Chemistry • Year 12 • Module 6 • Lesson 2
Nomenclature, Indicators & Predicting Acid Reactions
Build HSC Band 5–6 extended-response technique: evaluate real industrial data, explain indicator equilibria with Le Chatelier, and synthesise naming and reaction-pattern knowledge into a multi-criteria argument.
1. Data-based extended response — Indicator selection in Australian lead-acid battery manufacture (Band 5–6)
8 marks Band 5–6
Stimulus. Lead-acid batteries used in Australian mining vehicles (such as those operating in the Pilbara and Hunter Valley regions) use sulfuric acid (H&sub2;SO&sub4;) as the electrolyte at concentrations near 4.5 mol/L when fully charged. During a recharge-quality check, a technician dilutes a 0.1 mL sample 500-fold and titrates the resulting H&sub2;SO&sub4; solution against standardised NaOH. Two student technicians debate which indicator to use for the titration endpoint:
- Student A says phenolphthalein (PP; pH 8.3–10.0; colourless → pink) should be used because “PP always shows neutralisation.”
- Student B says methyl orange (MO; pH 3.1–4.4; red → yellow) should be used because “the acid is strong so the endpoint must be in the acidic range.”
The table below shows calculated equivalence point pH values for three titrations involving H&sub2;SO&sub4;.
| Titration | Acid | Base | Equivalence point pH |
|---|---|---|---|
| T1 | H&sub2;SO&sub4; (strong) | NaOH (strong) | 7.0 |
| T2 | H&sub2;SO&sub4; (strong) | Na&sub2;CO&sub3; (weak base) | ~8.5 |
| T3 | Acetic acid (weak) | NaOH (strong) | ~9.1 |
Data adapted from Harris, D.C. (2016) Quantitative Chemical Analysis, 9th ed. Table values for 0.1 mol/L solutions at 25 °C.
Q1. Evaluate both students’ claims about indicator selection, using the data in the table and your knowledge of indicator equilibria, acid nomenclature, and strong/weak acid-base classification. In your response you must:
- Name and classify H&sub2;SO&sub4; and NaOH using IUPAC conventions, and state their strong/weak classification.
- Use the indicator equilibrium HIn ⇌ H¹+ + Inˉ and Le Chatelier’s Principle to explain how an indicator detects an equivalence point.
- Evaluate Student A’s claim using data from T1 specifically, with reference to the phenolphthalein transition range.
- Evaluate Student B’s claim using data from T1, with reference to the methyl orange transition range and the equivalence point pH.
- Identify which indicator is most appropriate for titration T1, and use the data to justify why.
- Extend your analysis: if the titration were T3 (acetic acid vs NaOH) instead of T1, would your indicator choice change? Justify using the equivalence point data.
2. Source critique — errors in a published student summary (Band 5–6)
7 marks Band 5–6
Source. The following is an extract from a Year 12 student’s Module 6 study notes, shared on an online revision platform:
“Acids can be named using two simple rules. Rule 1: if the acid contains hydrogen, add ‘hydro’ to the start of the element name and ‘ic’ to the end, e.g. H&sub2;SO&sub4; = hydrosulfuric acid and HNO&sub3; = hydronitric acid. Rule 2: indicators change colour at exactly pH 7 to show when a solution is neutral. Phenolphthalein turns pink at pH 7, which is the sign of complete neutralisation. Rule 3: when an acid reacts with any metal you always get a salt and hydrogen gas, e.g. Cu + H&sub2;SO&sub4; → CuSO&sub4; + H&sub2;. The gas produced in acid–carbonate reactions is CO (carbon monoxide), which causes the solution to bubble.”
Q2. This source contains four significant scientific errors. For each error: (i) identify what is wrong, (ii) explain the correct chemistry using lesson content, and (iii) for at least two of the four errors, explain how the error could be detected experimentally or by reference to data.
Error 1:
Error 2:
Error 3:
Error 4:
Experimental detection (for two errors):
Q1 — Indicator selection evaluation (8 marks)
Naming and classification [1 mark]. H&sub2;SO&sub4; is an oxoacid (contains H, S and O; derived from the sulfate ion SO&sub4;²−, ending –ate → –ic acid = sulfuric acid). It is a strong acid (first ionisation complete: H&sub2;SO&sub4; → H¹+ + HSO&sub4;ˉ). NaOH is sodium hydroxide, an ionic compound and Arrhenius strong base (fully dissociates: NaOH → Na¹+ + OHˉ).
Indicator mechanism [1 mark]. An indicator is a weak acid (HIn) existing in equilibrium HIn ⇌ H¹+ + Inˉ where the two forms are different colours. At the equivalence point the [H¹+] changes sharply; by Le Chatelier’s Principle, the equilibrium responds to this shift, changing the dominant form and hence the colour. A suitable indicator has its transition range (pKa ± 1) centred on the equivalence point pH.
Evaluation of Student A [1 mark]. Student A’s claim is partially valid but inaccurate for T1. Phenolphthalein’s transition range is pH 8.3–10.0. The T1 equivalence point is pH 7.0 (strong acid + strong base). pH 7.0 falls below the PP transition range, so the colour change does NOT occur at the equivalence point — PP is still colourless at pH 7.0. PP would only start to change well past the equivalence point, introducing a significant titration error. Student A’s claim is incorrect for T1.
Evaluation of Student B [1 mark]. Student B’s reasoning is also flawed. The endpoint pH is not determined by the strength of the acid alone — it is determined by the nature of both acid and base and is the pH at the equivalence point. For T1 (strong acid + strong base) the equivalence point is pH 7.0, which lies above the methyl orange transition range (pH 3.1–4.4). MO would already be fully yellow before the equivalence point is reached, making it equally unsuitable. Student B’s claim is incorrect.
Correct indicator for T1 [1 mark]. The most appropriate indicator for T1 (equivalence point pH 7.0) is bromothymol blue (BTB; transition range pH 6.0–7.6), because its range spans the equivalence point and will give a sharp yellow-to-blue colour change at the correct pH.
Extension to T3 [1 mark]. For T3 (weak acid + strong base), the equivalence point is pH 9.1 because the acetate salt produced is basic (conjugate base of a weak acid undergoes hydrolysis). Bromothymol blue’s range (6.0–7.6) does not extend to pH 9.1, so it would be unsuitable. Phenolphthalein (pH 8.3–10.0) spans the T3 equivalence point and would be the appropriate choice. This confirms Student A’s claim is only valid for certain titration types, not universally.
Mark allocation: naming 1 + mechanism 1 + Student A evaluation 2 + Student B evaluation 1 + correct T1 indicator with justification 2 + T3 extension 1 = 8 marks.
Q2 — Source critique (7 marks)
Error 1 — Naming rule applied to oxoacids [2 marks]. The source applies the binary acid naming rule (“hydro-/ic”) to H&sub2;SO&sub4; and HNO&sub3;. Both compounds contain oxygen and are oxoacids. “Hydro-” is never used for oxoacids. The correct names are: H&sub2;SO&sub4; = sulfuric acid (from sulfate SO&sub4;²−, –ate → –ic); HNO&sub3; = nitric acid (from nitrate NO&sub3;ˉ, –ate → –ic). “Hydrosulfuric acid” is actually the name of H&sub2;S. Detectable experimentally: adding a test for SO&sub4;²− to “hydrosulfuric acid” solution would show no precipitate with BaCl&sub2; (H&sub2;S contains no sulfate), while H&sub2;SO&sub4; would give a white BaSO&sub4; precipitate.
Error 2 — Indicators and pH 7 [2 marks]. Indicators do not universally change colour at pH 7. The source claims phenolphthalein turns pink at pH 7; in fact phenolphthalein’s transition range is pH 8.3–10.0 — it is completely colourless at pH 7. Methyl orange completes its transition around pH 4.4, also not at pH 7. Only bromothymol blue spans pH 7. The correct statement is that an indicator changes colour within its specific transition range (pKa ± 1 of the indicator itself). Detectable: add phenolphthalein to a buffer at pH 7.0 — it remains colourless, disproving the source’s claim.
Error 3 — Any metal reacts with any acid [1 mark]. Not all metals react with dilute acids. The source claims Cu + H&sub2;SO&sub4; → CuSO&sub4; + H&sub2;, but copper is below hydrogen in the electrochemical activity series and does not react with dilute H&sub2;SO&sub4; by the standard acid–metal mechanism. Only metals above hydrogen (Mg, Al, Zn, Fe, etc.) displace H&sub2; from dilute acids.
Error 4 — Gas product in acid–carbonate reactions [1 mark]. The source states the gas produced is CO (carbon monoxide). This is incorrect. The gas produced is CO&sub2; (carbon dioxide), formed when the intermediate carbonic acid (H&sub2;CO&sub3;) immediately decomposes: H&sub2;CO&sub3; → H&sub2;O + CO&sub2;. CO is a toxic combustion product and has no role in acid–carbonate chemistry.
Mark allocation: Error 1 = 2 marks (identification + correct name); Error 2 = 2 marks (identification + correct mechanism); Error 3 = 1 mark; Error 4 = 1 mark; experimental detection for any two errors = 1 shared mark. Total = 7 marks.