A2 Chemistry Unit 6 Notes (transition metals)

Analysis of Transition Metal Compounds PRECIPITATES When 2 aqueous solutions are mixed together and an insoluble compound is formed, this is known as precipitate, not a suspension. The observation that a precipitate is formed should always be accompanied by the colour of the ppt even if it is white. Some reagents should be added until they are in excess. This may result in a ppt forming and then dissolving in excess reagent.

(I) SODIUM HYDROXIDE SOLUTION (NaOH) When dilute NaOH solution is added to a solution containing a metal ppt, the ppt of the insoluble hydroxide (e.g. Mn(OH)2 ) is formed. Ppt which are amphoteric (that acts both as base and acid) hydroxides will dissolve in excess NaOh to give a solution containg a complex ion (e.g. [Cr(OH)6]3- ). Students should assume that aqueous NaOH should be added until it is in excess even if it is not stated in instruction. OBS. ON ADDING DIL.NaOH

OBS. ON ADDING EXCESS NaOH

LIKELY ION

Green ppt

Ppt dissolves to give green solution.

Chromium (III)

Ppt is insoluble

Manganese (II)

Ppt is insoluble

Iron (II)

Brown ppt

Ppt is insoluble

Iron (III)

Green ppt

Ppt is insoluble

Nickel (II)

Blue ppt

Ppt is insoluble

Copper (II)

White ppt

Ppt dissolves to give colourless solution

Zinc (II)

White ppt

Ppt is insoluble

Mg2+/Ba2+/St2+/Ca2+

Off-white ppt which darkens to exposure to air Green ppt which turns brown on exposure to air

(II)

AMMONIA (NH3) SOLUTION

Dilute aqueous NH3 when added to a solution containing a cation that will form the same hydroxide as NaOH solution. Excess NH3(aq) may dissolve the ppt to form a complex ion [Cu(NH3)4(H2O)2]2+. Students must assume that NH3(aq) must be added until in excess.

OBS. ON ADDING DIL. NH3 (AQ) Green ppt

OBS. ON ADDING EXCESS DIL. NH3 (AQ) Ppt dissolves to give green solution.

Off-white ppt which darkens to exposure to air Green ppt which turns brown on exposure to air

LIKELY ION Chromium (III)

Ppt is insoluble

Manganese (II)

Ppt is insoluble

Iron (II)

Brown ppt

Ppt is insoluble

Iron (III)

Green ppt

Blue Solution

Nickel (II)

Blue ppt White ppt

Dissolves to give a darker blue solution (royal blue/deep blue) Ppt dissolves to give colourless solution

White ppt

Ppt is insoluble

Copper (II) Zinc (II) Magnesium

Negative ions – Refer AS Chemistry TEST FOR OXIDISING AND REDUCING AGENTS Reducing agents usually decolourise aqueous acidified KMnO4 (purple) and may also turn aqueous K2Cr2O7 from orange to green.Reducing agents:   

Iron (II) ions Iodide ions H2O2

Oxidizing agents usually liberate Iodine as a brown solution or black solid from aqueous Potassium Iodide (KI). Iodine solution gives a blue black colouration with starch. Oxidizing agents:  Acidified Manganate (VII) ions  Acidified Dichromate (VI) ions  H2O2 H2O2 SOLUTION (HYDROGEN PEROXIDE) Aqueous H2O2 can act both as an oxidizing agent and reducing agent often with an evolution of Oxygen. OBS ON ADDING H2O2

INFERENCE

Brown ppt

Manganate (VII), brown ppt – MnO2

Purple solution is decolourized

Manganate (VII) in acid solution

Pale green solution - yellow

Iron (II) to Iron (III) in acid solution

Green ppt – brown

Iron (II) Hydroxide

Green alkaline solution - Yellow

Chromium (III) to Chromate (VI)

Brown solution or black ppt

Iodine from Iodide in acid soluion