equilibrium constant

Examples of equilibrium constant in the following topics:

• Equilibrium Constant Expression

  • The equilibrium constant is an expression that gives the ratio of reactants and products at equilibrium.
  • This constant is known as the equilibrium constant.
  • Their activity is 1, so they do not need to be written in the equilibrium constant.
  • The larger the value of the equilibrium constant, the more the reaction proceeds to completion.
  • The equilibrium constant,denoted by K, is the ratio of products to reactants at equilibrium.

• Homogeneous versus Heterogeneous Solution Equilibria

  • The equilibrium constants for homogeneous and heterogeneous solutions need to be calculated differently.
  • The equilibrium constant K for a given reaction is defined as the ratio of the products of a reaction to the reactants, measured at equilibrium.
  • The reaction quotient measured at equilibrium is the equilibrium constant K.
  • When dealing with these equilibria, remember that solids and pure liquids do not appear in equilibrium constant expressions.
  • The equilibrium constant K is simply [Br2], with the concentration of the pure liquid Br2 excluded.

• Expressing the Equilibrium Constant of a Gas in Terms of Pressure

  • For gas-phase reactions, the equilibrium constant can be expressed in terms of partial pressures, and is given the designation KP.
  • Up to this point, we have been discussing equilibrium constants in terms of concentration.
  • For gas-specific reactions, however, we can also express the equilibrium constant in terms of the partial pressures of the gases involved.
  • Our equilibrium constant in terms of partial pressures, designated KP, is given as:
  • Note that in order for K to be constant, temperature must be constant as well.

• Specialized Equilibrium Constants

  • Common reactions, such as the self-ionization of water, have specially named equilibrium constants.
  • Many reactions are so common or useful that they have their own special equilibrium constants.
  • An acid dissociation constant, Ka, is the equilibrium constant for the dissociation of an acid in aqueous solution.
  • The equilibrium expression is given as:
  • The base dissociation constant, Kb, is analogous to the acid dissociation constant.

• Equilibrium Constant and Cell Potential

  • The equilibrium constant K can be calculated using the Nernst equation.
  • In equilibrium, the electrochemical potential will be constant everywhere for each species.
  • The cell equilibrium constant, K, can be derived from the Nernst equation:
  • The cell potential is zero at equilibrium (E=0), and Q (the reaction quotient) can now be designated as the equilibrium constant K.
  • Calculate the equilibrium constant, K, for a galvanic cell using the Nernst equation

• Reaction Quotients

  • By comparing the value of Q to the equilibrium constant, Keq, for the reaction, we can determine whether the forward reaction or reverse reaction will be favored.
  • Note that the reaction quotient takes the exact same form as the equilibrium constant, and is a function of concentrations and/or activities of the reactants and products.
  • Just as for the equilibrium constant, the reaction quotient can be a function of activities or concentrations.
  • Eventually, the concentrations become constant; at this point, the reaction is at equilibrium.
  • The equilibrium constant, Keq, can be expressed as follows:

• Predicting the Direction of a Reaction

  • Equilibrium constants and reaction quotients can be used to predict whether a reaction will favor the products or the reactants.
  • Equilibrium constants can be used to predict whether a reaction will favor the products or the reactants.
  • The equilibrium constant is only used when a reaction has reached equilibrium.
  • If you know the equilibrium constant for a reaction, and you know all the concentrations, you can predict in what direction the reaction will proceed.
  • Evaluate whether a chemical reaction has reached equilibrium from the reaction coefficient (Q) and the equilibrium constant (K), and use the latter to predict whether the reaction will favor the reactants or products

• Heterogeneous and Multiple Equilibria

  • Therefore, the equilibrium expression for this reaction will be written as:
  • Both steps must be included in the equilibrium constant equation.
  • K1 and K2 are examples the equilibrium constants for each step.
  • Notice that the equilibrium expression for the overall reaction, Keq, is equal to the product of the equilibrium expressions for the two reaction steps.
  • Calculate the equilibrium constant of a multiple-step reaction, given the equilibrium constant for each step

• Solubility Product

  • The solubility product is the equilibrium constant representing the maximum amount of solid that can be dissolved in aqueous solution.
  • The solubility product constant (Ksp) is the equilibrium constant for a solid that dissolves in an aqueous solution.
  • All of the rules for determining equilibrium constants continue to apply.
  • An equilibrium constant is the ratio of the concentration of the products of a reaction divided by the concentration of the reactants once the reaction has reached equilibrium.
  • The equilibrium equation can therefore be rearranged as:

• Acid Dissociation Constant (Ka)

  • The acid dissociation constant (Ka) is the measure of the strength of an acid in solution.
  • Ka is the equilibrium constant for the following dissociation reaction of an acid in aqueous solution:
  • As with all equilibrium constants, the value of Ka is determined by the concentrations (in mol/L) of each aqueous species at equilibrium.
  • The logarithmic constant (pKa) is equal to -log10(Ka).
  • Acetic acid is a weak acid with an acid dissociation constant $K_a=1.8\times 10^{-5}$ .