In physical chemistry, there are numerous quantities associated with chemical compounds and reactions; notably in terms of amounts of substance, activity or concentration of a substance, and the rate of reaction.
Theoretical chemistry requires quantities from core physics, such as time, volume, temperature, and pressure.
But the highly quantitative nature of physical chemistry, in a more specialized way than core physics, uses molar amounts of substance rather than simply counting numbers; this leads to the specialized definitions in this article.
Core physics itself rarely uses the mole, except in areas overlapping thermodynamics and chemistry.
Entity refers to the type of particle/s in question, such as atoms, molecules, complexes, radicals, ions, electrons etc.
[1] Conventionally for concentrations and activities, square brackets [ ] are used around the chemical molecular formula.
For an arbitrary atom, generic letters in upright non-bold typeface such as A, B, R, X or Y etc.
No standard symbols are used for the following quantities, as specifically applied to a substance: Usually the symbol for the quantity with a subscript of some reference to the quantity is used, or the quantity is written with the reference to the chemical in round brackets.
For example, the mass of water might be written in subscripts as mH2O, mwater, maq, mw (if clear from context) etc., or simply as m(H2O).
Another example could be the electronegativity of the fluorine-fluorine covalent bond, which might be written with subscripts χF-F, χFF or χF-F etc., or brackets χ(F-F), χ(FF) etc.
For the purpose of this article, the nomenclature is as follows, closely (but not exactly) matching standard use.
For general equations with no specific reference to an entity, quantities are written as their symbols with an index to label the component of the mixture - i.e. qi.
The labeling is arbitrary in initial choice, but once chosen fixed for the calculation.
If any reference to an actual entity (say hydrogen ions H+) or any entity at all (say X) is made, the quantity symbol q is followed by curved ( ) brackets enclosing the molecular formula of X, i.e. q(X), or for a component i of a mixture q(Xi).
No confusion should arise with the notation for a mathematical function.
is the average of the T masses mi(X) corresponding the T isotopes of X (i is a dummy index labelling each isotope):
where solv = solvent (liquid solution).
m(Xi) = mass of Xi m(Xi) = mass of Xi The defining formulae for the equilibrium constants Kc (all reactions) and Kp (gaseous reactions) apply to the general chemical reaction:
and the defining equation for the rate constant k applies to the simpler synthesis reaction (one product only):
where: The dummy indices on the substances X and Y label the components (arbitrary but fixed for calculation); they are not the numbers of each component molecules as in usual chemistry notation.
The units for the chemical constants are unusual since they can vary depending on the stoichiometry of the reaction, and the number of reactant and product components.
The general units for equilibrium constants can be determined by usual methods of dimensional analysis.
For the constant Kc; Substitute the concentration units into the equation and simplify:,
where Ni = number of molecules of component i.
Notation for half-reaction standard electrode potentials is as follows.
For the case of a metal-metal half electrode, letting M represent the metal and z be its valency, the half reaction takes the form of a reduction reaction:
where Def is the standard electrode of definition, defined to have zero potential.
φ = local electrostatic potential (see below also) zi = valency (charge) of the ion i
{\displaystyle \chi _{\rm {A}}-\chi _{\rm {B}}=({\rm {eV}})^{-1/2}{\sqrt {E_{\rm {d}}({\rm {AB}})-[E_{\rm {d}}({\rm {AA}})+E_{\rm {d}}({\rm {BB}})]/2}}}
Energies (in eV) Ed = Bond dissociation EI = Ionization EEA = Electron affinity