The following mechanism has been proposed for the gasphase...

The following mechanism has been proposed for the gasphase reaction of chloroform (CHCl_ ) and chlorine:$$\begin{array}{l}{\text { Step } 1 : \mathrm{Cl}_{2}(g) \frac{k_{1}}{k_{-1}} 2 \mathrm{Cl}(g) \text { (fast) }} \\ {\text { Step } 2 : \mathrm{Cl}(g)+\mathrm{CHCl}_{3}(g) \stackrel{k_{2}}{\longrightarrow} \mathrm{HCl}(g)+\mathrm{CCl}_{3}(g) \text { (slow) }}\end{array}$$ $$ { Step } \quad3 : \quad \mathrm{Cl}(g)+\mathrm{CCl}_{3}(g) \stackrel{k_{3}}{\longrightarrow} \mathrm{CCl}_{4} \quad(\text { fast })$$ (a) What is the overall reaction? (b) What are the intermedi- ates in the mechanism? (c) What is the molecularity of each of the elementary reactions? (d) What is the rate-determining step? (e) What is the rate law predicted by this mechanism? (Hint: The overall reaction order is not an integer.)

The following mechanism has been proposed for the gasphase reaction of chloroform (CHCl_ ) and chlorine:$$\begin{array}{l}{\text { Step } 1 : \mathrm{Cl}_{2}(g) \frac{k_{1}}{k_{-1}} 2 \mathrm{Cl}(g) \text { (fast) }} \\ {\text { Step } 2 : \mathrm{Cl}(g)+\mathrm{CHCl}_{3}(g) \stackrel{k_{2}}{\longrightarrow} \mathrm{HCl}(g)+\mathrm{CCl}_{3}(g) \text { (slow) }}\end{array}$$ $$ { Step } \quad3 : \quad \mathrm{Cl}(g)+\mathrm{CCl}_{3}(g) \stackrel{k_{3}}{\longrightarrow} \mathrm{CCl}_{4} \quad(\text { fast })$$ (a) What is the overall reaction? (b) What are the intermedi-
ates in the mechanism? (c) What is the molecularity of each of the elementary reactions? (d) What is the rate-determining step? (e) What is the rate law predicted by this mechanism? (Hint: The overall reaction order is not an integer.)

Key Concepts

Overall Reaction

The overall reaction is obtained by summing the elementary steps of a reaction mechanism while canceling out any species that appear on both sides, particularly intermediates. This concept is central to understanding reaction mechanisms since it provides the net chemical change occurring as a result of a series of elementary processes.

Reaction Intermediates

Reaction intermediates are species that are formed in one elementary step and consumed in a later step in the mechanism. They do not appear in the overall balanced equation but are crucial in connecting the individual steps. Identifying intermediates helps in analyzing the pathway and temporal sequence of chemical reactions.

Molecularity

Molecularity refers to the number of reactant species involved in an elementary reaction step. It defines the order of the reaction with respect to a single step and is determined by the number of molecules or ions that collide to bring about the transformation in that specific step. Understanding molecularity is essential for kinetic analysis at the molecular level.

Rate-Determining Step

The rate-determining step (RDS) is the slowest step in a reaction mechanism that governs the overall reaction rate. Identifying the RDS is crucial because it allows chemists to derive the rate law for the reaction, as the kinetics of the overall reaction are usually controlled by this single step.

Rate Law Prediction

The rate law expresses the relationship between the concentration of reactants and the rate of reaction. In complex mechanisms, it is often derived by assuming that the steps preceding the RDS reach a fast pre-equilibrium. This approximation allows the manipulation of the rate law to express it in terms of the concentrations of the reactants only, which is crucial in predicting and rationalizing reaction kinetics.

Transcript

00:01 Chapter 14, question 110.

00:04 The following mechanism has been proposed for a gas -phase reaction of chloroform and chlorine.

00:10 Now, what's the overall reaction? so to be able to do that, i want to add all these three steps, and left -hand sides to the left -hand sides, right -hand sides to the right -hand sides.

00:21 So let's start.

00:25 Cl2 plus cll plus chcl3 plus chcl3 plus cl plus cl plus cl plus ccl plus ccl 3.

00:45 These are all left -hand sides.

00:47 And right -hand sides, 2cl plus hcl plus ccl plus ccl 3 plus ccl 4.

01:07 Now, if you want to rearrange these, what we have is ccl 3 over here, a ccl 3 over here, so they cancel.

01:18 Do we have ccl4? we don't.

01:21 Do we have 8cl? we don't.

01:25 We have 2 cl on the right hand side, and we have 2 cl on the left hand side.

01:32 So they cancel to you.

01:34 So overall, what we're going to get is cl2 plus chcl3, which go to 8cl plus ccl plus ccl4.

01:57 Now, what are the intermediates in the mechanism? so the intermediates are each elementary step, we get something at the products, and if that whatever we get in the products is consumed in the other elementary step, that is an intermediate.

02:16 So chlorines over here are the intermediates, since we get them in the products and we consume them later on.

02:26 Over here, we use get a ccl3 over here in the products and we consume that too.

02:36 So ccl3 is also an intermediate.

02:40 So this is part a and part b would be ccl3 and cl.

02:54 Part c.

02:55 Part c, what is the molecularity of each of the elementary reactions? so the molecularity of this first one is going to be unimolecular.

03:16 The reason why is this is only one molecule.

03:21 And the second one is going to be there are two molecules...