Acids (OCR AS Chemistry A): Revision Note

Acids, bases & dissociation

• These are common acids and bases that appear frequently in OCR exam questions

  • Their behaviour in solution depends on whether they are strong or weak, as shown:

Strong acids dissociating 

• Strong acids fully dissociate in solution 

• For example, when hydrogen chloride (HCl) dissolves in water

  • 100% of the substance dissociates into ions:

HCl (aq) → H⁺ (aq) + Cl^- (aq)

• The dissociation of strong acids releases H⁺ (aq) 

Strong bases dissociating

• Strong bases fully dissociate in solution

• For example, when sodium hydroxide (NaOH) dissolves in water

  • 100% of the substance dissociates into ions:

NaOH (aq) → Na⁺ (aq) + OH^- (aq)

• The dissociation of strong bases releases OH^- (aq) 

Weak acids dissociating

• Unlike strong acids, weak acids do not fully ionise in water

  • Weak acids partially dissociate in solution

  • This means that only a small percentage of the products will be ions

• 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

CH₃COOH (aq) ⇌ CH₃COO^- (aq) + H⁺ (aq)

• The dissociation of weak acids releases some H⁺ (aq) 

Weak bases dissociating

• Unlike strong bases, weak bases do not fully ionise in water

  • Weak bases partially dissociate in solution

  • Only a small percentage of the products will be ions

• An equilibrium is established containing reactants and products

• For example, ethylamine (CH₃CH₂NH₂) is a weak base and will partially dissociate in solution and produce hydroxide ions:

CH₃CH₂NH₂ (aq) + H₂O (l) ⇌ CH₃CH₂NH₃⁺ (aq) + OH^- (aq) 

• The dissociation of weak bases releases some OH^- (aq) 

Neutralisation

What happens during neutralisation?

• A neutralisation reaction is one in which an acid (pH <7) and a base/alkali (pH >7) react together to form water (pH = 7) and a salt:

acid + base (alkali) → salt + water

• Specifically, neutralisation is the reaction between H⁺ ions from the acid and OH^- ions from the base or alkali to form water:

H⁺ (aq) + OH^- (aq) → H₂O (l)

• The remaining spectator ions form the salt

Example: HCl neutralisation by NaOH

• The overall equation for this reaction is:

Acid + base → salt + water

HCl + NaOH → NaCl + H₂O

• The two individual reactions taking place are:

H⁺ + OH^- → H₂O (neutralisation)

Na⁺ + Cl^- → NaCl (salt formation)

Naming salts

• The name of the salt produced can be predicted from the acid that has reacted:

Acid reactions with metals

• When a metal reacts with an acid, it produces a salt and hydrogen gas:

metal + acid → salt + hydrogen

• For example:

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

zinc + hydrochloric acid →  zinc chloride + hydrogen

• The extent of this reaction depends on:

  • The reactivity of the metal

  • The strength of the acid

• Very reactive metals can react dangerously with acids

  • So, these reactions are often avoided in the lab

• Unreactive metals, like copper, do not react with dilute acids

• Stronger acids react more vigorously with metals than weak acids

• When comparing strong and weak acids with the same metal, the stronger acid will:

  • Produce bubbles of hydrogen gas more rapidly

  • Cause the metal to dissolve faster

  • Release more heat during the reaction

Acid reactions with metal oxides

• Acids react with metal oxides to form a salt and water:

metal oxide + acid →  salt + water

• For example:

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

  calcium oxide + hydrochloric acid →  calcium chloride + water

Acid reactions with metal hydroxides

• Metal hydroxides react with acids in the same way as metal oxides, producing a salt and water:

metal hydroxide + acid →  salt + water

• For example

 Mg(OH)₂ (s) + H₂SO₄ (aq) → MgSO₄ (aq)  + 2H₂O (l)

 magnesium hydroxide + sulfuric acid →  magnesium sulfate + water

Acid reactions with metal carbonates

• Metal carbonates react with acids to produce a salt, water, and carbon dioxide gas:

metal carbonate + acid → salt + water + carbon dioxide

• For example:

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

copper carbonate + nitric acid → copper nitrate + water + carbon dioxide