Standard heats of formation (enthelphy, free energy)

In summary, the standard heats of formation for pure elements are 0 because, by definition, there is no change in enthalpy when a pure element (in standard state) is formed. This is because a pure element is made of itself, so there is no change in enthalpy when it is formed. This concept of enthalpy is relative and the choice of which compounds are chosen to be 0 does not imply anything about those compounds.
  • #1
renob
89
0
Quick question: why are the standard heats of formation 0 for pure elements?
 
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  • #2
Simply by definition.
 
  • #3
Ya, it pretty much means there is no change in enthalpy when a pure element (in standard state) is formed. And by definition the standard heat of formation of a substance has to be measured for the reaction where the substance is formed by its constituent elements in their standard states. Obviously a pure element is only made of itself so the change in enthalpy is non-existent.
 
  • #4
spectre101 said:
Ya, it pretty much means there is no change in enthalpy when a pure element (in standard state) is formed.

Sorry but that's gibberish. Formed from what?

Which is the point. There's no such thing as an absolute enthalpy (or any other energy). It's a relative quantity. If you want to talk about the energy required to form a certain compound, then it must always be relative whatever compound you're forming it from. What particular compounds are chosen to be zero isn't important and doesn't imply anything at all about those compounds.

Obviously a pure element is only made of itself so the change in enthalpy is non-existent.

That's totally misleading. If two atoms combine to form a molecule, then that certainly involves a change in enthalpy!
 

Related to Standard heats of formation (enthelphy, free energy)

1. What are standard heats of formation?

Standard heats of formation, also known as standard enthalpies of formation, are the change in enthalpy that occurs when one mole of a compound is formed from its constituent elements in their standard states at standard conditions (25°C and 1 atm).

2. How are standard heats of formation measured?

Standard heats of formation are usually measured using bomb calorimeters, which are devices that measure the heat released or absorbed during a chemical reaction. The enthalpy of the reaction is then divided by the number of moles of the compound formed to determine the standard heat of formation.

3. What is the significance of standard heats of formation?

Standard heats of formation are important in determining the stability and reactivity of compounds. They are also used in thermodynamic calculations to determine the energy changes involved in chemical reactions.

4. How do standard heats of formation relate to free energy?

Standard heats of formation and free energy are related through the Gibbs free energy equation: ΔG = ΔH - TΔS. This equation shows that the standard heat of formation is a factor in determining the change in free energy during a chemical reaction.

5. Can standard heats of formation be negative?

Yes, standard heats of formation can be either positive or negative. A negative value indicates that the formation of the compound releases heat, while a positive value indicates that heat is absorbed during the formation process.

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