Proton Transfer Reactions

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Brønsted–Lowry Acids and Bases

A Brønsted–Lowry acid is a species that donates a proton (H⁺).
A Brønsted–Lowry base is a species that accepts a proton using a lone pair of electrons.
The theory applies to proton transfer in any solvent, including gas-phase reactions.
In HCl(g) + H₂O(l) → H₃O⁺(aq) + Cl⁻(aq), HCl acts as the acid and water as the base.
The formation of ions from a molecular acid or base is called dissociation or ionisation.

Proton transfer between an acid and a base

Conjugate Acids and Bases

A conjugate acid–base pair consists of two species that differ by a single H⁺ ion.
In CH₃COOH(aq) + H₂O(l) ⇌ CH₃COO⁻(aq) + H₃O⁺(aq), CH₃COOH/CH₃COO⁻ and H₂O/H₃O⁺ are conjugate pairs.
The acid and its conjugate base are related by the loss of one proton; the base and its conjugate acid by the gain of one proton.
Strong acids have weak conjugate bases; weak acids have strong conjugate bases.
Strong bases have weak conjugate acids.

Amphiprotic Species

An amphiprotic species can both donate and accept a proton (H⁺).
Water is amphiprotic: it donates a proton to NH₃ and accepts a proton from HCl.
An amphoteric substance can act as an acid or a base in a chemical reaction.
All amphiprotic substances are amphoteric, but not all amphoteric substances are amphiprotic.
Aluminium oxide, Al₂O₃, is amphoteric but not amphiprotic because it does not donate or accept protons.

The pH Scale

pH is defined as pH = –log[H⁺], where [H⁺] is the concentration of hydrogen ions in mol dm⁻³.
The hydrogen ion concentration can be found from pH using [H⁺] = 10^(–pH).
The pH scale is logarithmic: a change of 1 pH unit corresponds to a tenfold change in [H⁺].
Acidic solutions have pH < 7, basic solutions have pH > 7, and neutral solutions have pH = 7 at 298 K.
The lower the pH, the more acidic the solution; the higher the pH, the more alkaline the solution.
A pH meter gives the most accurate pH reading; universal indicator paper is less accurate.

The pH scale

The Ion Product of Water

Water undergoes self-ionisation: H₂O(l) ⇌ H⁺(aq) + OH⁻(aq).
The ion product of water, Kw, is defined as Kw = [H⁺][OH⁻].
At 298 K, Kw = 1.00 × 10⁻¹⁴ mol² dm⁻⁶.
In pure water at 298 K, [H⁺] = [OH⁻] = 1.00 × 10⁻⁷ mol dm⁻³, so pH = 7.
The ionisation of water is endothermic; increasing temperature increases Kw and decreases the pH of pure water.
Pure water remains neutral at any temperature because [H⁺] = [OH⁻].

Strong and Weak Acids and Bases

A strong acid dissociates almost completely in aqueous solution (e.g. HCl, HNO₃, H₂SO₄).
A weak acid dissociates only partially in aqueous solution (e.g. CH₃COOH, HCN, H₂CO₃).
A strong base dissociates almost completely (e.g. Group 1 metal hydroxides).
A weak base dissociates only partially (e.g. NH₃, amines).
Strong acids have a higher [H⁺], lower pH, greater electrical conductivity, and react more vigorously with metals than weak acids of the same concentration.
For hydrogen halides, acid strength increases down Group 17: HF < HCl < HBr < HI, because the H–X bond becomes longer and weaker.

Neutralisation Reactions

A neutralisation reaction is: acid + base → salt + water.
The essential reaction is H⁺(aq) + OH⁻(aq) → H₂O(l).
The salt is formed from the spectator ions not involved in water formation.
Acid + metal → salt + hydrogen; acid + metal oxide → salt + water; acid + metal hydroxide → salt + water.
Acid + metal carbonate → salt + water + carbon dioxide; acid + metal hydrogencarbonate → salt + water + carbon dioxide.
The salt produced depends on the acid used: hydrochloric acid forms chlorides, nitric acid forms nitrates, sulfuric acid forms sulfates.

Universal indicator across the pH scale

pH Curves and Indicators

A pH curve shows how pH changes as acid or base is added during a titration.
All pH curves are S-shaped; the midpoint of the vertical section is the equivalence point.
Strong acid–strong base: equivalence point at pH 7.
Weak acid–strong base: equivalence point above pH 7; at half-equivalence, pH = pKa.
Weak base–strong acid: equivalence point below pH 7; at half-equivalence, pOH = pKb.
Weak acid–weak base: no sharp vertical section, so the equivalence point is difficult to determine and no suitable indicator exists.

The pOH Scale and Acid–Base Calculations (HL)

pOH is defined as pOH = –log[OH⁻].
The hydroxide concentration can be found from pOH using [OH⁻] = 10^(–pOH).
At 298 K, pH + pOH = 14.
The hydrogen ion concentration can be calculated from [OH⁻] using [H⁺] = Kw / [OH⁻].
For a strong acid, pH = –log[acid] (for monoprotic acids).
For a strong base, first find [OH⁻], then pOH, then pH = 14 – pOH.

Acid and Base Dissociation Constants (HL)

The acid dissociation constant, Ka, for HA(aq) ⇌ H⁺(aq) + A⁻(aq) is Ka = [H⁺][A⁻] / [HA].
The base dissociation constant, Kb, for B(aq) + H₂O(l) ⇌ BH⁺(aq) + OH⁻(aq) is Kb = [BH⁺][OH⁻] / [B].
pKa = –log Ka and pKb = –log Kb.
A larger Ka (smaller pKa) means a stronger acid; a larger Kb (smaller pKb) means a stronger base.
For conjugate acid–base pairs, Ka × Kb = Kw.
The range of pKa values for most weak acids is between 3 and 7.

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1.Using your knowledge of the Brønsted-Lowry theory, which of the following correctly describes ammonia?
Easy
- Aneutral
- Bacid
- Cbase
- Damphoteric
2.In the Brønsted–Lowry theory of acids and bases, the difference between a conjugate acid and its conjugate base is the presence of which of the following?
Easy
- Aa positive charge
- Ba pair of electrons
- Ca proton
- Da hydrogen atom
3.Which of the following ions or compounds is amphiprotic?
Medium
- APO₄³⁻
- BHCO₃⁻
- CAl₂O₃
- DP₄O₁₀
4.In the following reaction, identify which two species are acting as Brønsted–Lowry acids: H₃PO₄(aq) + OH⁻(aq) ⇋ H₂PO₄⁻(aq) + H₂O(l)
Medium
- AH₂PO₄⁻(aq) and OH⁻(aq)
- BH₃PO₄(aq) and H₂PO₄⁻(aq)
- CH₂PO₄⁻(aq) and H₂O(l)
- DH₃PO₄(aq) and H₂O(l)
5.Potassium hydrogen carbonate reacts vigorously with dilute sulfuric acid. Identify the correct formulas of the substances produced in the reaction.
Medium
- AK₂SO₄ + H₂O + CO₂
- BK₂SO₄ + CO₂
- CKSO₄ + H₂O + CO₂
- DKSO₄ + H₂CO₃
6.Copper(II) sulfate can be made by the reaction between dilute sulfuric acid and which of the following? I. Cu II. CuO III. CuCO₃
Easy
- AI and II only
- BI and III only
- CII and III only
- DI, II and III
7.Which statement is correct for the following equation? HSO₄⁻(aq) + OH⁻(aq) ⇌ SO₄²⁻(aq) + H₂O(l)
Medium
- AOH⁻ and H₂O are an acid and conjugate base pair
- BSO₄²⁻ is acting as Brønsted–Lowry acid
- CHSO₄⁻ and SO₄²⁻ are a base and conjugate acid pair
- DOH⁻ and H₂O are a base and conjugate acid pair
8.Which of the following statements about amphiprotic species are correct? (select all that apply)
Medium
- AThey can donate a proton.
- BThey can accept a proton.
- CAll amphiprotic species are also amphoteric.
- DAll amphoteric species are also amphiprotic.
- EThey must contain a metal ion.

Plus de questions

9.A Brønsted-Lowry base is a species that accepts a proton using its lone pair of electrons.
Easy
True or false?
10.All amphoteric substances are also amphiprotic.
Easy
True or false?
11.Which row shows the correct acid and base needed to make the salt specified?
Medium
- AAcid: NaHCO₃, Base: SO₂, Salt: Na₂SO₄
- BAcid: HNO₃, Base: SO₃, Salt: (NH₄)₂SO₄
- CAcid: H₂SO₄, Base: ZnO, Salt: ZnSO₄
- DAcid: H₂SO₄, Base: NH₄OH, Salt: (NH₄)₂SO₄
12.Which is a conjugate acid-base pair in the following reaction? HCO₃⁻(aq) + H₂O(l) ⇌ CO₃²⁻(aq) + H₃O⁺(aq)
Medium
- AHCO₃⁻ / H₃O⁺
- BHCO₃⁻ / CO₃²⁻
- CH₂O / CO₃²⁻
- DHCO₃⁻ / H₂O
13.Which coefficients balance the following acid-base equation? Al(OH)₃(s) + HNO₃(aq) → Al(NO₃)₃(aq) + H₂O(l)
Easy
- A3, 1, 3, 1
- B2, 2, 1, 3
- C1, 3, 1, 3
- D1, 2, 1, 2
14.Four 1.0 M solutions of HCl, NH₃, NaOH and CH₃COOH have been mislabelled, but a student has a pH meter to test the pH of the solutions at 25 °C. Arrange the solutions in order of increasing pH.
Medium
- AHCl (aq) < NH₃ (aq) < NaOH (aq) < CH₃COOH (aq)
- BCH₃COOH (aq) < HCl (aq) < NH₃ (aq) < NaOH (aq)
- CHCl (aq) < CH₃COOH (aq) < NH₃ (aq) < NaOH (aq)
- DNaOH (aq) < NH₃ (aq) < CH₃COOH (aq) < HCl (aq)
15.Below are four statements about acids and bases. Which is the correct one?
Medium
- AStrong acids are good proton donors and have weak conjugate bases.
- BStrong bases are good proton donors and have weak conjugate acids.
- CWeak acids are poor proton acceptors and have strong conjugate bases.
- DStrong acids are good proton donors and have strong conjugate bases.
16.Which statement is correct about the action of weak acids and their conductivity?
Medium
- AWeak acids are proton donors and their solutions are good conductors.
- BWeak acids are proton donors and their solutions are poor conductors.
- CWeak acids are proton acceptors and their solutions are good conductors.
- DWeak acids are proton acceptors and their solutions are poor conductors.
17.The correct expression for the ionic product of water at 25 °C is
Medium
- AKw = [H⁺][OH⁻] / [H₂O]
- BH₂O(l) ⇌ H⁺(aq) + OH⁻(aq)
- CKw = [H⁺][OH⁻]
- D1 x 10⁻¹⁴ mol² dm⁻⁶
18.Which is true about 1.0 mol dm⁻³ solutions of weak and strong acids?
Medium
- AMagnesium will react with strong acids but not weak acids
- BFewer moles of base are needed to neutralise weak acids
- CSodium carbonate reacts more slowly with weak acids
- DWeak acids have a lower pH than strong acids
19.Four solutions, K, L, M and N have the following properties: K: pH = 8; L: [H⁺] = 1 x 10⁻³ mol dm⁻³; M: pH = 5; N: [H⁺] = 1 x 10⁻⁷ mol dm⁻³. What is their correct order of increasing acidity?
Medium
- AK < L < M < N
- BN < L < M < K
- CK < N < M < L
- DN < M < L < K
20.Two beakers contain solutions of hydrochloric acid at pH = 2 and pH = 4. How does the concentration of hydrogen ions in the beaker at pH = 4 compare to the concentration in the beaker at pH = 2?
Medium
- ATwice as large
- BHalf as much
- C1/10 of the value
- D1/100 of the value
21.A sample of acid rain water is tested and found to be pH 4. How does its hydrogen ion concentration compare to pure water?
Medium
- A1000 times more concentrated
- B3 times more concentrated
- C1000 times less concentrated
- D3 times less concentrated
22.Which type of titration is indicated by a pH curve that starts around pH 1, has a vertical section at pH 7, and ends around pH 12?
Medium
- AStrong acid - strong base
- BWeak acid - weak base
- CStrong acid - weak base
- DWeak acid - strong base
23.Which mixture cannot act as a buffer?
Medium
- ACH₃COOH (aq) and CH₃COONa (aq)
- BCH₃CH₂COOH (aq) and CH₃CH₂COONa (aq)
- CHNO₃ (aq) and NaOH (aq)
- DNH₃ (aq) and NH₄Cl (aq)
24.The hydrolysis of ammonium chloride takes place via the following reaction: NH₄⁺ + H₂O → H₃O⁺ + NH₃. Which of the following is correct?
Medium
- AThe resultant pH is above 7
- BThe resultant pH is 7
- CThe resultant pH is below 7
- DAmmonium chloride is insoluble
25.Which of the following is the acid dissociation constant, Ka, for ethanoic acid?
Medium
- AKa = [CH₃COOH] / ([CH₃COO⁻][H⁺])
- BKa = [CH₃COO⁻][H⁺] / [CH₃COOH]
- CKa = [CH₃COO⁻] / [CH₃COOH]
- DKa = [H⁺] / [CH₃COOH]
26.At the same concentration, which acid would have the lowest pH?
Medium
- AC₆H₅COOH Ka = 6.3 x 10⁻⁵ mol dm⁻³
- BHCOOH Ka = 1.8 x 10⁻⁴ mol dm⁻³
- CHCN Ka = 4.9 x 10⁻¹⁰ mol dm⁻³
- DCH₃CH₂COOH Ka = 1.4 x 10⁻⁵ mol dm⁻³
27.The pKb value of ethylamine is 3.35 at 298 K. What is the value of the pKa of the ethylammonium ion?
Medium
- A2.99 x 10⁻¹⁴
- B10.65
- C4.18
- D2.24 x 10⁻¹⁰
28.Which of the following is the correct equation to calculate the concentration of hydroxide ions?
Medium
- AKw / pOH
- B-log [H⁺]
- C14 - pOH
- D10^(-pOH)
29.Which shows the correct relationship between Kw, Ka and Kb?
Medium
- AKw = Ka + Kb
- BKw = Ka x Kb
- CKw = Ka / Kb
- DKw = Kb / Ka
30.The following reaction occurs between concentrated sulfuric and nitric acids: H₂SO₄ + HNO₃ ⇋ H₂NO₃⁺ + HSO₄⁻. Identify the two species which are acting as Brønsted–Lowry bases.
Medium
- AH₂NO₃⁺ and HSO₄⁻
- BHNO₃ and H₂NO₃⁺
- CH₂SO₄ and HSO₄⁻
- DHNO₃ and HSO₄⁻
31.What role does each species play in the equilibrium below according to Brønsted-Lowry theory? CH₃COOH + HCl ⇌ CH₃COOH₂⁺ + Cl⁻
Medium
- ACH₃COOH: acid, HCl: base, CH₃COOH₂⁺: base, Cl⁻: acid
- BCH₃COOH: base, HCl: acid, CH₃COOH₂⁺: acid, Cl⁻: base
- CCH₃COOH: acid, HCl: acid, CH₃COOH₂⁺: base, Cl⁻: base
- DCH₃COOH: base, HCl: base, CH₃COOH₂⁺: acid, Cl⁻: acid
32.Match each term with its correct definition.
Medium
- Brønsted-Lowry acid
- Brønsted-Lowry base
- Amphiprotic species
- Conjugate acid-base pair
- Species that can donate a proton
- Species that can accept a proton
- Species that can both donate and accept a proton
- Two species differing by a single H⁺ ion
33.Arrange the following acids in order of increasing acid strength (weakest to strongest).
Medium
- HF
- HCl
- HBr
- HI
34.Arrange the following solutions in order of increasing pH (most acidic to most basic).
Medium
- 0.1 mol dm⁻³ HCl
- 0.1 mol dm⁻³ CH₃COOH
- 0.1 mol dm⁻³ NaOH
35.Which of the following statements about the Brønsted–Lowry theory are correct? (select all that apply)
Medium
- AA Brønsted–Lowry acid is a species that donates a proton.
- BA Brønsted–Lowry base is a species that accepts a proton using its lone pair of electrons.
- CThe Brønsted–Lowry theory can only be applied to reactions in aqueous solution.
- DA proton is a hydrogen atom that has lost its electron.
- EIn the reaction HCl(g) + H₂O(l) → H₃O⁺(aq) + Cl⁻(aq), water acts as a base.
36.Which of the following species are amphiprotic? (select all that apply)
Medium
- AHCO₃⁻
- BH₂O
- CPO₄³⁻
- DAl₂O₃
- EHSO₄⁻
37.Which of the following statements about strong and weak acids are correct? (select all that apply)
Medium
- AA strong acid dissociates almost completely in aqueous solution.
- BA weak acid is a poor proton donor and has a strong conjugate base.
- CAt the same concentration, a weak acid has a lower pH than a strong acid.
- DStrong acids conduct electricity better than weak acids at the same concentration.
- EA weak acid reacts with magnesium more vigorously than a strong acid at the same concentration.
38.Which of the following mixtures can act as a buffer solution? (select all that apply)
Hard
- ACH₃COOH(aq) and CH₃COONa(aq)
- BNH₃(aq) and NH₄Cl(aq)
- CHNO₃(aq) and NaOH(aq) in equal amounts
- DCH₃CH₂COOH(aq) and CH₃CH₂COONa(aq)
- EHCl(aq) and NaCl(aq)
39.Which of the following statements about the ion product of water, K_w, are correct? (select all that apply)
Medium
- AK_w = [H⁺][OH⁻]
- BAt 298 K, K_w = 1.00 × 10⁻¹⁴
- CThe ionisation of water is exothermic, so K_w decreases with increasing temperature.
- DIn pure water at any temperature, [H⁺] = [OH⁻].
- EAt temperatures above 298 K, the pH of pure water is greater than 7.
40.Match each acid with the salt it forms when it reacts with a base.
Medium
- Hydrochloric acid
- Nitric acid
- Sulfuric acid
- Ethanoic acid
- Ethanoates
- Chlorides
- Sulfates
- Nitrates
41.34.3 cm³ of 0.125 mol dm⁻³ sodium hydroxide solution reacts with 26.0 cm³ of sulfuric acid. What is the concentration of the acid in mol dm⁻³?
Medium
42.90 cm³ of water is added to 10 cm³ of hydrochloric acid with a pH of 2. What is the new pH of the acid?
Medium
43.What term describes a species that can act both as a proton donor and a proton acceptor?
Easy
44.What is the name of the ion formed when a Brønsted-Lowry base accepts a proton?
Easy
45.Complete the definition of a Brønsted-Lowry acid and base.
Medium
A Brønsted-Lowry acid is a species that ____ a proton, while a Brønsted-Lowry base is a species that ____ a proton.
46.Complete the sentence about conjugate acid-base pairs.
Medium
A conjugate acid-base pair is two species that differ by a single ____ ion.
47.Calculate the pH of a 0.010 mol dm⁻³ solution of hydrochloric acid at 298 K.
Medium
48.Calculate the pH of a 0.15 mol dm⁻³ solution of sodium hydroxide at 298 K. K_w = 1.00 × 10⁻¹⁴.
Medium
49.A solution of ethanoic acid has a pH of 3.10. Calculate the concentration of hydrogen ions, [H⁺], in mol dm⁻³. Give your answer in scientific notation to 2 decimal places.
Hard
50.Complete the definition of a Brønsted–Lowry acid and base.
Easy
A Brønsted–Lowry acid is a species that ____ a proton, while a Brønsted–Lowry base is a species that ____ a proton.
51.Complete the equation for the ion product of water.
Medium
K____w = [H⁺][____]
52.Explain why the pH of pure water decreases when the temperature is increased above 298 K, even though the water remains neutral.
Hard
53.Explain the difference between a strong acid and a weak acid in terms of dissociation and conductivity.
Hard
54.34.3 cm³ of 0.125 mol dm⁻³ sodium hydroxide solution reacts with 26.0 cm³ of sulfuric acid. What is the concentration of the acid in mol dm⁻³?
Medium
- A1.0825 mol dm⁻³
- B-0.9175 mol dm⁻³
- C0.0825 mol dm⁻³
- D2.0825 mol dm⁻³
55.90 cm³ of water is added to 10 cm³ of hydrochloric acid with a pH of 2. What is the new pH of the acid?
Medium
- A5
- B2
- C3
- D4
56.Calculate the pH of a 0.010 mol dm⁻³ solution of hydrochloric acid at 298 K.
Medium
- A1
- B2
- C4
- D3
57.Calculate the pH of a 0.15 mol dm⁻³ solution of sodium hydroxide at 298 K. K_w = 1.00 × 10⁻¹⁴.
Medium
- A14.18
- B15.18
- C13.18
- D12.18
58.A solution of ethanoic acid has a pH of 3.10. Calculate the concentration of hydrogen ions, [H⁺], in mol dm⁻³. Give your answer in scientific notation to 2 decimal places.
Hard
- A2.0008 mol dm⁻³
- B1.0008 mol dm⁻³
- C-0.9992 mol dm⁻³
- D0.000794 mol dm⁻³

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