Topic 2: Oxidation and Reduction

• Redox Reactions - involves a chemical to be reduced and oxidised, one cannot happen without another
• Oxidised - loses electrons
• Reduced - gains electrons
• Use OIL RIG to help remember the Oxidation and Reduction Rules
• OIL - Oxidation is Less
• RIG - Reduction is Gain
• Oxidising agents - cause oxidisation in other species. The oxidising agent is reduced in the process
• Reducing agents - reduce another species by donating electrons. They are themselves oxidised during this process.
• Reducing agents - cause reactions in other species, the reducing agent is then oxidised in the process
• Electron movement - can be shown using balanced half equations, these can be balanced within acidic solution
• Writing Half Equations

  Electrons on the right - reduction half equation

  Electrons on the left - oxidisation half equation
• Oxidisation Numbers and States - indicates the number of electrons tht a species has lost/gained
• Oxidation Numbers:

  Are written with the sign (+/-) and then the value

  Increased when Oxidised

  Decreased when Reduced
• Oxidation Numbers are increased when the molecule has been reduced
• Oxidation Numbers are decreased when the molecule has been oxidised
• Oxidation Number of elements

  Free (uncombined element) - Oxidation Number of O, No Exceptions

  Monatomic (Single Atom) Ions - same as degree of ion - no exceptions

  Polyatomic (many) ions - the sum of all oxidation numbers is equal to the overall charge

  Hydrogen in a compound - is usually H, +1. metal hydrides becomes -1

  Oxygen in a compound - -2, H2O2 where O = 1

  Group 17 elements - -1, where OCl- means Cl = +1
• The most reactive element becomes the cation in solution
• the least reactive element becomes the anion in solution
• Products of a Single- Displacement Reaction. - the MOST reactive element becomes the CATION in solution whilst least reactive element becomes the ANION in solution
• Two types of cells: Galvanic and Electrochemical cells
• Galavnic and Electrochemical cells both have:

  An anode - where OXIDATION occurs

  An cathode - where REDUCTION occurs

  A complete circuit

  The charge or polarity on the electrode depends on the type of cell, galvanic or electrolyte
• Mnemonic SOC SRA - Galvanic and Electrolytic cells 


  SOC - Strongest Oxidation at Cathode

  SRA - Strongest Reduction at Anode
• Galvanic cells such as alkaline batteries and button cells convert chemical energy + electrical energy
• To convert, there must be a difference in the oxidising (reducing strength of materials of the anode (reduction) and cathode (oxidising) that means they have different Eo values
• Spontaneous Reactions - Occurs when Eo is greater than Oo
• Electrons always travel from anode to cathode in a circuit.
• Anions move towards the anode in solution
• Cations move towards the cathode in solution
• Galvanic Cell Notation: Focus mainly on Mg, Zn, Fe and Cu half-cells
• Salt bridge - allows for the transfer of ions - mainly potassium chloride in a galvanic cell
• Standard Electrode Potential - SEP
• SEP is the relative strength of oxidising and reducing agents
• SEP are the voltages generated after redox-half equations under standard conditions
• Elements higher on the periodic table - they have stronger reducing agents, which means they will LOSE electrons faster
• Elements on the lower periodic table - Stronger oxidising agents, gain electrons more easily
• The difference in the amount of electricity consumed at the cathode and generated at the anode is the cell potential difference.
• Eo reduction half-cell - Eo oxidisied half-cell = EMF
• EMF = Eo Reduction + Eo Oxidiation
• Limitations of SEP

  
  

  Cells usually operate under non-standard conditions and this slightly affects the Eo values of each half-cell - Some of the half-cells may even switch positions

  Cannot predict the rate of reaction (no connection between two)

  Intermediate species that are insoluble may prevent the reaction from progressing
• Make sure you understand how to calculate SEP and the limitations associated with SEP