Question

Which substances have /_\Hf = 0 kJ/mol by definition? Select all that apply.
O2(g) N(g) H2O(l) Br2(l) Fe(s) He(g) S6(s)

Answer 1

Answer: O₂(g), Br₂(l) Fe(s), He(g)


Justification:

1) The state variable given, ΔHf°, is the standard enthalpy of formation. By definition it is the change in the enthalpy that accompanies the formation of a mole of compound in the standard state, from its constituent elements in the standard state.


2) Since the standard state of O₂(g), Br₂(l) Fe(s), He(g) is the very same shown, they do not evolve any heat for its formation. This is, that is the same state of the constituents, and no change is involved.


That is why, ΔHf°, of those elements is 0.

Answer 2

The substances that have a standard enthalpy of formation equal to 0 kJ/mol are:

• O₂(g)
• Br₂(l)
• Fe(s)
• He(g)

We want to know which substances have a standard enthalpy of formation (ΔH°f) equal to 0 kJ/mol.

What is the standard enthalpy of formation?

The standard enthalpy of formation is defined as the change in enthalpy when one mole of a substance in the standard state is formed from its pure elements under the same conditions.

By definition, the standard enthalpy of formation of pure elements in the standard state is zero.

Let's consider which of the following substances have a standard enthalpy of formation equal to 0 kJ/mol.

• O₂(g): Yes, it is a pure element in the standard state.
• N(g): No, the pure element in the standard state is N₂(g).
• H₂O(l): No, it is not a pure element but a compound.
• Br₂(l): Yes, it is a pure element in the standard state.
• Fe(s): Yes, it is a pure element in the standard state.
• He(g): Yes, it is a pure element in the standard state.
• S₆(s): No, the pure element in the standard state is S₈(s, rhombic).

The substances that have a standard enthalpy of formation equal to 0 kJ/mol are:

• O₂(g)
• Br₂(l)
• Fe(s)
• He(g)

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