Transition elements

Question 1

What is a transition element?

Answer 1

A d-block element that forms at least one ion with an incomplete d sub-shell.

Question 2

What are the rules for filling in electron configurations of (period 3) d-block elements?

Answer 2

1. S sub-shell of the level above always fills before d sub-shell.
2. For the 2 elements Cr and Cu, an electron from 4s shell shifts to 3d shell to form 3d⁵4s¹and 3d¹⁰4s¹ configurations respectively.
3. For an ion of a d-block element, electron are always lost from the s-shell first.

Question 3

What are the properties of transition elements and why?

Answer 3

1. Variable oxidation states: Even though 4s electrons are lost first to form 2+ ions, because 3d sub-shell is so close to 4s energy level, a number of electrons can also be lost from 3d sub-shell to form stable ions with range of oxidation number.
2. Used as catalysts: Due to having many oxidation states, transition elements are often used as catalysts for redox reactions.
3. Form coloured compounds: Due to electrons in partially filled sub-shells having ability to absorb certain wavelengths of visible light, the wavelenghts not absorbed are emitted. This results in transition element compounds often being coloured.
4. Form complex ions: Transition element ions are able to form complex ions with ligands when in solution .

Question 4

What are the main ways that transition metals act as catalysts?

Answer 4

1. Adsorption: The metals provide surface for reactants to adsorb onto. They are held in place which allows reaction to take place more readily and the products are desorbed from surface. E.g. Pt in catalytic onverters.
2. Forming intermediates: During redox reaction, transition metals are able to react with reactants and be converted to a different oxidation state, providing pathway with lower activation energy.

Question 5

What are examples of transition metal catalysts in industry?

Answer 5

1. Haber process: Iron metal used as catalyst to increase rate of reaction and decrease required temperature.
2. Contact process: V₂O₅ used as catalyst in converting SO₂ to SO₃ in production of sulfuric acid.
3. Hydrogenation of alkenes: Nickel metal catalyst used in halogenation of unsaturated compounds to saturated compounds (by adding H₂ across C=C bond). This is important in converting vegetable oil to margarine.
4. MnO₂ used to catalyse decomposition of H₂O₂ to O₂ and H₂O. This increases rate of decomposition and is often used to produce O₂ in laboratory.

Question 6

What are the changes observed when NaOH is added to aqueous Cu²⁺ ions?

Answer 6

Pale blue solution containing Cu²⁺(aq) reacts with hydroxide to form plae blue (gelatinous) precipitate.

Equation:

Cu²⁺(aq) + 2OH^-(aq) → Cu(OH)₂(s)

Question 7

What are the changes observed when NaOH is added to aqueous Co²⁺ ions?

Answer 7

Pink solution containing Co²⁺(aq) reacts with hydroxide to form blue (gelatinous) precipitate, which turns beige when exposed to air.

Equation:

Co²⁺(aq) + 2OH^-(aq) → Co(OH)₂(s)

Question 8

What are the changes observed when NaOH is added to aqueous Fe²⁺ ions?

Answer 8

Pale green solution containing Fe²⁺(aq) reacts with hydroxide to form a green (gelatinous) precipitate, which turns rusty brown when exposed to air.

Equation:

Fe²⁺(aq) + 2OH^-(aq) → Fe(OH)₂(s)

Question 9

What are the changes observed when NaOH is added to aqueous Fe³⁺ ions?

Answer 9

Pale yellow solution containing Fe³⁺(aq) reacts with hydroxide to form a rusty brown (gelatinous) precipitate.

Equation:

Fe³⁺(aq) + OH^-(aq) → Fe(OH)₃(s)

Question 10

What is a complex ion?

Answer 10

A transition metal ion bonded to one or more ligands by co-ordinate bonds (dative covalent bonds).

Question 11

What is a ligand?

Answer 11

A molecule or ion that can donate a pair of electrons to the transition metal ion to form a coordinate bond.

Question 12

What is the coordination number?

Answer 12

The total number of coordinate bonds formed between a central metal ion and its ligands.

Question 13

What are the common 3D shapes of ligands?

Answer 13

Number of ligands:

6 - Octahedral (bond angle = 90°)

4 - Tetrahedral (Bond angle = 109.5°)
Square plainer, for Ni and Pt (bond angle = 90°)

Question 14

What are stereoisomers?

Answer 14

Elements with the same structural formula but different arrangement of atoms in space.

Question 15

What types of stereoisomerism are associated with complex transition metal ions?

Answer 15

1. Cis-trans isomerism.
2. Optical isomerism.

Question 16

What are monodentate ligands?

Answer 16

A ligand which is able to donate up to 1 pair of electrons to the central metal ion to form 1 coordinate bond.

Question 17

What types of complex ions that contain only monodentate ligands display cis-trans isomerism?

Answer 17

• Octahedral complex ions that contain 2 of one ligand and 4 of the other.
• Square plainer complex ion that contains 2 of one ligand and 2 of the other.

Question 18

What are the properties of a cis isomer?

Answer 18

• 2 of the same ligands (square plainer) or the ligand which only 2 are present (octahedral) are located on adjacent corners.
• The ligand are at 90° to each other.

Question 19

What are the properties of a trans isomer?

Answer 19

• 2 of the same ligands (square plainer) or the ligand which only 2 are present (octahedral) are on opposite corners.
• The ligands are at 180º to each other.

Question 20

What are bidentate ligands?

Answer 20

Ligands which are able to donate up to 2 pairs of electrons to the central metal ion to form 2 coordinate bonds.

Question 21

What are some examples of bidentate ligands?

Answer 21

• Ethane-1,2-diame (NH₂CH₂CH₂NH₂), or ‘en’.
• Ethanedioate (C₂O₄^2-).

Question 22

What types of complex ions containing bidentate ligands show cis-trans isomerism?

Answer 22

Octahedral complex ions that contain 2 bidentate complex ions and 2 monodentate complex ions.

Question 23

What are the properties of a (bidentate) cis isomer?

Answer 23

• The 2 monodentate ligands are on adjacent corners.
• They are at 90° to each other.

Question 24

What are the properties of (bidentate) trans isomer?

Answer 24

• The 2 monodentate ligands are on opposite corners.
• They are at 180° to each other.

Question 25

What are hexadentate ligands?

Answer 25

Ligands that can donate up to 6 pairs of electrons to the central metal ion to form 6 coordinate bonds.

Question 26

What is an example of a hexadentate ligand?

Answer 26

Ethylenediaminetetraacetic acid, or EDTA. It exists as EDTA^2- ions when acting as ligands.

Question 27

What are optical isomers?

Answer 27

Stereoisomers that are non-superimposable mirror images of each other.

Question 28

What types of complex ions exhibit optical isomerism?

Answer 28

• Octahedral ion consisting of 3 bidentate ligands.
• Octahedral ion consisting of 2 bidentate ligands and 2 monodentate ligands in the cis isomeric form.
• Complex ion consisting of a hexadentate ligand.

Question 29

What is a ligand substitution reaction?

Answer 29

A reaction whereby one ligand in a complex ion is replaced by a different ligand.

Question 30

What are the obsevations when aqueous ammonia solution is added to a solution of aqueous Cu²⁺ ions?

Answer 30

1. On addition of small amounts of ammonia, a pale blue precipitate of Cu(OH)₂ is formed.

Cu^2+(aq) + 2OH^-(aq) → Cu(OH)₂(s)

2. On addition of excess ammonia, the blue precipitate disappears and the pale blue mixture turns into a dark blue solution.

[Cu(H₂O)₆]²⁺(aq) + 4NH₃(aq) ⇔ [Cu(NH₃)₄(H₂O)₂]²⁺(aq) + 4H₂O(l)

Question 31

What are the observations when HCl(aq) is added to aqueous Cu²⁺ ions?

Answer 31

The pale blue initially turns into a clear green solution and then into a yellow solution.

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

Question 32

What are the observation when excess distilled water is added to yellow [CuCl₄]^2- solution?

Answer 32

Yellow solution turns into a pale blue solution as the position of equilibrium in reaction:

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

is shifted to the left.

Question 33

What are the observations when conc. HCl(aq) is added to aqueous Co²⁺ solution?

Answer 33

Pale pink solution turns into a dark blue solution.

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

Question 34

Why are octahedral -(H₂O)₆ complex ions always substituted for tetrahedral/square plainer Cl₄ ions?

Answer 34

Cl^- ions are much larger than H₂O molecules and have a -ve charge, so they have stronger repulsions between each other. This results oin less being able to fit around a central metal ion.

Question 35

What is the structure of the complex ion formed around the Fe²⁺ ion in the haem groups of haemoglobin?

Answer 35

• An octahedral structure forms around each Fe²⁺ ion in each haem group.
• 4 bonds are made to nitrogen atoms.
• 1 bond is made to a globin protein.
• 1 bond is available to bind to O₂ atoms, which allows haemoglobin to carry O₂ molecule.

Question 36

How does carbon monoxide poisoning occur?

Answer 36

CO has the same binding site to haem as O₂ molecules, but are stronger ligands than O₂ molecules, so when CO present, more haemoglobin will prefer to carry CO than O₂. The process is irreversible so less haemoglobin is available in blood to carry O₂. Tissues are starved of oxygen and eventually die, leading to death.

Question 37

What is the stability constant, K_stab?

Answer 37

The equilibrium constant for an equilibrium existing between a transition metal ion surrounded by water ligands and the complex formed when the same ion has undergone a ligand substitution.

Question 38

What is the general formula for K_stab?

Answer 38

For equilibrium:

[M(H₂O)_n] + xL ⇔ [ML_x] + nH₂O

K_stab = [[ML_x]]/[[M(H₂O)_n]]*[L]^x

Question 39

What is the significance of the K_stab value?

Answer 39

The greater the K_stab value, the further the position of equilibrium lies to the right, the more stable the complex ion formed is (compared to H₂O ligands), the stronger the ligand (compared to H₂O).