Why is the heat of formation defined?

[member 392791]

It seems to me that the heat of reaction nullifies the concept of the heat of formation. The heat of formation is the enthalpy change associated with formation of 1 mole of a compound from its constituent elements in its standard state.

The heat of reaction is the enthalpy change that occur when stoichiometric quantities of reactants at some T and p react to form products at the same T and p.

Well, if you define T and p, then the heat of reaction should be the same as the heat of formation, right? Therefore, the standard heat of formation and standard heat of reaction is the same, provided that one mole of the compound is formed. However, the heat of reaction gives flexibility in that its not limited to standard state, and also it can be for any amount of moles formed, as long as the stoichiometric quantity is correct.

 

[pbuk]

How do you calculate heat of reaction?

Far from "nullifying the concept of heat of formation", the concept of heat of reaction is absolutely dependent on the concept of heat of formation.

 

[member 392791]

I am not able to differentiate the two. I know that the heat of reaction depends on the heat of formation, but conceptually I can't distinguish the two. Isn't the heat of reaction of a chemical reaction that forms a product the same as the heat of formation?

I'll use the following as an example

C + O2 ---> CO2

Is the heat of formation not the heat of reaction at standard state?

 

[Ygggdrasil]

Yes, the heat of formation of CO2 is the heat of reaction of the following reaction under standard contitions:

C (s, graphite) + O2 (g) --> CO2 (g)

However, not all heat of reactions are heats of formation. For example, the heat of reaction for the combustion of methane:

CH4 + 2 O2 --> CO2 + 2 H2O

is not the heat of formation of anything. (However, you can find the value for the heat of reaction from the heats of formation of methane, carbon dioxide, and water by using Hess's law).

 

[Yanick]

Enthalpy of formation describes a special reaction where you form some compound from "scratch." Of course we can't violate energy/matter conservation, so we simply assign a value of zero to the most stable elemental form of the elements which make our compound at some temperature and 1 bar pressure. Its a way for us to set an arbitrary benchmark from which to start measuring other things, like heats of reaction which can be calculated by summing the heats of formation of each term in the chemical equation weighted by their stoichiometric coefficients and where the coefficients are positive for products and negative for reactants (\sum _{i} \Delta H ^{°} _{f,i} \ast \nu _{i}). It helps to keep in mind the path independent nature of the state functions in thermodynamics, you imagine that any compound you are considering will have ultimately come from the constituents in there elemental form which have a defined enthalpy of zero and became transformed into your compound. Think Standard Hydrogen Electrode for defining E^°. These kinds of definitions allow us to set up scales and make useful measurements.