Question

What is pseudo first-order reaction? Give one example of pseudo first order.

Answer 1

Pseudo-first-order reaction

• Pseudo-first-order reactions are reactions that are not first-order but appear to be first-order due to larger concentrations of one or more reactants than the other reactants.
• Here pseudo means “fake”.
• The reaction is bimolecular but whose order is one.

Example

• One example is acid-catalyzed hydrolysis of ethyl acetate.
• Consider a reaction in which one reactant is in excess, say hydrolysis of ethyl acetate. During the hydrolysis of 0.01 mol of ethyl acetate with 10 mol of water, the reaction goes like this,
• $$\begin{gathered} {\mathrm{CH}}_3{\mathrm{COOC}}_2{\mathrm{H}}_5\left(\mathrm{l}\right)+{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)\to {\mathrm{CH}}_3\mathrm{COOH}\left(\mathrm{l}\right)+{\mathrm{C}}_2{\mathrm{H}}_5\mathrm{OH}\left(\mathrm{l}\right) \\ (\mathrm{Ethyl}\mathrm{acetate})\left(\mathrm{Water}\right)(\mathrm{Acetic}\mathrm{acid})(\mathrm{Ethyl}\mathrm{alcohol}) \end{gathered}$$
• As the concentration of water does not get altered much during the process ( because it is in excess), take it as constant.
• So in the rate equation,

Rate = ${\mathrm{k}}^{\text{'}}\left[{\mathrm{CH}}_3{\mathrm{COOC}}_2{\mathrm{H}}_5\right]\left[{\mathrm{H}}_2\mathrm{O}\right]$ and the term $\left[{\mathrm{H}}_2\mathrm{O}\right]$ can be taken constant. Thus, the equation becomes

Rate = $\mathrm{k}\left[{\mathrm{CH}}_3{\mathrm{COOC}}_2{\mathrm{H}}_5\right]$where $\mathrm{k}={\mathrm{k}}^{\text{'}}\left[{\mathrm{H}}_2\mathrm{O}\right]$

Therefore, the order of reaction now becomes one, that is the reaction is now a first-order reaction. Such reactions are called pseudo-first-order reactions.