Chemistry • Year 12 • Module 6 • Lesson 2
Nomenclature, Indicators & Predicting Acid Reactions
Apply acid-naming rules, indicator equilibrium reasoning and reaction-pattern logic to real data, authentic scenarios and writing balanced equations.
1. Interpret real indicator data — pH vs indicator colour in citrus processing
The graph below shows pH readings taken at six points in a Mildura citrus-juice processing line (from fresh-squeezed juice through to final pasteurised product). At each point, a technician also added three drops of methyl orange (transition range pH 3.1–4.4) and three drops of phenolphthalein (transition range pH 8.3–10.0) to separate aliquots of the sample. The technician recorded the indicator colours. 9 marks
MO = methyl orange (pH 3.1–4.4; red in acid, yellow above range). PP = phenolphthalein (pH 8.3–10.0; colourless below range, pink above). Hypothetical processing data based on citric acid content of Australian citrus fruit (Mildura, NSW). Adapted from Food Standards Australia New Zealand (FSANZ) pH benchmarks.
1.1 Describe the trend in pH across the six processing stages. 2 marks
1.2 Complete the table below by predicting the colour of methyl orange and phenolphthalein at each stage. Justify your prediction for Stage 3 for methyl orange only. 4 marks (1 for correct column + 1 for Stage 3 MO justification)
| Stage | pH | Methyl orange colour | Phenolphthalein colour |
|---|---|---|---|
| 1 | 2.4 | ||
| 2 | 3.1 | ||
| 3 | 3.8 | ||
| 4 | 4.2 | ||
| 5 | 5.5 | ||
| 6 | 6.8 |
Justification for Stage 3 methyl orange colour:
1.3 Explain why neither methyl orange nor phenolphthalein is a useful indicator for precisely monitoring the neutralisation of citric acid in citrus processing. Recommend a more appropriate indicator. 3 marks
2. Sequence the steps — writing and balancing an acid reaction
The seven steps below for writing a balanced equation for “nitric acid reacting with magnesium hydroxide” are shuffled. Write the correct order (1–7) in the “Order” column. 6 marks (1 per correctly placed step; 6 steps, ignore the final total)
| Order | Step (shuffled) |
|---|---|
| Identify the salt formula: Mg²+ from Mg(OH)&sub2;, NO&sub3;¹− from HNO&sub3; → Mg(NO&sub3;)&sub2; (need 2 NO&sub3;¹− for one Mg²+). | |
| Check: Mg = 1 each side; N = 2 each side; H = 4 each side; O = 2+6 = 8 left (Mg(OH)&sub2; + 2HNO&sub3;); O = 6+2 = 8 right (Mg(NO&sub3;)&sub2; + 2H&sub2;O). Balanced. | |
| Identify the reaction type: HNO&sub3; is an acid; Mg(OH)&sub2; is a hydroxide base. This is Pattern 1: acid + base → salt + water. | |
| Write the full balanced equation: 2HNO&sub3; + Mg(OH)&sub2; → Mg(NO&sub3;)&sub2; + 2H&sub2;O. | |
| Write the unbalanced equation with correct products: HNO&sub3; + Mg(OH)&sub2; → Mg(NO&sub3;)&sub2; + H&sub2;O. | |
| Balance: left has 2H (from Mg(OH)&sub2;) + 1H (from HNO&sub3;) but right needs 2H per H&sub2;O; need 2HNO&sub3; to supply enough H for 2H&sub2;O. Re-check N: 2 on each side. | |
| Identify the products: acid + base (hydroxide) → salt + water. Products are Mg(NO&sub3;)&sub2; and H&sub2;O. |
3. Cause-and-effect chain — indicator colour change
Complete the cause-and-effect chain below to explain what happens when excess NaOH is added to a methyl orange solution that is currently red (pH 2.0). Each arrow step is one causal link. 5 marks
| Cause (given) | → | Effect (student fills in) |
|---|---|---|
| Excess NaOH is added to the acidic methyl orange solution. | → | |
| OHˉ ions react with H¹+ ions in solution. | → | |
| [H¹+] in the solution falls significantly. | → | |
| The indicator equilibrium HIn ⇌ H¹+ + Inˉ shifts right. | → | |
| Inˉ (the yellow base form) dominates over HIn (the red acid form). | → |
Overall outcome: Starting from red at pH 2.0, the solution shifts to _______________________ colour as the pH rises above the methyl orange transition range.
4. Australian case study — Orica fertiliser production
Read the stimulus below, then answer the questions. 7 marks
Stimulus. Orica Limited operates one of Australia’s largest ammonium nitrate (NH&sub4;NO&sub3;) production facilities at Kooragang Island, near Newcastle, NSW. Ammonium nitrate is used as an explosive precursor and as a nitrogen-release fertiliser for Australian grain farms. It is manufactured by the neutralisation reaction: HNO&sub3;(aq) + NH&sub3;(g) → NH&sub4;NO&sub3;(aq). The reaction is strongly exothermic. During production, pH monitoring using bromothymol blue (BTB; transition range pH 6.0–7.6) ensures the neutralisation is complete before the solution is concentrated and prilled into pellets. The facility produces approximately 750,000 tonnes of ammonium nitrate annually.
4.1 Classify HNO&sub3; and NH&sub3; according to IUPAC nomenclature. For HNO&sub3;, state whether it is a binary or oxoacid, name the polyatomic ion it contains, and explain how this determines its name. 3 marks
4.2 Identify the reaction type (Pattern 1, 2 or 3) and explain, using the indicator equilibrium HIn ⇌ H¹+ + Inˉ, what colour change in bromothymol blue would indicate the reaction is approaching and then passing the equivalence point. 4 marks
Q1.1 — pH trend
pH increases progressively from Stage 1 (pH 2.4, very acidic fresh juice) to Stage 6 (pH 6.8, slightly acidic pasteurised product). Each stage from 1 to 6 shows a higher pH than the previous stage, indicating the juice becomes less acidic (more neutral) as processing proceeds. 1 mark for “increases”; 1 mark for identifying the numerical range or stating the direction of change across all stages.
Q1.2 — Indicator colour table
Methyl orange (range 3.1–4.4; red below, yellow above):
Stage 1 (2.4): red. Stage 2 (3.1): red/orange (at the lower edge of the transition range — just entering). Stage 3 (3.8): orange (within transition range — mixed forms). Stage 4 (4.2): orange/yellow (within transition range, approaching upper edge). Stage 5 (5.5): yellow. Stage 6 (6.8): yellow.
Phenolphthalein (range 8.3–10.0; colourless below): All six stages are below pH 8.3 → colourless at every stage.
Stage 3 MO justification: At pH 3.8 the methyl orange equilibrium HIn ⇌ H¹+ + Inˉ is within its transition range (3.1–4.4). Both the red HIn form and the yellow Inˉ form are present in similar concentrations, producing an intermediate orange colour. pH 3.8 > pKa (3.5), so Inˉ (yellow) is slightly dominant, giving orange-yellow. [1 mark for orange/transition-range colour; 1 mark for equilibrium/Le Chatelier explanation.]
Q1.3 — Indicator selection
Citric acid is a weak organic acid. Its neutralisation equivalence point would be above pH 7 (because the resulting citrate salt is basic by hydrolysis), roughly pH 8–9. Methyl orange’s transition range (3.1–4.4) does not include this equivalence point — it would be fully yellow before the endpoint is reached [1]. Phenolphthalein’s lower boundary (pH 8.3) may or may not align with the exact equivalence point depending on concentration [1]. A more appropriate indicator would be phenolphthalein (acceptable) or, ideally, a pH meter for precise monitoring, as neither common indicator is ideal for this particular acid–base pair [1].
Q2 — Sequence answers (correct order)
1. Identify reaction type: acid + base (hydroxide). 2. Identify the products: salt + water. 3. Identify the salt formula: Mg(NO&sub3;)&sub2;. 4. Write unbalanced equation. 5. Balance: 2HNO&sub3; and 2H&sub2;O. 6. Write full balanced equation: 2HNO&sub3; + Mg(OH)&sub2; → Mg(NO&sub3;)&sub2; + 2H&sub2;O. 7. Check atoms both sides. Award 1 mark per step in correct relative position.
Q3 — Cause-and-effect chain
Row 1: The solution pH rises (OHˉ neutralises H¹+, making the solution basic). Row 2: H&sub2;O forms; [H¹+] decreases significantly. Row 3: The indicator equilibrium HIn ⇌ H¹+ + Inˉ is disturbed by the decrease in [H¹+]. Row 4: By Le Chatelier’s Principle, the equilibrium shifts right to produce more H¹+ and more Inˉ. Row 5: The solution colour changes from red to yellow. Overall outcome: yellow.
Q4.1 — Nomenclature of HNO&sub3; and NH&sub3;
HNO&sub3;: oxoacid (contains H, N and O). The polyatomic ion it contains is NO&sub3;ˉ (nitrate), which ends in –ate → acid name ends in –ic → nitric acid. Strong acid (complete ionisation). [1 mark oxoacid identification; 1 mark naming logic.] NH&sub3;: ammonia; a Brønsted-Lowry base that accepts protons from water (NH&sub3; + H&sub2;O ⇌ NH&sub4;¹+ + OHˉ); weak base. [1 mark.]
Q4.2 — Reaction type and BTB colour change
Reaction type: Pattern 1 (acid + base → salt + water). HNO&sub3; is the acid, NH&sub3; is the Brønsted-Lowry base. [1 mark.] Before neutralisation, excess HNO&sub3; keeps [H¹+] high → HIn dominates → BTB is yellow [1 mark]. As HNO&sub3; is consumed, [H¹+] decreases → equilibrium shifts right toward Inˉ → solution turns green (within transition range 6.0–7.6, both forms present) [1 mark]. Past the equivalence point, excess NH&sub3; raises pH above 7.6 → Inˉ (blue) dominates → solution turns blue [1 mark]. The endpoint (green → blue) signals the approach and passage of the equivalence point.