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gracy
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substances in their elemental state have enthalpies of formation equal to zero,why?
so no reason or concept behind this?Bystander said:Convention. There has to be an agreed upon zero, and this is as good as any.
cseil said:It is not some sort of convention.
It is just that elements are not formed, they exist like they are. That's it.
You don't form an element, element just exists.
You take one mole of O2 and turn it into one mole of O2.cseil said:How do you form O2 from its constituent elements if the costituent element at the most stable form is O2?
Bystander said:You take one mole of O2 and turn it into one mole of O2.
cseil said:I am sorry but isn't standard enthalpy of formation defined as the change of enthalpy that comes from the formation of 1 mol of the compound from its elements at the most stable state at 1 atm?
Certainly --- and ΔH for such a process is zero. However, what would I accomplish by breaking the bond in the first place? The mole of O2 doesn't need its bonds broken before I use it in whatever process I have in mind --- it's the zero point from which all measurements are made.cseil said:But don't you obtain the same energy again when the two bonds are formed again?
The standard enthalpy of formation, also known as the heat of formation, is the change in enthalpy that occurs when one mole of a compound is formed from its constituent elements in their standard states at a given temperature and pressure.
The standard enthalpy of formation is measured using calorimetry, which involves measuring the heat released or absorbed during a chemical reaction at constant pressure.
The standard enthalpy of formation is a key thermodynamic property that helps determine the stability and energy content of a compound. It is also used to calculate the standard enthalpy of reaction for a given chemical reaction.
The standard enthalpy of formation is the change in enthalpy when a compound is formed from its constituent elements, while the standard enthalpy of combustion is the change in enthalpy when one mole of a compound is completely burned in excess oxygen to form carbon dioxide and water.
The standard enthalpy of formation is dependent on the temperature and pressure at which it is measured. As temperature and pressure increase, the standard enthalpy of formation may change due to changes in the energy content of the elements and compounds involved.