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Chapter 14: Problem 71

What are the differences between an intermediate and a transition state?

Short Answer

Expert verified
In summary, intermediate states are temporary species with defined geometries and lower energy levels, which can exist and be observed under certain conditions. On the other hand, transition states are high-energy barriers with no defined molecular structure, representing the critical point in the transformation between reactants and products. The main differences between the two lie in their stability, energy levels, existence, and molecular geometry.

Step by step solution

01

Definition

An intermediate state is a temporary species formed during a chemical reaction, which has a well-defined energy minimum. It exists for a discrete period and can further react to form products. On the other hand, a transition state represents the highest energy point along the reaction coordinate, which is between the reactants and products. It is not an actual molecular species but an energy barrier that needs to be crossed for the reaction to proceed.
02

Energy/state of stability

An intermediate has a lower energy than the transition state, indicating a greater stability than the transition state. An intermediate exists at an energy minimum and can be isolated under certain conditions, whereas a transition state exists at an energy maximum (called the activation energy), making it a high-energy, less stable entity that cannot be isolated.
03

Existence

Intermediate states can exist for a certain period and can be observed under specific experimental conditions. In contrast, a transition state exists for an extremely short period and cannot be directly observed due to its fleeting existence during the reaction.
04

Geometry

The geometry of molecules differs between intermediates and transition states. An intermediate has a defined geometry, which can often be determined using various spectroscopic techniques. A transition state, however, does not have a defined molecular structure, as it represents the highest energy point during the transformation from reactants to products.
05

Role in the reaction mechanism

In a reaction mechanism, intermediate states are actual chemical species that participate in reactions and can be involved in multiple steps of a reaction pathway. Transition states, however, represent the energy barriers between intermediates or reactants and products, and their role is to determine the activation energy and reaction rates. In conclusion, the main differences between intermediates and transition states lie in their stability, energy levels, existence, and molecular structure. While intermediate states are actual chemical species with a defined structure, transition states are high-energy barriers that need to be crossed for a reaction to proceed.

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Most popular questions from this chapter

Ozone in the upper atmosphere can be destroyed by the following two-step mechanism: $$ \begin{array}{l} \mathrm{Cl}(g)+\mathrm{O}_{3}(g) \longrightarrow \mathrm{ClO}(g)+\mathrm{O}_{2}(g) \\ \mathrm{ClO}(g)+\mathrm{O}(g) \longrightarrow \mathrm{Cl}(g)+\mathrm{O}_{2}(g) \end{array} $$ (a) What is the overall equation for this process? (b) What is the catalyst in the reaction? How do you know? (c) What is the intermediate in the reaction? How do you distinguish it from the catalyst?

The following is a quote from an article in the August 18,1998 , issue of The New York Times about the breakdown of cellulose and starch: "A drop of 18 degrees Fahrenheit [from \(77^{\circ} \mathrm{F}\) to \(\left.59^{\circ} \mathrm{F}\right]\) lowers the reaction rate six times; a 36 -degree drop [from \(77^{\circ} \mathrm{F}\) to \(\left.41^{\circ} \mathrm{F}\right]\) produces a fortyfold decrease in the rate." (a) Calculate activation energies for the breakdown process based on the two estimates of the effect of temperature on rate. Are the values consistent? (b) Assuming the value of \(E_{a}\) calculated from the 36 -degree drop and that the rate of breakdown is first order with a half-life at \(25^{\circ} \mathrm{C}\) of 2.7 years, calculate the half-life for breakdown at a temperature of \(-15^{\circ} \mathrm{C}\).

The \(\mathrm{NO}_{x}\) waste stream from automobile exhaust includes species such as \(\mathrm{NO}\) and \(\mathrm{NO}_{2}\). Catalysts that convert these species to \(\mathrm{N}_{2}\) are desirable to reduce air pollution. (a) Draw the Lewis dot and VSEPR structures of \(\mathrm{NO}, \mathrm{NO}_{2},\) and \(\mathrm{N}_{2} .(\mathbf{b})\) Using a resource such as Table 8.4 , look up the energies of the bonds in these molecules. In what region of the electromagnetic spectrum are these energies? (c) Design a spectroscopic experiment to monitor the conversion of \(\mathrm{NO}_{x}\) into \(\mathrm{N}_{2}\), describing what wavelengths of light need to be monitored as a function of time.

The rate of disappearance of HCl was measured for the following reaction: $$ \mathrm{CH}_{3} \mathrm{OH}(a q)+\mathrm{HCl}(a q) \longrightarrow \mathrm{CH}_{3} \mathrm{Cl}(a q)+\mathrm{H}_{2} \mathrm{O}(l) $$ The following data were collected: $$ \begin{array}{rl} \hline \text { Time (min) } & \text { [HCI] (M) } \\ \hline 0.0 & 1.85 \\ 54.0 & 1.58 \\ 107.0 & 1.36 \\ 215.0 & 1.02 \\ 430.0 & 0.580 \\ \hline \end{array} $$ (a) Calculate the average rate of reaction, in \(M / \mathrm{s}\), for the time interval between each measurement. (b) Calculate the average rate of reaction for the entire time for the data from \(t=0.0 \mathrm{~min}\) to \(t=430.0 \mathrm{~min} .\) (c) Graph [HCl] versus time and determine the instantaneous rates in \(M / \min\) and \(M / s\) at \(t=75.0 \mathrm{~min}\) and \(t=250\) min.

(a) What is meant by the term elementary reaction? (b) What is the difference between a unimolecular and a bimolecular elementary reaction? (c) What is a reaction mechanism?
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