chemical changes

Metal Oxides

  • metal + oxygen = metal oxide

    • oxidation because metals gain oxygen

    • reduction = loss of oxygen; oxidation = gain of oxygen

The Reactivity Series

  • when metals react, they form positive ions

    • the reactivity series arranges the metals in order of their tendency to form a positive ion (reactivity)

    • metals can be put in order from their reactivity with water and dilute acids

  • a more reactive metal can displace a less reactive metal from a compound

Oxidation & Reduction in terms of electrons

  • Oxidation is Loss of electrons; Reduction is Gain of electrons (OIL RIG)

    • sodium oxidised = Na → Na+ + e-

    • sodium+ reduced = Na+ + e- → Na

  • Identify elements which have been oxidised & reduced

eg. 2Na + 2HCl → 2NaCl + H2

  • write out ions

2Na + 2H+ + 2Cl- → 2Na+ + 2Cl- + H2

  • 2Na → 2Na+ + 2e- so oxidised

  • 2H+ + 2e- → H2 so reduced

  • 2Cl- → 2Cl- so neither

Reactions of acids with metals

  • acid + metal → salt + hydrogen

    • redox reaction = one substance reduced & one oxidised

eg. 2HCl + 2Na → 2NaCl + H2

Neutralisation of acids

  • acids are neutralised by alkalis and bases

    eg. metal hydroxides, metal carbonates & metal oxides

    • an alkali is a base dissolved in water

  • metal hydroxide + acid → salt + water

eg. 2NaOH + 2HCl → 2NaCl + 2H2O

  • metal carbonate + acid → salt + water + carbon dioxide

eg. CuCO3 + H2SO4 → CuSO4 + H2O + CO2

  • metal oxide + acid → salt + water

eg. Na2O + 2HCl → 2NaCl + H2O

The pH scale & neutralisation

  • H+ + OH- → H2O

    • acids produce hydrogen ions in aqueous solution

    • alkalis produce hydroxide ions in aqueous solution

Strong & Weak acids

  • strong/weak is not the same as concentrated/dilute

    • strong acids completely ionise in aqueous solution

      eg. hydrochloric, nitric and sulfuric acids

    • weak acids partially ionise in aqueous solution

      eg. ethanoic, citric and carbonic acids

  • the stronger the acid, the lower the pH

    • as pH decreases by one unit, H+ concentration increases by a factor of 10

  • concentration refers to the amount of substance in a given volume whereas strength refers to the concentration of H+ in aqueous solution

Electrolysis

  • when an ionic substance is melted/dissolved, the ions are free to move within the liquid

  • electrolysis is passing a current through a molten solution so it can be broken down into its elements; the molten solution is called an electrolyte

    • positive ions move to negatively charged electrode - cathode (reduction: gain of electrons)

    • negative ions move to positively charged electrode - anode (oxidation: loss of electrons)

  • ions are discharged at each electrode

  • when a simple ionic compound is electrolysed in its molten state using inert electrodes, the metal is produced at the cathode and the non-metal is produced at the anode

Using Electrolysis to Extract Metals

  • metals that are more reactive than carbon are too reactive to be extracted by reduction with carbon so they’re extracted by electrolysis of molten compounds

  • metals that react with carbon can also be extracted by electrolysis

  • large amounts of energy are used in extraction for melting the compounds and produce the electrical current

  • aluminium is manufactured by the electrolysis of aluminium and cryolite using carbon as the anode

    • aluminium oxide is mixed with cryolite to lower the melting point as aluminium oxide is too expensive to melt

    • the positive electrodes need to be continually replaced because oxygen is formed which reacts with the carbon forming carbon dioxide and causing the cathode to wear

Electrolysis of Aqueous Solutions

  • ions discharged when an aqueous solution is electrolysed with inert electrodes depend on the relative reactivity of the elements involved

  • at the negative cathode, hydrogen is produced unless the metal is less reactive than hydrogen because more reactive ions want to stay within the solution

  • at the positive anode, if hydroxide and halide ions are present, one of the haline ions will be produced - if no halide is present oxygen is formed

  • this happens in aqueous solution as water molecules break down producing H+ ions and OH- which get discharged

Representation of Reactions as Half Equations

  • negative electrode (cathode)

    • X+ + e- → X

  • positive electrode (anode)

    • X- → X + e-