Chemistry Practice Problems: Gibbs Free Energy
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Questions and Answers

What is the characteristic of a system in chemical equilibrium?

  • The reaction goes to completion and no reverse reaction occurs.
  • The rate of the forward reaction equals the rate of the backward reaction. (correct)
  • The rate of the forward reaction is greater than the rate of the backward reaction.
  • The concentrations of all reactants and products are zero.

    • What happens to the concentrations of reactants and products in a system at chemical equilibrium?

  • They fluctuate randomly with time.
  • They remain constant with time. (correct)
  • They increase with time.
  • They decrease with time.

    • What is the condition for a system to reach chemical equilibrium?

  • Any chemical reaction carried out in a closed vessel will reach equilibrium. (correct)
  • The reaction must be irreversible.
  • The reaction must be carried out in an open vessel.
  • The reaction must be reversible.

    • What is the characteristic of a reaction that does not go to completion?

    <p>The products react to form the original reactants once more.</p> Signup and view all the answers

    What is the direction of the reverse reaction in a system at chemical equilibrium?

    <p>From products to reactants.</p> Signup and view all the answers

    What is the consequence of a system reaching chemical equilibrium?

    <p>The concentrations of reactants and products remain constant with time.</p> Signup and view all the answers

    What is the primary goal of Le Chatelier's Principle?

    <p>To relieve the stress on a system at equilibrium</p> Signup and view all the answers

    What is the relationship between ∆G and K at equilibrium?

    <p>∆G = -RT ln K</p> Signup and view all the answers

    What is the role of the reaction quotient (Q) in the equation ∆G = ∆G° + RT ln Q?

    <p>It is the ratio of the concentrations of reactants and products at a given moment in time</p> Signup and view all the answers

    What is the unit of the ideal gas constant (R)?

    <p>J/K·mol</p> Signup and view all the answers

    What is the primary focus of chemical kinetics?

    <p>The study of the rate and mechanism of reactions</p> Signup and view all the answers

    What is the result of applying a stress to a system at equilibrium?

    <p>The equilibrium will shift in a direction to relieve the stress</p> Signup and view all the answers

    What is the law of mass action?

    <p>The rate of a reaction is proportional to the product of the molar concentrations of the reacting substances</p> Signup and view all the answers

    What is the equilibrium constant in terms of concentration?

    <p>[C]r[D]s / [A]p[B]q</p> Signup and view all the answers

    What happens to the equilibrium constant (K) if the concentration of reactants increases?

    <p>K remains the same</p> Signup and view all the answers

    What is the effect of increasing the pressure on the equilibrium constant (K) for a reaction involving gases?

    <p>K remains the same</p> Signup and view all the answers

    What happens to the equilibrium constant (K) if the temperature increases for an endothermic reaction?

    <p>K increases</p> Signup and view all the answers

    What is the equilibrium constant in terms of partial pressures?

    <p>pC pD / pA pB</p> Signup and view all the answers

    What happens to the equilibrium constant (K) if a catalyst is added to the reaction?

    <p>K remains the same</p> Signup and view all the answers

    What is the effect of decreasing the concentration of products on the equilibrium constant (K)?

    <p>K remains the same</p> Signup and view all the answers

    What is the equilibrium constant (K) for a reaction in which all reactants and products are gases?

    <p>pC pD / pA pB</p> Signup and view all the answers

    What is the effect of increasing the temperature on the equilibrium constant (K) for an exothermic reaction?

    <p>K decreases</p> Signup and view all the answers

    Study Notes

    Gibbs Free Energy

    • Gibbs free energy (∆G) can be expressed in several different equations:
      • ∆G° = ∑ G° (products) - ∑ G° (reactants)
      • ∆G° = -nFE°
      • ∆G = ∆G° + RT lnQ
    • ∆G° is the standard Gibbs free energy, and K is the equilibrium constant
    • At equilibrium, ∆G = 0, and Q = K

    Chemical Equilibrium

    • Chemical equilibrium is a state where the rate of the forward reaction equals the rate of the backward reaction
    • It is the state where the concentrations of all reactants and products remain constant with time at any given temperature
    • Any chemical reaction carried out in a closed vessel will reach equilibrium
    • Many reactions do not go to completion, and the products react to form the original reactants once more

    Le Chatelier's Principle

    • This principle states that if a stress is applied on a system at equilibrium, the equilibrium will tend to shift in a direction to relieve that stress
    • Examples of stresses include changes in concentration, pressure, and temperature
    • A shift is a net reaction of reactants to form products or of products to form reactants

    Chemical Kinetics

    • Chemical kinetics is the area of chemistry that deals with the study of the rate (speed) and mechanism of reactions
    • One of the main goals of chemical kinetics is to understand the steps by which a reaction takes place

    Law of Mass Action

    • The law of mass action states that the rate of a chemical reaction is proportional to the product of the molar concentrations of the reacting substances
    • For the general reaction: A + B ⇌ C + D, the rate of forward reaction (RF) is proportional to [A][B], and the rate of reverse reaction (RR) is proportional to [C][D]
    • RF = kF[A][B] and RR = kR[C][D], where kF and kR are constants of proportionality or rate constants

    Equilibrium Constant

    • The equilibrium constant (Kc) is a ratio of the product of the molar concentrations of the products to the product of the molar concentrations of the reactants
    • Kc = [C]r [D]s / [A]p [B]q, where p, q, r, and s are the numbers of moles of A, B, C, and D respectively
    • The equilibrium constant indicates the position of equilibrium, such that:
      • If K > 1, the equilibrium lies to the right, and products predominate
      • If K < 1, the equilibrium lies to the left, and reactants predominate
      • If K = 1, the equilibrium mixture contains equal amounts of products and reactants

    Equilibrium Expression for Gases

    • For gases, the equilibrium constant may be more usefully interpreted in terms of their partial pressures
    • Kp = 𝑝C 𝑝D / 𝑝A 𝑝B, where Kp is the equilibrium constant in terms of partial pressure

    Factors that Affect the Equilibrium Constant

    • Composition of equilibrium mixture:
      • Concentration of reactant increased: shift to right, no effect on K
      • Concentration of reactant decreased: shift to left, no effect on K
    • Pressure:
      • Increased or decreased: no effect on K, unless reaction involves a volume change
    • Temperature:
      • Increased or decreased: change in K depends on the sign of ∆H
    • Addition of catalyst: no effect on K
    • Solving equilibrium problems:
      • Write the balanced equation for the reaction
      • Write the equilibrium expression using the law of mass action
      • List the initial concentrations
      • Apply the changes to the initial concentrations
      • Substitute the equilibrium concentrations into the equilibrium expression
      • Solve for the unknown

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    Practice problems determining reaction feasibility using Gibbs free energy equations, including calculating ΔG values and evaluating reaction spontaneity.

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