Acid-Base Theory

Acid-base theory pervades the 0620 syllabus: the definitions appear directly in Paper 2 MCQs, the reactions underpin salt preparation, and the strong/weak distinction is a reliable 3-mark Supplement question. Getting the vocabulary precise is worth disproportionate marks.

Core definitions

Acid: A substance that produces hydrogen ions (H+) in aqueous solution.

Base: A substance that neutralises an acid, producing a salt and water. Bases are metal oxides and metal hydroxides.

Alkali: A base that is soluble in water, producing hydroxide ions (OH-) in solution.

The relationship: all alkalis are bases, but not all bases are alkalis. Copper(II) oxide is a base (neutralises acid) but not an alkali (insoluble in water). Sodium hydroxide is both a base and an alkali.

The three characteristic reactions of acids

Every acid reacts in these three patterns. Learn the general equations:

Acid +	Products	Example
Metal (above H in reactivity series)	Salt + hydrogen	Zn + H2SO4 -> ZnSO4 + H2
Base (metal oxide or hydroxide)	Salt + water	CuO + 2HCl -> CuCl2 + H2O
Carbonate	Salt + water + CO2	Na2CO3 + 2HCl -> 2NaCl + H2O + CO2

The salt formed depends on the acid used:

• Hydrochloric acid (HCl) produces chlorides
• Sulfuric acid (H2SO4) produces sulfates
• Nitric acid (HNO3) produces nitrates

This pattern is central to preparation of salts.

The pH scale

The pH scale runs from 0 to 14:

pH	Type	Example
0-3	Strong acid	HCl, H2SO4
4-6	Weak acid	CH3COOH, H2CO3
7	Neutral	Water, NaCl solution
8-10	Weak alkali	NH3 solution
11-14	Strong alkali	NaOH, KOH

pH measures the concentration of H+ ions. Lower pH = more H+ ions = more acidic.

Neutralisation

The ionic equation for neutralisation:

H+(aq) + OH-(aq) -> H2O(l)

This is one of the most important equations in the syllabus. Every acid-base neutralisation reduces to this ionic equation. Neutralisation is exothermic (releases heat).

Strong vs weak acids (Supplement)

This distinction is separate from concentration:

Strong acid: Completely dissociates (ionises) in water. Every molecule breaks into ions.

• HCl -> H+ + Cl- (100% ionised)
• H2SO4, HNO3 are also strong acids

Weak acid: Partially dissociates in water. Only a fraction of molecules ionise at any time. The reaction is reversible.

• CH3COOH ⇌ CH3COO- + H+ (partial ionisation)
• H2CO3 (carbonic acid) is also weak

Evidence that an acid is weak:

• Higher pH than a strong acid of the same concentration
• Slower reaction rate with metals (fewer H+ ions available)
• Lower electrical conductivity (fewer ions in solution)

Strong vs concentrated: the critical distinction

Term	Meaning	Example
Strong	Fully dissociated into ions	0.01 mol/dm3 HCl (dilute but strong)
Weak	Partially dissociated	10 mol/dm3 CH3COOH (concentrated but weak)
Concentrated	High amount of solute per volume	10 mol/dm3 HCl
Dilute	Low amount of solute per volume	0.01 mol/dm3 HCl

Strength is about the degree of ionisation. Concentration is about how much solute is dissolved. These are independent properties.

Proton transfer theory (Supplement)

The Extended definitions redefine acids and bases in terms of proton (H+) transfer:

• Acid = proton donor (gives away H+)
• Base = proton acceptor (receives H+)

Identifying donors and acceptors

In HCl + NH3 -> NH4Cl:

• HCl donates H+ to NH3, so HCl is the acid
• NH3 accepts H+ from HCl, so NH3 is the base

In HCl + H2O -> H3O+ + Cl-:

• HCl donates H+ to H2O, so HCl is the acid
• H2O accepts H+, so H2O is the base

The exam question format is: “Identify the acid and the base in the following reaction. Explain your answer.” You must name the species AND state whether it donates or accepts a proton.

Oxides and acid-base behaviour

Oxides are classified by their acid-base behaviour, covered in oxides:

Oxide type	Behaviour	Examples
Basic oxide	Reacts with acids, not alkalis	CuO, MgO, Fe2O3
Acidic oxide	Reacts with alkalis, not acids	CO2, SO2, SO3
Amphoteric oxide	Reacts with both acids and alkalis	Al2O3, ZnO, PbO
Neutral oxide	Reacts with neither	H2O, CO

The pattern: metal oxides are basic, non-metal oxides are acidic. Aluminium and zinc oxides are amphoteric (Supplement).

Common exam mistakes

1. Confusing strong with concentrated: “Concentrated sulfuric acid is a strong acid because there is a lot of it” — wrong reasoning. It is strong because it is fully dissociated.
2. Saying all bases dissolve in water: Only alkalis dissolve. Copper oxide is a base but insoluble.
3. Forgetting the proton transfer definition is Supplement only: Core students only need the H+/OH- definitions.
4. Writing “acids have a low pH”: More precise to say “acids have pH below 7.”

Worked exam questions

Ethanoic acid is a weak acid. Hydrochloric acid is a strong acid. Both have a concentration of 0.1 mol/dm3. Compare the pH values and explain the difference. [3 marks]

• Ethanoic acid has a higher pH than hydrochloric acid [1]
• HCl is a strong acid, fully dissociated, so it produces more H+ ions in solution [1]
• CH3COOH is a weak acid, only partially dissociated, so it produces fewer H+ ions in solution [1]

In the reaction NH3 + HNO3 -> NH4NO3, identify the acid and the base. Explain your answer in terms of proton transfer. [4 marks]

• HNO3 is the acid [1] because it donates a proton (H+) to the ammonia molecule [1]
• NH3 is the base [1] because it accepts a proton (H+) from the nitric acid [1]