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Reactivity and Redox (Iron chemistry (Rust can also be preventing using…

- - - - Expensive
      - These metals form an unreactive coating of their oxides on the surface of the steel, preventing exposure to water and oxygen
  - - - The iron ions in the rust would gain these electron and be reduced back to iron atoms
      - The sacrificial metal, having given away electrons, is oxidised
    - - Chromium plating — once scratched, steel beneath rusts faster than if it were not plated at all
- - - - Aluminium oxide has a melting point of >2000°c
      - Aluminium oxide is insoluble in water
    - - Cheaper and easier to carry out at this lower temp
    - - The negative electrode (KA the cathode) makes up the entire lining of the electrolysis cell
      - The anodes (positive electrodes) are large blocks of graphite dipped into the electrolyte
      - The molten aluminium, which forms on the cathode, sinks to the bottom of the cell, and can be tapped off
      - Hot oxygen gas produced at the anodes causes them to burn away as carbon dioxide
        
        Have to be replaced periodically
- - - - Reacts on standing in air to form a thin dull surface coating of magnesium oxide
      - Burns when heated with a brilliant white flame to form white magnesium oxide ash
    - - Forms a dull surface coating of iron (III) oxide if heated in a bunsen flame
      - Iron filings or wool will oxidise more vigorously
      - Iron will slowly rust if both oxygen and water are present, forming hydrated iron (III) oxide
    - - Will oxidise very slowly on standing in air or more rapidly if heated
      - Forms a black surface layer of copper (II) oxide
    - - Will burn when heated to form carbon dioxide (or carbon monoxide if oxygen supply is limited)
      - How readily this occurs depends on the form of the carbon
        
        Large surface area
        » combust readily
        
        Diamond and graphite are not readily flammable
    - - When heated in air will first melt, then ignite, burning with a blue flame
      - The product is sulfur dioxide –colourless, acidic gas
    - - (and all other hydrocarbons)
      - Burn to form carbon dioxide (or carbon monoxide in a limited oxygen supply) and water vapour
- - - - eg. Gold, platinum
      - May be found as native metals
      - Lack of reactivity
        » do not combine with other elements
        » occur naturally as the metal
      - They usually still have to be extracted from their ores
        
        Gold still must be removed from the rock, but not chemically broken down from a compound
    - - Below carbon in the reactivity series
      - Can be extracted cheaply and easily by heating ores with carbon to bring about a displacement reaction
      - Eg. copper, iron, lead
      - Not found native—combine with oxygen or moisture in the air to form compounds
    - - Extracted from ores by electrolysis
      - Eg. aluminium
      - Could be extracted by displacement using a more reactive metal, but this in turn would have to be extracted by electrolysis
  - - - Iron ore
        
        Often haematite (Fe2O3)
      - Coke
        
        Mainly consists of carbon
        
        Made by purifying coal
      - Limestone
        
        Calcium carbonate (CaCO3)
    - - Coke burns in this air to form carbon dioxide
        
        (s) + (g) –––> (g)
        
        Reaction is very exothermic—provides the high temperatures needed for the furnace to function
        
        C + O2 –––> CO2
        
        Higher up in the furnace the carbon dioxide reacts with more coke to form carbon monoxide
        
        CO2 + C –––> 2CO
        
        (g) + (s) –––> (g)
        
        Carbon monoxide reacts with the haematite to form carbon dioxide and molten iron
        
        Iron collects at the b ase of the furnace form where it can be tapped off
        
        Fe2O3 + 3CO –––> 2Fe + 3CO2
        
        (s) + (g) –––> (l) + (g)
      - Calcium carbonate is thermally decomposed in the heat, forming calcium oxide and carbon dioxide
        
        (s) –––> (s) + (g)
        
        CaCO3 –––> CaO + CO2
        
        Calcium oxide reacts with the main impurity in the iron ore, silicon dioxide, to make calcium silicate.
        
        This forms a slag, which floats on top of the molten iron, and can be tapped off separately
        
        CaO + SiO2 –––> CaSiO3
        
        (s) + (s) –––> (l)
        
        Slag can be used for road surfacing and cement-making
    - - The carbon monoxide which removes the oxygen is therefore the reducing agent
        
        The iron (III) oxide is reduced to iron – it loses oxygen
      - The iron (III) oxide from which it gains the oxygen is therefore the oxidising agent
        
        The carbon monoxide is oxidised – it gains oxygen – to form carbon dioxide