Acids & Bases

— Chemistry · AS Level (7405) · Brønsted–Lowry, strong/weak, pH titration curves —

★ Exam Tip: pH titration curves were newly added to the syllabus (2022 →) — sir calls them easy marks if you learn the few rules. Sketch questions ask for: start pH, end pH, equivalence point at 25 cm³, pH of equivalence point, and a narrow steep part.

1. Brønsted–Lowry Theory of Acids & Bases

DefinitionsAcid = H⁺ donor | Base = H⁺ acceptor

Remember: H⁺ is only a proton — an H atom is 1 proton + 1 electron, so once the electron is gone only the proton is left.

Common mistake: a base is an H⁺ acceptor — NOT an OH⁻ donor/acceptor. Acid gives H⁺, base takes H⁺. That is the whole theory.

Conjugate acid–base pairs

Every acid has its own conjugate base, every base has its own conjugate acid. An acid + a base react to give another acid + base.

HA gives H⁺ to H₂O → H₃O⁺ + A⁻ : two pairs, linked by one H⁺

H₂O behaved as a base (gained H⁺ → H₃O⁺); HA behaved as an acid (lost H⁺ → A⁻).
A⁻ is the conjugate base of HA; H₃O⁺ is the conjugate acid of H₂O.
Why? H₃O⁺ now carries an extra H⁺, so if anyone asks for H⁺ it hands it back → acts as an acid. A⁻ has lost its H⁺ and is negative, so it will accept an H⁺ very happily → acts as a base.

Trick: pairs differ by exactly one H⁺. Acid 1 ↔ base 1 and base 2 ↔ acid 2 — label them 1 and 2 to keep track.

Conjugate pair = acid / base differing by only 1 H⁺

Practice — complete & label (a → f)

a) HClacid 1 + H₂Obase 2 → H₃O⁺acid 2 + Cl⁻base 1

b) HNO₃acid 1 + H₂Obase 2 → H₃O⁺acid 2 + NO₃⁻base 1

c) CH₃COOHacid 1 + OH⁻base 2 → H₂Oacid 2 + CH₃COO⁻base 1

d) CH₃COOHacid 1 + NH₃base 2 → NH₄⁺acid 2 + CH₃COO⁻base 1

e) HNO₃acid 1 + OH⁻base 2 → H₂Oacid 2 + NO₃⁻base 1

f) H₂SO₄acid 1 + OH⁻base 2 → H₂Oacid 2 + HSO₄⁻base 1

Eq.	Conjugate pair (1)	Conjugate pair (2)
a	HCl / Cl⁻	H₃O⁺ / H₂O
b	HNO₃ / NO₃⁻	H₃O⁺ / H₂O
c	CH₃COOH / CH₃COO⁻	H₂O / OH⁻
d	CH₃COOH / CH₃COO⁻	NH₄⁺ / NH₃
e	HNO₃ / NO₃⁻	H₂O / OH⁻
f	H₂SO₄ / HSO₄⁻	H₂O / OH⁻

Common mistakes (from class):
• The other product is not always H₃O⁺ — OH⁻ + H⁺ → H₂O (−1 + 1 = 0; there is no “H₂O⁺”). Watch the + / − charges.
• H₂SO₄ loses only one H⁺ → HSO₄⁻. Losing both may still be a real reaction, but it is not the conjugate.
• In (f) students paired the wrong species — pairs are H₂SO₄/HSO₄⁻ and H₂O/OH⁻ (each pair differs by one H⁺).

★ Exam Tip: If the examiner says “write the equation showing how the conjugate forms”, gain/lose exactly ONE H⁺ — never more. (Only for conjugate equations — a full neutralisation can move more.)

2. Strong & Weak Acids and Bases

Strong acid: fully ionises (dissociates fully) in aqueous solution. e.g. nitric acid HNO₃, sulfuric acid H₂SO₄, hydrochloric acid HCl

Weak acid: partially ionises in aqueous solution. e.g. ethanoic acid CH₃COOH

	Strong acid (HCl, HNO₃, H₂SO₄)	Weak acid (CH₃COOH)
pH	around 1	around 3 (3–5)
Reaction with metals	reacts vigorously — lots of bubbles (effervescence of H₂ gas)	does not react vigorously — fewer bubbles
Conductivity	good conductor (fully dissociated → many ions)	bad conductor (few ions)

Note: Observation when an acid reacts with a metal = effervescence; the gas is H₂. The board lists only HNO₃, H₂SO₄ and HCl as strong. In class H₃PO₄ was also called “strong” — it is normally treated as a weak acid, so stick to the three on the board and check with sir/your syllabus.

Weak acids in AS: only carboxylic acids (ethanoic acid). Alcohols are NOT acidic — don't list them (A2 topic).

pH scale

where the usual suspects sit on the scale

Neutral pH (7): water — and salts formed from a strong acid + strong base (NaCl, KCl). Not every salt gives pH 7 — key idea for the titration curves later.

Strong base: sodium hydroxide (NaOH), potassium hydroxide (KOH) (also Ca(OH)₂ and Ba(OH)₂, per sir) Weak base: aqueous ammonia NH₃(aq) (Mg(OH)₂ is weak too)

Sir says: if the exam gives an unfamiliar species, they will tell you whether it is strong or weak.

Learn: NaOH & KOH (Group 1 hydroxides) as the strong bases, and NH₃ as the weak base you must know.

3. pH Titration Curves — Key Terms & Set-up

End point: when the indicator changes colour during titration.

Equivalence point: when an acid and base are mixed together in exactly the equation proportions (e.g. 1 mol acid needs 1 mol alkali → they have completely neutralised each other).

Careful: end point and equivalence point are not the same (the indicator need not change exactly at the equivalence point) — not all salts formed during titration have pH 7 at r.t.p. We simply use the end point to find the equivalence point.
• Titration is complete at the equivalence point.
• DO NOT use the term “neutral point”! pH 7 is neutral only at r.t.p. — at other temperatures/pressures the neutral value changes (A2).

Why 25? Same concentration + 1:1 ratio ⇒ volumes are equal ⇒ 25 cm³ needed. (If the ratio were different, the equivalence volume changes.)

Our set-up: we draw every curve for 1 mol dm⁻³ and 25 cm³ of acid or base already in the conical flask, then titrate it with the corresponding 1 mol dm⁻³ solution from the burette. Acid in flask → run alkali in; alkali in flask → run acid in. (Not necessarily the exact numbers in your exam — the method is the same.)

flask = what is already there; burette = what is added

four questions to answer before drawing ANY titration curve

General shape (all curves): not much change over most of the run → then one sudden steep jump around 25 cm³ (the pH “changes completely”). The steep part is narrow — a thin, near-vertical bit, never a wide sloping stretch. Draw curves, not straight lines.

Who wins? Stronger acid ⇒ salt is acidic (eq. pH < 7), steep part sits in acidic pHs. Stronger base ⇒ salt is basic (eq. pH > 7), steep part sits in alkaline pHs. Equal strength ⇒ salt pH 7.

Flipping the graph: “acid into alkali” and “alkali into acid” give the same curve flipped — only the start and end pH swap. Start at what is in the flask, end at what you added in excess.

4. Strong Acid + Strong Base (HCl + NaOH)

HCl + NaOH → NaCl + H₂O

1:1 ratio ⇒ 25 cm³ of one needs 25 cm³ of the other (same concentration). NaCl is a neutral salt ⇒ equivalence point at pH 7.

running acid into alkali

running alkali into acid (flipped)

Legend: yellow bands = indicator ranges. MO = methyl orange (3.2–4.3), PP = phenolphthalein (8.3–10). ✓ = colour change falls inside the steep part.

Steep part covers both alkaline AND acidic pHs (≈13 → 2): a strong acid and strong base are equally strong — neither “wins”. Bracket on the right of each graph.
pH changes only over a narrow domain — the steep part is very thin (not a long sloping stretch).
Acid into alkali: starts at 14 (NaOH, strong alkali in the flask) and ends at ~1 (excess HCl, strong acid). Flipped: starts at 1, ends at 14.
Equivalence point at 25 cm³, pH 7 — they neutralise each other perfectly, NaCl is neutral.
Both methyl orange ✓ and phenolphthalein ✓ change colour inside the steep part.

Common mistake: the x-axis of the flipped graph is volume of alkali added (the board reused “acid added” on some flipped graphs — correct it in your answers).

5. Strong Acid + Weak Base (HCl + NH₃)

HCl + NH₃ → NH₄Cl

Still 1:1 (25 cm³ ↔ 25 cm³). Can also write NH₄⁺ + Cl⁻. No water forms (NH₃ has no oxygen) — yet it is an acid–base reaction: acid loses H⁺, base gains H⁺. “Salt + water” from O-level is not always the result!

acid into alkali

alkali into acid (flipped)

Strong acid wins ⇒ salt NH₄Cl is slightly acidic ⇒ equivalence point at an acidic pH (~5; anything ~4–6 is fine — not as low as 2).
Steep part lies in the acidic pHs (≈ 8 → 2): only a little alkaline range (8–7) but lots of acidic range (7–2).
Start/end pH: whichever is in the flask starts it — weak base NH₃ ≈ 9–12 (sir used 10 in class, the board shows 12 / 8–9); strong acid = 1.
Indicator: methyl orange ✓ (3.2–4.3 is inside the steep part); phenolphthalein ✗ (8.3–10 is above it).

Sir's note: the slight bends at the start of these curves are due to buffer solutions (A2). At AS you need not show that bend — a gentle, nearly straight slope is fine. But the overall graph must still be a smooth curve with a narrow steep part, not ruler-drawn straight segments.

Remember: when the acid is strong, the salt is acidic — don’t try to memorise “NH₄Cl is acidic”. Use the rule: strong acid + weak base → strong acid wins → acidic salt.

6. Strong Base + Weak Acid (NaOH + CH₃COOH)

NaOH + CH₃COOH → CH₃COONa + H₂O

acid into alkali

alkali into acid (flipped)

Trick: Strong base + weak acid → strong base wins → salt is basic. Same idea, opposite side — no need to memorise pH of salts!

Strong base wins ⇒ salt CH₃COONa is slightly alkaline ⇒ equivalence point at pH ≈ 9.
Steep part lies in the alkaline pHs (strong base) — as much alkaline range as possible, as little acidic as possible.
Acid in flask starts at weak-acid pH (~4); acid added in excess ends at ~4–5 (sir: “3, 4, 5 are good estimates”).
Indicator: phenolphthalein ✓ (8.3–10 is inside the steep part); methyl orange ✗.

Other strong bases: Ca(OH)₂, Ba(OH)₂ (Group 2 hydroxides); Mg(OH)₂ is a weak base (studied in Group 2). Any unknown species will be described as strong/weak in the question.

7. Weak Acid + Weak Base (CH₃COOH + NH₃)

CH₃COOH + NH₃ → CH₃COO⁻ + NH₄⁺

A titration we never do — the curves show why it is a very bad titration (never titrate CH₃COOH with ammonia).

acid into alkali

alkali into acid (flipped)

No clear steep point ⇒ no suitable indicator (both MO ✗ and PP ✗).

Both weak ⇒ neither wins ⇒ salt CH₃COONH₄ (an ionic compound) is neutral ⇒ equivalence point at pH 7, 25 cm³.
The steep part is non-existent — it covers neither the alkaline nor the acidic pHs. Just a very short, gentle bump compared with the other curves.
Acid-into-alkali: from ~10 (weak base, NH₃) down to ~4–5 (weak acid). Flipped: ~4–5 up to ~10.

8. Indicators

colour-change ranges (shaded on the pH axis)

① Methyl orange changes colour between 3.2 – 4.3: below this red, above this yellow.

② Phenolphthalein changes colour between 8.3 – 10.0: below this colourless, above this pink/purple.

③ Universal indicator — colour chart below. We don’t use it for titrations: the colour change is gradual and indistinguishable, so it is hard to notice exactly what pH it is at.

very acidic ← slightly acidic | 7 neutral | slightly alkaline → very alkaline

★ Golden rule: Always choose an indicator which changes colour within the steep part. Use methyl orange for a strong acid (with weak base) • use phenolphthalein for a strong base (with weak acid).

Titration	Salt	Eq. pt pH	Steep part in	MO	PP
strong acid + strong base	NaCl (neutral)	7	acidic + alkaline	✓	✓
strong acid + weak base	NH₄Cl (acidic)	~5	acidic	✓	✗
strong base + weak acid	CH₃COONa (basic)	~9	alkaline	✗	✓
weak acid + weak base	CH₃COONH₄ (neutral)	7	none / no clear steep part	✗	✗

Note: the lecture recording supplied ends before the indicators page, so this section follows the board only. Weak + weak has no suitable indicator — that is exactly why it is a bad titration.

9. Past-paper Questions (from the board)

Past paper (d) · Fig 2.1Structured · 5 marks

25 cm³ of 0.10 mol dm⁻³ HCl(aq) is in a beaker, pH recorded. 50 cm³ of 0.10 mol dm⁻³ NH₃(aq) is added in 5 cm³ portions until all is added. HCl is a strong Brønsted–Lowry acid.

(i) [2] Describe what is meant by a strong Brønsted–Lowry acid.

A strong Brønsted–Lowry acid donates H⁺ and completely ionises in aqueous solution.

(ii) [1] NH₃ is a weak base — write an equation showing its behaviour as a weak B–L base in water.

NH₃ + H₂O ⇌ NH₄⁺ + OH⁻

board writes “→”; use ⇌ (weak base, only partial) — H₂O donates H⁺ (acid), NH₃ accepts (base).

(iii) [2] Sketch the pH change as HCl(aq) is titrated with NH₃(aq) (axes: pH 0–14, volume NH₃ added 0–50 cm³).

Fig 2.1 answer: slow rise → steep at 25 cm³ → levels off; equivalence point marked (dashed) at pH ≈ 5

Marks: strong acid + weak base ⇒ equivalence pt at acidic pH (~5) not 7; start at strong-acid pH (~1) in the beaker; end at weak-base pH (~9–10); steep part sits mostly in acidic pHs. Mark and label the equivalence point like the board does.

Past paper (d) · Fig 3.2Structured · 5 marks

The hydrogen halides dissolve in water to form strong Brønsted–Lowry acids. The concentration of a strong acid can be found by titration.

(i) [2] State what is meant by strong Brønsted–Lowry acid. → same answer as above (“Done”): donates H⁺ & completely ionises in aqueous solution.

(ii) [3] On Fig 3.2 sketch the pH curves when (1) 0.1 mol dm⁻³ NaOH(aq), (2) 0.1 mol dm⁻³ NH₃(aq) is added to 25 cm³ of 0.1 mol dm⁻³ HBr(aq), to excess.

NaOH + HBr: strong + strong ⇒ eq. pt pH 7, ends ~13–14

NH₃ + HBr: strong acid + weak base ⇒ eq. pt pH ≈ 5, ends lower (~10–11)

Compare the two curves: same start (~1) and same 25 cm³ — but the strong-base curve goes through pH 7 and finishes higher (~14); the weak-base curve goes through ~5 and finishes lower (~10–11), with its steep part ending lower. Mark the equivalence point on both, at 25 cm³.

10. Quick Recap — Full Chapter Cheat-Sheet

✓ Brønsted–Lowry

Acid = H⁺ (proton) donor; base = H⁺ acceptor (not OH⁻!)
Conjugate pair: acid ⇌ base differing by exactly 1 H⁺; every acid has a conjugate base and vice-versa
HA + H₂O → H₃O⁺ + A⁻ : acid 1/base 1 = HA/A⁻; base 2/acid 2 = H₂O/H₃O⁺ | OH⁻ + H⁺ → H₂O; H₂SO₄ → HSO₄⁻ (one H⁺ only)

✓ Strong / weak

Strong acid = fully ionises (HCl, HNO₃, H₂SO₄): pH ≈ 1, vigorous with metals (H₂ bubbles), good conductor
Weak acid = partially ionises (CH₃COOH): pH ≈ 3, slow with metals, poor conductor
Strong bases: NaOH, KOH (Group 1/2 hydroxides); weak base: NH₃(aq)
pH: acid 0–7 | neutral 7 | alkali 7–14. Only strong acid + strong base give a neutral salt

✓ Titration curves (1 mol dm⁻³, 25 cm³, 1:1 ⇒ eq. pt at 25 cm³)

Start = pH of solution in flask; End = pH of solution added in excess. Flip graph if reversed.
End point = indicator colour change; equivalence point = exact equation proportions. Never say “neutral point”.
SA + SB: eq. pt pH 7, steep spans alkaline + acidic, MO ✓ PP ✓
SA + WB: eq. pt ~5 (acidic salt), steep mostly acidic, use methyl orange
SB + WA: eq. pt ~9 (basic salt), steep mostly alkaline, use phenolphthalein
WA + WB: eq. pt 7, no clear steep part, no suitable indicator → never done
Steep part is narrow; draw curves, not straight lines. Indicator must change colour inside the steep part.

✓ Indicators

Methyl orange 3.2–4.3 (red → yellow) | Phenolphthalein 8.3–10.0 (colourless → pink/purple)
Universal indicator not used (gradual, indistinguishable colour change)