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In OCR A-Level Chemistry the acid-base model you must know is the Bronsted-Lowry model:
When an acid dissolves in water it donates a proton to a water molecule, forming the oxonium ion (also called hydronium), H3O+:
HCl(aq) + H2O(l) -> H3O+(aq) + Cl-(aq)
For convenience we usually abbreviate H3O+(aq) as H+(aq), but remember a "bare" proton cannot exist in aqueous solution; it is always hydrated.
You must know these acids by name, formula and whether they are strong or weak:
| Acid | Formula | Basicity | Type |
|---|---|---|---|
| Hydrochloric acid | HCl | Monoprotic | Strong |
| Sulfuric acid | H2SO4 | Diprotic | Strong (first dissociation) |
| Nitric acid | HNO3 | Monoprotic | Strong |
| Phosphoric(V) acid | H3PO4 | Triprotic | Weak |
| Ethanoic acid | CH3COOH | Monoprotic | Weak |
| Carbonic acid | H2CO3 | Diprotic | Weak |
| Methanoic acid | HCOOH | Monoprotic | Weak |
Basicity refers to the number of protons per acid molecule that can be donated. H2SO4 is diprotic because it can release two protons in two separate dissociation steps:
H2SO4(aq) -> H+(aq) + HSO4-(aq) (complete) HSO4-(aq) <=> H+(aq) + SO4^2-(aq) (partial)
A base accepts protons. An alkali is simply a base that is soluble in water to release OH- ions.
| Base / Alkali | Formula | Strong or Weak |
|---|---|---|
| Sodium hydroxide | NaOH | Strong alkali |
| Potassium hydroxide | KOH | Strong alkali |
| Calcium hydroxide | Ca(OH)2 | Strong (but sparingly soluble) |
| Ammonia | NH3 | Weak alkali (NH3 + H2O <=> NH4+ + OH-) |
| Copper(II) oxide | CuO | Insoluble base |
| Calcium carbonate | CaCO3 | Insoluble base |
Ammonia is a weak base because only a small proportion of NH3 molecules react with water to form OH- ions.
A strong acid is one that fully dissociates in aqueous solution. A weak acid only partially dissociates.
HCl(aq) -> H+(aq) + Cl-(aq) (strong; single arrow) CH3COOH(aq) <=> H+(aq) + CH3COO-(aq) (weak; equilibrium)
Crucially, "strong" and "concentrated" do not mean the same thing:
You can have a dilute strong acid (0.01 mol dm-3 HCl) and a concentrated weak acid (glacial ethanoic acid, ~17 mol dm-3). For this reason glacial ethanoic acid does not conduct electricity nearly as well as dilute hydrochloric acid even though it contains far more acid molecules.
Neutralisation is the reaction of an acid with a base to form a salt and water (and sometimes an additional product). You need to know four types:
Mg(s) + 2HCl(aq) -> MgCl2(aq) + H2(g) Zn(s) + H2SO4(aq) -> ZnSO4(aq) + H2(g)
Only metals above hydrogen in the reactivity series react in this way. Lead reacts extremely slowly because lead(II) chloride and lead(II) sulfate are insoluble and coat the surface.
CuO(s) + 2HCl(aq) -> CuCl2(aq) + H2O(l) MgO(s) + H2SO4(aq) -> MgSO4(aq) + H2O(l)
NaOH(aq) + HCl(aq) -> NaCl(aq) + H2O(l) 2KOH(aq) + H2SO4(aq) -> K2SO4(aq) + 2H2O(l)
The ionic equation for all strong acid + strong alkali neutralisations is simply:
H+(aq) + OH-(aq) -> H2O(l)
CaCO3(s) + 2HCl(aq) -> CaCl2(aq) + H2O(l) + CO2(g) Na2CO3(aq) + H2SO4(aq) -> Na2SO4(aq) + H2O(l) + CO2(g)
Effervescence (fizzing) and a "pop" or "glowing splint" test for CO2 confirm carbonate identification.
In the neutralisation equation NaOH + HCl -> NaCl + H2O, the Na+ and Cl- are spectator ions - they appear unchanged on both sides of the ionic equation. The true chemistry is just H+ + OH- -> H2O.
Naming the salt depends on the acid:
| Acid | Anion | Salt Name Ending |
|---|---|---|
| HCl | Cl- | chloride |
| HNO3 | NO3- | nitrate |
| H2SO4 | SO4^2- | sulfate |
| H3PO4 | PO4^3- | phosphate |
| CH3COOH | CH3COO- | ethanoate |
Q: Write a balanced symbol equation and an ionic equation for the reaction of sodium carbonate solution with dilute nitric acid.
Symbol equation: Na2CO3(aq) + 2HNO3(aq) -> 2NaNO3(aq) + H2O(l) + CO2(g)
Ionic equation: Na+ and NO3- are spectators. The carbonate ion is the active base: CO3^2-(aq) + 2H+(aq) -> H2O(l) + CO2(g)
In the Bronsted-Lowry model, every acid-base reaction features two conjugate acid-base pairs. When an acid donates a proton, the remaining species is the conjugate base. When a base accepts a proton, the resulting species is its conjugate acid.
HCl + H2O -> H3O+ + Cl- acid base conj.acid conj.base
Two pairs:
In the reverse direction, H3O+ would donate a proton back to Cl-, reforming HCl and H2O. For strong acids in water, however, this reverse reaction is negligible and we write a single arrow. For weak acids, the equilibrium lies further to the left and we use the equilibrium arrow.
Some species can behave as either an acid or a base, depending on the reaction partner. Water itself is the classic example:
Aluminium oxide, Al2O3, and zinc hydroxide, Zn(OH)2, are both amphoteric: they react with both acids and alkalis:
Al2O3 + 6HCl -> 2AlCl3 + 3H2O (acid behaviour of Al2O3 as a base) Al2O3 + 2NaOH + 3H2O -> 2NaAl(OH)4 (base behaviour of Al2O3 as an acid)
This is a useful diagnostic for identifying amphoteric oxides in inorganic chemistry.
Full quantitative pH calculations belong to Year 13 (OCR Module 5.1.3), but you should already know that:
A change of 1 pH unit corresponds to a tenfold change in H+ concentration. A dilute (0.1 mol dm-3) strong acid has pH 1; diluting by a factor of 10 gives pH 2.
Acids are proton donors; bases are proton acceptors. You must know common laboratory acids (HCl, H2SO4, HNO3 are strong; CH3COOH, H3PO4 are weak), common bases (NaOH, KOH strong; NH3 weak), conjugate pairs, amphoteric species and the four types of neutralisation reaction. The next lesson applies this to quantitative titrations.