different ways of calculating enthalpy of change/formation etc?

[sangria1986]

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Hey folks...so the different ways of calculating deltaH of a reaction are all basically giving the same result right?

in other words, if given the bond enthalpies(which are positive because its always the energy to break them up right?) The equation sum of bond energies of products - sum of bond energies of reactants

is the same as

using Hess' law from a variety of delta H values to determine the overall enthalpy

However, calculating the standard delta H is different from the above correct? because this is determined from the heats of formation which are determined at their standard states. The first two (bond energy, and hess law) do not use standard heats of formation right?

 

[sangria1986]

also when do you know to divide by the number of moles? will it be specified?

 

[Rabolisk]

Given bond enthalpies, which are positive as you stated, you would have to do sum of bond enthalpies of REACTANTS - that of PRODUCTS. This makes sense, because if you get a negative deltaH value, that corresponds to more stable products. More stable products have higher sum of bond enthalpies.

They give the same result as Hess's Law.

Standard delta H of a substance is the accompanying delta H for the formation of ONE mole of substance from its elements at their standard states.

To illustrate an example, if the delta H is given for the reaction
H2 + F2 -> 2HF (all gases) and you were asked to find the standard heat of formation for HF, you would have to divide the given value by two.