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:
- Write the unbalanced equation and identify the atoms which change in ox. no.
- Deduce the oxidation number changes
- Balance the oxidation number changes
- Balance the charges
- Balance the atoms
Worked example
Manganate(VII) ions (MnO4-) react with Fe2+ ions in the presence of acid (H+) to form Mn2+ ions, Fe3+ ions and water. Write the overall redox equation for the reaction
Answer:
- Write the unbalanced equation and identify the atoms which change in oxidation number
- Deduce the oxidation number changes
- Balance the oxidation number changes
- Balance the charges
- 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:
2S2O32– (aq) + I2 (aq) → 2I–(aq) + S4O62– (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
Worked example
A health supplement tablet containing iron(II)sulfate was analysed by titration. A tablet weighing 2.25 g was dissolved in dilute sulfuric acid and titrated against 0.100 mol dm-3 KMnO4.The titration required 26.50 cm3 for a complete reaction. Calculate the percentage by mass of iron in the table.
Answer:
- Write the balanced equation for the reaction
- oxidation: Fe2+ (aq) → Fe3+ (aq) + e-
- reduction: MnO4- (aq) + 8H+ (aq) + 5e- → Mn2+ (aq) + 4H2O (l)
- overall: MnO4- (aq) + 8H+ (aq) + 5Fe2+ (aq) → Mn2+ (aq) + 4H2O (l) + 5Fe3+ (aq)
- Determine the amount of MnO4- used in the titration
- moles of MnO4- = 0.0265 dm3 x 0.100 mol dm-3 = 0.00265 mol
- moles of MnO4- = 0.0265 dm3 x 0.100 mol dm-3 = 0.00265 mol
- Determine the amount of iron in the reaction
- From the equation for the reaction, we know the reacting ratio MnO4- : Fe2+ = 1: 5
- ∴ moles of Fe2+ = 0.00265 mol MnO4- x 5 = 0.01325 mol
- Convert moles into the mass of iron
- Mass of iron = 0.01325 mol x 55.85 gmol-1 = 0.740 g
- Mass of iron = 0.01325 mol x 55.85 gmol-1 = 0.740 g
- Find the percentage of iron in the tablet
- ∴ % Fe in the tablet = (0.740/ 2.25) x 100 = 32.9%