= Rate constant
{title2=$k$}
A <rate constant> is the coefficient in a specified <rate law>, held fixed under specified conditions. It can describe an empirical overall <chemical reaction>, as well as an <elementary reaction>. Fixed <temperature> matters: changing <temperature> can change $k$, even for the same <chemical reaction>. See https://goldbook.iupac.org/terms/view/O04322[IUPAC Gold Book: order of reaction and rate constants] and https://old.goldbook.iupac.org/html/R/R05137.html[IUPAC Gold Book: rate coefficient].
For $v=k[\mathrm A]^n$, dimensional consistency requires
$$
\operatorname{units}(k)=(\mathrm{mol\,L^{-1}})^{1-n}\,\mathrm{s^{-1}}.
$$
A <zero-order reaction> therefore uses $\mathrm{mol\,L^{-1}\,s^{-1}}$; a <first-order reaction> uses $\mathrm{s^{-1}}$; a <second-order reaction> uses $\mathrm{L\,mol^{-1}\,s^{-1}}$. Comparing bare numerical values of $k$ across different <orders of reaction> is meaningless without their units and <concentrations>. Even for the same <order of reaction>, state whether $k$ multiplies the normalized <reaction rate> $v$ or a particular <reactant> disappearance rate: a stoichiometric coefficient can change that convention.
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