Chemical Kinetics Overview
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Chemical Kinetics Overview

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Questions and Answers

What does the half-life of a reaction depend on?

  • The temperature of the environment
  • The volume of the reaction mixture
  • The initial concentration of the reactant (correct)
  • The pressure under which the reaction occurs

    • Why are higher order reactions (>3) considered rare?

  • They usually yield low product concentrations
  • The probability of simultaneous collision of all reacting species is low (correct)
  • They require extremely high temperatures to proceed
  • They are difficult to measure accurately

    • How much does the reaction proceed to completion in eight half-life durations?

  • 100%
  • 75%
  • 99.6% (correct)
  • Approximately 50%

    • Which statement is true regarding the Arrhenius equation?

    <p>Rate constant increases with a decrease in temperature.</p> Signup and view all the answers

    For a first order reaction, what does the half-life t1/2 depend on?

    <p>The rate constant of the reaction.</p> Signup and view all the answers

    What happens to the concentration of an organic compound undergoing first-order decomposition?

    <p>It decreases to 1/10 or 1/8 in total time</p> Signup and view all the answers

    If an organic compound takes time $t_{1/8}$ to decompose to 1/8 of its initial concentration, how would you denote this time mathematically?

    <p>$t_{1/8} = - rac{1}{k} imes ext{ln} rac{[A]_0}{[A]}$</p> Signup and view all the answers

    What is indicated by a reaction that proceeds to 99.6% completion in a particular time frame?

    <p>It is a first-order reaction</p> Signup and view all the answers

    What occurs to reactants if the equilibrium shifts towards them?

    <p>There is an increase in the concentration of the reactants</p> Signup and view all the answers

    Two reactions have identical pre-exponential factors, but R1 has a higher activation energy by 10 kJ mol–1. Which reaction would generally be faster at a given temperature?

    <p>R2, due to its lower activation energy.</p> Signup and view all the answers

    In what scenario would you observe the loss of active species on collision in a reaction?

    <p>When there are multiple reactants required to react simultaneously</p> Signup and view all the answers

    What is the unit of the rate constant for a zero-order reaction?

    <p>mol L–1 h–1</p> Signup and view all the answers

    If the rate constant of a zero-order reaction is 0.2 mol dm–3 h–1, which of the following represents the rate equation?

    <p>Rate = k</p> Signup and view all the answers

    What is the initial concentration of the reactant if its concentration after 30 minutes is 0.05 mol dm–3?

    <p>0.15 mol dm–3</p> Signup and view all the answers

    Which of the following is the correct formula to calculate the change in concentration over time in a zero-order reaction?

    <p>[A] = [A]0 - kt</p> Signup and view all the answers

    Which of the following concentrations indicates a higher initial concentration based on given final conditions?

    <p>4.6 × 10–1 mol dm–3</p> Signup and view all the answers

    According to the data provided, which set of values corresponds to a consistent rate decrease in concentration?

    <p>3.75 × 10–4, 1.25 × 10–4</p> Signup and view all the answers

    What does the negative sign in the rate constant equation signify?

    <p>The concentration is decreasing over time</p> Signup and view all the answers

    If the zero-order reaction starts with 0.15 mol dm–3, how much time will it take to reach 0.05 mol dm–3?

    <p>30 minutes</p> Signup and view all the answers

    If k1 is much smaller than k2, which rate expression is valid for the reaction A + B?

    <p>r = k2[A] [B]</p> Signup and view all the answers

    Which of the following correctly represents the qualitative plot of potential energy versus the reaction coordinate for the reaction in the given content?

    <p>A U-shaped curve depicting reactant and product stability</p> Signup and view all the answers

    What does a first-order reaction indicate about the change in concentration over time?

    <p>Concentration decreases exponentially</p> Signup and view all the answers

    What can be concluded about a second-order reaction based on the information provided?

    <p>Rate is proportional to the square of concentration</p> Signup and view all the answers

    What is the implication when a reaction is said to complete in infinite time?

    <p>Reactants will never be fully converted to products</p> Signup and view all the answers

    In the context of the qualitative plot, which option represents reaction rates after equal intervals of time?

    <p>The concentration decreases sharply at first and then slows down</p> Signup and view all the answers

    What characteristic differentiates the rate of the reaction based on concentration?

    <p>Rate depends on the concentration of reactants</p> Signup and view all the answers

    Which of the following correctly matches the column headings of 'Reaction complete in finite time'?

    <p>Second Order - Reaction complete in finite time</p> Signup and view all the answers

    What is the rate of formation of C when the rate of disappearance of A is 1.73 × 10 –4 M min –1?

    <p>3.47 × 10 –4 M min –1</p> Signup and view all the answers

    Which of the following statements about the half-life of a reaction is correct?

    <p>Half-life is a constant for first-order reactions</p> Signup and view all the answers

    Which statement correctly describes the unit of rate constant k?

    <p>k can have various units depending on the reaction order</p> Signup and view all the answers

    Which of the following statements is true regarding the value of k?

    <p>Value of k remains constant regardless of concentration</p> Signup and view all the answers

    If the rate of formation of product C is 3.47 × 10 –5 M min –1, what is the corresponding rate of disappearance of A?

    <p>3.47 × 10 –4 M min –1</p> Signup and view all the answers

    Which of the following correctly defines the relationship between the rates of a reaction?

    <p>Rate of formation of products is always equal to the rate of disappearance of reactants</p> Signup and view all the answers

    What does a concentration of $3.47 × 10 –4 M min –1$ suggest about the rate of a reactant's disappearance?

    <p>It indicates a fast reaction</p> Signup and view all the answers

    Which of the following claims regarding reaction rates is incorrect?

    <p>The rate of a reaction does not depend on the nature of the reactants</p> Signup and view all the answers

    Study Notes

    Chemical Kinetics

    • The reaction provided is a complex reaction consisting of two steps:

      • The first step is the slow step and the rate determining step.
      • The second step is a fast step.

    • The potential energy diagram for the reaction shows that the activation energy of the first step is higher than the activation energy of the second step.

    Reaction Order

    • The reaction is first order with respect to reactant A and first order with respect to reactant B.
    • The overall order of the reaction is second order.
    • The rate of reaction is dependent on the concentrations of the reactants.

    Half-Life

    • The half-life of a first-order reaction is constant and independent of the initial concentration of the reactant.
    • The half-life is the time it takes for the concentration of the reactant to decrease to half its initial value.

    Activation Energy

    • Activation energy is the minimum energy required for the molecules to react and form products.
    • Activation energy is usually a positive value and is measured in units of kilojoules per mole (kJ/mol).
    • The higher the activation energy, the slower the reaction.
    • The rate constant of a reaction increases with increasing temperature, which can be explained by the Arrhenius equation.

    Order of Reactions

    • The rate of a reaction can be determined by examining the effect of concentration on the rate.
    • The rate can be expressed as a function of the concentrations of the reactants raised to powers.
    • The order of a reaction is the sum of the exponents of the concentration terms in the rate law.

    Factors Affecting Reaction Rate

    • Temperature: Increasing temperature increases the rate of reaction because it provides more energy to the molecules, increasing the frequency of collisions and the number of collisions with enough energy to overcome the activation energy.
    • Concentration: Increasing concentration of reactants also leads to an increase in the rate of reaction since there are more colliding molecules available for reacting.
    • Catalyst: A catalyst is a substance that speeds up a reaction without being consumed in the process.

    Relationship between Rate Constant and Activation Energy

    • The Arrhenius equation provides a relationship between the rate constant and activation energy.
    • The pre-exponential factor in the Arrhenius equation is a measure of the frequency of collisions, irrespective of their energy.

    Zero Order Reactions

    • Zero order reactions are independent of the concentration of the reactant.
    • The rate of a zero order reaction is constant over time.
    • The units of the rate constant for a zero order reaction are mol dm-3 h-1.

    First Order Reactions

    • First order reactions have a rate proportional to the first power of the reactant concentration.
    • The half-life of a first order reaction is constant.
    • The units of the rate constant for a first order reaction are s-1.

    Second Order Reactions

    • Second order reactions have a rate proportional to the second power of the reactant concentration or the product of the concentrations of two reactants.
    • The half-life of a second order reaction depends on the initial concentration of the reactant.

    Higher Order Reactions

    • Higher order reactions (order > 3) are rare because the probability of simultaneous collision of all the reacting species is low.
    • The rate of a higher order reaction is very sensitive to the concentration of the reactants.
    • Higher order reactions are difficult to study experimentally.

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    Description

    This quiz covers the key concepts of chemical kinetics, including reaction order, half-life, and activation energy. Understand the complexities of reaction mechanisms, particularly slow and fast steps, and their impact on reaction rates. Test your knowledge on how these factors influence chemical reactions.

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