For example, in the case of non-dissociative adsorption, the rate of the forward process is given by

$$\frac{\mathrm{d}\theta}{\mathrm{d}t} = kCN(1−\theta)$$

where $C$ is the concentration, $N$ is the number of surface sites, and $\theta$ is the fraction of filled sites.

The reverse reaction is given by

$$\frac{\mathrm{d}\theta}{\mathrm{d}t} = −kN\theta.$$

Hence, it seems like the first reaction is second-order, while the second is first order. This seems strange to me.

  • $\begingroup$ But $\mathrm N$ is constant? $\endgroup$
    – DHMO
    Oct 16, 2016 at 13:55
  • $\begingroup$ N is constant, but $\theta$ is not $\endgroup$
    – Marcel
    Oct 16, 2016 at 14:10
  • $\begingroup$ The usage of \ce should be limited to typesetting chemicals and chemical equations - please don't use it to enclose mathematical expressions - these ought to be in italics. $\endgroup$ Oct 16, 2016 at 19:29
  • $\begingroup$ @orthocresol Thanks! What does the \mathrm you used mean? $\endgroup$
    – Marcel
    Oct 16, 2016 at 21:04
  • 1
    $\begingroup$ Math, roman type; i.e. no italics. In this case, it is only suitable for the $\mathrm{d}$ in differentials, which should be upright. $\endgroup$ Oct 16, 2016 at 21:04

1 Answer 1


Why is this strange?


$$\ce{2A <=>[$k_1$][$k_{-1}$] A_2}$$

Forward rate is second order: $k_1[\ce{A}]^{2}$.

Reverse rate is first order $k_{-1}[\ce{A2}]$.

  • 1
    $\begingroup$ IMHO it is very problematic to speak about 'the reaction order.' If you just say 'the order' of the reaction without specification against what the order is specified, this will lead to confusion very quickly. After all, there can be several reactants/products involved in the reaction. And at equilibrium the order of the reaction and reverse reaction should be the same against every component, respectively. $\endgroup$ Oct 16, 2016 at 20:33
  • $\begingroup$ It's not that bad when there's only one reactant, but yes, in general, we should specify the reaction order w.r.t. some reactant. $\endgroup$
    – Zhe
    Oct 17, 2016 at 14:57

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