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When calculating the enthalpy of a reaction using standard enthalpy of formation, should we assume an element is in their most stable form, or what should we do if the element that is given is not in their most stable form? How do you convert it into its most stable form?
An element is in its most stable form if it is in its most pure/stable state (For example Fe(s), Na(s), He(g), H2(g), C(s) are all n their elemental forms). In liquid and solid forms, an elemental state is a pure block or substance containing only that element (like a block of pure iron, or a block of pure Sodium (very rare naturally but in its pure form)). In gases, some elements like Oxygen, Hydrogen, Fluorine, Nitrogen and Chlorine exist as diatomic molecules (F2(g), O2(g),Cl2 (g) etc.). If an element is in one of these states, its enthalpy of formation is 0. Unfortunately though, only the chemical equation can determine whether or not they are needed/ can be used for the calculations.
From what I understand, you cannot assume that they are in their most stable state. You will need to know by looking at a chemical reaction whether or not, for instance, O2 as a gas is its most stable state. If any element is not in its stable form, you will need to write a phase change equation and then calculate your enthalpy. For example, if you are given water in it's gas state in a reaction, you would write out the phase change reaction of water from gas to liquid state and use those enthalpy values.
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