Redox and Electron Transfer

• Redox reactions can also be defined in terms of electron transfer
• Oxidation is a reaction in which an element, ion or compound loses electrons

  • The oxidation number of the element is increased
  • This can be shown in a half equation
• Reduction is a reaction in which an element, ion or compound gains electrons

  • The oxidation number of the element is decreased
  • This can be shown in a half equation
• The ions present (with state symbols) in the equation are:

  • Zn(s) + Cu2+(aq) + SO42-(aq) →Zn2+(aq) + SO42-(aq) + Cu(s)
• The spectator ions (those that do not change) are SO4^2-(aq

  • These can be removed and the ionic equation written as: Zn(s) + Cu2+(aq) → Zn2+(aq) + Cu(s)
• By analysing the ionic equation, we can split the reaction into two half equations by adding in the electrons to show how the changes in charge have occurred:

  • Zn(s) → Zn2+(aq) + 2e-
  • Cu2+(aq) +2e- → Cu(s)
  • It then becomes clear that zinc has been oxidised as it has lost electrons
  • Copper ions have been reduced as they have gained electrons
• Use the mnemonic OIL-RIG to remember oxidation and reduction in terms of the movement of electrons: Oxidation Is Loss –  Reduction Is Gain.
• Table of rules assigning oxidation numbers

  A) oxidation number

  B) element

  C) zero

  D) atoms

  E) ions

  F) fixed oxidation

  G) number

  H) compounds

  I) +1

  J) +2

  K) -1

  L) +1

  M) -1

  N) -2

  O) -1

  P) +2

  Q) oxidation number

  R) element

  S) mono-atomic

  T) ion

  U) charge

  V) +2

  W) +3

  X) -1

  Y) sum

  Z) oxidation number

  [) zero

  \) +1

  ]) -1

  ^) 0

  _) sum

  `) oxidation

  a) number

  b) ion

  c) equal

  d) charge on the ion

  e) +6

  f) 4 x -2

  g) -2

  
• Redox reactions can be identified by the changes in the oxidation number when a reactant goes to a product
• The tests for redox reactions involve the observation of a colour change in the solution being analysed

  • e.g. acidified potassium manganate(VII), and potassium iodide
• Potassium manganate(VII), KMnO4, is an oxidising agent which is often used to test for the presence of reducing agents

  • When acidified potassium manganate(VII) is added to a reducing agent its colour changes from purple to colourless

  A) unknown

  B) potassium

  C) manganate

  D) purple colour fades

  E) reducing

  F) agent

  
• Potassium iodide, KI, is a reducing agent which is often used to test for the presence of oxidising agents

  A) acidified

  B) hydrogen peroxide

  C) oxidising

  D) red-brown

  E) potassium

  F) iodide

  G) oxidised

  H) oxidising agent

  I) iodine

  J) potassium

  K) iodide

  
• When added to an acidified solution of an oxidising agent such as aqueous chlorine or hydrogen peroxide (H2O2), the solution turns a red-brown colour due to the formation of iodine, I2:

  • 2KI (aq) + H2SO4 (aq) + H2O2 (aq) →  I2 (aq) + K2SO4 (aq) + 2H20 (l)
• The potassium iodide is oxidised as it loses electrons and hydrogen peroxide is reduced, therefore potassium iodide is acting as a reducing agent as it will itself be oxidised:

  • 2I- →  I2 + 2e-
• Oxidising agent - a substance that oxidises another substance, and becomes reduced in the process

  • An oxidising agent gains electrons as another substance loses electrons
  • Common examples include hydrogen peroxide, fluorine and chlorine
• Reducing agent - A substance that reduces another substance, and becomes oxidised in the process

  • A reducing agent loses electrons as another substance gains electrons
  • Common examples include carbon and hydrogen
  • The process of reduction is very important in the chemical industry as a means of extracting metals from their ores