Topic 5

• Transition metals


  • High melting point
  • High density
  • Form coloured compounds
  • Can be used as catalysts
• Corrosion of metals


  Oxygen reacts with the metal to form a metal oxide
• Corrosion of a metal

  Is a redox reaction - the metal loses electrons so is oxidised, oxygen gains electrons so is reduced
• Rusting


  Corrosion of iron when it reacts with oxygen and water in the air
• Substances that need to be excluded to prevent rusting

  • Oxygen (O2)
  • Water (H2O)
• Preventing iron rusting


  1. Paint the metal

  2. Coat the metal in oil/grease

  3. Cover the metal in plastic

  4. Keep the metal in a vacuum container
• Sacrificial protection


  The metal being protected is galvanized with a more reactive metal, which will corrode first and prevent the corrosion of the inner metal
• Electroplating


  A process in which a metal is coated with a layer of another metal
• Reasons for using electroplating

  • To make a metal more resistant to corrosion
  • To improve the appearance of a metal (e.g. silver plated cutlery)
• Process of electroplating


  The metal being coated is the cathode, the metal that will form the exterior layer is the anode, the electrolyte solution must contain ions of the metal which will form the outer coating, a power supply is connected to both electrodes
• Malleability of pure metals


  The atoms lie in uniform rows which are able to slide over one another
• Alloy


  A mixture of two or more metals or a metal and another element
• How alloying increases strength


  Alloys contain several metals with different sized atoms, which distorts the regular arrangement of the atoms so the layers are unable to slide over one another very easily
• Reasons for alloying iron to produce alloy steels

  • Iron is relatively brittle, so it is combined with other elements such as carbon to produce a material with more desirable qualities - low carbon steel is malleable, high carbon steel is hard, stainless steel is corrosion resistant
• Property of copper that makes it suitable for electrical cables

  Very good electrical conductor
• Property of aluminium that makes it suitable for aircraft

  Low density
• Property of gold that makes it suitable for jewellery


  Very unreactive so resists corrosion, jewellery appearance will not be affected over time
• Magnalium


  An alloy of magnesium and aluminium, lighter and stronger than aluminium and more resistant to corrosion, used for cars and aircraft
• Brass


  An alloy of copper and zinc, hard and resistant to corrosion, used for decorative hardware such as plumbing fittings
• Calculating concentration of a solution in mol dm-3


  Concentration (mol dm-3) = moles ÷ volume (dm3)
• Converting concentration in g dm-3 to mol dm-3

  Concentration(mol dm-3) = concentration(g dm-3) ÷ Mr (molecular mass)
• Carrying out an acid-alkali titration to find the concentration of alkali


  Rinse the pipette with the solution of unknown concentration (alkali), use this to add a known volume of alkali to a conical flask

  2. Add a few drops of indicator

  3. Rinse and fill the burette with acid

  4. Gradually add the acid to the conical flask

  5. Record the volume in the burette when the indicator changes colour

  6. Repeat the experiment until you have concordant results

  7. Use results to calculate a mean titre and the concentration of alkali
• Calculating concentration of acid given alkali concentration and volume

  Calculate the number of moles of the alkali using the known volume and concentration (moles = concentration x volume)

  2. Use the chemical equation to work out the ratio of acid and alkali that react and hence work out how many mole of acid have reacted

  3. Divide the moles of acid by the volume used in neutralisation to find concentration
• If you know the volume of acid required to neutralise an alkali

  How could you calculate the concentration of the acid, given the alkali concentration and volume?
• Theoretical yield


  The amount of product that would be collected under perfect reaction conditions
• How to calculate percentage yield

  Percentage yield = (Actual yield ÷ Theoretical yield) x 100
• What is the percentage yield of NH3 if 40.5 g of NH3 is produced from 20.0 mol H2 and excess N2?
• Why could the actual yield of product be less than expected?

  • Incomplete reaction
  • Competing, unwanted side reactions
  • Practical losses, for example some solid may get lost when being transferred between beakers
• Atom economy


  A measure of the efficiency of the reaction. It looks at the amount of reactants that get turned into useful products
• How to calculate atom economy

  Atom economy = (Mr of desired product ÷ Mr of reactants) x 100
• The _____ the atom economy, the more sustainable and efficient the process
• Why might one reaction pathway be chosen over another?

  • Higher atom economy
  • Higher yield
  • Faster rate
  • Equilibrium position favours products more
  • By-products are more useful / less harmful
• Avogadro's Law


  At the same temperature and pressure, equal amounts of gas will occupy the same volume
• Molar volume of a gas

  The volume occupied by one mole of molecules of any gas at room temperature and pressure
• RTP


  Room temperature and pressure: 20oC, 1 atmosphere
• How to calculate the molar volume of a gas


  Molar volume = volume (dm3) ÷ moles of gas, Units: dm3 mol-1
• Haber process


  A reversible reaction that forms ammonia from nitrogen and hydrogen
• Steps in the Haber process

  1. Reactants are obtained (hydrogen from natural gas, nitrogen from air)

  2. The gases are compressed to 200 atm and heated to 450oC before being pumped into a tank containing layers of catalytic iron beads

  3. Ammonia forms

  4. Ammonia and unreacted hydrogen and nitrogen pass into a cooling tank where the ammonia is collected as a liquid

  5. The unreacted hydrogen and nitrogen are recycled back into the tank
• Dynamic equilibrium


  The rate of the forward reaction equals the rate of the backwards reaction. The concentration of reactants and products are constant
• Chemical equation for the formation of ammonia from hydrogen and nitrogen

  N2 + 3H2 ⇌ 2NH3