Lesson 5: Carboxylic Acids & Amines — Acid-Base Behaviour and Solubility
Use acid-base behaviour, solubility trends and diagnostic tests to distinguish carboxylic acids from amines and justify their reactions.
🎯 Ngā Whāinga Akoranga | Learning Intentions
Carboxylic acids (-COOH) as weak organic acids (turn blue litmus red, react with carbonates to produce CO2 gas) and amines (-NH2) as weak organic bases (turn red litmus blue, fishy odour, accept protons).
Write acid-base neutralisation equations, predict solubility trends (short-chain vs long-chain), and perform diagnostic tests with litmus paper and NaHCO3.
🎥 Media Anchor & Pedagogical Scaffold
Carboxylic Acids and Amines Chemistry
Video Clip: Alcohols, Carboxylic Acids and Esters — everything you need to know | GCSE & IGCSE Chemistry (Science with Hazel) (Runtime: 9m 52s).
🧠 1. Before Viewing (Activate & Predict)
Why do vinegar (ethanoic acid) and fish odour (methylamine) exhibit contrasting acidic and basic properties?
👁️ 2. During Viewing (Watch With a Job)
- Carboxylic acid structure: draw the -COOH functional group. Why does this group make carboxylic acids acidic? (What does it donate?)
- Solubility: the video compares short-chain and long-chain molecules in water. Which end of a carboxylic acid interacts with water, and which end does not? Predict what happens to solubility as the chain gets longer.
- Acid strength: are carboxylic acids strong or weak acids? How do you know? (What happens when you dissolve one in water?)
🗣️ 3. After Viewing & Kaiako Move (Process & Apply)
Kaiako Move: Connect short-chain carboxylic acids to food preservation (pickling and fermentation) and amines to natural breakdown products. Skip the ester section of the clip, or flag it as beyond the standard — AS91165 lists neither esters as a compound nor esterification as a reaction, so students who revise it are revising something that cannot be asked.
Immediate Task: Complete Section 5 of your Organic Portfolio: Acid-Base Diagnostic Flowchart & Litmus Test Record.
⚡ Whakaoho | Do Now: Organic Recall Challenge (10 mins)
Why lemon on fish works. The smell of old fish is amines evaporating off the surface. Squeeze lemon juice on it and the smell drops away.
Two minutes: amines are weak bases and lemon juice is an acid. Say what reaction happens, then explain the part that actually matters — why the product does not smell. (Clue: what makes a molecule able to reach your nose at all?)
⚠️ Hazard
Ethylamine is flammable and corrosive, and its vapour — the sharp ammonia-like smell you will notice straight away — is a severe irritant to eyes, skin and airways. Keep it in the fume hood as briefed, stoppered when you are not using it, and away from any flame or hotplate. Hydrochloric acid and sodium hydroxide are corrosive, and the carboxylic acid is an irritant; neutralising an acid with a base releases heat, so use only the small volumes your kaiako issues and do not mix larger quantities to see what happens. Wear safety glasses for the whole practical. Tell your kaiako immediately if anything contacts skin or eyes — rinse with water for at least 10 minutes. Wipe spills up as your kaiako directs rather than leaving them to evaporate, and dispose of all waste only as your kaiako directs, never down the sink.
📖 Activity 1: Core Reaction Mechanism & Structure Analysis (25 mins)
Test both (15 min). Using the samples provided: test a carboxylic acid with blue litmus and with sodium hydrogencarbonate solution; test an amine with red litmus. Record observations. The carbonate test produces a gas — identify it and write the equation. Then explain why this pair of tests distinguishes a carboxylic acid from every other functional group you have met.
Salts and solubility (10 min). React ethanoic acid with sodium hydroxide, and ethylamine with hydrochloric acid. Draw both salts. Predict which is more soluble in water — the neutral molecule or its salt — and connect your answer back to the fish.
💧 Activity 1b: Short Chain, Long Chain (10 mins)
Ethanoic acid mixes with water in any proportion. Decanoic acid, which is the same functional group on a ten-carbon chain, barely dissolves at all. On paper, in pairs:
- Draw ethanoic acid and decanoic acid side by side. Mark the part of each molecule that can hydrogen-bond to water, and the part that cannot.
- Write one sentence explaining the difference in solubility using what you have marked. The word "polar" on its own is not an explanation — say which part and what it does.
- Now predict, and give your reason: propylamine has almost the same molar mass as propan-1-ol, but boils about 50 °C lower. Both can hydrogen-bond. Which element is doing the hydrogen bonding in each, and why does that change the boiling point?
Kaiako answer key — Salts and solubility. CH3COOH + NaOH → CH3COO−Na+ + H2O (sodium ethanoate). CH3CH2NH2 + HCl → CH3CH2NH3+Cl− (ethylammonium chloride). The salt is far more soluble, because it is ionic and water surrounds ions strongly. That is the answer to the fish: the amine is volatile and reaches your nose; the ammonium salt the lemon juice makes is ionic, so it stays in solution and does not evaporate. The smell does not get destroyed, it gets pinned down.
Kaiako answer key — Short chain, long chain. (1) and (2): the –COOH end hydrogen-bonds to water; the hydrocarbon chain cannot, and has to push water molecules apart to make room for itself. In ethanoic acid the hydrogen-bonding end dominates a two-carbon molecule, so it mixes freely; in decanoic acid the same single –COOH is outnumbered by nine carbons of chain, so the molecule behaves mostly like an alkane. The trend is not a rule to memorise, it is a ratio. (3) Nitrogen is less electronegative than oxygen, so an N–H … N hydrogen bond is weaker than an O–H … O one. Less energy is needed to separate the amine molecules, so the amine boils lower even though both compounds hydrogen-bond and have similar molar masses. This is the shape of a full-marks answer in this standard: a structural feature, what it does, and the property that results — with no step left out.
📝 Activity 2: Level 2 Chemistry Portfolio Task & Merit/Excellence Scaffolding (20 mins)
Organic Portfolio — Section 5. Submit: (1) your test results table with equations; (2) both salt-formation equations drawn structurally; (3) a paragraph explaining the lemon-on-fish observation using volatility and acid–base chemistry together; (4) the annotated ethanoic/decanoic acid pair with your solubility explanation, and your propylamine/propan-1-ol boiling-point reasoning.
🏫 Kaiako Planning & Pedagogy Notes
Where this sits in the standard. Acid-base reactions of carboxylic acids and amines are the last item in the standard's reaction list, and solubility, melting and boiling points are in its property list. This lesson is the only place both are taught together, which is why the solubility activity is not optional extra.
Exit ticket (3 min, on a slip, collected). Ethanoic acid mixes with water in any proportion; decanoic acid barely dissolves. Same functional group. Explain the difference in one sentence.
What to watch for while they work. In the salts task, watch for students drawing the salt as a neutral molecule with the H still attached. In the solubility task, a student who says "because it is polar" has not yet said which part or what it does.
Differentiation. Support: Give the two litmus results and the carbonate result as a filled-in table and have students supply only the equations. The diagnostic logic is the point; the recall is not. Extension: Ask why a carboxylic acid boils higher than an alcohol of similar molar mass, and have them reason from how many hydrogen bonds each molecule can make at once.
The error to head off. Students think the lemon juice destroys the fish smell. It does not — it converts a volatile amine into a non-volatile ionic salt. The molecule is still there; it can no longer reach your nose.
Other teaching approach — Portfolio Mastery Course: Carboxylic Acids, Amines & Acid-Base Chemistry →