Core Concepts
In this article you will be able to understand the standard state of every element. After reading this article you’ll know the exact values of different standard sates, and its importance to chemistry.
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Standard State
In chemistry, a material’s standard state serves as a reference point for calculations of its characteristics under various circumstances. The standard state is the reference state for the thermodynamic state characteristics of a certain material or substance, including enthalpy, entropy, Gibbs free energy, and many other material standards. This approach allows for the calculation and tabulation of a wide range of additional thermodynamic variables since the standard enthalpy change of formation for an element in its standard state is zero.
It is not necessary for a material to exist in its normal state in nature; for instance, values for steam at 298.15 K and 105 Pa can be calculated even if steam does not exist at these temperatures and pressures. This method has the benefit of producing self-consistent tables of thermodynamic characteristics.
Conventional Standard States
Standard states, referred to as hypothetical states, are numerous but not actual. However, empirical measurements or extrapolation from a limiting condition—such as zero pressure or zero concentration—to a given condition typically yield their well-defined thermodynamic properties using an ideal extrapolating function, such as ideal solution or ideal gas behavior. It’s important to note that the definition of a standard condition does not specifically cover temperature. Nonetheless, most tables of thermodynamic quantities are generated at a designated temperature—usually 298.15 K or, much less commonly, 273.15 K.
Gases
A gas’s standard state is the condition it would have if it were a pure material that complied with the ideal gas equation at standard pressure. 105 Pa, or 1 bar, is the recommended standard pressure, according to IUPAC. While no actual gas behaves precisely as expected, this concept allows for systematic correction of non-ideality across all the various gases.
Liquids and Solids Standard State
When we expose the pure substance to a total pressure of 105 Pa, or 1 bar, it defines the standard state for liquids and solids. The most stable allotrope of an element, such as graphite in the case of carbon or the β-phase in the case of tin, is defined as the reference point for the majority of elements. White phosphorus, which is the most prevalent allotrope of phosphorus, stands as an exception and receives the designation of the standard state, despite being merely metastable.
Solutes Standard State
Typically, we choose the standard state for a substance in solution as the fictitious state it would have at the standard state molality or amount concentration. This exhibits infinite-dilution behavior, ideally where there are no solute-solute interactions, but there are present solute-solvent interactions. This concept is remarkable since equations that explain a solute’s behavior at the limit of infinite dilution are very similar to those that describe the behavior of perfect gases. Therefore, using infinite-dilution behavior as the reference state enables uniform non-ideality adjustments for all the various solutes.
The standard state molality is 1 mol/kg, while the standard state molarity is 1 mol/dm3.
Practice Problems
Problem 1
Define the concept of the standard state in chemistry and explain its significance in thermodynamic calculations.
Problem 2
Why do researchers typically choose a pressure of 1 bar as the reference pressure for the standard state in the case of a gases?
Problem 3
In the context of solutions, what do we mean by “standard state molality” and “standard state amount concentration”?
Problem 4
How does the concept of the standard state differ between gases, liquids, solids, and solutions?
Practice Problems Answers
Answer 1
The standard state in chemistry refers to the state of a substance at a defined set of conditions. This is often used as a reference point for thermodynamic calculations. These conditions include a pressure of 1 bar for gases, pure substances, and a specified concentration for solutions.
Answer 2
Scientists choose the standard pressure for gases as 1 bar (105 Pa) because it enables meaningful comparisons between substances. This choice closely approximates atmospheric pressure and represents a value commonly encountered in laboratory conditions.
Answer 3
Firstly, the “standard state molality” refers to the concentration of a solute in a solution at a specified molality under specific conditions. The “standard state amount concentration” refers to the concentration of a solute in a solution at a specified amount concentration under specific conditions.
Answer 4
The concept of the standard state varies for different states of matter:
- Gases: Pressure of 1 bar (105 Pa) and a specified temperature. This allows gases to be compared under consistent conditions.
- Liquids and Solids: Pure substance at a specified temperature and pressure. This serves as a baseline for comparing the behavior of substances in other states.
- Solutes: Includes standard state molality, which ensures that different solutions can be compared in terms of their solute concentrations. This is vital for understanding solute behavior and its impact on chemical reactions and equilibria.
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