14. introduction of acids

• Nitric acid is stored in brown bottles because it decomposes under sunlight
• Common acids in the laboratory

  • hydrochloric acid (HCl)
  • sulphuric acid (H2SO4)
  • nitric acid (HNO3)
• These three acids are often called mineral acids as they were derived from minerals in the past
• Dilute acids in the laboratory are usually in the form of dilute aqueous solutions
• Dilute acids


  • They exhibit some common characteristics
  • They have typical chemical reactions
• NEVER taste any acid in the laboratory
• Electrical conductivity of acids

  Acids are electrolytes, they conduct electricity when they dissolve in water to form an aqueous solution which contains mobile ions
• Reaction of dilute acids with metals

  1. Dilute acids react with metals which are higher than copper in the metal reactivity series to give salt and hydrogen

  2. Less reactive metals like copper and silver do not react with dilute acids
• Reaction of dilute acids with metals

  • Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g)
  • Fe(s) + 2HCl(aq) → FeCl2(aq) + H2(g)
  • Zn(s) + 2HCl(aq) → ZnCl2(aq) + H2(g)
• Magnesium reacts rapidly with dilute hydrochloric acid, while iron reacts slowly
• Reaction of dilute hydrochloric acid with calcium hydroxide

  CaOH(s) + 2HCl(aq) → CaCl2(aq) + 2H2O(l)
• In ionic equations, insoluble ionic compounds should not be written as separate ions
• Reaction of dilute sulphuric acid with copper(II) oxide

  CuO(s) + H2SO4(aq) → CuSO4(aq) + H2O(l)
• Neutralization reaction


  Reaction of dilute acids with metal hydroxides and metal oxides to form a salt and water
• Reaction of dilute acids with carbonates and hydrogencarbonates

  1. Carbonate + acid → salt + carbon dioxide + water

  2. Hydrogencarbonate + acid → salt + carbon dioxide + water
• Effervescence occurs when dilute acids react with carbonates or hydrogencarbonates due to the formation of carbon dioxide gas bubbles
• Carbon dioxide turns colourless limewater milky
• Reaction between ethanoic acid (vinegar) and a metal hydrogencarbonate

  Colourless gas bubbles form, sodium hydrogencarbonate powder dissolves
• Reaction between dilute sulphuric acid and calcium carbonate

  Insoluble calcium sulphate forms, coating the calcium carbonate and preventing further reaction, effervescence stops
• If dilute nitric acid is used instead of dilute sulphuric acid

  Colourless gas bubbles form, calcium carbonate dissolves completely
• Water must be present for acids to exhibit their acidic properties
• Hydrogen chloride exhibits different properties in water and in an organic solvent
• Reaction of sodium hydrogencarbonate with concentrated sulphuric acid

  Sodium hydrogencarbonate reacts with concentrated sulphuric acid to give hydrogen chloride gas
• Using concentrated sulphuric acid poses a potential hazard
• Tap funnel


  Used for adding acid to sodium hydrogencarbonate to prevent any splashing
• Hydrogen chloride is very soluble in water
• Hydrogen chloride does not exhibit acidic properties in the organic solvent</b>
• Zinc metal added to solution of hydrogen chloride in water

  Colourless gas bubbles formed
• Zinc metal does not react with solution of hydrogen chloride in organic solvent
• Dry blue litmus paper added to solution of hydrogen chloride in water


  Turned red
• Dry blue litmus paper remained blue when added to solution of hydrogen chloride in organic solvent
• Adding zinc metal to Solution P produced colourless gas bubbles
• Adding dry blue litmus paper to Solution P turned it red
• Adding zinc metal to Solution Q produced no observable change
• Adding dry blue litmus paper to Solution Q remained black
• Water is required for acids to exhibit their acidic properties
• Ionization of citric acid in water produces hydrogen ions which are responsible for the acidic properties of the solution
• Hydrogen chloride gas dissolves in water to form hydrogen ions and chloride ions
• Pure sulphuric acid and nitric acid do not exhibit acidic properties until they dissolve in water and form hydrogen ions
• Acid


  A molecular compound that forms hydrogen ions H(aq) as the only cation when dissolved in water