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Chapter 16: Problem 43

What is the role of the activated complex in a chemical reaction?

Short Answer

Expert verified
The activated complex, or transition state, is an unstable, high-energy configuration of atoms or molecules that exists briefly during a chemical reaction. Its formation represents the highest energy point along the reaction coordinate and determines the reaction rate. The activation energy required to reach the transition state directly affects the reaction rate, as expressed in the Arrhenius equation. A higher activation energy leads to a slower reaction, while a lower activation energy results in a faster reaction. The role of the activated complex is crucial in chemical reactions as it dictates the transition from reactants to products, and its energy barrier can be manipulated using catalysts to alter the reaction rate.

Step by step solution

01

Define the activated complex

The activated complex, also known as the transition state, is an unstable, high-energy configuration of atoms or molecules that exists for a brief moment during a chemical reaction. It represents the highest energy point along the reaction coordinate, where reactants transition into products. The activated complex is not a stable species, and it cannot be isolated or directly observed.
02

Discuss energy profile of a chemical reaction

A chemical reaction's energy profile describes how the potential energy of the system changes throughout the course of the reaction. The energy required to reach the transition state from the reactants is called the activation energy. Activation energy is the minimum amount of energy needed for the reaction to proceed. During the reaction, reactants transform into the activated complex before converting into products. The difference in energy between the reactants and the activated complex determines the overall reaction rate.
03

Relate the activated complex to the reaction rate

The relationship between the activated complex and the reaction rate can be described through the Arrhenius equation, which is: \(k = Ae^{-E_a/RT}\) Where k is the reaction rate constant, A is the pre-exponential factor (representing the frequency of successful collisions between reactants), \(E_a\) is the activation energy, R is the gas constant, and T is the absolute temperature. The formation of the activated complex is the rate-determining step of the reaction because it is the highest energy barrier that must be overcome. Thus, the activation energy directly affects the reaction rate.
04

Highlight the role of the activated complex in a chemical reaction

In summary, the activated complex plays a crucial role in a chemical reaction as it represents the transitional state between reactants and products. The energy barrier needed to form the activated complex dictates the reaction rate. A higher activation energy means the reaction is slower, while a lower activation energy corresponds to a faster reaction. In many reactions, the rate can be manipulated by using catalysts, which lower the activation energy by providing an alternative path with a lower energy barrier, thus promoting the formation of the activated complex more easily.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Transition State
The concept of the transition state is central to understanding how chemical reactions progress. It's often visualized as the peak point in an energy profile diagram of a reaction. When reactants start interacting, they don't instantly turn into products. Instead, they pass through an intermediate stage where their energy is at its highest. This stage is known as the transition state or activated complex. It exists only fleetingly, and due to its high energy, it cannot be directly observed.
  • It is the point of maximum energy in the reaction pathway.
  • The configuration during this state is high-energy and unstable.
  • It's the critical point where reactants have begun the transformation to products but are not yet completed as products.
Understanding this concept helps explain how reactions can be altered or controlled, for example, by employing catalysts. Catalysts lower the energy required to achieve this state, thus speeding up the reaction.
Activation Energy
Activation energy is a key component in the study of chemical kinetics. It is defined as the minimum energy required to convert reactants into products. This energy must be supplied to reach the activated complex, where a reaction either progresses to form products or reverts back to the original reactants.
Here's what you need to know:
  • It's symbolized as \(E_a\).
  • High activation energy indicates a slower reaction, as fewer reactant molecules have sufficient energy to reach the transition state.
  • Lower activation energies mean that more molecules can transform into products, making the reaction faster.
Knowing the activation energy is crucial for predicting how a reaction behaves under different conditions, such as changes in temperature or pressure.
Reaction Rate
The reaction rate is all about how quick or slow a reaction occurs. It is influenced by multiple factors, including the concentration of reactants, temperature, and the presence of catalysts. Most notably, it depends on how quickly the activated complex can form and convert to products.
Let's break it down:
  • The higher the activation energy, the slower the reaction rate.
  • Temperature increases typically boost the rate of reaction by giving reactant molecules more energy to reach the transition state.
  • Catalysts can accelerate the rate by providing a lower-energy pathway for the reaction.
Reaction rates are crucial for practical applications, such as industrial chemical production, where efficiency and speed are important.
Arrhenius Equation
The Arrhenius equation is a fundamental equation used in chemical kinetics to understand how temperature affects reaction rates. It links the chemical reaction rate constant \(k\) with the activation energy \(E_a\) and temperature \(T\). The equation is commonly written as:\[k = Ae^{-E_a/RT}\]
  • \(A\) is the pre-exponential factor, representing the number of times reactants collide with the correct orientation per unit time.
  • \(E_a\) is the activation energy, as previously discussed.
  • \(R\) is the universal gas constant.
  • \(T\) is the temperature in Kelvin.
The Arrhenius equation shows that even small temperature increases can significantly increase reaction rates by lowering the exponential factor \(e^{-E_a/RT}\). This equation is valuable for chemists and engineers who need to predict how changes in conditions will affect the speed of reactions.

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Most popular questions from this chapter

Use the rate law in Example Problem 16.2 and the concentrations given in Practice Problems 31 and 32 to calculate the instantaneous rate for the reaction between \(\mathrm{NO}\) and \(\mathrm{H}_{2} .\) $$[\mathrm{NO}]=0.00500 \mathrm{M} \text { and } \left[\mathrm{H}_{2}\right]=0.00200 \mathrm{M}$$

In the gas-phase reaction, \(\mathrm{I}_{2}+\mathrm{Cl}_{2} \rightarrow 2 \mathrm{ICl},\left[\mathrm{I}_{2}\right]\) changes from 0.400 \(\mathrm{M}\) at 0.00 \(\mathrm{min}\) to 0.300 \(\mathrm{M}\) at 4.00 \(\mathrm{min}\) . Calculate the average reaction rate in moles of I 2 consumed per liter per minute.

Explain why magnesium metal reacts with hydrochloric acid (\(\mathrm{HCl}\)) at a faster rate than iron does.

Suggest a reason why, when given the rate of a chemical reaction, it is important to know that the reaction rate is an average reaction rate

If a chemical reaction occurs at the rate of \(2.25 \times 10^{-2}\) moles per liter per second at 322 \(\mathrm{K}\) , what is the rate expressed in moles per liter per minute?
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