standard enthalpies of formation of the reactants.
In this approach, the standard enthalpy of formation (ΔHf) is the enthalpy change when one mole of a compound is formed from its elements in their standard states at a given temperature and pressure. These standard enthalpies of formation can be found in reference tables or databases. By calculating the total energy released or absorbed when forming the products and subtracting the total energy associated with the reactants, you can determine the overall change in enthalpy for the reaction.
In summary, the bond dissociation energy approach is used when dealing with covalent compounds and involves specific bond energies, while the standard enthalpy of formation approach is used when dealing with elements and their compounds and involves standard enthalpies of formation. Which equation to use depends on the nature of the reaction and the specific information provided.