Redox (SQA National 5 Chemistry): Revision Note

Reduction & oxidation

• Oxidation is a reaction in which an element, ion or compound loses electrons

  • This can be shown in an ion-electron equation, e.g. when silver reacts with chlorine, silver is oxidised to silver ions:

Ag → Ag⁺ + e^-

• Reduction is a reaction in which an element, ion or compound gains electrons

  • This can be shown in an ion-electron equation, e.g. when oxygen reacts with magnesium, oxygen is reduced to oxide ions:

O₂ + 4e^- → 2O^2-

• A good way to remember these definitions is the mnemonic OILRIG

  • Oxidation Is Loss (of electrons)

  • Reduction Is Gain (of electrons)

What is a redox reaction?

• In any reaction, where one substance loses electrons (is oxidised), another substance must gain them (is reduced)

  • Therefore, oxidation and reduction always happen at the same time

  • This is why they are called redox reactions

Example redox reaction

• The displacement reaction that occurs when a piece of magnesium ribbon is placed into a solution of blue copper(II) sulfate is:

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

• To understand what is happening with the electrons, we can split the reaction into two parts:

  1. The magnesium

  2. The copper

1. What happens to magnesium?

   • A neutral magnesium atom (Mg) turns into a magnesium ion (Mg²⁺) which is part of the magnesium sulfate.

   • To go from a neutral atom (Mg) to a 2+ ion (Mg²⁺), the atom must lose 2 electrons

   • Loss of electrons is oxidation (OILRIG)

   • Oxidation ion electron equation:

Mg (s) → Mg²⁺ (aq) + 2e^-

2. What happens to copper?

   • A copper(II) ion (Cu²⁺) from the copper(II) sulfate solution turns into a neutral copper atom (Cu), which is seen as a brown solid

   • To go from a 2+ ion (Cu²⁺) to a neutral atom (Cu), the ion must gain 2 electrons

   • Gain of electrons is reduction (OILRIG)

   • Reduction ion electron equation:

Cu²⁺ (aq) + 2e^- → Cu (s)

• Since magnesium is losing electrons (oxidation) and the copper ions are gaining electrons (reduction) in the same reaction, it is a redox reaction

• The 2 electrons lost by each magnesium atom are the same 2 electrons gained by each copper(II) ion

Redox ion-electron equations

• A redox reaction can be shown as one overall equation

• This equation is created by combining the two separate ion-electron equations for the oxidation and reduction processes

• The final, overall redox equation shows all the species that react but must not include electrons

Combining ion-electron equations

1. Identify the oxidised and reduced substances, using the Electrochemical Series (page 10 of the SQA Data Booklet)

   • The substance that is higher up in the series is the one that is oxidised (loses electrons)

     • Its ion-electron equation must be reversed

   • The substance that is lower down in the series is the one that is reduced (gains electrons)

     • Its ion-electron equation is used as written

2. Write the two ion-electron equations

   • Make sure to reverse the one for the substance higher up in the series

3. Balance the electrons

   • The number of electrons lost must equal the number of electrons gained

   • If needed, multiply one or both entire equations to make the electron numbers the same

4. Combine the equations

   • Add everything on the reactant (left) side together and everything on the product (right) side together

5. Cancel out the electrons

   • Cancel the electrons from both sides to get the final, overall redox equation