log10aH2

log10aH2 refers to the base-10 logarithm of the activity of molecular hydrogen, H2, in a given system. The symbol a_H2 denotes the activity of hydrogen gas, a dimensionless quantity that reflects its effective concentration in thermodynamic terms. log10 a_H2 is the common logarithm of that activity and is often used in reports and calculations that employ base-10 scales.

In chemical thermodynamics, the standard state for a gas uses a standard pressure, typically p°, and for an ideal gas the activity can be approximated by a_H2 ≈ f_H2 / f°, with f° commonly taken as p°. Since f_H2 ≈ p_H2 for an ideal gas, this leads to a_H2 ≈ p_H2 / p°, and thus log10 a_H2 ≈ log10(p_H2 / p°). This provides a convenient link between measurable pressure and the thermodynamic activity.

Applications of log10 a_H2 occur in electrochemistry, geochemistry, and catalysis. For example, in the hydrogen redox couple, the reaction quotient involves a_H2, and the Nernst equation can be written using log10 a_H2 alongside the activities of other species. More generally, log10 a_H2 is used to describe hydrogen fugacity and its influence on equilibria and kinetics in systems where hydrogen participates.

Limitations and notes: in non-ideal conditions, a_H2 should be derived from fugacity or activity coefficients rather than a simple pressure ratio. Different disciplines may prefer natural logarithms (ln) over log10, so it is important to note the base and standard state being used when interpreting log10 a_H2.