topic c4- chemical changes

• alkalis

  • soluble bases
  • form OH- ions in water
  • acids form H+ ions in water
• pH scale

  0-6= acids

  7= neutral

  8-12= alkalis
• ways to measure pH

  • universal indicator= a wide range indicator that changes colour depending on the pH - (approximate valve)
  • pH probe- gives an accurate value of the pH
• neutralisation reaction

  acid + base -> salt + water

  H+ (aq) + OH- (aq) -> H2O (l) ------> the products are neutral
• reactions of acids

  acid + metal carbonate -> salt + water + carbon dioxide

  acid + metal oxide -> salt + water

  acid + metal hydroxide -> salt + water
• soluble salts are made by adding metals or insoluble metal compounds to acids
• strong acid

  an acid that completely ionises in water to produce hydrogen ions. e.g. nitric acid
• weak acid

  an acid that partially ionises in water to produce hydrogen ions. e.g. citric acid
• pH

  • a measure of the concentration of H+ ions in a solution
  • as the acid strength increases, pH decreases
• acid strength

  the proportion of acid molecules that ionise in water
• acid concentration

  the number of acid molecules in a certain volume of water
• metal oxides and metal hydroxides are bases- these are alkalis

  acid + metal oxide -> salt + water/ acid + metal hydroxide -> salt + water
• acids and metal carbonates produce carbon dioxide

  acid + metal carbonate -> salt + water + carbon dioxide
• reactions of acids

  e.g.

  hydrochloric acid + calcium carbonate -> calcium chloride + water + carbon dioxide
• reactivity series part 1

  K- potassium )K- potassium )

  Na- sodium )- electrolysis= using electricity to break down

  Ca- calcium ) compounds

  Mg- magnesium )

  Al- aluminium )
• reactivity series part 2

  c- carbon---- l used to extract metals from their ores

  Zn- zinc l

  Sn- tin l = H+ (aq) in all acids. all metals

  Pb- lead l above react with acids

  H- hydrogen l

  Cu- copper l

  Ag- native= silver l

  Au- found as element- gold !
• reactivity series

  • acid + metal -> salt + hydrogen
  • metal + water -> metal hydroxide + hydrogen
  • example:
  • calcium: Ca (s) +2H2O -> Ca(OH)2 (aq) + H2
• separating metals from metal oxides

  formation of metal ore:

  oxidation- gain of oxygen

  e.g. magnesium is oxidised to make magnesium oxide

  2Mg + O2 -> 2MgO
• separating metals from metal oxides

  extraction of metal:

  reduction- loss of oxygen

  e.g. copper oxide is reduced to copper

  2CuO + C -> 2Cu + CO2
• redox reactions

  -a loss of electrons is called oxidation ) when both happen at the same

  -a gain of electrons is called reduction ) time it is called REDOX

  • 2H+ + 2e- -> H2
• displacement reactions are redox reactions

  • involve one metal kicking another one out of a compound
  • it's always the metal ion that gains electrons and is reduced, the metal atom always loses electrons and is oxidised
  • Fe (s) + CuSO4 (aq) -> FeSO4 (aq) + Cu (s)
• electrolysis 


  splitting up with electricity
• electrolyte

  a liquid of solution that can conduct electricity because ions are free to move
• Electrolysis

  electrolysis

  
• electrolysis of aqueous solutions

  • inside aqueous solutions= hydrogen ions (H+) and hydroxide ions from water= H20= H+ (aq) + OH- (aq)
  • at cathode, if H+ ions and metal ions are present, hydrogen gas will be produced if the metal ions form an elemental metal that is more reactive than hydrogen. if less reactive than hydrogen, solid layer of pure metal will form instead
  • at anode, if OH- and halide ions are present, molecules of chlorine, bromine or iodine will form. if no halide ions present, OH- ions are discharged and oxygen will be formed
• testing any gaseous products to work out what was produced:

  • chlorine bleaches damp litmus paper, turning it white
  • hydrogen makes a squeaky pop with a lighted splint
  • oxygen will relight a glowing splint
  • carbon dioxide- bubbling it through limewater- if the limewater turns cloudy, carbon dioxide is present
• half equations

  it shows the movement of electrons
• half equations- oxidation:

  Cu -> Cu+ + e-

  Al -> Al3+ + 3e-
• half equations- reduction:

  Fe + e- -> F-

  S + 2e- -> S2-
• oilrig

  Oxidation

  Is

  Loss of electrons

  Reduction

  Is

  Gain of electrons
• acids

  are reducing agents
• making salts

  • salts are substances made when acids react with metals, metal hydroxides, metal oxides and metal carbonates; used in: medicines, fertilisers, toothpaste and food additives
  • salts are made up of a metal ion combined with ion left over from the acid when the H+ ions react
• making salts

  Mg + 2HCl -> MgCl2 + H2

  2NaOH + H2SO4 -> Na2SO4 + 2H2O