Q.The enthalpies of all the elements in their standard states are

Enthalpy has no absolute zero point, so chemists fix a reference: every element in its stable standard state is assigned an enthalpy of exactly zero, by definition/convention.

Enthalpy (H) cannot be measured in absolute terms — only CHANGES in enthalpy (Delta H) between states can be measured experimentally. To make enthalpy values comparable and usable in calculations (like Hess's law), a reference baseline is needed.

By international convention, the standard enthalpy of formation (Delta_f H-degree) of every element in its most thermodynamically stable form, at standard state (298 K, 1 bar), is defined as zero. For example:

• O2(g), N2(g), H2(g) — zero …

The standard enthalpy of formation (ΔfH°) of an element in its standard state is defined as zero by convention — the reference point against which all other enthalpies of formation are measured.

Enthalpy has no absolute zero point that can be measured directly, so thermodynamics uses a convention: the standard enthalpy of formation of every element in its most stable form at standard state (298 K, 1 bar) is taken as zero. For example, ΔfH°(O2, g) = 0, ΔfH°(C, graphite) = 0, ΔfH°(H2, g) = 0. This lets us …

The enthalpy of an element in its standard state is defined as zero — it is the reference baseline for all enthalpy of formation values.

Enthalpy (H) cannot be measured in absolute terms, only changes in enthalpy (ΔH) can be measured. To build a consistent scale, chemists adopt a convention: the standard enthalpy of formation of every element in its most stable physical form at standard state (298 K, 1 bar) is defined as zero (e.g. Hf° …

Graphite has the lowest energy content among carbon's allotropes, so it is chosen as carbon's standard reference state (ΔfH° = 0).

An allotrope's thermodynamic stability is judged by its energy content: the allotrope with the lowest internal energy/enthalpy at standard conditions is the most stable. Diamond (sp$^3$, rigid 3-D lattice) and fullerenes are both higher in energy than graphite (sp$^2$, layered planar sheets); converting diamond to graphite is thermodynamically favourable (though kinetically it does not happen at room temperature because the activation energy is enormou …

By definition, the standard enthalpy of formation ($\Delta_fH^{\circ}$) of an element in its most stable form at standard conditions (298 K, 1 bar) is assigned the value zero.

Step 1 — What is standard enthalpy of formation? It is the enthalpy change when 1 mole of a compound is formed from its constituent elements in their standard (most stable) states.

Step 2 — The reference-point convention: Since enthalpy (like all internal energy quantities) cannot be measured in absolute terms, chemists need a reference point. By international convention, elements in their standard states (e.g. $O_2(g)$, $H_2(g)$, $C(\text{graphite})$, $N_2(g)$) are assigned $\Delta_fH^{\circ} = 0$, because "forming" an element from itself involves no chemical change.

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Graphite is carbon's most stable allotrope at standard conditions, which is exactly why it — not diamond — is taken as carbon's standard reference state with $\Delta_fH^{\circ} = 0$.

Step 1 — Recall the standard-state convention (Thermodynamics): The standard enthalpy of formation of an element in its most stable form at 298 K, 1 bar is defined as zero. For carbon, this reference form is specifically graphite, not diamond, coal, or fullerenes.

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$\Delta_f H^\circ$ is the heat change when exactly one mole of a compound is made from its elements, each in its normal stable form, at 298 K and 1 bar.

The standard enthalpy of formation, $\Delta_f H^\circ$, of a compound is defined as the enthalpy change that accompanies the formation of one mole of the compound from its constituent elements, with all substances in their standard states (most stable form/state of aggregation at 298 K and 1 bar pressure). For example, $\Delta_f H^\circ$ of $\text{CO}_2(g)$ is the enthalpy change for the reaction $\text{C(graphite)} + \text{O}_2(g) \rightarrow \text{CO}_2(g)$. By convention, the …

Among carbon's allotropes, graphite has the lowest standard Gibbs free energy at 298 K, 1 atm, making it the reference (most stable) form — this is why ΔfH° of graphite is defined as zero.

Thermodynamic stability of an allotrope is judged by its standard Gibbs free energy (or, at the reference level, its standard enthalpy of formation): the form with the lowest free energy under given conditions is the most stable one, and other allotropes will (in principle, even if very slowly) tend to convert toward it.

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