OxidatIon & Reduction in Terms of Electrons (OCR GCSE Chemistry A (Gateway)) : Revision Note

Oxidation & Reduction in Terms of Electrons

Higher Tier Only

  • For reactions not involving oxygen, we can look at oxidation and reduction in terms of losing and gaining electrons

  • Oxidation is the loss of electrons

  • Reduction is the gain  of electrons 

  • We can use half and ionic equations to determine which species has been oxidised and reduced in terms of electrons 

  • A displacement reaction occurs between magnesium and copper sulfate: 

Mg (s) +  CuSO4 (aq) →   MgSO4 (aq) + Cu (s) 

  • The  ionic equation for this reactions is:

Mg (s) + Cu2+ (aq)  → Mg2+ (aq)  + Cu (s)

  • This can be split into two half equations which demonstrate oxidation and reduction individually:

Mg → Mg2+ + 2e

Cu2+ + 2e→ Cu

  • The magnesium atoms are oxidised as they lose electrons

  • The copper ions are reduced as they gain electrons

Worked Example

Zinc displaces copper from a solution of copper(II)sulfate.

Using ionic equations, determine which species undergoes oxidation and which species undergoes reduction

Answer:

  • Write the overall balanced symbol equation:

Zn (s) + CuSO4 (aq)→ ZnSO4 (aq) + Cu (s)

  • Write out the ions separately:

Zn (s) + Cu2+(aq) + SO42- (aq) →Zn2+(aq) + SO42-(aq) + Cu (s)

  • Eliminate ions that occur on both sides (spectator ions):

Zn (s) + Cu2+(aq) →  Zn2+(aq) + Cu (s)

  • Identify which species has lost or gained electrons

    • Zinc has lost electrons and been oxidised:

Zn →  Zn2+  +  2e-

  • Copper ions have gained electrons and been reduced:

Cu2+ +  2e-  →  Cu

Examiner Tips and Tricks

A handy way of remembering definitions for oxidation and reduction for electrons is:

OIL RIG: Oxidation Is Loss of electrons, Reduction Is Gain

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