chemical change (c4)

• 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
• Extraction of metals

  • Gold is very unreactive and found in the Earth as the metal itself
  • Most metals are found as compounds that require chemical reactions to extract the metal
  • Metals less reactive than carbon can be extracted from their oxides by reduction with carbon
• Reduction

  Involves the loss of oxygen
• Oxidation

  loss of electrons
• Reduction

  Gain of electrons
• The charges on each side of an ionic equation should add up to the same number
• Determining what has been oxidised and reduced in an equation

  1. Look at the changes for each element

  2. Sodium: 2Na -> 2Na+ + 2e-, so sodium has been oxidised

  3. Chlorine: 2Cl- -> 2Cl-, so chlorine has not been oxidised or reduced

  4. Hydrogen: 2H+ + 2e- -> H2, so hydrogen 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 reduced and oxidised 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
• Calculating concentration from titration

  Convert volumes to dm3

  1. Work out moles of NaOH
  2. Use mole ratio from equation to work out moles of HCl
  3. Calculate concentration = moles / volume
• 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)
• pH decreases by 1 unit

  H+ concentration of the solution increases by a factor of 10
• Strong/weak and concentrated/dilute are not the same - strong/weak refers to H+ ion concentration, concentrated/dilute refers to amount of substance in a given volume
• 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
• Aluminium extraction by electrolysis

  • Molten mixture of aluminium oxide and cryolite is electrolysed
  • Carbon is used as the positive electrode (anode)
  • Oxygen reacts with the carbon electrodes, forming carbon dioxide, so they need to be continually replaced
• Metals that react with carbon can also be extracted by electrolysis