transition metals

whats the difference between transition metals and d-block elements?

• transition metals form stable ions that have partially filled d-subshell

• d-block elements just have outer electron in d orbital

e.g. scandium and zinc are d-block elements but are not transition metals

why is zinc not a transition metal?

zinc is not a transition metal because zinc can only form a +2 ion, Zn²⁺which has a complete d-orbital

4 chemical properties of transition metals

1. form complex ions

2. form coloured ions

3. variable oxidation states e.g. Fe(II) and Fe(III)

4. acts as catalysts

suggest why ions from s block elements do not usually act as catalysts

bc they only exist in one oxidation state

Why do transition metals have variable oxidation states?

because the electrons sit in the 4s and 3d subshells

- which are very close in energy

types of ligands

1. monodentate - forms one coordinate bond per ligand

   • e.g. H2O and NH3 and Cl-

   • H2O and NH3 ligands are uncharged, therefore the exchange of H2O and NH3 occur without a change in oxidation state

   • however, the Cl- ligand has a charge and is larger than the H2O and NH3 ligands, so the exchange with a Cl- ligand changes the coordination number

2. bidentate - coordinate bonds from 2 atoms

   • e.g. (C2O4)2- and NH3CH2CH2NH3

3. multidentate - forms 6 coordinate bonds per ligand

   • e.g. EDTA4-

total oxidation state of metal equation

total oxidation state of metal = the total oxidation state - total oxidation state of ligands

what happens if a compound has a full (3)d subshell?

cannot absorb visible light

• therefore compound is white

In terms of bonding, explain the meaning of the term complex.

an atom/ion/transition metal bonded to one or more ligands by coordinate bonds

a central metal atom/ion surrounded by ligands.

coordination number definition

the number of coordinate bonds formed to a central metal ion

coordinate bond definition

Co-ordinate bonding is when the shared pair of electrons in the covalent bond come from only one of the bonding atoms.

• where both electrons are donated by one atom

Explain why coordinate bonds can be formed between transition metal ions and water molecules

• The oxygen in the water molecule has a lone pair of electrons which it can donate to the transition metal ion

• The transition metal ion can accept electron pairs

explain how a coordinate bond is is formed between a transition metal ion and a ligand

An electron pair on the ligand is donated from the ligand to the central metal ion

define ligand

an atom/ion/molecule that has atleast 1 l.p of electrons and donates an electron pair to a central metall ion

• forms a co-ordinate bond with a transition metal by donating a pair of electrons.

what allows a molecule to behave as a ligand?

they have a lone pair of electrons

explain why the chloride ions in [Co(NH3)₆]Cl₃ are not considered to be acting as ligands in this complex

because:

• the chloride ions are not bonded to the cobalt ion

• they dont donate a lone pair of electrons to the cobalt ion

Give an example of a linear complex ion

[Ag(NH3)₂]⁺

Give an example of a tetrahedral complex ion

[CuCl₄]^2-

Give an example of a square planar complex ion

(NH₃)₂PtCl₂

structure of ethanedioate

structure of ethane-1,2-diamine

How oxygen is transported in the blood

oxygen forms a coordinate bond to the Fe(II) in the haemoglobin

Why CO is toxic to humans

CO can form a strong coordinate bond with haemoglobin

• this bond that CO makes is stronger than the bond that oxygen makes with haemoglobin

• so the CO replaces the oxygen

what does the complex shape of ions depend on?

1. size of ligands

2. coordination number

The Chelate effect

The substitution of a monodentate ligands with a bidentate or multidentate ligand, resulting in a more stable complex

• positive entropy change, because there are more moles of products than there are moles of reactants

  • more particles are formed

  • so therefore disorder increases

• the greater the entropy change, the more negative the free energy change (ΔG)

  • therefore, the reaction is more favourable

• the enthalpy change (ΔH) for a ligand substitution reaction is very small

  • this is because the bonds being formed are very similar to the bonds that are broken

  • therefore, the enthalpy change is near 0

Cisplatin and Transplatin structure

cisplatin: used as an anti-cancer drug

transplatin: no current medical use

explain why complexes formed from transition metal ions are coloured

• absorbs wavelengths of light to excite electrons in d-orbital

• complementary wavelength of light transmitted to give colour seen

how a calibration graph is produced and used to find the concentration of a complex

1. add appropriate ligand to intensify colour if necessary

2. make solutions of known conc.

3. measure absorbance from known conc.

4. plot the graph of calibration curve: plot graph of conc. vs absorbance

5. read value of conc. for the measured absorbance from this graph

How to calculate ΔE (change in energy)

ΔE = hv = hc / wavelength

ΔE : energy absorbed by the electron, from ground to excited state

h: planks constant

v: frequency of light (Hz)

c: speed of light (m/s)

how does ΔE determine colour

ΔE affects frequency of absorbed photons

• determines colour

what affects the colour of transition metal compounds

1. oxidation state of metal

2. no. of ligands

3. type of ligands

4. shape

5. coordination no.

Complete ligand substitution of hexaaquacobalt with ammonia

[Co(H2O)6] 2+ + 6NH3 -----> [Co(NH3)6]2+ + 6H2O

when do incomplete ligand substitution of hexaaquacopper reactions occur?

when you use excess ammonia with copper

incomplete ligand substitution of hexaaquacopper and colour change

[Cu(H2O)6]²⁺ + 2NH3 —> Cu(H2O)₄(OH)₂ + 2NH₄⁺

Cu(H2O)₄(OH)₂ + 4NH3 —> [Cu(NH3)₄(H2O)₂]²⁺ + 2OH^- + 2H₂O

blue ppt. dissolves to form deep blue solution

• colour change from blue solution to dark blue solution

ligand substitution reaction of hexaaquacopper and Cl- ligand, including colour change

[Cu(H2O)6]2+ + 4Cl- ----> [CuCl4]2- + 6H2O

colour change from blue solution to yellow solution

ligand substitution reaction of hexaaquacobalt and Cl- ligand, including colour change

[Co(H2O)6]2+ + 4Cl- ----> [CoCl4]2- + 6H2O

colour change from pink solution to blue solution

ligand substitution reaction of hexaaquairon (II) and Cl- ligand, including colour change

[Fe(H2O)6]2+ + 4Cl- ----> [FeCl4]2- + 6H2O

colour change from purple solution to yellow solution

Ligand subsitution reaction of ethane-1,2-diamine w/ hexaaquacopper

[Cu(H2O)6]2+ + 3NH2CH2CH2NH2 -----> [Cu(NH2CH2CH2NH2)3]2+ + 6H2O

Ligand substitution reaction of ethanediaote w/ hexaaquacopper

[Cu(H2O)6]2+ + 3C2O42- -----> [Cu(C2O4)3]4- + 6H2O

when does the partial substition of ethanedioate ions occur?

• may occur when a dilute aqueous solution containing ethanedioate ions is added to a solution containing aqeous copper (II) ions

partial substitution of ethanedioate ions reaction equation

[Cu(H₂O)₆]²⁺ + 2C₂O₄²⁻ ——> [Cu(C₂O₄)₂(H₂O)₂]²⁻ + 4H₂O

vanadium chemistry

	oxidation state	colour
VO2⁺	+5	yellow
VO²⁺	+4	blue
V³⁺	+3	green
V²⁺	+2	violet

ratio between between MnO₄⁻ and Fe²⁺

1:5

ratio between between MnO₄⁻ and C₂O₄²⁻

2:5

ratio between between C₂O₄²⁻ and Fe²⁺

1:2

role of catalysts

catalysts speed up reaction and lowers Ea for reaction and is left unchanged

heterogenous catalyst

catalyst in different phase from reactants

homogenous catalyst

catalyst in same phase as reactant

how platinum acts as a catalyst by providing an alternative route

• platinum provides an active site

• reaction on the surface: bonds weakening/breaking

• desorption of the product

catalyst poisoning

sulfur poisons/binds to catalyst

• active sites blocked

why reaction b/w peroxodisulfate (VI) ions and iodide ions is slow before catalyst is added

• 2 negative ions repel

• therefore Ea is high

Write the equation for reaction between iodide ions and peroxodisulfate ions

Explain why the activation energy for each of these reactions is low

Explain why Fe3+ ions are as effective as Fe2+ ions in catalysing this reaction

• Ea is low bc each reaction has oppositely charged ions which attract

• Fe3+ ions as effective as Fe2+ ions since equations 1&2 can occur in any order

autocatalysis reaction

catalytic converter

uses solid Rh catalyst

contact process

uses solid V2O5 as a catalyst

haber process

uses solid iron catalyst