Acids Bases and Salts

• Acid


  A chemical species that donates protons or hydrogen ions (H+) and/or accepts electrons
• Acid anhydride


  A compound that reacts with water to form an acid
• Base


  A chemical species that donates electrons or hydroxide ions (OH-) or that accepts protons
• Alkali


  A base that dissolves in water to form a solution containing OH- ions
• Salt


  A compound formed when some or all of the H+ ions in an acid are replaced by metal or NH4+ ions
• Salts are made when an acid reacts with a base, carbonate or metal. The name of the salt formed depends on the metal in the base and the acid used. E.g. salts made using hydrochloric acid are called chlorides
• Acidic Oxide


  Oxides of certain non-metals that react with alkalis to form a salt and water
• Basic Oxide


  Oxides of metals which react with acids to form a salt and water
• Amphoteric compound


  A substance that can react with both acids and with strong alkalis
• Amphoteric oxide


  Oxides of certain metals which react with both acids and strong alkalis to form a salt and water
• Neutral oxide


  Oxides of certain non-metals which do not react with acids or alkalis
• General properties of acids

  • Change blue litmus red
  • pH value of less than 7
  • Corrosive
  • Sour taste
  • Electrolytes, i.e. they conduct an electric current
• Ionisation of acids in water

  1. Acid molecules ionise to form hydrogen ions, H+, and negative anions

  2. H+ ions become attached to polar water molecules forming hydronium ions or H3O+ ions (hydroxonium ions)
• Ionisation of acids in water

  • HCl (g) + H2O (l) → H3O+ (aq) + Cl- (aq)
  • HNO3 (aq) → H+ (aq) + NO3- (aq)
  • H2SO4 (aq) → 2H+ (aq) + SO42- (aq)
  • H3PO4 (aq) → 3H+ (aq) + PO43- (aq)
  • CH3COOH (aq) ⇌ CH3COO- (aq) + H+ (aq)
• Hydrogen ion (H+)


  A single proton since it is formed by a hydrogen atom losing its one valence electron, leaving only the nucleus containing the one proton
• ALWAYS ADD ACID TO WATER! When you mix acid with water, it's extremely important to add the acid to the water rather than the other way around. Acid and water react in a vigorous exothermic reaction, releasing heat, sometimes boiling the liquid.
• pH scale


  A numeric scale with a range of 0 - 14 used to measure the strength of an aqueous acid or alkali
• pH values


  • Acidic solutions have a pH of less than 7
  • Bases have a pH of more than 7
  • Neutral solution (distilled water) has a pH of 7
• Strong acid


  Has a pH close to 1, e.g. HCl, HNO3 and H2SO4
• Weak acid


  Has a pH of about 4 or 5, e.g. CH3COOH
• Neutral substance


  Has a pH of 7, e.g. distilled water
• Weak alkali


  Has a pH of about 9 or 10, e.g. aqueous ammonia, NH4OH
• Strong alkali


  Has a pH close to 14, e.g. NaOH and KOH
• There are two main methods used in the lab to measure the pH of a solution: Universal Indicator and pH meters
• Common indicators and their colours in acidic and alkaline solutions

  • Litmus: Red in acids, Blue in alkalis (pH 5-8)
  • Methyl orange: Red in acids, Yellow in alkalis (pH 3-5)
  • Screened methyl orange: Red in acids, Green in alkalis (pH 3-5)
  • Phenolphthalein: Colourless in acids, Pink in alkalis (pH 8.2-10)
  • Bromothymol blue: Yellow in acids, Blue in alkalis (pH 6.0-7.6)
• Chemical Reactions of Acids

  1. Acids react with reactive metals to produce a salt and hydrogen

  2. Acids react with metal carbonates and hydrogencarbonates to produce a salt, carbon dioxide and water

  3. Acids react with bases (neutralisation reactions) to produce a salt and water
• When reactive metals react with acids, effervescence (bubbling) is seen as hydrogen gas is evolved and the reactions become hotter because they produce heat, i.e. the reactions are described as being exothermic
• When nitric acid reacts with metals, it produces oxides of nitrogen, e.g. nitrogen dioxide (NO2) and not hydrogen
• Acids react with metal carbonates and hydrogencarbonates


  1. Metal carbonate + acid → salt + carbon dioxide + water

  2. Metal hydrogencarbonate + acid → salt + carbon dioxide + water
• Acids react with bases (neutralisation reactions)

  1. Metal oxide + acid → salt + water

  2. Metal hydroxide + acid → salt + water
• When bases react with acids, the reactions become warmer because they are exothermic reactions
• Classification of Acids


  • Inorganic and Organic Acids
  • Monobasic, dibasic and tribasic acids
  • Dilute and concentrated acids
  • Strong and weak acids
• Inorganic acid


  Contains a non-metallic element or polyatomic group as well as hydrogen, e.g. HCl, HNO3, H2SO4, H3PO4, HNO2, H2SO3, H2CO3
• Organic acid


  Most contain the carboxyl group, COOH, e.g. HCOOH (methanoic acid), CH3COOH (ethanoic acid), CH3CH(OH)COOH (lactic acid), C6H8O6 (ascorbic acid), C6H8O7 (citric acid)
• Monobasic acid


  Produces one H+ ion per molecule when it dissolves in water, e.g. HCl, HNO3, CH3COOH
• Dibasic acid


  Produces two H+ ions per molecule when it dissolves in water, e.g. H2SO4
• Tribasic acid


  Produces three H+ ions per molecule when it dissolves in water, e.g. H3PO4
• Dilute acid


  Contains a lot of water, e.g. a solution of HCl with a concentration of 0.1 mol dm-3
• Concentrated acid


  Contains very little water, e.g. an HCl solution with a concentration of 12 mol dm-3
• Strong acid


  Fully ionised when it dissolves in water