chemical changes

• Metal oxides


  Metals + oxygen -> metal oxides
• Oxidation


  Gain of oxygen
• Reduction


  Loss of oxygen
• Reactivity series


  • When metals react with other substances, metal atoms form positive ions
  • Reactivity of a metal is related to its tendency to form positive ions
  • Metals can be arranged in order of their reactivity in a reactivity series
• Metals in order of reactivity

  • Potassium
  • Sodium
  • Lithium
  • Calcium
  • Magnesium
  • Zinc
  • Iron
  • Copper
• Reactions of metals with water

  • Potassium: violent
  • Sodium: very quick
  • Lithium: quick
  • Calcium: more slow
• Reactions of metals with dilute acid

  • Calcium: very quick
  • Magnesium: quick
  • Zinc: fairly slow
  • Iron: more slow
  • Copper: very slow
• Non-metals hydrogen and carbon are often included in the reactivity series
• Displacement


  A more reactive metal can displace a less reactive metal from a compound
• Gold is very unreactive and is found in the Earth as the metal itself
• Most metals are found as compounds that require chemical reactions to extract the metal
• Reduction


  Involves the loss of oxygen
• Oxidation


  Loss of electrons
• Reduction


  Gain of electrons
• Writing ionic equations


  1. If sodium is oxidised, it has lost an electron, leaving it with a +1 charge, so the ionic equation is: Na -> Na+ + e-

  2. If sodium +1 ion is reduced, it has gained an electron, leaving it with a charge of zero, so the ionic equation is: Na+ + e- -> Na

  3. The charges on each side of the equation should add up to the same number
• Identifying oxidation and reduction in an equation

  1. e.g. 2Na + 2HCl -> 2NaCl + H2

  2. HCl is made up of H+ and Cl- ions & NaCl is made up of Na+ and Cl- ions

  3. Looking at just sodium: 2Na -> 2Na+, so the ionic equation must be: 2Na -> 2Na+ + 2e-, meaning sodium has lost electrons & has been oxidised

  4. Looking at just chlorine: 2Cl- -> 2Cl-, meaning chlorine has not been oxidised or reduced

  5. Looking at just hydrogen: 2H+ -> H2, so the ionic equation must be: 2H+ + 2e- -> H2, meaning hydrogen has gained electrons so has been reduced
• Reactions of acids with metals

  1. Acid + metal -> salt + hydrogen

  2. These are redox reactions - one substance is reduced and another is oxidised

  3. Identify which substances are oxidised and reduced by looking at electrons gained and lost (following OIL RIG)
• Redox reaction


  A reaction where one substance is reduced and another is oxidised
• Neutralisation of acids and salt production

  1. Acid + alkali -> salt + water

  2. Acid + base -> salt + water

  3. Acid + metal carbonate -> salt + water + carbon dioxide
• Salt produced in alkali and base reactions

  • Depends on the acid used:
  • Hydrochloric acid (HCl) produces chlorides (XCl)
  • Nitric acid (HNO3) produces nitrates (XNO3)
  • Sulfuric acid (H2SO4) produces sulfates (XSO4)
  • Also depends on the positive ions in the base, alkali or carbonate (the metal X)
• The charges on the positive ion from the base/alkali/carbonate and the negative ion from the acid must add up to zero
• Making soluble salts


  1. Add the chosen solid insoluble substance to the acid, the solid will dissolve

  2. Keep adding until excess solid sinks to the bottom, indicating the acid has been neutralised

  3. Filter out excess solid, evaporate some water, then leave the rest to evaporate slowly (crystallisation)
• pH scale


  • Measures the acidity or alkalinity of a solution
  • pH 7 is neutral
  • pH < 7 is acidic
  • pH > 7 is alkaline
• Neutralisation reaction


  H+(aq) + OH-(aq) -> H2O(l)
• Titration


  1. Wash burette with dilute HCl and water

  2. Fill burette to 100cm3 with acid

  3. Use 25cm3 pipette to add 25cm3 of alkali to conical flask

  4. Add indicator to conical flask

  5. Add acid from burette to alkali until end-point is reached (as shown by indicator)

  6. The titre is the difference between the first and second burette readings

  7. Repeat to gain more precise results
• Titration calculations


  • 1dm3 = 1000cm3
  • One mole of a substance in grams is the same as its relative atomic mass in grams
• Example titration calculation: 25cm3 of dilute HCl is neutralised by 20cm3 of 0.5 mol/dm3 NaOH. What is the concentration of the HCl?
• Strong acid


  Completely ionised in aqueous solution (e.g. HCl, HNO3, H2SO4)
• Weak acid


  Partially ionised in aqueous solution (e.g. ethanoic, citric, carbonic acids)
• Strength of acid


  The stronger the acid, the lower the pH (for a given concentration of aqueous solutions)
• As the pH decreases by one unit, the H+ concentration of the solution increases by a factor of 10
• Strong and weak is not the same as concentrated and dilute - the latter refers to the amount of substance in a given volume, whereas the former refers to the H+ ion concentration in aqueous solutions
• Electrolysis


  The process of breaking down an ionic substance into its elements by passing an electric current through it
• The process of electrolysis

  1. Ionic substance is melted or dissolved

  2. Ions are free to move about

  3. Current is passed through the molten or solution

  4. Substance is broken down into elements
• Electrolyte


  The substance being broken down during electrolysis
• What happens during electrolysis

  1. Positively charged ions move to the negative electrode (cathode)

  2. Negatively charged ions move to the positive electrode (anode)

  3. Ions are discharged at the electrodes producing elements
• Electrolysis of molten ionic compounds

  1. Metal is produced at the cathode

  2. Non-metal is produced at the anode
• This is because the metal is the positive ions and the non-metal is the negative ions
• Extracting metals by electrolysis

  • Metals more reactive than carbon are extracted by electrolysis of molten compounds
  • Large amounts of energy are used to melt the compounds and produce the electrical current
• Extracting aluminium by electrolysis

  • Aluminium oxide is mixed with cryolite to lower the melting point
  • Carbon is used as the positive electrode (anode) but needs to be continually replaced as it reacts with oxygen and burns away