3.2.5 Transition Metals Flashcards
Explain the chelate effect
Bidentate and multidentate ligands replace monodentate ligands from complexes
Increase in number of species present so increase in entropy
What causes colours of transition metals
Colour arises when some of the wavelengths of visible light are absorbed and the remaining wavelengths of light are transmitted or reflected.
d electrons move from the ground state to an excited state when light is absorbed.
What causes change in colour of transition metal complexes
Change in oxidation state, co-ordination number and ligand alter change E
What causes Vanadium species in oxidation states IV, III and II
Reduction of V ions by zinc in acidic solution
What influences redox potential for a transition metal ion changing from high to lower oxidation state
pH and the ligand
Heterogeneous catalyst
In different phase from the reactants and the reaction occurs at active sites on the surface
Homogeneous catalyst
In the same phase as reactants. When catalyst and reactants are in the same phase, the reaction proceeds through an intermediate species
The contact process reactions
SO2 + V2O5 —> SO3 + V2O4 (redox)
2V2O4 + O2 —> 2V2O5
Overall : 2SO2 + O2 <=> 2SO3
Each step has lower activation energy than one step reaction
Catalyst in contact process
V2O5 acts as heterogeneous catalyst
Catalyst in Haber process
Fe used as heterogeneous catalyst
Pea size lumps to increase surface area
Heterogeneous catalyst negatives
Can be poisoned by impurities that block active sites
How do Fe2+ ions catalyse the reaction between I− and S2O82–
S2O82- (aq) + 2I-(aq) → 2SO42-(aq) + I2(aq)
Aqueous iron(II) ions catalyse the reaction:
Step1: S2O82-(aq) + 2Fe2+(aq) → 2SO42-(aq) + 2Fe3+(aq)
Step2: 2Fe3+(aq) + 2I-(aq) → 2Fe2+(aq) + I2(aq)
How do Mn2+ ions autocatalyse the reaction between C2O42– and MnO4–
2MnO4-(aq) + 16H+(aq) +5C2O42-(aq) → 2Mn2+(aq) + 10CO2(g) + 8H2O(l)
Step1: 4Mn2+(aq) + MnO4-(aq) + 8H+(aq) → 5Mn3+(aq) + 4H2O(l)
Step2: 2Mn3+(aq) + C2O42-(aq) → 2CO2 (g) + 2Mn2+(aq)
State what is meant by the term bidentate ligand.
Two atoms or two points of attachment
Each donating a lone electron pair
Define coordinate bond
A shared electron pair or a covalent bond
Both electrons from one atom
Meaning of the term autocatalytic
Product of the reaction acts as a catalyst
4 oxidation states of vanadium + ion colours
+5 (VO2^+) yellow
+4 (VO^2+) blue
+3 (V^3+) green
+2 (V^2+) violet
Describe the characteristic properties of the elements titanium to copper + explain in terms of electronic structure
Variable oxidation states
Catalysts
Form complex ions
Form coloured compounds
Transition elements are d-block elements which form one or more stable ions with an incomplete d subshell
Define ligand
a molecule or ion that forms a co-ordinate bond with a transition metal by donating a pair of electrons
Define co-ordinate bond
A covalent bond where both electrons come from the same atom
Define co-ordination number
number of co-ordinate bonds to the central metal atom or ion
Describe the origin of the colour of a transition metal complex ion
Colour arises when some of the wavelengths of visible light are absorbed and the remaining wavelengths of light are transmitted or reflected
Describe how the concentration of iron(ii) ions in aqueous solution can be found
Titrating with either potassium manganate(VII) solution or potassium dichromate(VI) solution
Describe how heterogeneous catalysts can be made more efficient
Make the particles as small as possible to increase surface area
Explain how a homogeneous catalyst works
The catalyst forms an intermediate with the reactant with a lower activation energy. The intermediate often has a different oxidation state to the original transition metal. At the end of the reaction the original oxidation state will reoccur.
D-block elements not classed as transition metals
Scandium- only forms 3+ ion- no electrons in 3d sub shell
Zinc- only forms 2+ ion- complete 3d sub shell
Define complex
central metal atom or ion surrounded by ligands
What complexes are formed when a solution of ammonium vanadate(V) (NH4VO3) is reduced by zinc metal in acid conditions
[VO2(H2O)4]^+
[VO(H2O)5]^2+
[V(H2O)6]^3+
[V(H2O)6]^2+
State the observations when Na2CO3(aq) is added to a solution containing
[Al(H2O)6]3+(aq) ions
White ppt
Bubbles/effervescence
What are some characteristic physical properties of transition metals
Metallic, good conductors of heat and electricity, hard, shiny, high melting + boiling, low reactivity
Uses of iron
Vehicle bodies, to reinforce concrete
Uses of titanium
Jet engine parts
Uses of copper
Water pipes
Give some examples of transition metals catalysts and the processes/reactions they catalyse
Iron- Haber process
Vanadium (V) oxide - Contact process
MnO2 - decomposition of H2O2
Which electrons do transition metals lose first when forming ions
4s
Define the term multidente ligand
A ligand that forms 3 or more co-ordinate bonds to the central metal ion
How many co-ordinate bonds does EDTA^4- form
6
Shape of platin (cisplatin)
Square planar
4 ligands (breaks rule)
What conditions are needed for a complex ion to display optical isomerism
Usually octahedral molecules with 2 or more bidente ligands
Mirror images are non-superimposable
What happens to Co^2+, Cu^2+, Fe^3+ coordination numbers when Cl^- ligands replace NH3/H2O ligands
Decrease from 4 to 6 as Cl^- much larger ligand
What is haem’s metal ion, coordination number and ligands
Molecule that makes up protein chains
Fe^2+ central metal ion
Coordination number 6
4 bonds are to a ring system called porphyrin
1 is to nitrogen of a glob in molecule
1 is to oxygen of O2 molecule
How does haemoglobin transport oxygen
O2 forms weak coordinate bond to the metal ion
Transported around the body
Bond breaks when haemoglobin reaches cells and oxygen is released
Why is CO toxic
Coordinately bonds to Fe^2+ and is a better ligand
Stops O2 from bonding and being transported around body
What can you use to reduce Vanadium
Zinc
What does a colorimeter do
Measures the absorbance of a particular wavelength of light by a solution
Why can transition metals have variable oxidation states
Have partially filled d-orbitals so can lose 4s and 2d electrons
When oxidation state is high, how do transition metals exist
Covalently bond to other species
After oxidation state of around 3
What colour is MnO4^-
Deep purple
What colour is Mn^2+
Pink
What colour is Cr2O7^2- and Cr^3+
Orange
Green
What happens to aqua metal ions in acidic conditions and alkaline conditions
Reduced acid
Oxidised alkaline
What metals are used in catalytic converted and which reactions do they catalyse
Pt, Rd, Pd
2CO + 2NO —> 2CO2 + N2
Alkane + oxygen —> H2O + CO2
How do heterogeneous catalysts work
Reactants adsorb to the catalyst’s surface at active sites
Weakens bond within reactants + hold them close together on surface
Once reaction has occurred, products desorb from active sites
What properties make a good catalyst
Can’t adsorb too strongly as products won’t desorb
Can’t adsorb too weakly as reactant wouldn’t be held in place long enough and bonds wouldn’t be sufficiently weakened
How to increase efficiency of heterogeneous catalysts
Increase surface area
Spread onto inert support medium like ceramic honeycomb or mesh
What is catalyst poisoning
Unwanted impurities adsorb to catalyst’s active sites and don’t desorb
How can catalyst be degraded other than impurities poisoning
Finely divided catalysts can be gradually lost from their support medium
How long does catalyst in Haber process last + what is it poisoned by
5 years
Sulphur impurities
Why does the reaction between S2O8^2- ions and I^- ions have high activation energy
2 negative ions
Repel each other
Conc of reactant against time graph for auto catalysed reaction shape
Initially show - not much catalyst formed
Rate increases as catalysts made
Slows down as reactants are used up
How can you monitor the concentration of MnO4^- ions
Colorimeter
