Question

I have seen several questions on standard states, but none quite get at the issue I'm having in understanding them. In Elements of Physical Chemistry by Atkins and de Paula, they describe standard states for pure substances as the state of the substance at 1 bar pressure at a given temperature and in a given phase of matter. As I read their explanation, standard states exist for all phases of matter at a given temperature, regardless of whether they are thermodynamically stable or not. For example, Argon would have three standard states at 100 K: one for the solid at 1 bar and 100 K, another for the liquid at the same p and T, and another for the gas at the same p and T. This makes sense to me, because even though two of these states are unstable, they still can be defined in principle; these standard states, while having the same p and T have different energies, densities, enthalpies, etc. In my mind, this also agrees with the fact that we have non-zero enthalpies of phase transitions. However, Wikipedia states The standard enthalpy change of formation for an element in its standard state is zero which seems to contradict the idea that standard states exist for all of the phases of matter for a given temperature. As I read this, it seems to imply that only the thermodynamically stable phase at the given temperature is the standard state, and other phases of matter for the same element at 1 bar and the same temperature do not have a standard state. I have seen a similar definition in Principles of Chemistry by Munowitz. Can someone clarify what standard state actually means? Every time I read a new definition I feel less certain I really know. Is this just a case where different authors use the same term to refer to different things? [OP] Can someone clarify what standard state actually means? Standard state has a common definition across the field of thermodynamics. Each species, in combination with the physical state, has a standard state; often, these standard states are hypothetical (e.g. a solute at 1 mol/L concentration behaving ideally like an infinitely diluted solute). [OP] As I read this, it seems to imply that only the thermodynamically stable phase at the given temperature is the standard state, and other phases of matter for the same element at 1 bar and the same temperature do not have a standard state. This is imprecise language, skipping one step. For the definition of enthalpy of formation, you need the elements in a defined state (without giving the chemical equation). This should be called reference state (IUPAC Gold Book entry) of the element, and it includes which allotrope of the element is used, and in which physical state. For ions, it also includes using the aqueous hydronium ion as reference state. Once you know which reference state you agree on (i.e. white or red phosphorus, iodine in the solid or gaseous state, hydronium ion rather than another ion), you also stipulate that this reference species is at standard state, and is at the temperature for which the thermodynamic data is given. The term reference state is mentioned explicitly in aIUPAC recommendation from 1981: The use of subscript f was discussed in paragraph 3.3. When ° is additionally used, as inΔfH°ΔfH°, the implication is that both the compound in question and its constituent elements are in standard states and that the elements, moreover, are in their reference states; for any given temperature the reference states of the elements will normally be those that are stable at the chosen standard-state pressure and at that temperature. Stable refers to the state (allotrope and physical state) with the lowest Gibbs energy, in this context.

Answer 1

The standard state in thermochemistry is a reference condition used for the comparison of thermodynamic properties, typically at 1 bar pressure and for solutions at 1 M, generally assuming a temperature of 298.15 K unless stated otherwise.

Step-by-step explanation:

The concept of standard state is fundamental in thermochemistry and refers to a reference point used for reporting thermodynamic properties. The IUPAC standard state specifies materials under a pressure of 1 bar and solutions at 1 Molar concentrations.

Standard enthalpies of formation (ΔH°) are measured under these conditions, and by definition, the value for any element in its most stable form is zero. This implies that at the reference pressure and temperature, the most stable phase of an element is considered its standard state, like O₂ (g), H₂ (g), and graphite for carbon.