Question [(a) (i) ]2023-May-Chemistry_paper_2__TZ1_HL
Topic: Acid–base properties.
Discuss: the ionic equation, indicating the states of matter, for the interaction between hydrogen chloride gas and water.
▶️Answer/Explanation
Ans:
The reaction of hydrogen chloride gas (HCl) with water (H₂O) involves the HCl molecule donating a proton (H⁺) to the water molecule, resulting in the formation of hydronium ion (H₃O⁺) and chloride ion (Cl⁻). The ionic equation for this reaction, including state symbols, is as follows:
\[ \text{HCl(g)} + \text{H₂O(l)} \rightarrow \text{H₃O⁺(aq)} + \text{Cl⁻(aq)} \]
This equation represents the dissociation of hydrogen chloride gas in water to produce hydronium ions and chloride ions in aqueous solution.
Question [(a) (ii) ]2023-May-Chemistry_paper_2__TZ1_HL
Topic:pH Calculation
Calculate: the pH of 0.50mol \(dm^{-3}\) hydrochloric acid.
▶️Answer/Explanation
Ans:
Hydrochloric acid (\(HCl\)) is a strong acid that dissociates completely in water to produce \(H^+\) ions and \(Cl^-\) ions. Since \(HCl\) is a strong acid, the concentration of \(H^+\) ions in the resulting solution will be equal to the initial concentration of hydrochloric acid (\(0.50 \, \text{mol} \, dm^{-3}\)).
The pH of a solution is defined as the negative logarithm of the hydrogen ion concentration (\(H^+\)) in moles per liter (\(mol \, dm^{-3}\)). Mathematically, it is expressed as:
\[ \text{pH} = -\log[H^+] \]
Given that the concentration of \(H^+\) ions in the hydrochloric acid solution is \(0.50 \, mol \, dm^{-3}\), we can calculate the pH as follows:
\[ \text{pH} = -\log(0.50) \]
Using a calculator:
\[ \text{pH} = -\log(0.50) \]
\[ \text{pH} \approx -(-0.301) \]
\[ \text{pH} \approx 0.301 \]
Therefore, the pH of a \(0.50 \, mol \, dm^{-3}\) hydrochloric acid solution is approximately \(0.301\).
Question [(a) (iii) ]2023-May-Chemistry_paper_2__TZ1_HL
Topic:pH Properties
Discuss:Why does a solution containing ethanoic acid exhibit a higher pH compared to a solution of hydrochloric acid with the same concentration?
▶️Answer/Explanation
Ans:
Ethanoic acid (also known as acetic acid) is a weak acid, while hydrochloric acid is a strong acid. The difference in pH between solutions of ethanoic acid and hydrochloric acid of the same concentration arises from their different dissociation behaviors.
1. Weak Acid Dissociation:
- Ethanoic acid (\(CH_3COOH\)) is a weak acid, meaning it only partially dissociates in water to produce \(H^+\) ions and \(CH_3COO^-\) ions. The dissociation equilibrium is represented by the equation:
\[ CH_3COOH(aq) + H_2O(l) \rightleftharpoons CH_3COO^-(aq) + H_3O^+(aq) \] - Because ethanoic acid only partially dissociates, the concentration of \(H^+\) ions in the resulting solution is lower compared to a solution of the same concentration of a strong acid like hydrochloric acid.
2. Strong Acid Dissociation:
- Hydrochloric acid (\(HCl\)) is a strong acid, which means it completely dissociates in water to produce \(H^+\) ions and \(Cl^-\) ions. The dissociation of hydrochloric acid is complete, as represented by the equation:
\[ HCl(aq) \rightarrow H^+(aq) + Cl^-(aq) \] - Because hydrochloric acid completely dissociates, the concentration of \(H^+\) ions in the resulting solution is equal to the concentration of the acid, which in this case is \(0.50 \, mol \, dm^{-3}\).
3. pH Calculation:
- The pH of a solution is determined by the concentration of \(H^+\) ions. Since ethanoic acid only partially dissociates, the concentration of \(H^+\) ions in the resulting solution is lower compared to hydrochloric acid of the same concentration. As a result, the pH of a solution of ethanoic acid is higher than the pH of hydrochloric acid of the same concentration.
In summary, a solution of ethanoic acid has a higher pH than hydrochloric acid of the same concentration because ethanoic acid is a weak acid and only partially dissociates, leading to a lower concentration of \(H^+\) ions in the solution.
Question [(a) (iv) ]2023-May-Chemistry_paper_2__TZ1_HL
Topic:pH Properties
Discuss: A pH probe is capable of distinguishing between the acids discussed in part (a)(iii). Propose another straightforward instrumental technique that could be employed in a school laboratory to differentiate between the two acids.
▶️Answer/Explanation
Ans:
Another simple instrumental method that could be used in a school laboratory to distinguish between the two acids is conductivity measurement.
Principle:
- Conductivity is a measure of a solution’s ability to conduct an electric current. Strong acids, which dissociate completely into ions in solution, result in a higher conductivity compared to weak acids, which only partially dissociate.
- Hydrochloric acid, being a strong acid, will produce a higher conductivity compared to ethanoic acid, which is a weak acid.
Experimental Setup:
- Two conductivity meters or conductivity probes can be used to measure the conductivity of solutions of hydrochloric acid and ethanoic acid.
- The solutions are prepared with the same concentration of each acid.
- The conductivity of each solution is measured using the conductivity meter or probe.
Observation:
- Hydrochloric acid, being a strong acid, will show a higher conductivity due to the higher concentration of ions in solution resulting from complete dissociation.
- Ethanoic acid, being a weak acid, will show a lower conductivity due to the lower concentration of ions in solution resulting from partial dissociation.
Interpretation:
- Based on the conductivity measurements, the solution with higher conductivity is identified as hydrochloric acid, while the solution with lower conductivity is identified as ethanoic acid.
Using conductivity measurement in this manner provides a simple and straightforward way to distinguish between strong and weak acids in a school laboratory setting.