Presence of one of the reactants in excess, as in such a condition, its concentration remains constant and rate of such reaction depends upon concentration of one reactant only and reaction is known as pseudo first order reaction e.g., acid catalysed hydrolysis of ethyl acetate.

$$\underset{\text { Ethyl acetate }}{\mathrm{CH}_3 \mathrm{COOC}_2 \mathrm{H}_5}+\mathrm{H}_2 \mathrm{O} \xrightarrow[\text { Acetic acid }]{\mathrm{H}^{+}} \mathrm{CH}_3 \mathrm{COOH}+\underset{\text { Ethyl alcohol }}{\mathrm{C}_2 \mathrm{H}_5 \mathrm{OH}}$$

This reaction is bimolecular but is found to be of first order as experimentally it is observed that rate of reaction depends upon the concentration of ethyl acetate not on water as it is present in excess.

For reaction $2 A+B \longrightarrow C$ if the rate of reaction is zero then it can be represented as

$$\text { Rate }=k[A]^0[B]^0=k$$

i.e., rate of reaction is independent of concentration of $A$ and $B$.

We can determine the rate of this reaction as a function of initial concentrations either by keeping the concentration of one of the reactants constant and changing the concentration of the other reactant or by changing the concentration of both the reactants. e.g., for the given reaction,

(i) Keeping $\left[\mathrm{O}_2\right]$ constant, if the concentration of NO is doubled, rate is found to become four times. This shows that,

$$\text { Rate } \propto[\mathrm{NO}]^2$$

(ii) Keeping [NO] constant, if the concentration of $\left[\mathrm{O}_2\right]$ is doubled, rate is also found to become double. This shows that,

$$\text { Rate } \propto\left[\mathrm{O}_2\right]^2$$

Hence, overall rate law will be

$$\begin{aligned} \text { Rate } =k\left[\mathrm{NO}^2\left[\mathrm{O}_2\right]\right. \\ \text{Rate law expression}\quad -\frac{1}{2} \frac{\Delta[\mathrm{NO}]}{\Delta t} & =-\frac{\Delta\left[\mathrm{O}_2\right]}{\Delta t} \\ & =\frac{1}{2} \frac{\Delta\left[\mathrm{NO}_2\right]}{\Delta t} \end{aligned} $$