Question

A three-step mechanism has been suggested for the formation of carbonyl chloride: Step I: \(\mathrm{Cl}_{2} \rightarrow 2 \mathrm{Cl}\) (fast, equilibrium) Step II: \(\mathrm{Cl}+\mathrm{CO} \rightarrow \mathrm{COCl}\) (fast, equilibrium) Step III: \(\mathrm{COCl}+\mathrm{Cl}_{2} \rightarrow \mathrm{COCl}_{2}+\mathrm{Cl}\) (slow) What is the molecularity of the rate-determining step? a. Termolecular b. Unimolecular c. Bimolecular d. None of these

Short answer

c. Bimolecular

Step-by-step solution

Identify the Rate-Determining Step

The rate-determining step is identified as the slowest step in the mechanism. From the given steps, Step III is stated to be the slow step: \(\mathrm{COCl} + \mathrm{Cl}_{2} \rightarrow \mathrm{COCl}_{2} + \mathrm{Cl}\).

Determine the Molecularity

The molecularity of a reaction step refers to the number of reactant molecules involved in the step. In Step III, there are two reactant molecules: \(\mathrm{COCl}\) and \(\mathrm{Cl}_{2}\). Therefore, the molecularity for Step III is bimolecular.

Conclusion

Since Step III is the rate-determining step and involves two reactant species, \(\mathrm{COCl}\) and \(\mathrm{Cl}_{2}\), the molecularity of this step is bimolecular.

Key concepts

Reaction Mechanism

A reaction mechanism provides a detailed account of the step-by-step process by which reactants transform into products. It breaks down the overall chemical equation into simpler, elementary steps. Each step details the specific reactants and products involved at that stage. In understanding reaction mechanisms, it's crucial to identify all intermediate species and the sequence in which bonds are broken and formed. This breakdown helps chemists understand the entire transformation process and pinpoint where control might be needed to enhance efficiency or selectivity.

One important aspect of a reaction mechanism is the rate-determining step. This is the slowest step in the sequence, often acting as a bottleneck that controls the overall rate of the reaction. By studying the rate-determining step, scientists can propose conditions that may speed up the overall reaction or increase yield by focusing on this step specifically. For instance, improving conditions for the slower third step in our example mechanism can lead to a more efficient process overall.

Molecularity

Molecularity refers to the number of molecules coming together to react in an elementary step of a reaction mechanism. It tells us how many reactant molecules are involved in that step. Although often confused with reaction order, molecularity is a theoretical concept and specifically applies to elementary steps, whereas reaction order is an empirical expression derived from experimental data.

In the given exercise, we focus on the third step, which is the rate-determining step among the three. This step involves two molecules:

• One molecule of carbonyl chloride COCl
• One molecule of chlorine Cl₂.

Since two molecules collide and react, the step is termed 'bimolecular'. It's essential to note that the possible values of molecularity are integers: unimolecular (one reacting species), bimolecular (two reacting species), and termolecular (three reacting species), with bimolecular steps being the most common in reactions due to the low probability of three molecules colliding simultaneously.

Chemical Kinetics

Chemical kinetics is the branch of chemistry that studies the rates of chemical reactions and the factors affecting them. It helps chemists understand how different conditions or modifications can speed up or slow down reactions. The area of chemical kinetics is vast and includes aspects such as reaction rates, rate laws, and activation energy.

The reaction rates are influenced by several factors, including temperature, concentration, and the presence of a catalyst. In our three-step mechanism exercise, the emphasis on the slowest step (rate-determining step) is a direct application of kinetics principles. By focusing on this particular step, we can examine how variables like concentration and temperature impact the reaction rate and thus adjust conditions effectively.

Chemical kinetics is crucial for designing and optimizing industrial chemical processes. By manipulating the factors that affect kinetics, industries can make reactions safer, faster, and more cost-effective, a goal that's increasingly critical in today’s resource-sensitive world.