Acids & Bases (OCR A Level Chemistry A): Revision Note

Brønsted-Lowry Acids & Bases Theory

• A Brønsted acid is a species that can donate a proton

  • For example, hydrogen chloride (HCl) is a Brønsted acid as it can lose a proton to form a hydrogen (H⁺) and chloride (Cl^-) ion

            HCl (aq) → H⁺ (aq) + Cl^- (aq)

• A Brønsted base is a species that can accept a proton

  • For example, a hydroxide (OH^-) ion is a Brønsted base as it can accept a proton to form water

   OH^- (aq) + H⁺ (aq) → H₂O (l)

Weak acids dissociating

• In an equilibrium reaction, the products are formed at the same rate as the reactants are used

• This means that at equilibrium, both reactants and products are present in the solution

• For example, ethanoic acid (CH₃COOH) is a weak acid that partially dissociates in solution

• When equilibrium is established there are CH₃COOH, H₂O, CH₃COO^- and H₃O⁺ ions present in the solution

• The species that can donate a proton are acids and the species that can accept a proton are bases

 CH₃COOH (aq)   +  H₂O (l)    ⇌  CH₃COO^- (aq)     +   H₃O⁺ (aq)

 acid                   base            conjugate base      conjugate acid 

• The reactant CH₃COOH is linked to the product CH₃COO^- by the transfer of a proton from the acid (CH₃COOH) to the base (CH₃COO^-)

• Similarly, the H₂O molecule is linked to H₃O⁺ ion by the transfer of a proton

• These pairs are therefore called conjugate acid-base pairs

• A conjugate acid-base pair is two species that are different from each other by an H⁺ ion

  • Conjugate here means related

  • In other words, the acid and base are related to each other by one proton difference

Monobasic, dibasic & tribasic acids

• Acids can be classified by the number of bases that they can donate protons to in a reaction, which depends on how many H⁺ per molecule that they can give up in a reaction

• Acids such as HCl, HNO₃, and HCN that contain one ionisable hydrogen atom in each molecule are called monobasic (or monoprotic) acids

  • When HCl reacts with NaOH we can see that one hydrogen is replaced by a sodium atom

HCl (aq) + NaOH (aq) → NaCl (aq) + H₂O (aq) 

• Dibasic (or diprotic) acids contain two ionisable hydrogen atoms per molecule, for example H₂SO₄

  • Ionisation of such acids occurs in two steps

  • When H₂SO₄ reacts with NaOH we can see that two hydrogens are replaced by two sodium atoms

H₂SO₄ (aq) + 2NaOH (aq) → Na₂SO₄ (aq) + 2H₂O (aq) 

• Tribasic (or triprotic) acids contain three ionisable hydrogen atoms per molecule, for example H₃PO₄

  • Ionisation of such acids occurs in three steps

  • When H₃PO₄ reacts with NaOH we can see that three hydrogens are replaced by three sodium atoms

H₃PO₄ (aq) + 3NaOH (aq) → Na₃PO₄ (aq) + 3H₂O (aq) 

The Role of Hydrogen Ions in Equations

• As we have seen previously, metals, alkalis, metal oxides and metal carbonates react with acids to form salts

• We can represent the active species, H⁺ (aq), by writing ionic equations

• Once we have written all the ions in the reaction, we can then cancel them out

Acids and metals

• The typical reaction of a metal and an acid can be summarised as

acid + metal  →  salt + hydrogen

• For example:

2HCl (aq) + Zn (s)  → ZnCl₂ (aq) +   H₂ (g)

Becomes 

2H⁺ (aq) + 2Cl^- (aq) + Zn (s) → Zn²⁺ (aq) + 2Cl^- (aq) + H₂ (g)

2H⁺ (aq) + Zn (s) → Zn²⁺ (aq) + H₂ (g)

Acids and metal oxides 

• The reaction of an acid with a metal oxide forms two products:

acid + metal oxide →  salt + water

• For example:

2HCl (aq) + CaO (s) → CaCl₂ (aq) + H₂O (l)

Becomes 

2H⁺ (aq) + 2Cl^- (aq) + CaO (s) → Ca²⁺ (aq) + 2Cl^- (aq) + H₂O (l)

2H⁺ (aq) +  CaO(s) → Ca²⁺ (aq) + H₂O (l)

Metals and carbonates

• The reaction between a metal carbonate and an acid produces three products:

acid + metal carbonate → salt + water + carbon dioxide

• For example:

    2HNO₃ (aq) + CuCO₃ (s) → Cu(NO₃)₂ (aq) + H₂O (l) + CO₂ (g)

Becomes

2H⁺ (aq) + 2NO₃^- (aq) + CuCO₃ (s) → Cu²⁺ (aq) + 2NO₃^- (aq) + H₂O (l) + CO₂ (g)

2H⁺ (aq) + CuCO₃ (s) → Cu²⁺ (aq) + H₂O (l) + CO₂ (g)

• If the carbonate is soluble, e.g. Na₂CO₃ 

2H⁺ (aq) + 2NO₃^- (aq) + 2Na⁺ (aq) + CO₃^2– (aq) → 2Na⁺ (aq) + 2NO₃^- (aq) + H₂O (l) + CO₂ (g)

2H⁺ (aq) + CO₃^2– (aq) → H₂O (l) + CO₂ (g)

Acids and alkalis

acid + alkali →  salt + water

• For example:

HCl (aq) + NaOH (aq) → NaCl (aq) + H₂O (l)

Becomes 

H⁺ (aq) + Cl^- (aq) + Na⁺ (aq) + OH^- (aq) → Na⁺ (aq) + Cl^- (aq) + H₂O (l)

H⁺ (aq) + OH^- (aq) → H₂O (l)