dissociation

Examples of dissociation in the following topics:

• Acid Dissociation Constant (Ka)

  • The acid dissociation constant (Ka) is the measure of the strength of an acid in solution.
  • Acid dissociation constants are most often associated with weak acids, or acids that do not completely dissociate in solution.
  • The larger the value of pKa, the smaller the extent of dissociation.
  • Acetic acid is a weak acid with an acid dissociation constant $K_a=1.8\times 10^{-5}$ .
  • The acetic acid partially and reversibly dissociates into acetate and hydrogen ions.

• Calculating Percent Dissociation

  • To determine percent dissociation, we first need to solve for the concentration of H+.
  • However, because the acid dissociates only to a very slight extent, we can assume x is small.
  • As we would expect for a weak acid, the percent dissociation is quite small.
  • However, for some weak acids, the percent dissociation can be higher—upwards of 10% or more.
  • Calculate percent dissociation for weak acids from their Ka values and a given concentration.

• Diprotic and Polyprotic Acids

  • With any polyprotic acid, the first amd most strongly acidic proton dissociates completely before the second-most acidic proton even begins to dissociate.
  • Dissociation does not happen all at once; each dissociation step has its own Ka value, designated Ka1 and Ka2:
  • This first dissociation step of sulfuric acid will occur completely, which is why sulfuric acid is considered a strong acid; the second dissociation step is only weakly dissociating, however.
  • A triprotic acid (H3A) can undergo three dissociations and will therefore have three dissociation constants: Ka1 > Ka2 > Ka3.
  • The following formula shows how to find this fractional concentration of HA-, in which pH and the acid dissociation constants for each dissociation step are known:

• Calculating Equilibrium Concentrations of Polyprotic Acids

  • The first proton's dissociation may be denoted as Ka1 and the constants for successive protons' dissociations as Ka2, etc.
  • Because the first dissociation is so strong, we can assume that there is no measurable H2SO4 in the solution, and the only equilibrium calculations that need be performed deal with the second dissociation step only.
  • At a pH equal to the pKa for a particular dissociation, the two forms of the dissociating species are present in equal concentrations, due to the following mathematical observation.
  • When a weak diprotic acid such as carbonic acid, H2CO3, dissociates, most of the protons present come from the first dissociation step:
  • The above concentration can be used if pH is known, as well as the two acid dissociation constants for each dissociation step; oftentimes, calculations can be simplified for polyprotic acids, however.

• Specialized Equilibrium Constants

  • The value of the dissociation constant of water, KW, is $1.0\times 10^{-14}$.
  • An acid dissociation constant, Ka, is the equilibrium constant for the dissociation of an acid in aqueous solution.
  • Recall that strong acids dissociate completely or almost completely into their ions.
  • The base dissociation constant, Kb, is analogous to the acid dissociation constant.
  • A ball-and-stick model of the dissociation of acetic acid to acetate.

• Weak Acids

  • On average, only about 1 percent of a weak acid solution dissociates in water in a 0.1 mol/L solution.
  • The generalized dissociation reaction is given by:
  • The strength of a weak acid is represented as either an equilibrium constant or a percent dissociation.
  • The first Ka refers to the first dissociation step:
  • The second Ka is 4.69×10−11 (pKa2 = 10.329) and refers to the second dissociation step:

• The Acid Dissociation Constant

  • The acid dissociation constant measures the strength of an acid and is essential for understanding acid-base equilibria in solution.
  • This gives rise to a special equilibrium constant known as theacid dissociation constant, Ka.
  • It is the equilibrium constant for a chemical reaction known as dissociation in the context of acid-base reactions.
  • Therefore, the larger the value of pKa, the smaller the extent of dissociation.
  • A ball-and-stick model of the dissociation of acetic acid to acetate.

• Electrolyte and Nonelectrolyte Solutions

  • This is because when a salt dissolves, its dissociated ions can move freely in solution, allowing a charge to flow.
  • It is important to keep in mind, however, that CO2 is not an electrolyte, because CO2 itself does not dissociate into ions.
  • Only compounds that dissociate into their component ions in solution qualify as electrolytes.
  • As mentioned above, when an ionizable solute dissociates, the resulting solution can conduct electricity.
  • By contrast, if a compound dissociates to a small extent, the solution will be a weak conductor of electricity; a compound that only dissociates weakly, therefore, is known as a weak electrolyte.

• Base Dissociation Constant

  • The base dissociation constant, Kb, is a measure of basicity—the base's general strength.
  • It is related to the acid dissociation constant, Ka, by the simple relationship pKa + pKb = 14, where pKb and pKa are the negative logarithms of Kb and Ka, respectively.
  • The base dissociation constant can be expressed as follows:
  • Calculate the Kw (water dissociation constant) using the following equation: Kw = [H+] x [OH−]  and manpulate the formula to determine [OH−] = Kw/[H+] or [H+]=Kw/[OH-]

• Bond Enthalpy

  • Bond enthalpy, also known as bond dissociation energy, is defined as the standard enthalpy change when a bond is cleaved by homolysis, with reactants and products of the homolysis reaction at 0 K (absolute zero).
  • For instance, the bond enthalpy, or bond-dissociation energy, for one of the C-H bonds in ethane (C2H6) is defined by the process:
  • Each bond in a molecule has its own bond dissociation energy, so a molecule with four bonds will require more energy to break the bonds than a molecule with one bond.
  • As each successive bond is broken, the bond dissociation energy required for the other bonds changes slightly.
  • Bond dissociation energies for different element pairings are listed.