Unit 5: Further Reactions
Exploring amphoterism, acid-salt reactions, and the hydrolysis of salts.
5.11 Amphoterism
Most substances are clearly acidic or basic. However, some substances, known as amphoteric substances, can act as either an acid or a base depending on the chemical environment.
This dual nature is common among the oxides and hydroxides of certain metals, such as aluminium, zinc, and lead. Their bonding is intermediate between ionic and covalent, allowing them to react with both strong acids and strong bases.
Example: Zinc Oxide ($ZnO$)
- Acting as a base (reacting with an acid):
$ZnO(s) + 2HCl(aq) \rightarrow ZnCl_2(aq) + H_2O(l)$
Here, zinc oxide neutralises the acid to form a salt and water. - Acting as an acid (reacting with a strong base):
$ZnO(s) + 2NaOH(aq) + H_2O(l) \rightarrow Na_2[Zn(OH)_4](aq)$ (Sodium Zincate)
Here, zinc oxide reacts with the strong alkali to form a complex salt.
Water itself is the most common amphoteric substance, as it can donate a proton (act as an acid) or accept a proton (act as a base).
Solved Examples:
- Define an amphoteric substance.
Solution: A substance that can act as both an acid and a base. - Aluminium hydroxide, $Al(OH)_3$, is amphoteric. Write an equation for its reaction with hydrochloric acid ($HCl$).
Solution: $Al(OH)_3(s) + 3HCl(aq) \rightarrow AlCl_3(aq) + 3H_2O(l)$. (Acting as a base). - Write an equation for the reaction of aluminium hydroxide, $Al(OH)_3$, with sodium hydroxide ($NaOH$).
Solution: $Al(OH)_3(s) + NaOH(aq) \rightarrow Na[Al(OH)_4](aq)$. (Acting as an acid). - Is calcium oxide ($CaO$) amphoteric?
Solution: No, it is a basic oxide. It reacts with acids but not with bases. - Name an amphoteric metal oxide.
Solution: Zinc oxide ($ZnO$) or Aluminium oxide ($Al_2O_3$). - Show how the hydrogencarbonate ion ($HCO_3^-$) can act as an acid.
Solution: It can donate a proton to a base: $HCO_3^-(aq) + OH^-(aq) \rightarrow CO_3^{2-}(aq) + H_2O(l)$. - Show how the hydrogencarbonate ion ($HCO_3^-$) can act as a base.
Solution: It can accept a proton from an acid: $HCO_3^-(aq) + H^+(aq) \rightarrow H_2CO_3(aq)$. - Lead(II) oxide ($PbO$) is amphoteric. What would it react with?
Solution: It would react with both strong acids (like $HNO_3$) and strong bases (like $KOH$). - What distinguishes an amphoteric oxide from a neutral oxide (like CO or $N_2O$)?
Solution: An amphoteric oxide reacts with both acids and bases, while a neutral oxide reacts with neither. - Why is water considered amphoteric?
Solution: It can donate a proton to become $OH^-$ (acting as an acid) or accept a proton to become $H_3O^+$ (acting as a base).
5.12 Acid-Salt Reactions
A more reactive or stronger acid can displace a less reactive or weaker acid from its salt. This is a type of displacement reaction where the anion of the salt acts as a base, accepting a proton from the stronger acid.
This principle is often used to prepare volatile acids (acids that easily turn into gas).
Example: Preparation of Hydrogen Chloride
Concentrated sulphuric acid (a strong, non-volatile acid) is reacted with a solid chloride salt, like sodium chloride. The stronger sulphuric acid forces a proton onto the chloride ion, displacing the weaker (and more volatile) hydrochloric acid, which is released as a gas.
$$H_2SO_4(l) + NaCl(s) \rightarrow NaHSO_4(s) + HCl(g)$$
Similarly, an acid can react with the salt of a weaker acid if a gas is produced, driving the reaction forward according to Le Chatelier's principle. For example, any acid will react with a carbonate salt to displace the very weak carbonic acid, which immediately decomposes into water and carbon dioxide gas. $$2HCl(aq) + CaCO_3(s) \rightarrow CaCl_2(aq) + H_2O(l) + CO_2(g)$$
Solved Examples:
- Why can sulphuric acid displace hydrochloric acid from sodium chloride?
Solution: Sulphuric acid is a stronger and less volatile acid than hydrochloric acid. - Complete the reaction: $H_2SO_4(conc) + KNO_3(s) \rightarrow$
Solution: $H_2SO_4(conc) + KNO_3(s) \rightarrow KHSO_4(s) + HNO_3(g)$. Nitric acid is displaced. - What gas is produced when nitric acid is added to sodium sulphite ($Na_2SO_3$)?
Solution: Sulphur dioxide ($SO_2$). Nitric acid displaces the weak sulphurous acid ($H_2SO_3$), which decomposes into $SO_2$ and $H_2O$. - Can hydrochloric acid displace sulphuric acid from sodium sulphate?
Solution: No, because hydrochloric acid is a weaker acid than sulphuric acid. - In the reaction $H_2SO_4 + 2KCl \rightarrow K_2SO_4 + 2HCl$, what role does the chloride ion ($Cl^-$) play?
Solution: It acts as a Brønsted-Lowry base by accepting a proton from $H_2SO_4$. - What observation would be made if you add any acid to marble chips ($CaCO_3$)?
Solution: Effervescence (fizzing) would be observed as carbon dioxide gas is produced. - Write an equation for the reaction of concentrated phosphoric acid ($H_3PO_4$) with potassium bromide ($KBr$).
Solution: $H_3PO_4(l) + KBr(s) \rightarrow KH_2PO_4(s) + HBr(g)$. - Why must the sulphuric acid be concentrated in the preparation of $HCl$ gas?
Solution: The presence of water would cause the gaseous $HCl$ to dissolve, preventing it from being collected as a gas. - Which acid is being displaced in the reaction: $2H^+(aq) + CO_3^{2-}(aq) \rightarrow H_2O(l) + CO_2(g)$?
Solution: Carbonic acid ($H_2CO_3$). - Can ethanoic acid ($CH_3COOH$) react with sodium chloride ($NaCl$)?
Solution: No. Ethanoic acid is a much weaker acid than hydrochloric acid and cannot displace it.
5.13 Salt Hydrolysis (Acidic, Basic, Neutral Salts)
While we think of salt solutions as being neutral, this is not always the case. Salt hydrolysis is a reaction in which an ion from a salt reacts with water to produce either an acidic or a basic solution.
The pH of a salt solution depends on the strength of the acid and base from which it was formed.
- Salt of Strong Acid + Strong Base (e.g., $NaCl$, $KNO_3$): Neither the cation (e.g., $Na^+$) nor the anion (e.g., $Cl^-$) reacts with water. The solution is neutral (pH = 7).
- Salt of Weak Acid + Strong Base (e.g., $CH_3COONa$, $KCN$): The anion is the conjugate base of a weak acid and will react with water (hydrolyse) to produce $OH^-$ ions. The solution is basic (pH > 7).
Example: $CH_3COO^-(aq) + H_2O(l) \rightleftharpoons CH_3COOH(aq) + OH^-(aq)$ - Salt of Strong Acid + Weak Base (e.g., $NH_4Cl$, $AlCl_3$): The cation is the conjugate acid of a weak base and will react with water to produce $H^+$ ions. The solution is acidic (pH < 7).
Example: $NH_4^+(aq) + H_2O(l) \rightleftharpoons NH_3(aq) + H_3O^+(aq)$ (or simply $NH_4^+(aq) \rightleftharpoons NH_3(aq) + H^+(aq)$) - Salt of Weak Acid + Weak Base (e.g., $NH_4CH_3COO$): Both ions hydrolyse. The pH depends on the relative strengths of the weak acid and weak base. It can be acidic, basic, or neutral.
Solved Examples:
- Will a solution of potassium nitrate ($KNO_3$) be acidic, basic, or neutral? Explain.
Solution: Neutral. It is formed from a strong base ($KOH$) and a strong acid ($HNO_3$). Neither ion hydrolyses. - Predict the pH of a sodium ethanoate ($CH_3COONa$) solution.
Solution: Basic (pH > 7). The ethanoate ion ($CH_3COO^-$) is the conjugate base of a weak acid and will hydrolyse water to produce $OH^-$ ions. - Write the hydrolysis equation for the ammonium ion ($NH_4^+$).
Solution: $NH_4^+(aq) + H_2O(l) \rightleftharpoons NH_3(aq) + H_3O^+(aq)$. - Is a solution of ammonium chloride ($NH_4Cl$) acidic, basic, or neutral?
Solution: Acidic (pH < 7). The $NH_4^+$ ion is the conjugate acid of a weak base ($NH_3$) and hydrolyses water to produce $H^+$ ions. - What is salt hydrolysis?
Solution: A reaction in which an ion from a salt reacts with water, causing the solution to become acidic or basic. - A solution of sodium carbonate ($Na_2CO_3$) turns red litmus paper blue. Explain this observation.
Solution: $Na_2CO_3$ is the salt of a weak acid ($H_2CO_3$) and a strong base ($NaOH$). The carbonate ion hydrolyses water ($CO_3^{2-} + H_2O \rightleftharpoons HCO_3^- + OH^-$), producing $OH^-$ ions and making the solution basic. - From which acid and base is the salt lithium chloride ($LiCl$) derived? Will its solution be acidic, basic, or neutral?
Solution: Derived from strong base $LiOH$ and strong acid $HCl$. The solution will be neutral. - Aluminium chloride ($AlCl_3$) forms an acidic solution. Write an equation to show why.
Solution: The highly charged $Al^{3+}$ ion polarises water molecules, causing hydrolysis: $[Al(H_2O)_6]^{3+}(aq) \rightleftharpoons [Al(H_2O)_5(OH)]^{2+}(aq) + H^+(aq)$. - Predict the nature of a solution of potassium cyanide ($KCN$). (HCN is a weak acid).
Solution: Basic. $KCN$ is the salt of a strong base ($KOH$) and a weak acid ($HCN$). The cyanide ion ($CN^-$) will hydrolyse water. - Why is a solution of $NaCl$ neutral?
Solution: Because $Na^+$ is the cation of a strong base and $Cl^-$ is the anion of a strong acid, neither ion reacts with water.