The transition metals - Chapter 23

Question 1

Define a transition metal

Answer 1

A metal that forms at least one stable ion with a partially full d-shell of electrons.

Question 2

Chemical properties of transition metals

Answer 2

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

Question 3

What is a ligand?

Answer 3

An ion or molecule with a lone pair that forms a co-ordinate bond with a transition metal.

Question 4

What is the co-ordination number?

Answer 4

The number of co-ordinate bonds surrounding the d-block metal ion

Question 5

Give an example of a square planar ion

Answer 5

[Ni(CN)4]2-

Question 6

Aqua ions

Answer 6

When you dissolve the salt of a transition metal in water, the positively charged metal ion becomes surrounded by water molecules acting as ligands. If this is six water molecules in an octahedral arrangement then this is an aqua ion.

Question 7

Multidentate ligands

Answer 7

Have more than one atom with a lone pair of electrons e.g. EDTA4-

Question 8

Chelate effect

Answer 8

If you add a hexadentate ligand (e.g. EDTA) to a solution with a ligand, the EDTA will replace the water.

Question 9

Why are transition metal ion complexes coloured?

Answer 9

1. They absorb some wavelengths of visible light
2. To promote electrons in d-orbitals
3. The remaining wavelengths of light in combination give the colour seen

Question 10

Energy difference equation

Answer 10

ΔE = hv
E - Energy
v - Frequency
h - Planck’s constant

Question 11

Colours of complexes in oxidation states of 2+ and 3+

Answer 11

Oxidation state 2+ 3+
Iron complexes Green Pale brown
Chromium complexes Blue Red-Violet
Cobalt complexes Brown Yellow

Question 12

How does colorimetry work?

Answer 12

A detector measures the amount of light of a particular wavelength that passes through a coloured solution. The more concentrated the solution, the less light is transmitted through.

Question 13

Why do transition metals have greater variable oxidation states?

Answer 13

A typical transition metal can use it’s 3d electrons as well as it’s 4s electrons in bonding, meaning it can have a higher variety of oxidation states in different compounds.

Question 14

Redox titration

Answer 14

1. Using a burette, gradually add potassium manganate (VII) solution to a solution containing Fe2+ ions that is acidified with excess dilute sulfuric acid.
2. The MnO4- ions from the potassium manganate are converted to Mn2+ ions, turning the solution colour from purple to pale pink.
3. Once enough potassium manganate has been added to react with all the Fe2+ ions one more drop will turn the solution purple. This is the titrations end point.
   MnO4- + 8H+ + 5Fe2+ → Mn2+ + 4H2O + 5Fe3+

Question 15

Why can’t HCL be used in place of H2SO4 in a redox titration?

Answer 15

Cl- ions are oxidised by MnO4- ions. This will affect the titration because the MnO4- ions are only supposed to oxidise the Fe2+ ions.

Question 16

What are the two types of catalyst?

Answer 16

Heterogenous

Homogenous

Question 17

What is a heterogenous catalyst?

Answer 17

A catalyst present in a reaction that is in a different phase than the reactants. (Usually present as solids, the reactants pass over the catalyst surface)

Question 18

How do you make heterogenous catalysts more efficient?

Answer 18

• Increase their surface area

- Spread the catalyst onto a support medium, this increases the surface to mass ratio and is less expensive

Question 19

Why do catalysts not last forever?

Answer 19

• Over time their surfaces become covered in impurities, this is called poisoning
• A catalyst may gradually be lost from it’s support medium

Question 20

The Haber process and catalyst

Answer 20

N2 (g) + 3H2 (g) ⇌ 2NH3 (g)

Iron lumps = catalyst

Question 21

What is a common impurity that causes poisoning in catalysts?

Answer 21

Sulfur compounds

Question 22

The Contact process

Answer 22

2SO2 + O2 ⇌ 2SO3

SO2 + V205 —-> SO3 + V204

2V204 + O2 —-> 2V205

Question 23

What is the contact process a good example of?

Answer 23

How the variability in transition metal oxidation states makes them useful as catalysts

Question 24

What is a homogenous catalyst?

Answer 24

A catalyst present in a reaction that is in the same phase as the reactants. When catalysts and reactants are in the same phase, the reaction proceeds through an intermediate species. (Usually gas or liquid)

Question 25

Homogenous catalyst example

Answer 25

S2O8 2- (aq) + 2Fe 2+ (aq) —-> 2SO4 2- (aq) + 2Fe 3+ (aq)
2Fe 3+ (aq) + 2I - (aq) —-> 2Fe 2+ (aq) + I2 (aq)

The Fe ions are the catalysts.
This reaction has a high activation energy as it is between two negative ions that repel.

Question 26

What is autocatalysis? And what does the rate of reaction do in a reaction with an autocatalyst?

Answer 26

When one of the products of the reaction is the catalyst. This means the reaction rate initially speeds up as more catalyst is generated, and then slows down.

Question 27

Give an example of an autocatalysis reaction

Answer 27

2MnO4- (aq) + 16H+ + 5C204 2- (aq) —-> 2Mn 2+ (aq) + 8H20 (l) + 10CO2 (g)

Catalyst - Mn2+ ions

Question 28

In terms of bonding, explain what is meant by the term complex

Answer 28

A transition metal bonded to ligands by co-ordinate bonds.

Question 29

Why do ligands with Cl only form four co-ordinate bonds?

Answer 29

Because chlorine is a large molecule