The transition metals - Chapter 23 Flashcards
Define a transition metal
A metal that forms at least one stable ion with a partially full d-shell of electrons.
Chemical properties of transition metals
- Variable oxidation states
- Colours
- Catalysts
- Form complex ions
What is a ligand?
An ion or molecule with a lone pair that forms a co-ordinate bond with a transition metal.
What is the co-ordination number?
The number of co-ordinate bonds surrounding the d-block metal ion
Give an example of a square planar ion
[Ni(CN)4]2-
Aqua ions
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.
Multidentate ligands
Have more than one atom with a lone pair of electrons e.g. EDTA4-
Chelate effect
If you add a hexadentate ligand (e.g. EDTA) to a solution with a ligand, the EDTA will replace the water.
Why are transition metal ion complexes coloured?
- They absorb some wavelengths of visible light
- To promote electrons in d-orbitals
- The remaining wavelengths of light in combination give the colour seen
Energy difference equation
ΔE = hv
E - Energy
v - Frequency
h - Planck’s constant
Colours of complexes in oxidation states of 2+ and 3+
Oxidation state 2+ 3+
Iron complexes Green Pale brown
Chromium complexes Blue Red-Violet
Cobalt complexes Brown Yellow
How does colorimetry work?
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.
Why do transition metals have greater variable oxidation states?
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.
Redox titration
- Using a burette, gradually add potassium manganate (VII) solution to a solution containing Fe2+ ions that is acidified with excess dilute sulfuric acid.
- The MnO4- ions from the potassium manganate are converted to Mn2+ ions, turning the solution colour from purple to pale pink.
- 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+
Why can’t HCL be used in place of H2SO4 in a redox titration?
Cl- ions are oxidised by MnO4- ions. This will affect the titration because the MnO4- ions are only supposed to oxidise the Fe2+ ions.
What are the two types of catalyst?
Heterogenous
Homogenous
What is a heterogenous catalyst?
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)
How do you make heterogenous catalysts more efficient?
- Increase their surface area
- Spread the catalyst onto a support medium, this increases the surface to mass ratio and is less expensive
Why do catalysts not last forever?
- Over time their surfaces become covered in impurities, this is called poisoning
- A catalyst may gradually be lost from it’s support medium
The Haber process and catalyst
N2 (g) + 3H2 (g) ⇌ 2NH3 (g)
Iron lumps = catalyst
What is a common impurity that causes poisoning in catalysts?
Sulfur compounds
The Contact process
2SO2 + O2 ⇌ 2SO3
SO2 + V205 —-> SO3 + V204
2V204 + O2 —-> 2V205
What is the contact process a good example of?
How the variability in transition metal oxidation states makes them useful as catalysts
What is a homogenous catalyst?
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)
Homogenous catalyst example
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.
What is autocatalysis? And what does the rate of reaction do in a reaction with an autocatalyst?
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.
Give an example of an autocatalysis reaction
2MnO4- (aq) + 16H+ + 5C204 2- (aq) —-> 2Mn 2+ (aq) + 8H20 (l) + 10CO2 (g)
Catalyst - Mn2+ ions
In terms of bonding, explain what is meant by the term complex
A transition metal bonded to ligands by co-ordinate bonds.
Why do ligands with Cl only form four co-ordinate bonds?
Because chlorine is a large molecule
