AQA A Level Chemistry复习笔记1.9.3 Redox Equations

• Balancing equations using redox principles is a useful skill and is best illustrated by following an example
• It is important to follow a methodical step-by-step approach so that you don't get lost:

Answer

Step 1: Write the unbalanced equation and identify the atoms which change in oxidation state

Step 2: Deduce the oxidation state changes

Step 3: Balance the oxidation state changes

Step 4: Balance the charges

Step 5: Finally, balance the atoms

Oxidation

• Oxidation is the gain of oxygen, eg:

Cu + H2O → CuO + H2

(Cu has gained an oxygen and is oxidised)

• Oxidation is also the loss of hydrogen, eg:

2NH3 + 3Br2 → N2 + 6HBr

(NH3 has lost hydrogen and is oxidised)

• Oxidation is also the loss of electrons, eg:

Cu2+ + Mg → Mg2+ + Cu

(Mg has lost two electrons and is oxidised)

• Oxidation causes an increase in oxidaiton state, eg:

Cu2+ + Mg → Mg2+ + Cu

(change in ox. no. of Mg is +2 thus Mg is oxidised)

Reduction

• Reduction is the loss of oxygen, eg:

Cu+ H2O → 2CuO + H2

(H2O has been reduced)

• Reduction is also the gain of hydrogen, eg:

2NH3+ 3Br2 → N2 + 6HBr

(Br has been reduced)

• Reduction is also the gain of electrons, eg:

Cu2+ + Mg → Mg2+ + Cu

(Cu has been reduced)

• Reduction causes a decrease in oxidation number, eg:

Cu2+ + Mg → Mg2+ + Cu

(the change in oxidation state of Cu is -2 thus Cu is reduced)

Redox reactions

• Redox reactions are reactions in which oxidation and reduction take place simultaneously
• While one species is oxidising, another is reducing in the same reaction, eg:

Cu2++ Mg → Mg2+ + Cu

(Cu has been reduced and Mg has been oxidised)

Answer

Answer 1:

• • Oxidised: Cl- as the oxidation state has increased by 1
  • Reduced: Na+ as the oxidation state has decreased by 1

   

Answer 2:

• • Oxidised: Mg as the oxidation state has increased by 2
  • Reduced: Fe2+ as the oxidation state has decreased by 2

   

Answer 3:

• • Oxidised: C as it has gained oxygen
  • Reduced: Ag as it has lost oxygen

   

Disproportionation reactions

• A disproportionation reaction is a reaction in which the same species is both oxidised and reduced

Example of a disproportion reaction in which the same species (chlorine in this case) has been both oxidised and reduced

Answer

Step 1: Write the unbalanced equation and identify the atoms that change in oxidation state:

Step 2: Deduce the oxidation state changes:

Step 3: Balance the oxidation state changes:

Step 4: Balance the charges

Step 5: Balance the atoms