6.2.6 Redox Systems

Redox System: Ferrous & Permanganate

• The changes in oxidation states of transition metal ions involve redox reactions
• Redox reaction = involves oxidation and reduction
• To find the concentration of metal ions in solution, titration can be performed
• Examples of such titration reactions are the following redox systems:
• Ferrous (Fe2+) and permanganate (MnO4-) in the acid solution given suitable data
• Ferrous (Fe2+) and dichromate (Cr2O72-) in the acid solution given suitable data

Reaction of MnO4– and Fe2+ in acid

• The concentration of Fe2+ ions can be determined by titrating a known volume of Fe(II) ions with a known concentration of MnO4 ions
• During the reaction of MnO4 with Fe2+ the purple colour of the manganate(VII) ions disappears
• The end-point is when all the iron ions have reacted with the MnO4 ions and the first purple colour appears in the flask
• At this point, the MnO4 is in excess
• The two half-reactions that are involved in this redox reaction are as following:

• The half equations are combined to get the overall equation

Worked Example

Determining the concentration of Fe(II) ions

The overall redox equation for the oxidation of Fe(II) ions by manganate(VII) ions is as following:

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

When a metal ore is placed in acid and left to dissolve, all the metal that was originally present in the ore are then present as ions in the solution

0.570 g of iron ore were placed in acid and the obtained solution was then titrated against 0.0650 mol dm-3 KMnO4 (aq) and the titre of this titration was 0.0240 dm3

Calculate the percentage mass of iron in the iron ore

Step 1: Calculate the number of moles of manganate (MnO4) ions

Number of moles = volume x concentration

= 0.0240 x 0.0650

= 0.00156 mol of manganate ions

Step 2: Determine the number of moles of Fe(II) ions that have reacted

The ratio of MnO4 to Fe2+  is 1:5

Therefore, the number of moles of Fe2+ that has reacted is 5 x 0.00156

= 0.0078 mol of Fe2+ ions

Step 3: Determine the mass of Fe(II) ions that have reacted

Mass = mol x relative atomic mass

= 0.0078 x 55.8

= 0.435 g of Fe2+ ions

Step 4: Determine the percentage mass of the iron in the iron ore

The mass of the ore was 0.570 g

The mass of iron in the ore was 0.435 g

= 76%

Author: Francesca

Fran has taught A level Chemistry in the UK for over 10 years. As head of science, she used her passion for education to drive improvement for staff and students, supporting them to achieve their full potential. Fran has also co-written science textbooks and worked as an examiner for UK exam boards.
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