Transition Elements and Qualitative Analysis

OCR A Level Chemistry

Transition element

A d block element that forms at least one stable ion with partially filled d subshell.

Location of transition metals in the periodic table

In the middle block from Ti to Cu.

Elements in d block not considered as transition metals

Scandium and zinc.

Reason why scandium is not a transition metal

Scandium only forms Sc3+, where the d orbitals are empty.

Reason why zinc is not a transition metal

Zinc only forms Zn2+ where the d orbitals are full.

Noble gas configuration of chromium

[Ar]4s13d5.

Noble gas configuration of copper

[Ar]4s13d10.

Electrons lost first by transition metals when forming ions

4s.

Characteristic physical properties of transition metals

●Metallic ●High density ●High melting and boiling point ●Shiny ●Good conductors of heat and electricity.

Characteristic chemical properties of transition metals

●Variable oxidation states → take part in many redox reactions ●Coloured compounds/ions in solution ●Good catalysts ●Form complex ions.

Oxidation number of Mn in potassium manganate (VII)

Mn = +7.

Ways transition metals act as effective catalysts

●They provide a surface on which reaction can take place ●They change oxidation states to form intermediates required for pathways with lower activation energy.

Examples of transition metal catalysts and their processes

●Iron - Haber process ●Vanadium (V) oxide - contact process ●Nickel - hydrogenation of alkenes ●Manganese (IV) oxide - decomposition of hydrogen peroxide ●Copper sulfate - hydrogen production.

Complex ion

Transition metal ion bonded to one or more ligands by coordinate bonds.

Ligand

Molecule or an ion that can donate a pair of electrons to the metal to form a coordinate bond.

Coordinate bond

A bond in which one of the atom provides both the electrons required for bonding, also known as dative bonding.

Coordination number

The number of coordinate bonds formed between the metal ion and its ligands.

Mono/unidentate ligand

A ligand that forms one co-ordinate bond to the central metal ion (one lone pair to donate).

Bidentate ligand

A ligand that forms two coordinate bonds to the central metal ion.

Multidentate ligand

A ligand that forms three or more coordinate bonds to the central metal ion.

Monodentate ligands

Cl-, H2O, NH3, CN-.

Most common bidentate ligand

ethane-1,2-diamine.

Ethanedioate

Can form 2 coordinate bonds to a transition metal ion.

Benzene-1,2-diol

Can form 2 coordinate bonds to a transition metal ion.

Ethane-1,2-diamine

Forms 2 coordinate bonds, shortened name = en.

EDTA4-

Forms six coordinate bonds.

Chelating agent

EDTA decreases the concentration of metal ions in the solution by binding to them and forming complex ions.

Aqua ion

6 H2O ligands around the central metal ion; forms an octahedral complex ion.

Shape with 2 ligands

Linear.

Shape with 4 ligands

Tetrahedral.

Exception to tetrahedral rule

Platin is square planar → forms cisplatin.

Shape with 6 ligands

Octahedral.

E-Z or cis-trans isomerism

Ligands differ in the way in which they are arranged in space; applies to square planar and octahedral complex ions.

Conditions for optical isomerism

Usually applies to octahedral molecules with 2 or more bidentate ligands, so that the mirror images are non-superimposable.

Cis-platin

Used for cancer treatment.

Mechanism of action of anti-cancer drug

It binds to DNA of fast growing cancer cells and prevents cell division, hence reducing the growth of cancer.

Metal ion in a haem group

Fe2+

Coordination number of a haem group

6

Ligands in haem group

4 nitrogen forming the porphyrin ring, 1 oxygen, 1 globin.

Why is CO toxic?

CO also coordinately bonds to the Fe2+, and bonds more strongly to Fe2+ than O2. Stops O2 from bonding to haemoglobin, so O2 cannot be transported around the body.

Colour of [Cu(H2O)6]2+ solution

Pale blue.

Colour of [Cu(NH3)4(H2O)2]2+ solution

Deep blue.

Colour of [CuCl4]2- solution

Yellow.

Colour of [Cr(H2O)6]3+ solution

Violet

What is a precipitation reaction?

A reaction where soluble ions in separate solutions are mixed to form an insoluble compound.

Colour of Fe2+'s aqua ion

Green.

Colour of Fe3+'s aqua ion

Yellow

Colour of Cr3+'s aqua ion

Violet.

Colour of Mn2+'s aqua ion

Pink.

Colour of precipitate when Cu2+ reacts with NH3 or OH-

Blue.

Colour of precipitate when Fe2+ reacts with NH3 or OH-

Green.

Colour of precipitate when Mn2+ reacts with NH3 or OH-

Brown.

Cr(OH)3

Green precipitate.

Colour of precipitate when Fe3+ reacts with NH3 or OH-

Brown.

Colour of MnO4-

Purple.

Why are redox titrations with transition metal compounds said to be self-indicating?

They usually involve a colour change as the metal is changing oxidation state; sometimes an indicator is still needed/useful.

Colour of Cr2O7 2-

Orange.

Cr(OH)6 3-

Dark green SOLUTION WHY???

Describe how to test for metal ions

Fill half of the test tube with the sample. Add aqueous ammonia or sodium hydroxide drop by drop. Colour of precipitate indicates the ion present.

Method to test for ammonium ions

Fill half the test tube with sample. Add sodium hydroxide and warm gently. Smelly gas is produced; damp red litmus paper turns blue.

Why does lime water turn milky in the presence of carbon dioxide?

When carbon dioxide is bubbled in calcium hydroxide (lime water), calcium carbonate precipitate is formed.