12: Chemical Kinetics and Catalysts
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Questions and Answers

What is the main purpose of a catalyst in a chemical reaction?

  • To change the reactants into different products
  • To increase the activation energy
  • To decrease the temperature of the reaction
  • To lower the activation energy (correct)

    • The Arrhenius equation suggests that increasing temperature will always increase the reaction rate.

    True

    What does the reaction N2 + 3H2 ⇌ 2NH3 signify in the context of the Haber–Bosch process?

    It shows the synthesis of ammonia from nitrogen and hydrogen.

    In kinetics, ______ represents the energy barrier that must be overcome for reactants to turn into products.

    <p>activation energy</p> Signup and view all the answers

    Match the following concepts to their descriptions:

    <p>Activation energy = Energy needed to start a reaction Catalysts = Substances that speed up reactions without being consumed Rate law = Equation that relates the rate of reaction to the concentration of reactants Equilibrium = State where the rates of forward and reverse reactions are equal</p> Signup and view all the answers

    What does the activity of a species in an ideal mixture refer to?

    <p>Ratio of its concentration or pressure to a reference concentration or pressure</p> Signup and view all the answers

    The activity of pure liquids and pure solids is always less than 1.

    <p>False</p> Signup and view all the answers

    What happens when the activity quotient, !, is much greater than 1?

    <p>The equilibrium lies to the right, and products dominate.</p> Signup and view all the answers

    The activity of species j, given the reference concentration or pressure, is expressed as a ______________.

    <p>unitless number</p> Signup and view all the answers

    Match the following activity conditions to their outcomes:

    <p>! ≈ 1 = Equilibrium is reached ! ≪ 1 = Reactants dominate ! ≫ 1 = Products dominate Ratef = Rater = No net change at equilibrium</p> Signup and view all the answers

    At equilibrium, what is true about the rate of the forward reaction compared to the rate of the reverse reaction?

    <p>Rates of forward and reverse reactions are equal</p> Signup and view all the answers

    The reaction quotient, !, is calculated using concentrations or partial pressures of reactants and products.

    <p>True</p> Signup and view all the answers

    Define the term 'activity' in the context of chemistry.

    <p>Activity is a measure of the effective concentration of a species in a mixture.</p> Signup and view all the answers

    What does Kc represent in the context of the Law of Mass Action?

    <p>Equilibrium constant for molarity concentrations</p> Signup and view all the answers

    Kc is always equal to Kp.

    <p>False</p> Signup and view all the answers

    Who postulated the Law of Mass Action?

    <p>Goldberg and Waage</p> Signup and view all the answers

    The equilibrium constant in terms of partial pressures is denoted as _____.

    <p>Kp</p> Signup and view all the answers

    Match the following terms with their definitions:

    <p>Kc = Equilibrium constant in terms of molarity concentrations Kp = Equilibrium constant in terms of partial pressures Elementary reaction = Reaction where stoichiometric coefficients can be used as exponents Rate law = Expression that relates the rate of a reaction to the concentrations of reactants</p> Signup and view all the answers

    Which statement is true regarding rate laws and elementary reactions?

    <p>All reactions are considered elementary when at equilibrium.</p> Signup and view all the answers

    Exponents in rate laws come from the stoichiometric coefficients when the reaction is elementary.

    <p>True</p> Signup and view all the answers

    In the expression for the difference in Gibbs free energy, ΔG is equal to the difference in _____.

    <p>free energies</p> Signup and view all the answers

    Which type of equilibria involves all substances being in the same phase?

    <p>Homogeneous equilibria</p> Signup and view all the answers

    In heterogeneous equilibria, pure solids and pure liquids are included in the equilibrium expression.

    <p>False</p> Signup and view all the answers

    What is the term used for the product of the concentrations of dissolved ions in a saturated solution at equilibrium?

    <p>solubility product</p> Signup and view all the answers

    Le Châtelier’s Principle describes how a system at equilibrium can be disturbed by changes in _____.

    <p>temperature, pressure, or concentration</p> Signup and view all the answers

    Match the equilibrium terms with their meanings:

    <p>Homogeneous equilibria = All substances in same phase Heterogeneous equilibria = Substances in different phases Solubility product = Product of concentrations of dissolved ions at equilibrium Le Châtelier’s Principle = Response of equilibrium to disturbances</p> Signup and view all the answers

    Which statement correctly describes a closed system at equilibrium?

    <p>Equilibrium sets a maximum amount of product that can be obtained.</p> Signup and view all the answers

    The concentration of pure solids and pure liquids does change during a chemical reaction.

    <p>False</p> Signup and view all the answers

    How do changes in temperature, pressure, or concentration affect a system at equilibrium according to Le Châtelier’s Principle?

    <p>The system will shift to counteract the change.</p> Signup and view all the answers

    What happens to the equilibrium expression when a reaction is written in reverse?

    <p>It is inverted.</p> Signup and view all the answers

    The equilibrium constant for a reaction multiplied by a constant 'n' is also multiplied by 'n'.

    <p>False</p> Signup and view all the answers

    How do you calculate the equilibrium constant for a multistep reaction?

    <p>It is the product of the equilibrium constants for each individual step.</p> Signup and view all the answers

    The equilibrium expression for the reaction A ⇌ B is given by [B]/[A], while the expression for B ⇌ A is given by ______.

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

    Match the following reactions with their corresponding equilibrium expressions:

    <p>A ⇌ B = [B]/[A] B ⇌ A = [A]/[B] 2A ⇌ B = [B]/[A]^2 3B ⇌ C = [C]/[B]^3</p> Signup and view all the answers

    Which of the following statements correctly describes the effect of changing concentrations on the direction of a reaction?

    <p>The direction of the reaction can be predicted given a specific set of concentrations.</p> Signup and view all the answers

    Equilibrium concentrations can vary depending on how the reaction is written.

    <p>False</p> Signup and view all the answers

    If the reaction A ⇌ B has a certain value of K, then the reaction 3A ⇌ 3B will have a value of K equal to ______.

    <p>K^3</p> Signup and view all the answers

    If the standard change in Gibbs free energy, ∆G°, is less than zero, what can be concluded about the equilibrium constant, K?

    <p>K is greater than 1</p> Signup and view all the answers

    At equilibrium, the rates of the forward and reverse reactions are different.

    <p>False</p> Signup and view all the answers

    What is the relationship between the reaction quotient, Q, and the equilibrium constant, K, when a system is at equilibrium?

    <p>Q equals K</p> Signup and view all the answers

    If ∆G° > 0, then K is __________.

    <p>less than 1</p> Signup and view all the answers

    Study Notes

    Lecture 12 Announcements

    • Topics Covered: Chemical Equilibrium, the Concept of Equilibrium, the Equilibrium Constant, Understanding and Working with Equilibrium Constants, Calculating Equilibrium Constants, Le Châtelier's Principle, Free Energy and the Equilibrium Constant
    • Assigned Readings: Brown Chapter 15
    • Problem Sets: Problem Set 11 due before Exercise #12 tomorrow, Problem Set 12 due before Exercise #13 next week. Both problem sets are posted on Moodle
    • Study Center: Wednesdays, 18:00-20:00 in ETA F 5
    • Office Hours: Prof. Norris and Brisby, Thursdays, 17:00-18:00 in LEE P 210
    • Final Exam: Monday, February 3, 2025, 8:30-10:30

    Lecture 13

    • Topics Covered: Acid-Base Equilibria, the Autoionization of Water, The pH Scale, Strong Acids and Bases, Weak Acids, Weak Bases, Additional Aspects of Aqueous Equilibria, The Common-Ion Effect, Buffers
    • Assigned Readings: Brown Chapter 16 and Chapter 17

    Red Thread (Last Three Weeks)

    • Concepts: Properties, Kinetics, Catalysis, Acid-Base, Equilibrium, Christmas!, Batteries

    Review of Lecture 11

    • Kinetics: Role of temperature in kinetics, the Arrhenius equation (k(T) = A exp(-Ea/RT)), activation energy (Ea), factors affecting reaction rate (Ea can be large or small
    • Spontaneous Reactions: Why some spontaneous reactions are fast and others are slow (activation energy, independent of enthalpy change)
    • Rate Laws: Rate = k[A]m[B]n[C]p, what do m, n, p represent (stoichiometric coefficients for multistep reactions of elementary reactions), molecularity, elementary reactions

    Why High Pressure is Needed to Produce NH3 (Haber-Bosch Process)

    • Haber-Bosch Process: Reaction to convert N2 and H2 to NH3.
    • Spontaneity vs. Kinetics: Reaction is spontaneous at room temperature, but slow due to high activation energy.
    • Catalysts: Catalysts lower activation energy, but still require high temperature to get reasonable reaction rates.
    • Pressure: Increasing pressure shifts the reaction towards products

    Today: Equilibrium (Dynamic Equilibrium in Chemical Reactions)

    • Definition of dynamic equilibrium: reaction and reverse processes balance, no net change in concentrations
    • Examples of dynamic equilibrium: saturated solutions, Henry's Law, vapor pressure.
    • Dynamic Equilibrium: Molecules leaving/entering gas phase in equilibrium.
    • Equilibrium is dynamic: even though the concentrations are stable, molecules are still reacting in both directions.
    • Assumption of Reactions: Reaction that go only forward or reverse reactions only.
    • Equilibrium is reached in both directions simultaneously

    Chemical Equilibrium

    • Equilibrium occurs when forward and reverse rates are equal.
    • Rate constants: kf (forward) and kr (reverse)
    • Equilibrium Constant: The ratio of product concentrations to reactant concentrations at equilibrium, which is written mathematically as K.
    • [B]/[A] = K

    Equilibrium Constant (K)

    • Equilibrium constant, K, relates the concentrations of products to reactants.
    • K depends on temperature (T).
    • K for Reaction in Reverse: K is 1/K for the reverse reaction
    • Multiple reaction: K is the product of the individual reaction constants.

    Equilibrium Constant and Pressure

    • The equilibrium constant in terms of partial pressure, Kp
    • General case Kp ≠ Kc;Kp = Kc * (RT)Δn, where R= gas constant ,T= absolute temperature. Δn = The difference between the number of moles of gaseous products and the number of moles of gaseous reactants

    Important Considerations About Kc and Kp

    • K is unitless.
    • Relating K to Equilibrium concentration, Activity: The ratio of concentration to a reference concentration or pressure.
    • Units of K, depend on stoichiometry and whether pressure measurements are in bars or atmospheres.
    • Magnitude of equilibrium constant K:
      -If K > 1: Equilibrium lies towards products- products dominate the system. -If K < 1: Equilibrium lies towards reactants -reactants dominate the system.
    • Reaction Quotient (Q) : For calculating position of reaction away from Equilibrium.

    Equilibrium Math

    • Equilibrium constant K for a reaction written in reverse is inverted. (if a reaction is written in reverse , then the equilibrium constant for the new reaction is 1/original reaction equilibrium constant).
    • If reaction written with multiple of a constant n, the equilibrium constant will be raised to the power of n. (Example; if a reaction is multiplied by 3, then equilibrium constant is raised to power 3 )
    • Equilibrium constant K for a multistep reaction is the product of the equilibrium constants of the individual reactions

    Equilibrium Considerations: Solids and Pure Liquids

    • In Heterogeneous Equilibria: Removing pure solids and pure liquids from K expressions.
    • Equilibrium Constant for Solid and/or Liquid: Concentrations of pure solids and pure liquids are considered to be constant; therefore are omitted from Equilibrium constant expressions.
    • Solubility product: for saturate solutions of low solubility ionic solids

    Implications of Equilibrium in a Closed System

    • Product yield is limited by closed system, open system might yield more products.
    • Equilibrium sets maximum amount product obtainable.

    Le Chatelier's Principle

    • If system at equilibrium is disturbed by changes in temperature (T), pressure (P), or concentration, the system will shift its equilibrium to counteract the change, to approach back to new equilibrium in a closed system.

    Concentration Changes at Equilibrium

    • Adding more reactant or product shifts system to re-establish equilibrium
    • Adding more Reactant pushes the reaction to the right, creating more product
    • Adding more Product pushes the reaction to the left, making more reactant.

    Temperature Changes at Equilibrium

    • In Exothermic reaction, increasing temperature will cause the equilibrium to shift toward the reactants and vice-versa.
    • In endothermic reaction, increasing temperature will cause the equilibrium to shift toward the products and vice versa.

    Catalysts and Equilibrium

    • Catalyst: A substance that increases the rate of a chemical reaction without itself undergoing a permanent chemical change.
    • Effect on Equilibrium: Catalysts do not affect the equilibrium constant, K; they only speed up the rate by which equilibrium is achieved.

    Equilibrium, Thermodynamics & Free Energy

    • Relation between K (Equilibrium constant) and Thermodynamic parameters: The relationship of equilibrium constant K to Gibbs free energy G.
    • Equilibrium and Spontaneity: Spontaneous reactions, when ∆G is negative.
    • Equilibrium and Rate: Equilibrium constant and reaction rates

    Gibbs Free Energy, AG

    • ∆G = Free energy change, a product of T (Temperature), and R (Ideal gas constant), and ∆G°
    • Standard Free Energy Change:∆ G°
    • Equilibrium constant and Gibbs free energy AG: AG = −RT ln K

    Relationship Between ∆G° and K

    • ∆G° and K relationship: If ∆G° is negative (-), K is greater than 1 (equilibrium lies toward products).
    • ∆G° and K relationship: If ∆G° is positive (+), K is less than 1 (equilibrium favor reactants).

    Summary of Key Concepts in Equilibrium

    • Dynamic equilibrium
    • Equilibrium constant (Kc,Kp)
    • Reaction quotient (Q)
    • Le Chatelier's Principle
    • Catalyst effects
    • Relation between ∆G and K
    • Equilibrium, Thermodynamics, and free Energy
    • Significance of K

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    Description

    Test your understanding of chemical kinetics, catalysts, and the Haber–Bosch process. This quiz covers key concepts, such as the role of catalysts, reaction rates, and the activity of species in chemical reactions. Dive into the principles that govern the energy barriers and equilibrium in reactions.

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