Unit 5: Strong and Weak Acids & Bases
Distinguishing between strong and weak acids and bases based on their degree of dissociation.
5.8 Strong and Weak Acids
The strength of an acid is determined by how completely it dissociates (or ionises) in water to produce hydrogen ions ($H^+$).
A strong acid is an acid that dissociates completely in aqueous solution. This means that nearly all the acid molecules react with water to form ions. The reaction is shown with a forward arrow ($\rightarrow$).
Examples:
- Hydrochloric Acid: $HCl(aq) \rightarrow H^+(aq) + Cl^-(aq)$
- Sulphuric Acid: $H_2SO_4(aq) \rightarrow 2H^+(aq) + SO_4^{2-}(aq)$
- Nitric Acid: $HNO_3(aq) \rightarrow H^+(aq) + NO_3^-(aq)$
A weak acid is an acid that only partially dissociates in aqueous solution. Only a small fraction of the acid molecules donate their protons to water. This creates a dynamic equilibrium, which is represented by a reversible arrow ($\rightleftharpoons$).
Examples:
- Ethanoic Acid: $CH_3COOH(aq) \rightleftharpoons H^+(aq) + CH_3COO^-(aq)$
- Carbonic Acid: $H_2CO_3(aq) \rightleftharpoons H^+(aq) + HCO_3^-(aq)$
Solved Examples:
- Classify hydrochloric acid ($HCl$) as a strong or weak acid. Explain why.
Solution: Strong acid. It completely dissociates into $H^+$ and $Cl^-$ ions in water. - Vinegar contains ethanoic acid ($CH_3COOH$). Is it a strong or weak acid?
Solution: Weak acid. It only partially ionises in water. - Write the dissociation equation for the strong acid, nitric acid ($HNO_3$).
Solution: $HNO_3(aq) \rightarrow H^+(aq) + NO_3^-(aq)$. - Why is a reversible arrow used for the dissociation of a weak acid?
Solution: It indicates that the reaction is an equilibrium; both the forward reaction (dissociation) and the reverse reaction (association) are occurring simultaneously. - Which would have a lower pH, $0.1 M$ $HCl$ or $0.1 M$ $CH_3COOH$?
Solution: $0.1 M$ $HCl$. As a strong acid, it produces a higher concentration of $H^+$ ions, resulting in a lower pH. - Name two common strong acids.
Solution: Sulphuric acid ($H_2SO_4$) and Nitric acid ($HNO_3$). - What does "degree of dissociation" mean for an acid?
Solution: It refers to the fraction or percentage of acid molecules that have separated into ions in solution. - Is phosphoric acid ($H_3PO_4$) generally considered strong or weak?
Solution: Weak. Although it can donate three protons, each dissociation step is partial and establishes an equilibrium. - What are the main species present in a solution of hydrofluoric acid ($HF$), a weak acid?
Solution: Mainly undissociated $HF$ molecules, with small amounts of $H^+$ and $F^-$ ions. - A student needs a solution with a high concentration of hydrogen ions. Should they choose a strong or a weak acid?
Solution: A strong acid, as it dissociates completely.
5.9 Strong and Weak Bases
Similar to acids, the strength of a base depends on its ability to produce hydroxide ions ($OH^-$) in solution.
A strong base is a base that dissociates completely in aqueous solution to yield hydroxide ions. The metal hydroxides of Group 1 and Group 2 (lower down) are typically strong bases.
Examples:
- Sodium Hydroxide: $NaOH(aq) \rightarrow Na^+(aq) + OH^-(aq)$
- Barium Hydroxide: $Ba(OH)_2(aq) \rightarrow Ba^{2+}(aq) + 2OH^-(aq)$
A weak base is a base that only partially ionises in aqueous solution. It establishes an equilibrium where most of the base remains as undissociated molecules.
Example:
- Ammonia: $NH_3(aq) + H_2O(l) \rightleftharpoons NH_4^+(aq) + OH^-(aq)$
Solved Examples:
- Is potassium hydroxide ($KOH$) a strong or weak base?
Solution: Strong base. It is a Group 1 hydroxide and dissociates completely. - Ammonia ($NH_3$) is a common cleaning agent. Is it a strong or weak base?
Solution: Weak base. It reacts with water in an equilibrium to produce a low concentration of $OH^-$ ions. - Write the dissociation equation for the strong base, calcium hydroxide ($Ca(OH)_2$).
Solution: $Ca(OH)_2(aq) \rightarrow Ca^{2+}(aq) + 2OH^-(aq)$. - Why is sodium carbonate ($Na_2CO_3$) considered a weak base?
Solution: The carbonate ion ($CO_3^{2-}$) reacts with water in an equilibrium ($CO_3^{2-} + H_2O \rightleftharpoons HCO_3^- + OH^-$) to produce a low concentration of $OH^-$ ions. - Which solution would have a higher pH: $0.5 M$ $NaOH$ or $0.5 M$ $NH_3$?
Solution: $0.5 M$ $NaOH$. As a strong base, it produces a higher concentration of $OH^-$ ions, resulting in a higher pH. - Name a common weak base.
Solution: Ammonia ($NH_3$). - What are the main particles present in an aqueous solution of ammonia?
Solution: Mainly undissociated $NH_3$ molecules, water molecules, and small amounts of $NH_4^+$ and $OH^-$ ions. - Is magnesium hydroxide ($Mg(OH)_2$) a strong or weak base?
Solution: It is considered a weak base primarily due to its very low solubility in water. - What makes a base "strong"?
Solution: Its ability to completely dissociate or ionise in water to produce a high concentration of hydroxide ions. - A solution has a high concentration of $OH^-$ ions. Is it a strong or weak alkali?
Solution: It is a strong alkali.
5.10 Comparing Properties of Strong & Weak (Conductivity, Enthalpy)
The difference in the degree of dissociation between strong and weak acids/bases leads to measurable differences in their properties.
Electrical Conductivity
Electrical conductivity in solutions depends on the concentration of mobile ions.
- Strong acids and bases are strong electrolytes. They dissociate completely, producing a high concentration of mobile ions. Therefore, their solutions conduct electricity well (a conductivity probe bulb would shine brightly).
- Weak acids and bases are weak electrolytes. They only dissociate partially, producing a low concentration of mobile ions. Their solutions conduct electricity poorly (a bulb would shine dimly or not at all).
Enthalpy of Neutralisation ($\Delta H_{neut}$)
The standard enthalpy of neutralisation is the enthalpy change when one mole of water is formed from the reaction of an acid and a base under standard conditions.
- For any strong acid and strong base, the net ionic reaction is always the same: $H^+(aq) + OH^-(aq) \rightarrow H_2O(l)$. The enthalpy change is therefore constant, approximately -57 to -58 kJ/mol. This is a highly exothermic reaction.
- When a weak acid or weak base is involved, the enthalpy of neutralisation is less exothermic (e.g., -50 to -56 kJ/mol). This is because some of the energy released from the formation of water is used to dissociate the weak acid or weak base molecules first (an endothermic process). For example, for $CH_3COOH + NaOH$:
- $CH_3COOH(aq) \rightleftharpoons H^+(aq) + CH_3COO^-(aq)$ (Energy absorbed)
- $H^+(aq) + OH^-(aq) \rightarrow H_2O(l)$ (Energy released)
Solved Examples:
- Which solution is a better electrical conductor: $1 M$ $HNO_3$ or $1 M$ $H_2CO_3$?
Solution: $1 M$ $HNO_3$. It is a strong acid and a strong electrolyte, while carbonic acid ($H_2CO_3$) is a weak acid and a weak electrolyte. - The enthalpy of neutralisation for $HCl$ and $NaOH$ is -57.3 kJ/mol. Would you expect the value for $HCN$ (a weak acid) and $NaOH$ to be more or less negative?
Solution: Less negative (less exothermic). Energy is required to dissociate the weak acid $HCN$ before it can be neutralised. - A student tests two acidic solutions of the same concentration. Solution A makes a bulb glow brightly, while Solution B makes it glow dimly. Classify the acids.
Solution: Solution A contains a strong acid. Solution B contains a weak acid. - Why is the enthalpy of neutralisation for all strong acid-strong base reactions nearly identical?
Solution: Because the actual chemical reaction is always the same: the formation of one mole of water from its ions ($H^+ + OH^- \rightarrow H_2O$). - What is a weak electrolyte? Give an example.
Solution: A substance that only partially ionises in solution, resulting in poor electrical conductivity. Example: Ammonia ($NH_3$). - The neutralisation of ammonia (weak base) with nitric acid (strong acid) has $\Delta H_{neut} = -52.2$ kJ/mol. Why is this value less exothermic than -57.3 kJ/mol?
Solution: Energy is absorbed from the surroundings to break the N-H bond in ammonia and form ions before the neutralisation reaction can occur. - Does a higher concentration of ions lead to higher or lower electrical conductivity?
Solution: Higher electrical conductivity. - If you neutralise sulphuric acid ($H_2SO_4$) with sodium hydroxide ($NaOH$), would the enthalpy change per mole of water be approximately -57 kJ/mol or a different value?
Solution: Approximately -57 kJ/mol, because both are strong (a strong acid and a strong base). - Explain in terms of ions why a solution of ethanoic acid is a weak electrolyte.
Solution: Ethanoic acid only dissociates partially, so the concentration of mobile ions ($H^+$ and $CH_3COO^-$) is very low compared to the concentration of undissociated $CH_3COOH$ molecules. - The reaction of a weak acid and a weak base has a very small enthalpy of neutralisation. Why?
Solution: Because energy is required to dissociate both the weak acid and the weak base, which offsets most of the energy released by the formation of water.
Knowledge Check (20 Questions)
Answer: The degree of dissociation in water (strong acids dissociate completely, weak acids only partially).
Answer: $CH_3COOH$ (ethanoic acid).
Answer: Hydroxide ions ($OH^-$).
Answer: Because they have a low concentration of mobile ions.
Answer: Weak base.
Answer: less.
Answer: The reversible arrow ($\rightleftharpoons$).
Answer: Barium hydroxide ($Ba(OH)_2$) or Calcium Hydroxide ($Ca(OH)_2$).
Answer: A solute that completely dissociates into ions in solution, leading to high electrical conductivity.
Answer: Energy is required to break the bonds in the undissociated weak acid molecules to release the $H^+$ ions.
Answer: $1 M$ $KOH$, because it is a strong base.
Answer: -57 to -58 kJ/mol (strong acid + strong base).
Answer: Low concentration.
Answer: Strong electrolyte.
Answer: Degree of dissociation/ionisation.
Answer: $H^+$ ions, $Br^-$ ions, and water molecules. (Essentially no undissociated $HBr$).
Answer: It is generally considered a weak base because its low solubility results in a low concentration of $OH^-$ ions in the overall solution.
Answer: $1 M$ $Ca(OH)_2$. It produces three moles of ions ($Ca^{2+}$ and $2OH^-$) per mole of solute, whereas $NaOH$ only produces two.
Answer: No. "Strong" refers to the degree of dissociation, while "concentrated" refers to the amount of solute per unit volume of solution.
Answer: $H^+(aq) + OH^-(aq) \rightarrow H_2O(l)$.