practice questions

• Formation of carbonic acid from water and carbon dioxide

  H2O + CO2 → H2CO3
• Reaction of acids with carbonates

  2H+ + CO3 2- → H2O + CO2
• Oxidation number of sulfur increases from 0 to +1, so it is oxidation

  
• Oxidation number of silver decreases from +2 to +1, so it is reduction
• Fluorine does not change from -1, so its redox reaction
• Oxidation number for chlorine changes from 0 to -1, so it is reduced

  
• Oxidation number for chlorine changes from 0 to -5, so it is oxidised
• Oxidation number of Br is +5 for BrO3-, -1 for Br-, and 0 for Br2

  
• Calcium is oxidised as it loses electrons
• Chlorine is reduced as it gains electrons
• PO3 3-

  Phosphate (III) ion
• This reaction is not disproportionation because two different species are oxidised and reduced

  
• Disproportionation is simultaneously oxidation and reduction of an element in the same species
• Chlorine changes from 0 to -1 AND +1
• Chlorine is oxidised from 0 to +1
• Chlorine is reduced from 0 to -1
• Chlorine is a stronger oxidising agent than iodine

  Chlorine oxidises sulfur from +2 to +6, whereas iodine only oxidises sulfur from +2 to +2.5
• Molecular shape of NH3

  • Trigonal pyramidal
  • Angle is 107°
  • 3 bonding pairs and 1 lone pair
  • Electron pairs repel each other for maximum separation
• Decomposition of magnesium nitrate on heating

  2Mg(NO3)2 → 2MgO + 4NO2 + O2
• The reaction of potassium chloride with H2SO4 is not a redox reaction because the oxidation number does not change for any element
• On descending Group 7, the hydrogen halides become better reducing agents


  Sulfuric acid is reduced further by hydrogen iodide than hydrogen bromide and hydrogen chloride
• A reducing agent increases in oxidation number and loses electrons
• Electrons are transferred when reduction and oxidation occurs
• Oil rig

  Oxidation is loss of electrons, Reduction is gain of electrons
• Reducing agents

  Electrons, they are oxidized themselves
• Oxidizing agents


  Gain electrons and are reduced themselves
• Oxidation numbers


  • Rules for assigning oxidation numbers to elements and compounds
• Oxidation numbers

  • Uncombined elements are always zero
  • Ions have oxidation number equal to charge
  • Group 1 is +1
  • Group 2 is +2
  • Aluminium is +3
  • Hydrogen is +1 except in hydrides where it's -1
  • Chlorine is -1 except with F and O where it's positive
  • Fluorine is always -1
  • Oxygen is -2 except in peroxides and OF2 where it's different
• Reduction

  Decrease in oxidation number
• Oxidation

  Increase in oxidation number
• Disproportionation reaction is where an element is simultaneously oxidized and reduced
• Writing half-equations

  1. Write species before and after

  2. Balance atoms other than O and H

  3. Balance O with H2O

  4. Balance H with H+

  5. Balance charge with e-
• Manganate (MnO4-) is a classic oxidizing agent
• Combining half-equations to get full ionic equation


  1. Multiply one half-equation to balance electrons

  2. Cancel out electrons and combine the two half-equations
• The full ionic equation shows both oxidation and reduction, it is a redox reaction