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Chemistry LibreTexts

15.S: Chemical Equilibrium (Summary)

  • Page ID
    70555
  • chemical equilibrium – condition where the concentration of products and reactants do not change with time

    15.1: The Concept of Equilibrium

    at equilibrium

    \[k_f[A] = k_r[B] \nonumber\]

    there for the ratio

    \[\displaystyle\frac{[B]}{[A]} = \frac{k_f}{k_r} = \textit{constant} \nonumber\]

    15.2: The Equilibrium Constant

    equilibrium condition can be reached from either forward or reverse direction

    Cato Maximillian Galdberg (1836-1902), and Peter Wauge (1833-1900)

    • Law of mass action – relationship between concentrations of reactants and products at equilibrium

    If \(aA + bB\rightleftharpoons pP+qQ\)

    then an equilibrium expression can be constructed

    \[\displaystyle K_c=\frac{[P]^p[Q]^q}{[A]^a[B]^b} \nonumber\]

    • equilibrium expression depends only on stoichiometry of reaction and not mechanisms
    • equilibrium constant:
    • does not depend on initial concentrations
    • does not matter if other substances present as long as they do not react with reactants or products
    • varies with temperatures
    • no units

    15.2.1 Expressing Equilibrium Constants in Terms of Pressure, \(K_p\)

    \[\displaystyle K_p=\frac{(P_P)^p(P_Q)^q}{(P_A)^a(P_B)^b}\]

    15.2.2 The Magnitude of Equilibrium Constants

    • \(K\gg 1\); equilibrium lies to the right; products favored
    • \(K \ll 1\); equilibrium lies to the left; reactants favored

    15.2.3 The Direction of the Chemical Equation and \(K\)

    • equilibrium expression written in one direction is the reciprocal of the one in the other direction

    15.4: Heterogeneous Equilibria

    • homogeneous equilibria – substances in the same phase
    • heterogeneous equilibria – substances in different phases
    • concentration of pure liquid or solid
    • \[\displaystyle \frac{\textit{density}}{M} = \frac{\textit{mol}}{cm^3}\]
    • density of pure liquid or solid is constant at any temperature
    • if pure solid or liquid is involved in a reaction, its concentration is excluded from equilibrium expression
    • pure solids must be present for equilibrium to be reached even through they are excluded from equilibrium expression

    15.5: Calculating Equilibrium Constants

    determining unknown equilibrium concentrations

    • tabulate known initial and equilibrium concentrations
    • calculate change in concentration that occurs as system reaches equilibrium
    • use stoichiometry to determine change in concentration of unknown species
    • from initial concentrations and changes in concentrations, calculate equilibrium concentrations

    15.5.1 Relating Kc and Kp

    \[PV = nRT\]

    \[P = (n/V)RT = MRT\]

    \[PA = [A](RT)\]

    \[K_p=K_c(RT)D^n\]

    • D n = change in moles from reactants to products

    15.6: Applications of Equilibrium Constants

    • equilibrium constant:
      1. product direction reaction mixture will proceed
      2. calculate concentrations of reactants and products once equilibrium is reached

    15.6.1 Predicting the Direction of Reaction

    • reaction quotient
    • at equilibrium Q=K
    • Q>K; reaction moves right to left
    • Q<K; reaction moves left to right

    15.6.2 Calculating of Equilibrium Concentrations

    15.7: Le Chatelier's Principle

    If system at equilibrium is disturbed by change in temperature, pressure or concentration then system will shift equilibrium position

    15.7.1 Change in Reactant or Product Concentration

    • addition of substance will result in consummation of part of added substance
    • if substance removed, reaction will move to produce more of the substance

    15.7.2 Effects of Volume and Pressure Changes

    • reducing volume, reaction shifts to reduce number of gas molecules
    • increase volume, reaction shifts to produce more gas molecules
    • increase pressure, decrease volume reduces total number of moles
    • pressure volume changes do not affect K as long as temperature is constant
    • changes concentrations of gaseous substances

    15.7.3 Effect on Temperature Change

    • endothermic: reactants + heat « products
    • exothermic: reactants « products + heat
    • increase temperature, equilibrium shifts in direction that absorbs heat
    • endothermic: increase T, increase K
    • exothermic: increase T, decrease K
    • cooling shifts equilibrium to produce heat

    15.7.4 The Effect of Catalysts

    • catalysts increase rate at which equilibrium is obtained
    • does not change composition of equilibrium mixture