Chemistry Rate Constants and Molecularity

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Questions and Answers

What does the unit L mol–1 s–1 indicate about a reaction?

It represents a first order reaction.

It is associated with unimolecular reactions.

It indicates a second order reaction. (correct)

It denotes a ternary reaction.

Which of the following best describes bimolecular reactions?

They involve collisions between three reacting species.

They require simultaneous collision between two species. (correct)

They are always first order reactions.

They can only occur if one species is involved.

What is molecularity in chemical kinetics?

The number of steps in a reaction mechanism.

The number of reacting species colliding simultaneously. (correct)

The overall order of the reaction.

The rate constant of the reaction.

Which reaction is an example of a trimolecular reaction?

2NO + O2 → 2NO2 Signup and view all the answers

What does the term 'rate determining step' refer to in a reaction mechanism?

It is the step that controls the overall rate of the reaction. Signup and view all the answers

For a reaction with molecularity greater than three, what can be inferred?

It will generally occur through multiple steps. Signup and view all the answers

What is the rate determining step in a reaction?

The slowest step in the reaction Signup and view all the answers

Which of the following statements about order and molecularity is true?

Order can be zero; molecularity cannot be zero. Signup and view all the answers

If a reaction has a rate constant, k, with units of s–1, what can be inferred about its order?

It is a first order reaction. Signup and view all the answers

In a reaction with the stoichiometric equation KClO3 + 6FeSO4 + 3H2SO4 → KCl + 3Fe2(SO4)3 + 3H2O, what is the actual order of the reaction?

Second order reaction. Signup and view all the answers

In the reaction where H2O2 decomposes in the presence of I–, what role does IO– play?

It is an intermediate formed during the reaction Signup and view all the answers

For a reaction with the rate law r = k [A]^(1/2) [B]^2, what is the overall order of the reaction?

1.5 Signup and view all the answers

If the concentration of X is tripled in a second order reaction converting X to Y, how will the rate of formation of Y change?

It will increase ninefold Signup and view all the answers

Which of these statements is correct about bimolecular reactions?

They can occur in complex reactions as well Signup and view all the answers

What does the integrated rate equation do for a first-order reaction?

It relates concentration to time Signup and view all the answers

In a rate law, if the concentration of a reactant is raised to a power of zero, what does this imply?

The reactant has no effect on the reaction rate Signup and view all the answers

What is the primary assumption of the collision theory regarding reactant molecules?

They are assumed to be hard spheres. Signup and view all the answers

In the rate expression for a bimolecular reaction, which variable represents the collision frequency of reactants?

ZAB Signup and view all the answers

What role does the probability or steric factor (P) play in the rate of a chemical reaction?

It accounts for proper orientation of molecules during collisions. Signup and view all the answers

What factor besides activation energy is essential for a reaction to proceed according to collision theory?

Proper orientation of the reactant molecules Signup and view all the answers

Which of the following statements accurately describes effective collisions?

Only certain collisions lead to product formation. Signup and view all the answers

How does collision theory describe the relationship between the rate of reaction and temperature?

Rate increases with increased temperature due to more collisions with sufficient energy. Signup and view all the answers

According to collision theory, which equation represents the rate of a bimolecular reaction considering activation energy and steric factor?

Rate = PZAB e^(-Ea/RT) Signup and view all the answers

What limitation does collision theory have regarding the nature of molecules?

It considers atoms/molecules as hard spheres and ignores their structural aspects. Signup and view all the answers

Study Notes

Rate Constant Units

The unit of a second order rate constant is L mol⁻¹ s⁻¹
The unit of a first order rate constant is s⁻¹

Molecularity of a Reaction

Molecularity: The number of reacting molecules in an elementary reaction that must collide simultaneously to cause a reaction
Unimolecular: One reacting species is involved. Example: the decomposition of ammonium nitrite (NH₄NO₂ → N₂ + 2H₂O)
Bimolecular: Two reacting species are involved. Example: the decomposition of hydrogen iodide (2HI → H₂ + I₂)
Trimolecular (Termolecular): Three reacting species are involved. Example: the reaction between nitric oxide and oxygen (2NO + O₂ → 2NO₂)
Reactions with molecularity higher than three are rare because the probability of more than three molecules colliding simultaneously is very low

Rate Determining Step

Complex reactions with multiple steps can be understood by their rate determining step.
The rate determining step is the slowest step in a reaction and controls the overall rate of the reaction. This is analogous to the slowest runner in a relay race determining the overall speed of the team.
Example: The decomposition of hydrogen peroxide catalyzed by iodide ions in an alkaline medium:
- 2H₂O₂ → 2H₂O + O₂
- Rate = - d[H₂O₂] / dt = k[H₂O₂][I⁻]. This reaction is first order with respect to both H₂O₂ and I⁻.
- The reaction proceeds in two steps:
  - (1) H₂O₂ + I⁻ → H₂O + IO⁻ (slow, rate-determining step)
  - (2) H₂O₂ + IO⁻ → H₂O + I⁻ + O₂ (fast)
- The intermediate, IO⁻, is formed during the reaction but does not appear in the overall balanced equation.

Order vs. Molecularity

Order of a reaction is determined experimentally. It can be zero, fractional, or an integer.
Molecularity is a theoretical concept and can only be an integer. It cannot be zero or fractional.
Order applies to both elementary and complex reactions.
Molecularity only applies to elementary reactions. For complex reactions, it has no meaning.
For complex reactions, the order is determined by the slowest step, and the molecularity of the slowest step is the same as the order of the overall reaction.

Collision Theory

Based on the kinetic theory of gases, which assumes molecules are hard spheres.
Reactions occur when molecules collide with each other.
Collision frequency (Z): The number of collisions per second per unit volume of the reaction mixture.
Successful collisions require:
- Sufficient kinetic energy (threshold energy)
- Proper orientation of the molecules
Effective collisions lead to the formation of products.
The probability factor (P) or steric factor accounts for the fact that not all collisions result in products.
Rate = PZABe^(-Ea / RT)
- ZAB represents the collision frequency of reactants A and B.
- e^(-Ea/RT) represents the fraction of molecules with energies equal to or greater than the activation energy (Ea).

Summary

The rate of a chemical reaction is determined by the rate determining step in a complex reaction, and the molecularity of the slowest step is the same as the order of the overall reaction.
Collision theory explains how reactions occur based on collisions between molecules, but not all collisions result in products.
The probability of successful collisions depends on both kinetic energy and the proper orientation of the molecules.

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Description

Test your knowledge on rate constants, their units, and the molecularity of reactions in this engaging chemistry quiz. Understand the differences between unimolecular, bimolecular, and trimolecular reactions, and explore the concept of the rate determining step in complex reactions.