Redox and Electron Transfer

Cards (16)

  • 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