Transition Metals (copy)

5.3.1 and 5.3.2

Transition element

A metal that can form one or more stable ions with an incomplete d sub-level

Which d block elements are not transition elements?

Zn and Sc

Chemical properties of transition metals

• Form complex ions
• Variable oxidation states
• Coloured ions
• Catalysts

Complex ion

A central metal ion surrounded by ligands

Ligand

A molecule or ion that forms a dative covalent bond with a central metal ion by donating a pair of electrons

Co-ordinate bond

A covalent bond where the shared pair of electrons have been donated by the same atom/molecule

Co-ordination number

The number of dative bonds to the central metal

Monodentate ligands

Ligands which donate a lone pair of electrons to form a co-ordinate bond with the central metal ion

Bidentate ligands

Ligands which donate a lone pair of electrons from 2 atoms to form 2 co-ordinate bonds with the central metal ion

Multidentate ligands

A molecule which can donate lone pairs of electrons from more than 2 atoms forming multiple co-ordinate bonds

Co-ordination number of an octahedral

6

Co-ordination number of a tetrahedral

4

Chloride ions and other large ions form

Tetrahedral complexes with a coordination number of 4

Smaller ligands such as H2O, NH3 can usually form

Octahedral complexes with co-ordination number of 6

Silver ions form

Linear complexes with a coordination number of 2 (eg [Ag(NH3)]+)

Which 2 ions form a square planar shape?

Platinum and Nickel

Stereoisomerism

A molecule with the same structural formula but a different arrangement of atoms in space

Optical isomer

These are non superimposable mirror images

Cis ligands

2 ligands are at 90 degrees
to each other

Trans ligands

2 ligands are at
180 degrees to each other

Optical isomers occur with what shape of molecule?

Octahedral with 3 bidentate ligands

Ligand substitution

Where one ligand substitutes another.

If a ligand is substituted for a ligand of a similar size…

Co-ordination number and shape don’t change

If a ligand is substituted for a ligand of a different size (eg Cl- is larger than H2O/NH3)…

Change in co-ordination number and shape

Why is CO toxic?

CO can bind to haemoglobin sites, the same way oxygen can. However, CO forms stronger co-ordinate bonds so the CO cannot be removed. Thus O2 cannot be transported around the body.

When monodentate ligands are replaced with bi/multidentate ligands, there is an increase in…

Entropy

Explain the chelate effect.

Bidentate and multidentate ligands always substitute monodentate ligands because the entropy change is large and positive.

Explain why transition metal complexes are coloured.

When ligands bond to the ions, the d orbitals are split into different energy levels. Electrons move to higher energy orbitals (move to excited state). Certain frequencies of visible visible light are absorbed and colour seen is from the colours which are transmitted.

Energy absorbed equation?

Change in energy (J) = hv (plancks const. x frequency Hz) = hc/ \lambda (plancks const. x speed of light / wavelength (m))

What factors affect the colour of the TM ion?

• The oxidation state of the ion

• Type of ligand

• The co-ordination number

• The metal

How can calorimetry be used to determine the conc. of a coloured compound?

• Measure the absorbance for a range of known concentrations

• Plot a calibration curve (of absorbance and concentration)

• Measure absorbance of the coloured complex and find concentration from graph

Metal aqua ions

• Has 6 water ligands
• Octahedral shape
• Has the same charge as the metal (as water is neutral)

Vanadium exists as… (you better get vanadium!)

• +5 (yellow VO2 +)

• +4 (blue VO 2+)

• +3 (green V 3+)

• +2 (violet V 2+)

+ charged species tend to…

• form in acidic conditions

• be easier to reduce

• harder to oxidise

- charged species tend to…

• form in acidic conditions

• be easier to reduce

• harder to oxidise

How to reduce vanadium to lower oxidation states?

Add a reducing agent (eg by adding zinc metal in acidic solution)

Tollens reagent

When added to aldehydes, gives silver mirror. It cannot reduce ketones

Redox potential is…

How easily an ion/atom is reduced to a lower oxidation state

The redox titration between Fe2+ with MnO4- and colour change

MnO4 + 8H+ + 5Fe2+ → Mn2+ + 4H20 + 5Fe3+

Purple —> colourless

Why is acid added to Fe2+ and which acid is it?

The acid is needed to supply the 8H+ ions. We use dilute sulphuric acid for manganate titrations.

Titration of C2O4 2- with MnO4- overall equation

2MnO4- + 16H+ + 5C2O4 2- → 10C02 + 2Mn2+ + 8H20

Why do transition metals make good catalysts? + catalyst definition

Because they have variable oxidation states as they can gain and lose e-s from d orbital. A catalyst speeds up the RoR w/o being used up in the reaction by providing an alternate pathway w/ a lower Ea

Heterogeneous Catalyst

Present in a different phase from the reactants (usually solid with gas reactants)

Homogeneous Catalyst

Present in the same phase as the reactants (usually in solution)

How do heterogeneous catalysts work?

Reactants are adsorbed onto surface of catalyst, weakening the bonds. The reaction takes place and then the products are desorbed.

Contact process (Overall equation: 2SO2 + O2 → 2SO3)

Step 1: SO2 + V2O5 → SO3 + V2O4

Step 2: 2V2O4 + O2 → 2V2O5

Catalytic poisoning

When heterogeneous catalysts have impurities, the active sites are blocked on the surface of the catalyst so the reactants are no longer absorbed

How do homogeneous catalysts work?

Works by forming an intermediate which then reacts to form the products and reform the catalyst.

Example for homogeneous catalysts (between iodine ions and peroxodisulfate S2O82-, catalysed by Fe2+ ions)

Overall: S2O8 2- + 2I- → 2SO4 2- + I2

stage 1: S2O8 2- + 2Fe 2+ → 2Fe 2+ + I2

stage 2: 2Fe3+ + 2I- → 2Fe2+ + I2

Autocatalysis

When one of the products of a reaction is a catalyst for the reaction

Example for autocatalysis

Overall: 2 MnO4 + 5C2O4 2- + 16H+ → 2Mn2+ + 10 CO2 + 8 H2O

The autocatalysis by Mn2+ in titrations of C2O4 2- with MnO4-

Step 1: 4Mn2+ + MnO4- + 8 H+→ 5Mn3+ + 4 H2O

Step 2: 2Mn3+ + C2O4 2- → 2Mn+2 +2CO2

Cu2+ metal aqua ion with OH- colour

Blue precipitate

Fe2+ metal aqua ion with OH- colour

Green precipitate

Fe3+ metal aqua ion with OH- colour

Brown precipitate

Al3+ metal aqua ion with OH- colour

White precipitate

Cu2+ metal aqua ion with NH3 colour

Blue precipitate (initially)

Cu2+ metal aqua ion with excess NH3 colour

Deep blue precipitate

Fe2+ metal aqua ion with NH3 colour

Green precipitate [Fe(H2O)4(OH)2]

Fe3+ metal aqua ion with NH3 colour

Brown precipitate [Fe(H2O)3(OH)3]

Al3+ metal aqua ion with NH3 colour

White precipitate [Al(H2O)3(OH)3]

Amphoteric

A molecule that is able to react both as a base and as an acid

Testing for presence of a carbonate

• Add any dilute acid (ideally nitric acid)
• Observe for effervescence

Testing for presence of a sulphate

• Add BaCl2
• If present, white precipitate will form

Testing for presence of halide ions

• Add silver nitrate (AgNO3)

• Precipitates will form if present

• Chloride: white precipitate

• Bromide: cream precipitate

• Iodide: yellow precipitate

Silver chloride will dissolve in

Dilute ammonia

Silver bromide will dissolve in

Concentrated ammonia

Sequence of testing for negative ions

Carbonate, sulphate, halide

Test for ammonium ions

• React with warm NaOH
• Ammonia gas formed can be identified by turning red litmus paper blue