Chemical Changes

• Reactivity of metals


  Metals react with oxygen to produce metal oxides. The reactions are oxidation reactions because the metals gain oxygen (but lose electrons).
• Reactivity series of metals

  • Potassium
  • Sodium
  • Lithium
  • Calcium
  • Magnesium
  • Zinc
  • Iron
  • Copper
  • Hydrogen
  • Carbon

  
• A more reactive metal

  Can displace a less reactive metal from a compound
• Comparing reactivity of metals

  1. Add equal volumes of dilute acid to test tubes

  2. Add equal mass of metal to each test tube

  3. Count number of bubbles produced in a given time

  4. Confirm bubbles are hydrogen using lit splint

  5. Faster bubbles = faster reaction rate = more reactive metal
• Potassium, magnesium, zinc in dilute sulfuric acid

  Potassium reacts explosively, magnesium bubbles vigorously, zinc forms bubbles slowly
• Reactions of metals with water

  1. Reactive metals (potassium, sodium, lithium, calcium) react rapidly in cold water

  2. Less reactive metals (magnesium, zinc, iron) react with water vapour (steam)

  3. Unreactive metals (copper, silver, gold) don't react with cold water or steam
• Aluminium should be below magnesium but above carbon in the reactivity series
• More reactive metals

  Lose outer electrons more readily and form a positive ion
• Reactions of metals with water and acids are limited to room temperature and do not include reactions with steam
• Unreactive metals


  Less reactive than hydrogen, found in the Earth as the metal itself
• Extraction of metals less reactive than carbon

  Involves reduction with carbon, which is the loss of oxygen
• Oxidation

  Loss of electrons
• Reduction


  Gain of electrons
• Reactions of acids

  1. Acids react with some metals to produce salts and hydrogen

  2. Redox reaction

  
• Acid + Metal


  Produces salt + hydrogen

  
• Oxidation half equation


  Metal -> Metal^(n+) + n e^(-)

  
• Reduction half equation

  n H^(+) + n e^(-) -> n/2 H2

  
• Neutralisation reactions


  1. Acid + Alkali -> Salt + Water

  2. Metal Carbonate + Acid -> Salt + Water + Carbon Dioxide
• The particular salt produced depends on the acid used and the positive ions in the base, alkali or carbonate
• Making soluble salts

  1. Solid insoluble substance (metal, metal oxide, hydroxide or carbonate) added to acid until no more reacts

  2. Excess solid filtered off to produce salt solution

  3. Salt solution can be crystallised to produce solid salt
• Acids

  Produce hydrogen ions (H+) in aqueous solutions

  
• Alkalis

  Aqueous solutions contain hydroxide ions (OH-)
• pH scale


  Measure of acidity or alkalinity, 0-14, 7 is neutral, acids <7, alkalis >7
• Neutralisation reactions


  Hydrogen ions react with hydroxide ions to produce water

  
• Titration

  Volumes of acid and alkali solutions that react can be measured using a suitable indicator
• Strong acid


  Completely ionised in aqueous solution (e.g. hydrochloric, nitric, sulfuric)
• Weak acid


  Only partially ionised in aqueous solution (e.g. ethanoic, citric, carbonic)
• Concentration of acid

  Stronger acid = lower pH = more H+ ions
• Concentrated acid


  Larger amount of substance in a given volume
• Dilute acid


  Lesser amount of substance in a given volume
• Electrolyte

  Ionic compound melted or dissolved in water, ions free to move, conducts electricity
• Electrolysis


  1. Electric current passed through electrolyte causes ions to move to electrodes

  2. Positive ions to negative electrode (cathode), negative ions to positive electrode (anode)

  3. Ions discharged at electrodes producing elements
• Electrolysis of simple ionic compound (e.g. lead bromide)


  Metal produced at cathode, non-metal produced at anode

  
• Electrolysis for metal extraction


  • Used if metal too reactive to extract by reduction with carbon, or reacts with carbon
  • Large amounts of energy used to melt compounds and produce electrical current
• Electrolysis of aluminium


  1. Molten mixture of aluminium oxide and cryolite used, cryolite lowers boiling point

  2. Carbon used as positive electrode (anode), reacts with oxygen to form CO2, so electrodes need replacing
• Electrolysis of aqueous solutions


  1. Ions discharged depend on reactivity of elements

  2. At cathode, hydrogen produced if metal more reactive than hydrogen

  3. At anode, oxygen produced unless solution contains halide ions when halogen is produced

  4. Reactions at cathode are reductions, reactions at anode are oxidations

  
• Half equations for electrolysis

  Cathode (reduction): Positive ion + electrons -> Element

  Anode (oxidation): Element - electrons -> Positive ion