Proton Transfer, Strong & Weak Acids
Section: 7. Acids, Bases & Salts | Syllabus: Cambridge AS Level Physics 9702
Proton Transfer Proton transfer is the movement of hydrogen ions (H⁺) from one substance to another during an acid-base reaction. According to the Brønsted-Lowry theory : An acid is a proton (H⁺) donor A base is a proton (H⁺) acceptor Example of Proton Transfer When hydrochloric acid reacts with ammonia: HCl + NH₃ → NH₄⁺ + Cl⁻ HCl donates a proton (H⁺) → acts as an acid NH₃ accepts a proton (H⁺) → acts as a base The proton transfers from HCl to NH₃ Neutralization as Proton Transfer During neutralization, protons transfer from the acid to the base: H⁺(aq) + OH⁻(aq) → H₂O(l) The H⁺ ion (proton) from the acid is accepted by the OH⁻ ion from the base to form water.
Key Concept All acid-base reactions involve the transfer of protons from the acid to the base. This is the fundamental definition of an acid-base reaction. Strong Acids vs Weak Acids Strong acids completely ionize (dissociate) in water, releasing all their hydrogen ions.
Weak acids only partially ionize in water, releasing only some of their hydrogen ions. Comparison of Strong and Weak Acids Property Strong Acids Weak Acids Ionization Complete (100%) Partial ( H⁺ concentration High Low pH (same concentration) Lower pH (more acidic) Higher pH (less acidic) Conductivity High (more ions) Low (fewer ions) Rate of reaction Faster with metals/carbonates Slower with metals/carbonates Equation arrow → (complete) ⇌ (reversible) Examples of Strong Acids Hydrochloric acid (HCl) - used in stomach acid Sulfuric acid (H₂SO₄) - used in car batteries Nitric acid (HNO₃) - used in fertilizers HCl(aq) → H⁺(aq) + Cl⁻(aq) (complete ionization) Examples of Weak Acids Ethanoic acid (CH₃COOH) - found in vinegar Carbonic acid (H₂CO₃) - found in fizzy drinks Citric acid (C₆H₈O₇) - found in citrus fruits CH₃COOH(aq) ⇌ H⁺(aq) + CH₃COO⁻(aq) (partial ionization) Important!
The reversible arrow (⇌) shows that weak acids establish an equilibrium - most molecules remain intact, while only a small percentage release H⁺ ions. Strong Bases vs Weak Bases Strong bases completely ionize in water, releasing all available hydroxide ions (OH⁻).
Weak bases only partially ionize in water, releasing only some hydroxide ions. Examples of Strong Bases Sodium hydroxide (NaOH) Potassium hydroxide (KOH) Calcium hydroxide (Ca(OH)₂) NaOH(aq) → Na⁺(aq) + OH⁻(aq) (complete ionization) Examples of Weak Bases Ammonia (NH₃) - used in cleaning products Ammonium hydroxide (NH₄OH) NH₃(aq) + H₂O(l) ⇌ NH₄⁺(aq) + OH⁻(aq) (partial ionization) Property Strong Bases Weak Bases Ionization Complete Partial OH⁻ concentration High Low pH (same concentration) Higher pH (more alkaline) Lower pH (less alkaline) Conductivity High Low Concentration vs Strength Common Mistake!
Concentration and strength are NOT the same thing! Concentration Concentration refers to the amount of acid (or base) dissolved in a given volume of solution. Measured in mol/dm³ or g/dm³ Concentrated solution = lots of acid molecules per unit volume Dilute solution = few acid molecules per unit volume Strength Strength refers to the degree of ionization (dissociation) in water.
This is a fixed property of the acid or base. Strong acid/base = complete ionization Weak acid/base = partial ionization All Possible Combinations Type Example Description Concentrated strong acid 2 mol/dm³ HCl Lots of acid, all ionized → very high [H⁺] Dilute strong acid 0.01 mol/dm³ HCl Little acid, all ionized → moderate [H⁺] Concentrated weak acid 2 mol/dm³ CH₃COOH Lots of acid, partially ionized → moderate [H⁺] Dilute weak acid 0.01 mol/dm³ CH₃COOH Little acid, partially ionized → low [H⁺] Example Comparison A dilute strong acid can have the same pH as a concentrated weak acid if they have the same [H⁺] concentration!
For example: 0.01 mol/dm³ HCl (pH ≈ 2) might have a similar pH to 1 mol/dm³ ethanoic acid (pH ≈ 2.4) Comparing pH Values Effect of Strength on pH For acids of the same concentration : Strong acids have lower pH (more H⁺ ions released) Weak acids have higher pH (fewer H⁺ ions released) Example: Both at 0.1 mol/dm³ HCl (strong) → pH ≈ 1 CH₃COOH (weak) → pH ≈ 3 Effect of Concentration on pH For the same acid : Higher concentration → lower pH (more acid present) Lower concentration → higher pH (less acid present) Example: HCl at different concentrations 1 mol/dm³ HCl → pH ≈ 0 0.1 mol/dm³ HCl → pH ≈ 1 0.01 mol/dm³ HCl → pH ≈ 2 Remember When concentration increases by 10 times, pH changes by 1 unit (for strong acids) Practical Observations Reactions with Metals When reacting with magnesium ribbon: Strong acid : vigorous reaction, rapid bubbling, metal disappears quickly Weak acid : slow reaction, gentle bubbling, metal takes longer to disappear Mg(s) + 2H⁺(aq) → Mg²⁺(aq) + H₂(g) Rate depends on [H⁺] concentration Reactions with Carbonates When reacting with calcium carbonate: Strong acid : rapid fizzing, fast CO₂ production Weak acid : slow fizzing, slow CO₂ production CaCO₃(s) + 2H⁺(aq) → Ca²⁺(aq) + H₂O(l) + CO₂(g) Electrical Conductivity When testing with a co…
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