Acids and Bases

Acids

Definition:
An acid is a substance that produces hydrogen ions, H+, when dissolved in water.

Acids have pH values less than 7.

  • The more acidic the solution, the lower the pH value (the further the pH from 7). Example, acids with pH of 0 is a strong acid. Acids with pH of 6 is a weak acid.
  • The more acidic the solution, the higher the concentration of hydrogen ions present in the solution.

Strong acids VS Weak Acids:

  • A acid is an acid that is completely ionised in aqueous solution. (Eg. Sulfuric acid, Hydrochloric acid, Nitric acid)Example: HCl (aq) → H+ (aq) + Cl (aq)

Strong acid dissociation

  • A weak acid is an acid that is partially ionised in aqueous solution. (Eg. Ethanoic acid, Citric acid)Example:  CH3COOH (aq)  ⇌ CH3COO (aq) + H+ (aq)

Note: ⇌ should be used in for the ionisation of a weak acid. Symbol indicates that reaction is reversible (occur in both directions).

Weak acid dissociation

  • Concentrated acid does not determine the strength of the acid. Weak acid can be concentrated. Strong acid can be diluted.

Measuring pH of a given solution:

  • Acids turn blue litmus paper red.
  • Colour of universal indicator in weak acid: yellow/orange
  • Colour of universal indicator in strong acid: red

Common Acids and their Uses:

  • Sulfuric acid (H2SO4) – Manufacture of detergent, Manufacture of fertilizers, In car batteries
  • Nitric acid (HNO3) – Manufacture of dyes, Manufacture of fertilizers
  • Hydrochloric acid (HCl) – Clean metals and aluminium alloys(get rid of rust or scale), In leather processing
  • Phosphoric acid (H3PO4) – Give sour taste in food and beverages
  • Ethanoic acid (CH3COOH) – In vinegar to preserve food and enhance flavour, In adhesives

Alkalis

Definition:
An alkali is a substance that produces hydroxide ions, OH, when dissolved in water.

Properties:
Alkalis have pH values more than 7.

  • The more alkaline the solution, the higher the pH value (the further the pH from 7). Example, alkali with pH of 14 is a strong alkali. Alkali with pH of 8 is a weak alkali.
  • The more alkaline the solution, the higher the concentration of hydroxide ions present in the solution.

Measuring pH of a given solution:

  • Alkali turn red litmus paper blue.
  • Colour of universal indicator in weak alkali: blue
  • Colour of universal indicator in strong alkali: purple

Common Alkalis and their Uses:

  • Magnesium hydroxide (Mg(OH)2) – Used in toothpaste, used in antacids to relieve gastric pain, Manufacture of refractory bricks
  • Sodium hydroxide (NaOH) – Manufacture of soaps and detergents, Manufacture of industrial cleaning detergents
  • Calcium hydroxide (Ca(OH)2) – Used to reduce acidity in soil

Calcium hydroxide is also known as slaked lime. It is used to control the pH of soil. Most plants grow well when the soil is neutral (pH 7) or slightly acidic. Process of adding calcium hydroxide to control the pH is known as liming the soil. Other than adding slaked lime, quick lime can also be added. Quick lime is also known as calcium oxide.

  • Aqueous ammonia (NH3/NH4OH) – In window cleaning solutions, In fertilisers

Chemical Properties of Acids and Alkalis

Acid + Metal → Salt + Hydrogen gas

Acid + Carbonate →  Salt + Carbon dioxide gas + Water

Acid + Alkali →  Salt + Water (Neutralisation reaction)
Ionic Equation: H+ + OH → H2O

Alkali + Ammonium salt → Salt + Water + Ammonia gas

Alkali + salt (of metal B) → metal hydroxide + salt (of metal A)

Bases

  • A base is a substance that reacts with an acid to give salt and water only.
    Base + Acid  Salt + Water (Neutralisation Reaction)
  • A base is any metal oxide (contains oxide ions, O2-) or metal hydroxide (contains hydroxide ions,   OH).
  • Soluble base is called alkali.
    Examples of alkali: Group I hydroxides, Calcium hydroxide, Barium hydroxide
    All other bases are insoluble in water.

Types of Oxides

Non-metal oxides – Either Neutral Oxides or Acidic Oxides

Neutral oxides: Water, Nitrogen monoxide, Carbon monoxide
Acidic Oxides: All other non-metal oxides

Neutral oxides do not react with acids and alkalis.
Acidic oxides react with alkalis to form salt and water

Metal oxides – Either Amphoteric Oxides or Basic Oxides

Amphoteric oxides: Zinc Oxide, Aluminium Oxide, Lead(II) Oxide
Basic Oxides: All other metal oxides

Amphoteric oxides react with both acids and alkalis to form salt and water.
Basic oxides react with acids to form salt and water