14.3 Relative Strengths of Acids and Bases

14.3 Relative Strengths of Acids and Bases

  • The strengths of acids can be ranked by how much they ionize in the solution.
    • A strong acid can yield 100% of H3O+ and A- when it ionizes in water.
    • Small amounts of H3O+ and A- can be given by a weak acid.
  • The strong acids and bases are listed here.
  • The relative strengths of acids can be determined by measuring their equilibrium constants.
    • Stronger acids ionize to a greater extent in solutions of the same concentration.
  • We don't include H2O in the equation because water is a reactant as well.
    • A stronger acid has a bigger ionization constant than a weaker acid.
    • As the acids' strengths increase, the ionization constants increase.

  • The percent ionization is a measure of the strength of an acid.
  • The percent ionization for a solution of a given weak acid varies depending on the original concentration of the acid, and actually decreases with increasing acid concentration.
  • The logarithm 2.09 shows a hydronium ion concentration with only two figures.
  • The strengths of bases can be ranked by their tendency to form hydroxide ion in solution.
    • A strong base can give 100% of OH- and HB+ when it reacts with water.
    • A weak base has a small amount of hydroxide ion.
    • NaOH is considered a strong base because it is dissolved in water.
  • Stronger bases ionize to a greater extent in solutions of the same concentration.
    • A stronger base has a larger constant than a weaker one.
    • We don't include H2O in the equation because water is the solvent.

  • A table of ionized constants of weak bases can be found in Appendix I, with a partial list in As with acids.

  • The base conjugate base of acetic acid (CH3COOH) is 5.6 x 10 and the acid ion constant is 1.8 x 10.
  • The strength of the conjugate base of the acid affects the amount of protons donated to water.
    • If A- is a strong base, any protons that are donated to water are captured by A-.
    • The acid, HA, is weak and there is relatively little A- and H3O+ in solution.
    • The acid is strong if A- is a weak base and the solution contains mostly A- and H3O+.
    • Strong acids form strong conjugate bases.
  • The relative strengths of conjugate acid-base pairs are shown in the diagram.
  • The decreasing strengths of the acids are listed in order of the increasing strengths of the bases.
    • The acid and base are conjugate to each other.
  • The chart shows the strengths of the pairs.
  • The acids are in a solution.
    • The conjugate bases of these acids are not as strong as water.
    • When one of these acids is dissolved in water, their protons are transferred to the stronger base.
  • In a solution of one of these acids, there are both hydronium ion and nonionized acid molecule present.
    • If any conjugate base were formed, it would react with water to re-form the acid.
  • A strong base that accepts protons from water can give 100% of the conjugate acid and hydroxide ion.
    • A mixture of the hydroxide ion and the base results in protons being accepted from water.
    • Bases that are weaker than water have no observable basic behavior in the solution.

  • The constant of HCN is given in Appendix H.

Which is the stronger acid, HCN or NH4?

  • Many acids and bases are weak.
    • A solution of a weak acid in water is a mixture of the nonionized acid, hydronium ion, and the conjugate base of the acid, with the nonionized acid present in the greatest concentration.
    • A weak acid increases the hydronium ion concentration in the solution, but not as much as a strong acid.
  • A weak acid is acetic acid.
  • This equilibrium is similar to other equilibria in that acetic acid molecule give hydrogen ion to water molecule and form hydronium ion and acetate ion at the same rate.
    • If we measure the pH of the solution, we can see that the majority of the weak acid is nonionized.

  • Table 14.2 gives the ionized constants for several weak acids; additional ionized constants can be found in Appendix H.
  • A solution of a weak base in water is a mixture of the nonionized base, the conjugate acid of the weak base, and hydroxide ion with the nonionized base present in the greatest concentration.
    • A weak base increases the ion concentration in the solution but not as much as a strong base.
  • This equilibrium is similar to the one described for weak acids.
  • We can confirm by measuring the pH of the solution of a weak base of known concentration that only a fraction of the base reacts with water.
  • It tastes sour becauseacetic acid is the main ingredient.
  • A weak acid is acetic acid.
  • We are asked to calculate an equilibrium constant.
  • C8H10N4O2 is a weak base.

  • We approximate the initial concentration of H3O+ to be zero because it is so much less than the final concentration.
  • The equilibrium concentration of HNO2 is the same as its initial concentration.
  • Check your learning.
  • The irritant that causes the body is formic acid.
  • An ant's sting is caused by formic acid.

  • 3 CO2 is the fraction of acetic acid that is ionized x 100.
  • The concentration of products produced by the ionization of a weak base can be determined by the same series of steps used with a weak acid.

  • This problem requires us to calculate an equilibrium concentration by determining concentration changes as the base goes to equilibrium.

  • The assumption is justified because the change is less than 5% of the initial concentration.

  • As we solve for the equilibrium concentrations in such cases, we will see that we cannot neglect the change in the initial concentration of the acid or base.
  • The HSO4 ion is a weak acid and it is used in some household cleanser.

  • The equation can be solved using a formula.

  • The strength of the acids in water is the same.
    • The water molecule is a strong base compared to the conjugate bases, which makes it easy to ionize strong acids.
    • The tendency of the solvent to give up a protons is different in different solvents.
    • HI is shown to be the strongest of the acids when dissolved in a weaker base than water.
  • Acid-Base Equilibria Water has a leveling effect on the strengths of strong bases.
  • As the H-A bond strength decreases down a group in the periodic table, the acid strength of hydrogen with nonmetals increases.
    • Group 17 has an order of increasing acidity.
    • The order of increasing acid strength is H2O H2S H2Se.
  • Across a row in the periodic table, the acid strength of hydrogen compounds increases when the H-A bond is increased.
    • The acid strength increases when you move from left to right.
    • The base strength increases as you move.
  • Oxygen and one or more OH groups can be acidic, basic, or amphoteric, depending on the position in the periodic table of the central atom E. The general formula OnE(OH)m is used for these compounds, which include sulfuric acid, O2S(OH)2, sulfurous acid, OS(OH)2, nitric acid, O2NOH, perchloric acid, O3ClOH, aluminum hydroxide, and The more metallic elements have lower electronegativity and form ionic hydroxides that are basic compounds.
  • The more nonmetallic elements have high electronegativities.
  • Increasing the oxidation number of the central atom E increases the attraction of E for the electrons it shares with oxygen and weakens the O-H bond.
    • H2SO4 or O2S(OH)2 is more acidic than sulfurous acid, H2SO3 or OS(OH)2 with a sulfur oxidation number of +4.
    • nitric acid, HNO3 or O2NOH, is more acidic than nitrous acid, HNO2, or ONOH.
    • The oxidation number of the central atom E increases.
  • Hydroxy compounds of elements with intermediate electronegativities and relatively high oxidation numbers are usually amphoteric.
    • The hydroxy compounds act as acids when they react with strong bases and as bases when they react with strong acids.
    • The amphoterism of aluminum hydroxide can be seen in both strong acids and strong bases.
  • Under these conditions, the Al(H2O)3(OH)3 compound acts as an acid.