(see → the experiment)
$Zn$ $+$ $2H^+$ $\longrightarrow$ $Zn^{2+}$ $+$ $H_2(g)$
The rate of this reaction can be analysed by measuring the volume of hydrogen gas in function of time, or by titrating the molarity of hydrogen ions.
Such a quantitative study shows that the instantaneous rate of this reaction obeys following law: $v=k[H^+]^{\alpha}$ with $\alpha=2$, $[H^+]$ = instantaneous molarity of hydrogen ions and $k$ a constant which depends on the temperature, the contact surface of the reagents and eventually the presence of catalysts.
It can be shown that at any time,
For a given chemical reaction: $aX$ $+$ $bY$ $\longrightarrow$ $cC$ $+$ $dD$ ($X,Y$ reagents; $C,D$ products; $a,b$ coefficients of the reagents; $c,d$ coefficients of the products) we have: $v$ $=$ $k[X]^{\alpha}[Y]^{\beta}$ ($\alpha$ , $\beta$ called "orders" with respect to $X$ ,$Y$)
- Orders must not be equal to $a, b$ called "molecularities" of $X,Y$ $k$ , called rate constant, depends on . the temperature, . the nature of the contact surface of the reagents and . eventually the presence of catalysts.