Transition Metals Flashcards
What is the difference between a ‘transition metal’ and ‘d-block element’?
Transition metal: An element that forms one or more stable ions with a partially filled d subshell.
D block element: an element whose last electron goes into the d-subshell when electronic structure is built.
State four properties of transition metals
- Have variable oxidation states (numbers)
- Form complex ions
- Form coloured compounds
- Show catalytic activity
Define ‘ionisation energy’, as applied to d block elements.
The energy required to remove one mole of electrons from one mole of atoms of an element in the gaseous state. First removed from 4s, then 3d.
What are the trends in first and second ionisation energy across the d-block elements?
Values: 1st IE: 631-906 kjmol-1
2nd IE: 1235-1733 kjmol-1
1st IE: increases across period as effective nuclear charge increases, but mainly similar due to electron being removed from same inner shell (3d electron shielding)
2nd IE: increases steadily except for Cr & Cu which involve removing from stable half-full & full sub-shells.
The more highly charged an ion, the more electrons you have to remove and the greater the IE.
Why do d-block elements have high melting points?
Melting point: attraction between positive metal ions and delocalised electrons.
Both the d and s electrons are used in metallic bonding so the melting points are higher than s-block metals.
Why do transition metals show a variety of oxidation states?
There is similarity in the energy of the 4s and 3d electrons. Energy required to promote electrons must be regained from energy released. Higher oxidation states are shown when there is covalent bonding.
State the colours and complexes of the following vanadium species and state the reagents you can use to make each of them:
a) Vanadium (II)
b) Vanadium (III)
c) Oxovanadium (IV)
d) Dioxovanadium (V)
If KI was used as a reducing agent, I- ions would be oxidised to I2. The colour given would mask the colour of the vanadium ion.
a) Purple: V(H2O)2+
b) Green: V(H2O)3+
c) Blue: VO(H2O5)2+
d) Yellow: VO2(H2O4)+
Dissolve NH4VO3 in NaOH and acidify with H2SO4; shake with powdered zinc for yellow -> green -> blue, then green -> purple.
Why can transition metals act as catalysts?
They show variable oxidation states.
Are able to oxidize and reduce (donate and receive e’s)
Their ions contain partially filled d subshells.
How is vanadium (V) oxide used as a catalyst in the contact process?
- Reduction of V2O5: SO2 + V2O5 -> SO3 + V2O4
- Oxidation of V2O4: V2O4 + 1/2O2 -> V2O5
Overall: SO2 + 1/2O2 -> SO3 (SO3 reacts with H2SO4 to make oleum)
How does a heterogeneous catalyst work?
- Adsorption: reactant adsorbed onto catalyst active site surface (held by covalent, H bonds or London forces)
- Reaction: bonds are weakened within reactants (lowers Ea) & rate of reaction is increased. Changes orientation
- Desorption: products molecules desorbed from surface
How does a catalyst converter work?
It uses redox reactions and platinum catalyst to remove pollutant gases (NOx, CO, hydrocarbons) and convert them to less harmful substances as the fuel is burnt.
Why are new catalysts being developed?
- To lead to more efficient processes - products made quicker and higher yield
- To use less energy and produce higher atom economy
What is a complex ion?
A central metal ion that is surrounded by dative covalently bonded ligands.
What is a ligand and state the difference between:
a) monodentate ligand
b) bidentate ligand
c) polydentate ligand
A molecule or ion that donates a lone pair of electrons to a central metal ion, forming a dative covalent bond.
a) monodentate: donates one lone pair (Cl-, H2O, CN-)
b) bidentate: donates two lone pairs (1,2-diaminoethane)
c) polydentate: donates more than two lone pairs (EDTA)
Why do transition metals form coloured ions?
They have incomplete d subshells, so electrons can be promoted to higher energy levels.
Ligands cause repulsion and the d subshell to split into 2 upper and 3 lower orbitals (if octahedral).
When light shines through, electrons absorb a photon of light and are promoted to upper orbitals (d-d transitions).
A complimentary colour to the light absorbed is reflected and seen as visible light.
What factors affect the colour of a complex ion?
Why do some complexes show no colour?
- Oxidation state - high charge density = strong attraction
- Type of ligand - different ligands split to different extent
- Frequency of light absorbed
Complexes show no colour if: 3d subshell is full or there are no d electrons for promotion
What is the difference between ligand exchange and deprotonation?
Ligand exchange: one ligand replaces another ligand. Can’t be reversed if new ligand is more stable. Chelate effect: multidentate replaces unidentate = total entropy increases (more moles of product in system)
Deprotonation: loss of a proton by a water molecule. High charge density on metal ion draws ligands closer, weakening the O-H bond and allowing H+ to be released. [Fe(H2O)6]2+ + H2O => [Fe(H2O)5(OH)]+ + H3O+
How does charge on a metal ion affect acidity and stability? (E.g. why are [V(H2O)6]5+, [Cr(H2O)6]6+ and [Mn(H2O)6]7+ not stable?)
The higher the charge density, the greater the polarising power and deprotonation, thus the greater the acidity.
Charges greater than 3+ make the cation too polarising, so even after deprotonation the complex is unstable.
(V5+ would distort the electron clouds of water).
What is a stereospecific drug and why are stereospecific routes prefered?
Drug: only one isomer is active (one is beneficial)
Stereospecific routes:
1. Are more effective - no need to separate enantiomers, lower amounts (dosages) required.
2. Avoids waste products - unwanted isomers may have bad effects
Suggest why Cr2+ ions are readily oxidised to Cr3+ ions.
Cr2+ ions are unstable because:
When their 3d sub shells are split into 2 upper and 3 lower orbitals, the Cr3+ ions can have all its electrons in the lower set but Cr2+ ions must have at least one 3d electron in the upper set.
How can a sample of chromium (II) ethanoate be prepared?
a) What is the colour of this complex?
b) Why is this compound unusual (think oxidation states)
c) Why is it a neutral complex?
- Reduce chromium (III) compound (e.g. Dichromate) with zinc and H2 gas (from HCl) to form a blue solution
- Add sodium ethanoate to = red ppt
a) Red precipitate
b) Chromium is usually stable in +3 and +6 oxidation states
c)
What does a metal ion require for complex ion formation?
Low-lying, easily available orbitals that are close to outer electrons.
Needs to form dative covalent bonds
High charge density
