15.9
Ions as Acids and Bases
Salts with Acidic Ions
Salts are ionic compounds composed of cations and anions, either of which may be capable of undergoing an acid or base ionizati…
Acids, bases, and ionic compounds form anions and cations when they dissolve in water.
Anions that are the conjugate base of strong acids, like chloride formed by the dissociation of hydrochloric acid, are too weak to accept a proton from water. Therefore, chloride ions are pH neutral; that is, they are neither acidic nor basic.
In contrast, anions formed by weak acids, like acetate, the conjugate base of acetic acid, acts as a weak base as it can accept a proton from water.
Cations that are the conjugate acid of strong bases, like sodium ions formed by sodium hydroxide, cannot accept protons and, therefore, are also pH neutral.
In contrast, cations produced by weak bases, like ammonium, the conjugate acid of the ammonia, acts as a weak acid as it can donate protons to water.
When they ionize, salts can produce acidic and basic solutions if they contain a conjugate acid or a conjugate base of a weak acid or base.
Ammonium bromide produces ammonium and bromide ions in water. Bromide ions are pH neutral, whereas ammonium ions act as a weak acid as they can donate protons.
When sodium acetate is dissolved in water, the sodium ions do not react with water; however, the acetate ion can accept a proton—forming a basic solution.
Salts that contain pH neutral cations and anions form neutral solutions. For example, sodium chloride dissociates into sodium ions and chloride ions when dissolved in water. As both of these ions can neither accept nor donate protons, they form a neutral solution.
Small and highly charged metal ions, like iron(III) and aluminium(III), can also act as weak acids when they become hydrated.
When aluminum(III) is hydrated, it acts as a weak acid and transfers protons from its waters of hydration to the free water molecules, leading to the production of hydronium ions.
The smaller the metal ion and the higher the charges present on it, the greater is its tendency to act as an acid. For example, the Ka for Fe(III) is 6.3 × 10−3, whereas the Ka for Ni(II) is 2.5 × 10−11.
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Q1: Why are chloride ions pH neutral when hydrochloric acid dissolves in water?
Chloride ions are the conjugate base of the strong acid hydrochloric acid. Since strong acids dissociate completely, their conjugate bases are too weak to accept protons from water molecules. Therefore, chloride ions neither donate nor accept protons, making them pH neutral and unable to affect solution acidity or basicity.
Q2: How do acetate ions make a sodium acetate solution basic?
Acetate ions are the conjugate base of the weak acid acetic acid. Because acetic acid is weak, acetate ions retain the ability to accept protons from water molecules. This proton acceptance produces hydroxide ions, creating a basic solution. Sodium ions do not react with water, so they have no effect on pH.
Q3: What determines whether ammonium bromide solution is acidic or basic?
Ammonium bromide produces ammonium and bromide ions in water. Bromide ions are pH neutral because they are the conjugate base of the strong acid hydrobromic acid. Ammonium ions act as weak acids by donating protons to water. Since only ammonium ions affect pH, the solution becomes acidic.
Q4: Why do small, highly charged metal ions like aluminum(III) act as weak acids?
When aluminum(III) ions dissolve in water, they form hydrated complex ions with six bonded water molecules. The positive charge on the metal ion polarizes these bonded water molecules, making their O-H bonds more polar and prone to donating hydrogen ions. This proton transfer produces hydronium ions, making the solution acidic.
Q5: How does ion charge and size affect the acid strength of hydrated metal ions?
The acid strength of hydrated metal ions increases with higher charge and smaller size. For example, Fe(III) with Ka = 6.3 × 10−3 is a stronger acid than Ni(II) with Ka = 2.5 × 10−11. Smaller, more highly charged ions create stronger polarization of bonded water molecules, increasing their tendency to donate protons and making them stronger acids.
Q6: Why does sodium chloride form a neutral solution in water?
Sodium chloride dissociates into sodium and chloride ions. Sodium ions are the conjugate acid of the strong base sodium hydroxide and cannot accept protons. Chloride ions are the conjugate base of the strong acid hydrochloric acid and cannot donate protons. Since neither ion reacts with water, the solution remains neutral.
Q7: What is the difference between conjugate bases of strong acids and weak acids?
Conjugate bases of strong acids, like chloride, are too weak to accept protons from water and remain pH neutral. Conjugate bases of weak acids, like acetate, retain significant ability to accept protons from water, acting as weak bases. This difference determines whether salt solutions containing these ions will be acidic, basic, or neutral.