Half Equations (DP IB Chemistry)

Half Equations

How to balance a redox equation

• Oxidation numbers can be used to balance chemical equations

• Go through these steps to balance a redox equation:
  

  1. Write the unbalanced equation and identify the atoms which change in ox. no.

  2. Deduce the oxidation number changes

  3. Balance the oxidation number changes

  4. Balance the charges

  5. Balance the atoms

Redox titrations

• In titrations, the concentration of a solution is determined by titrating with a solution of known concentration.

• In redox titrations, an oxidising agent is titrated against a reducing agent

  • Electrons are transferred from one species to the other

• Indicators are sometimes used to show the endpoint of the titration

• However, most transition metal ions naturally change colour when changing the oxidation state

• There are two common redox titrations you should know about:

  • Manganate(VII) titrations

  • Iodine-thiosulfate titrations

Manganate(VII) titrations

• A redox reaction occurs between acidified manganate(VII) ions and iron(II) ions:

MnO₄^– (aq) +  8H⁺ (aq) +  5Fe²⁺ (aq)  → Mn²⁺ (aq) +  5Fe³⁺ (aq) + 4H₂O (l)

• This reaction needs no indicator as the manganate(VII) is a strong purple colour which disappears at the endpoint, so the titration is self-indicating

• This reaction is often used for the analysis of iron for example in iron tablets (health supplements)

Iodine-thiosulfate titrations

• A redox reaction occurs between iodine and thiosulfate ions:

2S₂O₃^2– (aq) + I₂ (aq) → 2I^–(aq) + S₄O₆^2– (aq)

• The light brown/yellow colour of the iodine turns paler as it is converted to colourless iodide ions

• When the solution is a straw colour, starch is added to clarify the endpoint

• The solution turns blue/black until all the iodine reacts, at which point the colour disappears.

• This titration can be used to determine the concentration of an oxidising agent, which oxidises iodide ions to iodine molecules

• The amount of iodine is determined from titration against a known quantity of sodium thiosulfate solution

• This reaction can be used for the analysis of chlorine in bleach