Virtual CrezLab - Qualitative Analysis

Hydrogen peroxide is an oxidising agent that is usually used to oxidise colourless potassium iodide to brown iodine solution and pale green iron(II) ions to yellow iron(III) ions.

3. Add acidified hydrogen peroxide solution to the unknown colourless solution, U. Observe.	View video
Observations	Colourless solution U, turned brown. Black deposits are formed.
Chemical reaction	Redox reaction. Hydrogen peroxide has oxidised the reducing agent present in solution U to iodine solution. The colourless U is likely to be potassium iodide, KI, solution. Iodide ions lose electrons to become oxidised to iodine molecules. 2I^- (aq) -----> I₂ (aq) + 2e^- Solution U is a reducing agent.
4a) Add hydrogen peroxide to unknown pale green solution, V. 4b) Add aqueous sodium hydroxide until no further change is observed.	Digital image for 4a View video 4b
Observations	Pale green solution turned yellow or yellow brown. A dirty green ppt. insoluble in excess NaOH formed.
Chemical reaction in 4a	Redox reaction. Hydrogen peroxide has oxidised the reducing agent present in solution V to a yellow solution. Solution V is likely to to contain pale green Fe²⁺ ions. Fe²⁺ ions lose electrons to become oxidised to Fe³⁺ ions. Fe²⁺ (aq) -----> Fe³⁺ (aq) + e^-
Chemical reaction in 4b	Precipitation reaction. Fe³⁺ ions formed reacted with hydroxide ions to form insoluble Fe(OH)₃. Fe²⁺ (aq) + 3OH^- (aq) ---> Fe(OH)₃ (s)

It is important to note that:

Hydrogen peroxide, however, behaves as a reducing agent when added to stronger oxidising agent such as acidified potassium dichromate(VI) and acidified potassium manganate(VII).
In this case, hydrogen peroxide is oxidised to form oxygen gas.
Effervescence would be observed and the glowing splint placed at the mouth of test tube rekindles.

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