Transition Metals

Question 1

Transition Metals

Answer 1

Transition metals are metals that form one or more stable ions with an incomplete d sub-shell

Characteristics

• hard metals with high melting and boiling points
• variable oxidation numbers
• form coloured ions in solution
• act as catalysts as elements and in compounds
• form complex ions

Variable Oxidation Numbers

The transition metal elements from titanium to copper have electrons of similar energy in both the 3d and 4d levels so the elements can form ions of roughly the same stability in aqueous solution or in crysatalline solid by losing different numbers of electrons

When an ion is more highly charged, there are more electrons that must be removed, so the ionisation energy is higher, and more energy is released

Question 2

Complex Ion formation and Ligands

Answer 2

A ligand is a species that uses a lone pair of electrons to form a dative bond with a metal ion and can be in the form of atoms, ions or molecules. They form dative bonds by donating a lone pair of electrons onto empty orbitals of the transition metal ion.

A complex is a central metal ion surrounded by ligands and a complex ion is a complex with an overall positive or negative charge

As dative bonds are used to form ligands, complexes are essentially central metal ions surrounded by dative bonds

The coordination number is the number of dative bonds formed to the central metal ion

Monodente Ligands: only use one lone pair of electrons to form a co-ordinate bond e.g. H₂O, OH^-, NH₃

Bidentate Ligands: molecule that donates 2 lone pairs of electrons, NH₂CH₂CH₂NH₂

Multidente Ligands: can from several dative covalent bonds, EDTA^4-

Question 3

Colour of Transition Metals

Answer 3

Fe²⁺ is GREEN

Fe³⁺ is BROWN

Co²⁺ is PINK

Cu²⁺ is BLUE

Cr³⁺ is DARK BLUE

Question 4

Shapes of Complexes

Answer 4

• co-ordination number of 6, the ligands usually have take an octahedral shape so that the position of the six electron pairs around the centre can repel as much as possible
• co-ordination number of 4, the ligands usually take a tetrahedral position ([CuCl₄]^2- and [CoCl₄]^2-) , although some have a square planar structure ([Pt(NH₃)Cl₂])
• co-ordination number of 2, the ligands take a linear structure

Question 5

Cis-Platin

Answer 5

4 bonding pairs

Used as an anti-cancer drug

• cis version only works if 2 Cl^- ions are displaced and the molecule joins to the DNA through dative and hydrogen bonding
  
  • Stops the replication of cancer cells
• supplied as a single isomer and not in a mixture with the trans form
  
  • increases the risk of it attaching to healthy cells

Question 6

Identifying Ligands

Answer 6

Bidentate Ligands: molecule that donates 2 lone pairs of electrons, NH₂CH₂CH₂NH₂

Multidente Ligands: can from several dative covalent bonds, EDTA^4-

Question 7

Haemoglobin

Answer 7

It is an iron (II) complex containing a multidentate ligands

When an oxygen molecule is bound to haemoglobin is replaced by a carbon dioxide molecule - this is ligand exchange

Question 8

Deprotonation

Answer 8

The hydroxides and the ammonia are acting as bases (proton acceptors) and removing protons from water ligands

e. g. Deprotonation of hexaaquacopper reacting with NaOH
1. The hydroxides remove hydrogens from 2 waters leaving 2OH^- and 2 ammonias

Question 9

Ligand Exchange

Answer 9

The ammonia moelcules are replacing the water ligands and donating a lone pair of electrons to form a dative covalent bond

Question 10

Why are colours seen?

Answer 10

Colours arise when the electrons are promoted from their ground state to excited state between different d orbitals

1. When visible light passes through a substance, a d-electron is promoted to an excited state as it absorbs light energy
2. The frequency of light that causes the promotion is absorbed and removed from the white light
3. The remaining frequencies of light and seen and a colour is observed

The colour of light absorbed depends on:

• the nature and oxidation state of the transition metal ion
• the ligand
• the coordination number

They change the energy split between the d orbitals and hence affect the frequency of light absorbed

The colour of the ions results from the splitting of the energy levels of the d-orbitals by ligands

Question 11

Colours of Vanadium Oxidation States

Answer 11

(+5) [VO2]⁺ → Yellow

(+4) [VO]²⁺ → blue

(+3) [V]³⁺ → green

(+2) [V]²⁺ → violet

Learn redox equations for the interconversion of the oxidation states of vanadium

Question 12

Reduction of Cr³⁺ to Cr²⁺ using Zinc in Acidic Conditions

Answer 12

Cr³⁺ and Cr²⁺ are formed by the reduction of Cr₂O₇^2- by zinc in HCl (acidic conditions)

Zinc has a more negative electrode potential than all chromium half equations so zinc will reduce Chromium ions

[Cr2O7]2- + 3Zn + 14H+ –> 2Cr3+ + 7H2O + 3Zn2+

[Cr2O7]2- + 4Zn + 14H+ –> 2Cr2+ + 7H2O + 4Zn2+

Question 13

Oxidation of Cr³⁺ to Cr₂O₇^2- using Hydrogen Peroxide in Alkaline Conditions

Answer 13

Transition metals in low oxidation states are more easily oxidised in alkaline solutions than in acidic solutions

• It is easier to remove an electron from a negatively charged ion
• The chromium ions can be oxidised using hydrogen peroxide in alkaline conditions

[Cr(OH)6]3- → [CrO4]2-

Reduction :

H2O2 + 2e- → 2OH-

Oxidation:

[Cr(OH)6]3- + 2OH- → CrO42-+ 3e- + 4H2O

2[Cr(OH)6]3- + 3H2O2 → 2CrO42 +2OH- + 8H2O

Question 14

Conversion of Dichromate Ions into Chromate Ions

Answer 14

The chromate and dichromate ions can be converted from one to the other by the following equilibrium reaction:

2CrO₄^2- + 2H⁺ → Cr₂O₇^2- + H₂O

Yellow → orange

1. The addition of acid, pushes the equilibrium to the dichromate
2. The addition of alkali, removes the H+ ions and pushes the equilibrium to the chromate

This is not a redox reaction as they both have an oxidation number of +6

Question 15

Catalysts

Answer 15

Catalysts increase the rate of the reaction by providing an alternative pathway for the reaction with a lower activation energy

Transition metals and their compounds can act as heterogeneous (different phase) and homogeneous catalysts (same phase)

Catalysts provide economic benefits for a reaction as they require less energy to cause a reaction so there are lower energy costs

Question 16

Heterogeneous Catalysts

Answer 16

A heterogeneous catalyst is in a different phase from the reactants (the catalyst is normally the solid e.g. the contact process)

The reaction occurs at the surface of the catalyst on the active sites

1. Adsorption of reactants at active sites may lead to catalytic action
2. This adsorption weakens bonds and the molecules are held in a more reactive configuration
3. There will be a higher concentration of reactants on the surface leading to a higher collision frequency
4. The reactants react and the products are then desorbed into the stream of gases

The strength of adsorption determines the effectiveness of the catalytic activity

• Too strong adsorption means the products cannot be released
• Too weak adsorption means the reactants do not absorb in high enough concentration
• Nickel and platinum have the right strength and are the most useful catalysts