Test For Oxidizing and Reducing Agents


The table below shows the common and important oxidizing agents:

Oxidizing AgentHalf EquationColour change when added to reducing agentApplication
Acidified potassium manganate (VII)

 

$\text{KMnO}_{4}$

$\text{MnO}_{4}^{-} (\text{aq}) + 8 \text{H}^{+} (aq) + 5\text{e}^{-} \rightarrow \text{Mn}^{2+} (\text{aq}) + 4 \text{H}_{2}\text{O}(\text{l})$Purple to pale pink (or colourless) Used to test for reducing agent
 Acidified potassium dichromate(VI)

 

$\text{K}_{2}\text{Cr}_{2}\text{O}_{7}$

$\text{Cr}_{2}\text{O}_{7}^{2-} (\text{aq}) + 14\text{H}^{+}(\text{aq}) + 6 \text{e}^{-} \rightarrow 2\text{Cr}^{3+} + 7\text{H}_{2}\text{O}$ Orange to green Oxidizes alcohol to acids; used to test for $\text{SO}_{2}$ gas
 Chlorine

 

$\text{Cl}_{2}$

 $\text{Cl}_{2} (\text{g}) + 2\text{e}^{-} \rightarrow 2\text{Cl}^{-} (\text{g})$ Greenish yellow to colourless Oxidizes bromide to bromine and iodide to iodine

Testing for presence of reducing agent:

  • Add an oxidising agent, e.g. Aqueous potassium manganate (VII) to the reducing agent
  • Shake the mixture
  • The aqueous potassium manganate (VII) is decolourised

The table below shows the common and important reducing agents.

Reducing AgentHalf EquationColour change when added to oxidizing agentApplication
Aqueous potassium iodide

 

$\text{KI}$

$2\text{I}^{-} (\text{aq}) \rightarrow \text{I}_{2} (\text{aq}) + 2\text{e}^{-}$Colourless to brownUsed to test for oxidizing agent
Aqueous iron(III) sulphate

 

$\text{FeSO}_{4}$

$\text{Fe}^{2+} (\text{aq}) \rightarrow \text{Fe}^{3+} (\text{aq}) + \text{e}^{-}$Green to brown
Carbon Monoxide

 

$\text{CO}$

Used to reduce iron oxides to iron in blast furnace
Hydrogen$\text{H}_{2} (\text{g}) \rightarrow 2\text{H}^{+} (\text{aq}) + 2\text{e}^{-}$Reduces copper(II) oxide to copper
Metals

 

E.g. $\text{Mg}$, $\text{Na}$

Displacement of less reactive metals

Testing for presence of oxidising agent:

  • Add a reducing agent, e.g. Aqueous potassium iodide to the oxidising agent.
  • Shake the mixture.
  • A brown solution of iodine is produced.
  • The presence of iodine can be confirmed by adding starch solution.
  • A dark blue coloration is obtained.

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