SPEC 4.7.2.3 & 4.7.2.4 • PAPER 2
⬢ CHEMISTRY ONLY ⬢
Alcohols & Carboxylic Acids
Two organic functional groups defined by what they contain — not just by name. Three reactions for each, plus fermentation conditions and one HT-only weak acid extension.
Two functional groups
A functional group is the reactive part of an organic molecule. The whole homologous series shares the same functional group — and reacts in the same way.
Alcohols
Functional group: –OH
The –OH (hydroxyl) is bonded to a carbon. The four members in the spec: methanol, ethanol, propanol, butanol.
Recognise from name: ends in -anol. Recognise from formula: –OH attached to a C that is otherwise bonded only to C or H.
Carboxylic acids
Functional group: –COOH
The –COOH (carboxyl) is one functional group — a C double-bonded to O and single-bonded to O–H. Members: methanoic, ethanoic, propanoic, butanoic acid.
Recognise from name: ends in -anoic acid. Recognise from formula: –COOH at the end of a chain.
Why this matters
If you can identify the functional group in any molecule you've never seen before, you can predict its reactions. That's the entire idea behind organic chemistry.
Alcohols: the first four members
A homologous series — same functional group, each next member differs by one –CH₂– unit.
NameMolecular formulaStructural formulaMain use
MethanolCH₃OHCH₃OHSolvent; fuel
EthanolC₂H₅OHCH₃CH₂OHAlcoholic drinks; solvent; fuel
PropanolC₃H₇OHCH₃CH₂CH₂OHSolvent
ButanolC₄H₉OHCH₃CH₂CH₂CH₂OHSolvent; fuel
Note on formula columns: the structural formula (e.g. CH₃CH₂OH) shows the order of atoms but compresses C–H bonds. The fully displayed formula (shown below for ethanol) draws every atom and every bond.
All four alcohols share the same –OH group — so they share the same set of reactions.
Reaction 1 — added to water
The first four alcohols are soluble in water.
Result
Alcohol + water → neutral solution (pH ≈ 7).
No fizzing. No colour change. Universal indicator stays green. Methanol, ethanol, propanol, and butanol all behave the same way.
Mark-scheme language
Examiners want to see "neutral" or "pH ≈ 7". Writing "not acidic" alone is not enough — it doesn't say what the solution is. State the property positively.
Don't mix this up
Carboxylic acids dissolve in water to give an acidic solution (we'll see this on slide 11). The –OH in –COOH behaves completely differently from the –OH in an alcohol — that's coming up.
Reaction 2 — burn in air
Alcohols are flammable. They burn in oxygen to give carbon dioxide and water — releasing heat. This is why alcohols, especially ethanol, are used as fuels.
General word equation
alcohol + oxygen → carbon dioxide + water
Worked balanced equations (combustion is the only alcohol reaction the spec asks you to balance)
Ethanol
C₂H₅OH + 3 O₂ → 2 CO₂ + 3 H₂O
Check: C: 2 = 2 ✓ H: 6 = 6 ✓ O: 1 + 6 = 7 = 4 + 3 ✓
Butanol
C₄H₉OH + 6 O₂ → 4 CO₂ + 5 H₂O
Check: C: 4 = 4 ✓ H: 10 = 10 ✓ O: 1 + 12 = 13 = 8 + 5 ✓
Mark-scheme rule (P2 Nov21 Q10.7)
When asked to balance a combustion equation for a higher alcohol, the mark scheme accepts the simplest balanced version or any whole-number multiple of it. Half-marks are typical when formulae are correct but balancing numbers (multipliers) are missing. Always double-check by counting C, H and O on both sides — a one-line check costs no time.
Reaction 3 — react with sodium
A small piece of sodium dropped into an alcohol behaves like sodium in water — but more slowly.
See
Sodium fizzes / effervesces and gradually disappears.
Hear
The fizzing comes from a gas — collect it and it gives a squeaky pop with a lit splint.
Identify
Squeaky pop = hydrogen gas.
Word summary
alcohol + sodium → hydrogen gas + a salt
The reaction is similar to sodium + water — but slower, because the alcohol is a less reactive source of the H that gets displaced.
No equation needed
The spec says: "Students are not expected to write balanced chemical equations for the reactions of alcohols other than for combustion reactions." A correct description — fizzing, hydrogen gas given off — is what scores marks here.
Reaction 4 — oxidising agent
Add an oxidising agent to an alcohol and the alcohol is oxidised — the –OH is converted into –COOH.
Outcome
alcohol + oxidising agent → carboxylic acid (water is also produced)
Specific example: ethanol + [O] → ethanoic acid. This is the same chemistry that turns wine into vinegar over time, when oxygen in the air slowly oxidises the ethanol.
Naming pattern
Alcohol→ Carboxylic acid produced
MethanolMethanoic acid
EthanolEthanoic acid
PropanolPropanoic acid
ButanolButanoic acid
Examiner alert — common confusion (P2 Jun23 Q02.6)
In a multiple-choice question on what reacts with ethanol to make ethanoic acid, examiners reported "'an alkali metal' was a strong distractor". The trap: students remember that sodium reacts with ethanol — and pick "alkali metal". But sodium gives hydrogen gas, not a carboxylic acid.
Lock this in: only an oxidising agent turns ethanol into ethanoic acid. Sodium gives hydrogen. Halogens don't react with the –OH group at GCSE level. Water just dissolves the alcohol.
Producing ethanol — fermentation
Aqueous ethanol can be made by fermenting sugar solution with yeast. The yeast contains enzymes that break sugar down into ethanol and carbon dioxide.
Word equation
sugar (glucose) → ethanol + carbon dioxide(catalysed by yeast enzymes)
The three conditions — every one is markable
1 · Yeast
Source of the enzymes that break down the sugar.
2 · Warm
Around 30 °C. Mark scheme accepts any value 25–45 °C. Too cold = enzymes too slow. Too hot = enzymes denatured.
3 · Anaerobic
No oxygen / no air. If oxygen is present, ethanol is oxidised to ethanoic acid (vinegar) and the product is ruined.
How marks are awarded
If a question asks for the conditions open-ended, all three (yeast + warm + anaerobic) earn a mark each. If the stem already states "yeast is added", only warm and anaerobic remain — that's exactly what happened in the next slide's question.
Examiner alert: fermentation
P2 Nov21 Q10.5 — verbatim examiner report
Students were asked to give the conditions for a fermentation-style reaction. The mark scheme awarded warm (allow any value 25–45 °C) and anaerobic (allow "without oxygen / air") — 1 mark each.Examiner report: "Around half of students scored one mark by extending their knowledge of fermentation to this unfamiliar context, 'warm' was seen much more often than 'anaerobic'."
Lesson learned
Most students remember warm. Almost half forget anaerobic. Train yourself to give both every time fermentation conditions come up — and add yeast as a third if the source of enzymes hasn't already been stated in the question.
Three-word memory cue
Yeast — Warm — Anaerobic. Drill it. The examiner has flagged this exact gap multiple times.
Carboxylic acids: the first four
Same logic as alcohols — a homologous series with one shared functional group, –COOH, on the end of every member.
NameMolecular formulaStructural formulaWhere it's found
Methanoic acidHCOOHHCOOHAnt stings; nettles
Ethanoic acidCH₃COOHCH₃COOHVinegar (~5% in water)
Propanoic acidC₂H₅COOHCH₃CH₂COOHFood preservative
Butanoic acidC₃H₇COOHCH₃CH₂CH₂COOHRancid butter (the smell)
%3C%2Ftext%3E%0A%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%3C%2Fsvg%3E)
Reaction 1 — dissolve in water
Carboxylic acids dissolve in water — and the solution is acidic (pH < 7). This is the opposite of an alcohol's neutral solution.
Alcohol + water
Neutral pH ≈ 7
Universal indicator stays green.
Carboxylic acid + water
Acidic pH < 7
Universal indicator turns yellow / orange / red. Litmus turns red.
Why is it acidic?
In water, some –COOH groups release H⁺ ions into solution. The presence of H⁺ is what makes any solution acidic.
But not all of the –COOH groups release H⁺. Carboxylic acids are weak acids — only a fraction ionise. We come back to this on slide 15 (HT only).
Reaction 2 — with carbonates
Carboxylic acids behave like any other acid when they meet a metal carbonate.
General word equation
carboxylic acid + metal carbonate → salt + water + carbon dioxide
Specific example
ethanoic acid + sodium carbonate → sodium ethanoate + water + carbon dioxide
See
Effervescence (fizzing); the carbonate dissolves.
Test the gas
Bubble through limewater — turns cloudy / milky.
Identify
Cloudy limewater = carbon dioxide.
Common matching trap
In matching-style questions where ethanoic acid reacts with several different compounds, the two products students must distinguish are ethyl ethanoate (when an alcohol is the partner) and carbon dioxide (when a metal carbonate is the partner). Mix these up and you lose both marks at once.
Reaction 3 — with alcohols
A carboxylic acid reacts with an alcohol to make an ester — a sweet-smelling organic compound. Esters are responsible for many fruit smells and flavours.
General word equation
carboxylic acid + alcohol → ester + water
The ester you must know by name
ethanoic acid + ethanol → ethyl ethanoate + water
Naming the ester: the alcohol gives the first part (ethyl — from ethanol), the carboxylic acid gives the second part (ethanoate — from ethanoic acid).
Spec scope: only one ester to name
The spec says: "Students do not need to know the names of esters other than ethyl ethanoate." If a question gives you a different ester, you'll be told its name. Memorise this one — it appears in past papers regularly.
Examiner alert: circle the right thing
Functional-group questions are worth 1 mark each — but examiners flag the same mistakes year after year.
P2 Nov21 Q04.5 — examiner report
"Some students seem to think that any -OH is an alcohol group even if it is part of a carboxylic acid group. Other students circled whole molecules rather than functional groups." Two distinct mistakes: (1) circling the –OH inside a –COOH group as if it were an alcohol; (2) drawing a circle so wide it includes the entire molecule.
P2 Jun24 Q08.3 — examiner report
"A variety of areas in the molecule were circled in an attempt to answer this question. A mistake some students made who were not awarded this mark was incorporating one of the C atoms either side of the ester bond in their circle. Around 40% of students gained credit." Same lesson: the circle must contain the functional group only — not the carbon chain attached to it on either side.
Rule
Alcohol –OH: attached to a C that has only H or other Cs attached to it.
Carboxylic acid –COOH: the C has a double-bond to one O and a single bond to a separate O–H. The whole –C(=O)–O–H counts as one functional group — circle all four atoms.
Weak acids HT ONLY
The HT extension to carboxylic acids: explaining why they are described as weak acids — and what this does to their pH.
Definition
A weak acid is one that is only partially ionised in aqueous solution.
In a solution of ethanoic acid, only a small fraction of the CH₃COOH molecules release an H⁺ ion. Most stay intact. (Compare: HCl, a strong acid, is fully ionised — every molecule splits.)
Strong acid (e.g. HCl)
Fully ionised: every molecule splits into H⁺ + anion. High concentration of H⁺ per dm³.
Weak acid (e.g. ethanoic)
Partially ionised: only a fraction of molecules release H⁺. Lower concentration of H⁺ per dm³.
Effect on pH
For solutions of equal concentration, a weak acid has fewer H⁺ ions per dm³ than a strong acid → so the weak acid has a higher pH.
Mark scheme — exact wording (P1 Jun23 Q05.1)
For 1 mark: "(the acid is only) partially ionised (in aqueous solution)". The phrase "in aqueous solution" must be there. "H⁺ ions are ionised" loses the mark — the molecule ionises and H⁺ is what's released.
Don't confuse
Weak ≠ dilute. Weak/strong = degree of ionisation. Dilute/concentrated = how much acid per dm³. They are independent properties (this is covered in depth in lesson 4.06).
Practice question
3 MARKS
A student leaves a glass of wine open on a kitchen counter for several days. The wine starts to taste sour, like vinegar.
Explain, using ideas about functional groups and reactions of alcohols, why the wine has turned sour.
P
Point. The wine has turned sour because the ethanol it contains has been oxidised to ethanoic acid — the carboxylic acid found in vinegar.
E
Evidence. Ethanol has the functional group –OH. Oxygen from the air acts as an oxidising agent: ethanol + oxidising agent → ethanoic acid (which has the functional group –COOH).
E
Explain. Ethanol gives a neutral solution (pH ≈ 7), but ethanoic acid dissolves in water to give an acidic solution. The acidic ethanoic acid is what produces the sour taste — wine vinegar is around 5% ethanoic acid in water.
Lesson summary
The two functional groups
–OH (alcohols: methanol, ethanol, propanol, butanol) · –COOH (carboxylic acids: methanoic, ethanoic, propanoic, butanoic)
ReactionAlcoholCarboxylic acid
+ waterNeutral solution (pH ≈ 7)Acidic solution (pH < 7)
+ sodiumFizzes; gives off H₂—
burn in airCO₂ + H₂O (balance!)—
+ oxidising agent→ carboxylic acid—
+ carbonate—Salt + H₂O + CO₂
+ alcohol—Ester (e.g. ethyl ethanoate) + water
Memorised facts
Fermentation: yeast + warm (~30 °C) + anaerobic → ethanol + CO₂.
Combustion balance to know: C₂H₅OH + 3 O₂ → 2 CO₂ + 3 H₂O.
HT only: carboxylic acids are weak — partially ionised in aqueous solution → higher pH than a strong acid of equal concentration.
Next lesson
Lesson 7.06 — Addition Polymerisation. How alkenes (those C=C bonds from 7.04) join together to make polymers like poly(ethene) and poly(propene).
Checkpoint
Before you move on, you should be able to do all of the following without help:
Self-check
Identify alcohols and carboxylic acids from a name or a displayed formula — and circle only the functional group.
Describe what happens when any of the first four alcohols (a) burn in air, (b) react with sodium, (c) are added to water, (d) react with an oxidising agent.
Write a balanced equation for the combustion of any of the first four alcohols (e.g. ethanol).
State the three conditions for fermentation: yeast, warm, anaerobic.
Describe what happens when any of the first four carboxylic acids (a) dissolve in water, (b) react with a carbonate, (c) react with an alcohol — naming ethyl ethanoate.
HT only: explain why carboxylic acids are described as weak acids in terms of partial ionisation in aqueous solution and the resulting pH.
Final reminder
If a question asks you to circle the functional group, your circle should contain that group only — no carbon chain on either side, no extra hydrogens, and certainly no whole molecule. The –COOH group is four atoms (one C, two O, one H) bonded together — circle all four.
Next: Two functional groups