10 min readConcept sheet1-mark MCQ and assertion-reason, 2-mark 'how many moles of AgCl' questions, and a 3-mark answer stating the postulates

Werner's theory and double salts

Werner said a metal in a complex shows two valences: an ionisable primary valence (oxidation state) and a non-ionisable secondary valence (coordination number).

Primary valenceIonisable; satisfied by negative ions; equals the oxidation state of the metal

Secondary valenceNon-ionisable; satisfied by neutral molecules or negative ions; equals the coordination number, which is fixed for a metal

Test for ionisable Cl⁻Only chloride outside the square bracket gives a white precipitate of AgCl with AgNO₃

Double salt vs complexA double salt splits fully into simple ions in water; a complex ion such as [Fe(CN)₆]⁴⁻ stays whole

What Werner found

Alfred Werner, a Swiss chemist, made many complexes of cobalt and platinum. He studied them with two simple tests. First, how much AgCl each gave with silver nitrate. Second, how well its solution conducted electricity, which shows how many ions it gives. He explained the results with two kinds of valence for the metal. Werner received the Nobel Prize in Chemistry in 1913.

Postulates of Werner's theory

In a coordination compound the metal shows two types of linkage (valence): primary and secondary.
Primary valences are normally ionisable. They are satisfied by negative ions. Today we call this the oxidation state of the metal.
Secondary valences are non-ionisable. They are satisfied by neutral molecules or negative ions. Their number is the coordination number, and it is fixed for a metal.
The groups held by secondary valences have a definite arrangement in space around the metal, for example octahedral, tetrahedral or square planar. This shape is the coordination polyhedron.
In a modern formula, groups held by secondary valences go inside the square bracket. Ions held only by primary valence go outside it.

Werner's cobalt(III) ammine series

Old formula	Colour	Mol AgCl per mol	Modern formula	Ions in water
CoCl₃·6NH₃	Yellow	3	[Co(NH₃)₆]Cl₃	4: [Co(NH₃)₆]³⁺ + 3Cl⁻
CoCl₃·5NH₃	Purple	2	[CoCl(NH₃)₅]Cl₂	3: [CoCl(NH₃)₅]²⁺ + 2Cl⁻
CoCl₃·4NH₃	Green	1	trans-[CoCl₂(NH₃)₄]Cl	2: [CoCl₂(NH₃)₄]⁺ + Cl⁻
CoCl₃·4NH₃	Violet	1	cis-[CoCl₂(NH₃)₄]Cl	2: [CoCl₂(NH₃)₄]⁺ + Cl⁻

Finding the formula from AgCl data

Problem

A solution containing 0.010 mol of a cobalt(III) compound, CoCl₃·5NH₃, is treated with excess silver nitrate. 2.87 g of AgCl (molar mass 143.5 g mol⁻¹) is formed. How many chloride ions are ionisable? Write the formula of the complex.

Working

Moles of AgCl = mass ÷ molar mass = 2.87 g ÷ 143.5 g mol⁻¹ = 0.020 mol.
Each free Cl⁻ gives one AgCl: Ag⁺ + Cl⁻ → AgCl.
Ionisable Cl⁻ per formula unit = 0.020 mol ÷ 0.010 mol = 2.
Co³⁺ has coordination number 6, so five NH₃ and the third Cl⁻ fill the six secondary valences.

Answer

2 chloride ions are ionisable. The complex is [CoCl(NH₃)₅]Cl₂, which gives 3 ions in water.

Double salt vs complex compound

Double saltTwo salts crystallised together in a fixed ratio: Mohr's salt FeSO₄·(NH₄)₂SO₄·6H₂O, potash alum K₂SO₄·Al₂(SO₄)₃·24H₂O, carnallite KCl·MgCl₂·6H₂O. In water it splits completely into simple ions (Fe²⁺, NH₄⁺, SO₄²⁻), so it gives the test of every one of them.

Complex compoundContains a complex ion that keeps its identity in water. K₄[Fe(CN)₆] gives K⁺ and [Fe(CN)₆]⁴⁻, but no free Fe²⁺ or CN⁻, so it does not give the usual tests for Fe²⁺.

Both are made by combining stable compounds in a fixed ratio. The difference shows only when they dissolve.

Activity: prepare a double salt (ferrous ammonium sulphate or potash alum)

Mohr's salt is FeSO₄·(NH₄)₂SO₄·6H₂O and potash alum is K₂SO₄·Al₂(SO₄)₃·24H₂O. Each is made by crystallising two salts from one solution in equal numbers of moles.

1
Weigh the two salts in a 1 : 1 mole ratio.
For Mohr's salt: ferrous sulphate crystals, FeSO₄·7H₂O (278 g mol⁻¹), and ammonium sulphate (132 g mol⁻¹). For alum: potassium sulphate and aluminium sulphate.
2
Dissolve them in the minimum amount of warm water and add 2 to 3 mL of dilute sulphuric acid.
The acid stops hydrolysis of Fe²⁺ or Al³⁺, which would make the solution turbid.
3
Filter the warm solution to remove any insoluble matter.
4
Heat the filtrate gently until it reaches the crystallisation point.
Test: a drop on a glass rod forms crystals as it cools. For Mohr's salt, do not boil for long, or Fe²⁺ is oxidised to yellow Fe³⁺.
5
Cover the dish and let it cool slowly without disturbing it.
6
Pour off the mother liquor, wash the crystals with a little 1 : 1 mixture of cold water and alcohol, and dry them between filter papers.
7★
Record the colour, shape and yield.
Mohr's salt: pale green crystals. Potash alum: colourless octahedral crystals.

★ key step

In real life

Potash alum (phitkari) is used in many Indian homes to settle mud in drinking water and as an aftershave stone. It works because, like every double salt, it gives simple Al³⁺ ions in water.

Exam angle

1 marks: “How many ions does [Co(NH₃)₆]Cl₃ give in water?” Four: one [Co(NH₃)₆]³⁺ and three Cl⁻.

2 marks: “One mole of CoCl₃·4NH₃ gives one mole of AgCl with excess AgNO₃. Write its formula and state the primary and secondary valence of cobalt.” [CoCl₂(NH₃)₄]Cl. Primary valence 3 (oxidation state +3). Secondary valence 6 (two Cl⁻ and four NH₃ inside the bracket).

3 marks: “State the postulates of Werner's theory. How does a double salt differ from a complex? Give one example of each.” Two valences: primary (ionisable, anions, oxidation state) and secondary (non-ionisable, neutral or anionic ligands, coordination number, fixed); secondary groups have a fixed spatial arrangement. Double salt (Mohr's salt) ionises fully; complex (K₄[Fe(CN)₆]) keeps its complex ion.

Remember it

Primary = plus charge (oxidation state), ionisable. Secondary = sphere (coordination number), stays put. Outside the bracket reacts with AgNO₃; inside does not.