Acids (OCR AS Chemistry)

Strong acids dissociating 

• Strong acids will fully dissociate in solution 
• For example, when hydrogen chloride (HCl) is dissolved in water 100% of the product will be ions. 
  • HCl (aq) → H⁺ (aq) + Cl^- (aq)
  • In this case hydrogen ions are released, H⁺ (aq) 
  • The same applies with strong bases

Strong bases dissociating

• Strong bases will fully dissociate in solution
  • NaOH (aq) → Na⁺ (aq) + OH^- (aq)
  • In this case hydroxide ions are released, OH^- (aq) 

Weak acids dissociating

• Weak acids only partially dissociate in solution, 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 same applies with weak bases

Weak bases dissociating

• Weak bases only 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) 

• 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

• The proton of the acid reacts with the hydroxide of the base to form water

• The spectator ions which are not involved in the formation of water, form the salt

The diagram shows a neutralisation reaction of HCl and NaOH and the two individual reactions that take place to form the water and salt

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

Acid Reacted & Salt Table

Metals and acids

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

acid + metal  →  salt + hydrogen

• For example:

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

hydrochloric acid + zinc →  zinc chloride + hydrogen

• Clearly, the extent of reaction depends on the reactivity of the metal and the strength of the acid
• Very reactive metals would react dangerously with acids and these reactions are not usually carried out
• Metals low in reactivity do not react at all, for instance copper does not react with dilute acids
• Stronger acids will react more vigorously with metals than weak acids. What signs of reaction would be expected to be different between the two?
  • Faster reaction, seen as
    • more effervescence
    • the metal dissolves faster
    • more exothermic

Metals and 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)

  hydrochloric acid + calcium oxide →  calcium chloride + water

Metals and hydroxides

• The reaction with a metal hydroxide and an acid follows the same pattern as an oxide:

acid + metal hydroxide →  salt + water

• For example

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

 sulfuric acid  + magnesium hydroxide →  magnesium sulfate + water

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)

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