Transition metals Flashcards
What are some metallic properties of transition metals?
- All of the d-block elements are metals
- They have strong metallic bonds (high mpts + bpts)
- Small size and close packing (high density) of the ions result in strong electrostatic forces of attraction between the positive metal ions and the delocalised electrons
- They have higher melting and boiling points and higher densities than s-block
What is the electronic configuration of chromium?
[Ar] 4s1 3d5
What is the electronic configuration of copper?
[Ar] 4s1 3d10
What is the trend in first ionisation across the d-block elements?
There’s only a slight increase in 1st IEs across the d-block elements
- The nuclear charge increases
- and the shielding also increases, as the additional electron is added to 3d subshell, which shields the 4s electrons from the positive nucleus
- These two factors almost cancel each other out
Compare the successive ionisation energies of Calcium and Vanadium
For Ca, the first two IEs are relatively low as the electrons are being removed from 4s orbital. There’s a big jump between the 2nd and 3rd ionisation energies because the third electron is removed from the 3p orbital. The e- in the 3p orbital is closer to the attraction of the nucleus and experiences less shielding so more energy is needed to remove it.
For V, the 4s and 3d electrons are very close in energy so there’s no big jump in successive IEs for vanadium until the sixth electron is removed
Why are there variable oxidation states?
This is because the five inner d orbitals are at similar energy to the outer s orbital
Which compounds tend to be covalent?
Compounds and ions containing transition metals in the higher oxidation states tend to be covalent eg/MnO4- is a covalently bonded anion containing Mn in an oxidation state of +7.
Which compounds tend to be ionic?
Lower oxidation states tend to involve ionic bonding eg/MnCl2. Ionic bonding in transition metals form when 4s and then 3d electrons are lost to form positively charged ions.
Define transition metal:
A transition metal is a d-block element that forms at least one stable ion with a partially filled d-subshell
What does the stable version of Scandium look like?
Sc3+ [Ar] - no partially filled d-subshell, as the d-subshell is empty
What does the stable version of Zinc look like?
Zn2+ [Ar] 3d10 - full 3d subshell
Give the structure, shape, bond angle, coordination number and ligand of [Cu(H2O)6]2+ ?
Name: hexaaquacopper (II)
Shape: Octahedral
Bond angles: 90
Coordination number: 6
Ligand: H2O
What does the coordination number tell us?
The number of electron pairs donated to the central metal ion
Give the structure, shape, bond angle, coordination number and ligand of [Ag(NH3)2] + ?
Name: diamminesilver (I)
Shape: Linear
Bond angles: 180
Coordination number: 2
Ligand: NH3
Give the structure, shape, bond angle, coordination number and ligand of [CuCl4]2- ?
Name: tetrachlorocuprate (II)
Shape: tetrahedral
Bond angles: 109.5
Coordination number: 4
Ligand: Cl-
Give the structure, shape, bond angle, coordination number and ligand of [Ni(CN)4]2- ?
Name: tetracyanonickelate (II)
Shape: square planar
Bond angles: 90
Coordination number: 4
Ligand: CN-
What coordination number do transition metals only have?
2,4 or 6
What shape is the most common for transition metals?
Tetrahedral ones are by far the most common
When will transition metals have a square planar shape?
When it has a d8 configuration such as Ni2+, Pd2+, Pt2+
What are ligands?
A species with a lone pair of electrons that it can donate to form a dative covalent bond with a transition metal ion
What can ligands be described as?
Lewis bases because they are electron pair donors.
This means they must a lone pair of electrons. They can be neutral or negatively charged
What are bidenate ligands?
Donate two pairs of electrons to the central metal ion
Draw an example of a bidenate ligand with H2NCH2CH2NH2 (1,2-diaminoethane) group:
Draw an example of a bidenate ligand with C2O4 2- (ethanedioate ion) group:
What are multidentate/polydentate ligands?
Ligands that donate more than two pairs of electrons to the central metal ion
Give an example of a multidentate ligand?
EDTA 4- (ethylenediaminetetraacetate)
Why are transition metals/ions coloured?
- When surrounded by ligands the d-subshell splits into two
- Electrons get promoted/excited from the lower orbitals to the higher orbitals by absorbing frequencies of light within the visible region
- The colour of the complex is complementary to the absorbed light
What does the energy gap correspond to?
Frequencies within the visible spectrum
Describe how the d-to-d transition occurs?
When an ion is surrounded by ligands, some d orbitals are closer than others to the ligands, because of their distribution in space. The ligands may be roughly regarded as clouds of negative charge, which push the orbitals closest to them to slightly higher energy levels
What does the energy gap allow to happen?
For an electron to be excited from a lower energy to a higher energy d orbital as it absorbs light, provided that there is at least one electron present to be excited and that there is space for it in one of the orbitals of higher energy.
What is the condition in order for d-to-d transitions to occur, and hence for an ion to be coloured is?
The ion must have a partially filled d subshells
What transitions metals aren’t coloured?
Zn2+ and Sc3+
What does the energy difference between the split d orbitals (and hence the colour) depend on?
- The oxidation state of the metal ion
- The type of ligands
- The number of ligands
What colour of the transition metal will appear as compared to the frequency absorbed?
The colour of the transition metal complex will appear as the complementary colour to the frequency absorbed
Give the colour of the hexaaquametal ion solution of Co2+, Cu2+, Fe2+, Fe3+ and Cr3+:
Co2+ : pink solution
Cu2+ : blue solution
Fe2+ : pale green solution
Fe3+ : orange solution
Cr3+ : green solution
Give the observations when NaOH is added dropwise for Co2+, Cu2+, Fe2+, Fe3+ and Cr3+:
Co2+ : blue ppt
Cu2+ : blue ppt
Fe2+ : green ppt
Fe3+ : red-brown ppt
Cr3+ : green ppt
Give the observations when NaOH is added in excess for Co2+, Cu2+, Fe2+, Fe3+ and Cr3+:
Co2+ : ppt remains
Cu2+ : ppt remains
Fe2+ : ppt remains
Fe3+ : ppt remains
Cr3+ : ppt dissolves to give a green solution
What happens to the green ppt of Fe2+?
The green ppt starts to turn into red-brown ppt as the O2 in the air is oxidising the Fe2+ ions into Fe3+ ions. This is known as aerial oxidation
Give the observations when NH3 is added dropwise for Co2+, Cu2+, Fe2+, Fe3+ and Cr3+:
Co2+ : blue ppt
Cu2+ : blue ppt
Fe2+ : green ppt
Fe3+ : red-brown ppt
Cr3+ : green ppt
Give the observations when NH3 is added in excess for Co2+, Cu2+, Fe2+, Fe3+ and Cr3+:
Co2+ : ppt dissolves to give a brown solution
Cu2+ : ppt dissolves to give a deep blue solution
Fe2+ : ppt remains
Fe3+ : ppt remains
Cr3+ : ppt dissolves (slowly) to give a green solution
What does a deprotonation (NaOH) reaction look like? ([Cu(H2O)6]2+
[Cu(H2O)6]2+ (aq) + 2OH- (aq) –> [Cu(OH)2(H2O)4] (s) + 2H2O(l)
What is a deprotonation reaction?
When any base is added to an aqueous solution containing transition metal ions, dropwise addition always causes deprotonation
How can deprotanation be reversed?
By adding a dilute acid eg/dil. HCl
What does a protonation (NaOH) reaction look like? ([Cu(OH)2(H2O)4] (s)
([Cu(OH)2(H2O)4] (s) + 2H+(aq) –> [Cu(H2O)6]2+(aq)
What does a deprotonation (NaOH) reaction look like? ([Cr(H2O)6]3+?
([Cr(H2O)6]3+(aq) + 3OH- (aq) –> [Cr(OH)3(H2O)3] (s) + 3H2O (l)
What does adding excess NaOH determine?
If the precipitate is amphoteric. If a substrate is amphoteric it can act as both an acid and a base
What is the only amphoteric precipiate?
[Cr(OH)3(H2O)3] (s)
Give an example of a further deprotanation reaction? [Cr(OH)3(H2O)3] (s)
[Cr(OH)3(H2O)3] (s) + 3OH-(aq) –> [Cr(OH)6]3- (aq) + 3H2O(l)
Give an example of reprotanation reaction? [Cr(OH)3(H2O)3] (s)
[Cr(OH)3(H2O)3] (s) + 3H+ (aq) –> [Cr(H2O)6)]3+ (aq)
What does a deprotonation (NH3) reaction look like? (Fe(H2O)6]2+
(Fe(H2O)6]2+ (aq) + 2NH3(aq) –> [Fe(OH)2(H2O)4] (s) + 2NH4+
Why can’t ammonia further deprotanate any of the precipiate?
Weak base
What happens when excess ammonia is added?
Sometimes the precipitate does dissolve but this is due to a different type of reaction called ligand exchange (aka ligand substitution)
Give an example of a ligand exchange reaction: ([Co(OH)2(H2O)4] (s)
([Co(OH)2(H2O)4] (s) + 6NH3(aq) –> [Co(NH3)6]2+ (aq) + 2OH- (aq) + 4H2O (l)
Give the deprotonation reaction for [Cu(H2O)6]2+ (aq)
(NH3)
[Cu(H2O)6]2+ (aq) + 2NH3(aq) –> [Cu(OH)2(H2O)4] (s) + 2NH4+ (aq)
Give the ligand exchange reaction for [Cu(OH)2(H2O)4] (s)
[Cu(OH)2(H2O)4] (s) + 4NH3(aq) –> [Cu(NH3)4(H2O)2]2+ (aq) + 2OH-(aq) + 2H2O(l)
Give the reaction for the dropwise addition of CH3NH2 to [Cu(H2O)6]2+
[Cu(H2O)6]2+ (aq) + 2CH3NH2 –> [Cu(OH)2(H2O)4] (s) + 2CH3NH3+
Give the reaction for the excess addition of CH3NH2 to [Cu(OH)2(H2O)4] (s)
[Cu(OH)2(H2O)4] (s) + 4CH3NH2 –> [Cu(CH3NH2)4(H2O)2]2+ (aq) + 2OH- (aq) + 2H2O (l)
Give the reaction for when conc. HCl is added to [Cu(H2O)6]2+:
[Cu(H2O)6]2+(aq) + 4Cl- ⇌ [CuCl4]2- (aq) + 6H2O(l)
Give the reaction for when conc. HCl is added to [Co(H2O)6]2+
[Co(H2O)6]2+ (octahedral) + 4Cl- ⇌ [CoCl4]2- (tetrahedral) + 6H2O
What does cis-platin consist of?
[PtCl2(NH3)2]
What is cis-platin used for?
Chemotherapy drug in the treatment of certain cancers.
Trans-platin is ineffective against cancer
What is the oxidation state of platinum?
2+
What is the shape of cis-platin and why?
Square planar
Pt2+, d8, [5d8]
How does cis-platin work?
The drug binds with the DNA of the tumour cell, preventing replication
What are the side effects of using cis-platin?
Kidney failure, hair loss
What is the use of haemoglobin?
Transport oxygen in the blood
What is inside each of the 4 polypeptide chains of haemoglobin?
Porphyrin ligand surrounding an Fe2+ ion. Each porphyrin ligand forms 4 dative covalent bonds to the central Fe2+ ion
What shape is haemoglobin?
Octahedral - one of the other two sites is bonded to the protein structure leaving one site to which oxygen can bond as a ligand
Why is oxygen not a good ligand?
Easily comes off the iron when needed to transfer to cells for respiration
Why are carbon monoxide and cyanide compounds toxic?
Better ligands than oxygen and so will bind to haemoglobin in preference to oxygen (ligand exchange occurs) - prevent the transportation of oxygen around the body
Draw out an equation for the reaction between [Cr(NH3)6-3+ and 1,2-diaminoethane
4 particles (all aq) –> 7 product particles (all aq)
Does the reaction between [Cr(NH3)6-]3+ and 1,2-diaminoethane have a positive ∆S sys?
This reaction occurs because of an increase in the entropy of the system. All the particles in the equation are aqueous and 4 reactant particles are converted into 7 product particles. The ∆Sys therefore becomes more positive as the disorder increases
Why is enthalpy not the driving force for this ligand exchange reaction?
In this reaction, 6 Cr-N bonds are broken in the reactants and 6 Cr-N bonds are formed in the products. The ∆H for this reaction would therefore be roughly equal to zero. Enthalpy is not the driving force behind this ligand exchange reaction.
What is another example of a ligand exchange reaction?
[Fe(H2O)6]2+ (aq) + edta4- (aq) –> [Feedta)]2- (aq) + 6H2O (l)
What would the ∆Sys for the reaction of edta4- be?
The ∆Sys would be more positive in this example
- The more positive the value of ∆Sys, the more negative the value of ∆G and the more thermodynamically feasible the reaction is
What is green Cr3+ oxidised to?
Yellow chromate ions, Cr2O4 2- (aq) using a strong oxidising agent eg/H2O2
Why are transition metal ions with low oxidation more readily oxidised in alkane conditions over acidic?
As it is easier to remove an electron from a negatively charged ion
Give the equation of a water molecule attacking [Cr(H2O)6]3+
An aqueous solution of Cr3+ ions would be naturally acidic, as the 3+ ion is polarising, distorting the electron density of the water ligands, weakening the O-H bonds in the water ligands, allowing the solvent water molecules to act as bases
[Cr(H2O)6]3+ (aq) –> [Cr(OH)(H2O)5] 2+ (aq) + H3O+ (aq)
Therefore, rather than starting with [Cr(H2O)6 3+, better to start with [Cr(OH)6] 3-
Give the half equations and overall equations in alkaline conditions ([Cr(OH6)]3-
([Cr(OH)6]3- + 2OH- –> Cr2O42- + 4H2O + 3e- ) x2
(H2O2 + 2e- –> 2OH- ) x3
2[Cr(OH)6]3- + 3H2O2 –> 2CrO42- + 8H2O + 2OH-
What happens when dilute acid is added to yellow Cr2O42-
It converts to the orange Cr2O7 2-
2CrO4 2- (aq) + 2H+ (aq) ⇌ Cr2O7 2- (aq) + H2O (l)
How can the equilibrium be reversed?
By adding a base
What colour is VO2+ (+5)
Yellow
What colour is VO2+ (+4)
Blue
What colour is V3+?
Green
What colour is V2+?
Purple
What is a catalyst?
Substance that increases the rate of a chemical but is not used up in reaction
How do catalysts speed up reactions?
Providing an alternative route with a lower activation energy
How do catalysts act in reversible reactions?
Catalysts increase the rate at which equilibrium is reached by speeding both the forward and reverse reactions - do not alter the position of equilibrium nor the value of K
What is the advantage of using a catalyst to increase rate of reaction over other methods?
Increasing temp or increasing pressure is that it reduces energy costs and also save resources (fossil fuels)
What is a heterogeneous catalyst?
A catalyst that is in a different phase to the reactants
- Usually a solid and the reaction takes place on the surface of the solid
What are some industrial processes that involve heterogeneous catalysis
- Hydrogenation (Ni catalyst)
- Making oxygen (MnO2 catalyst) ( 2H2O2(aq) –> 2H2O(l0 + O2(g))
- Haber process (Fe catalyst) ( 3H2(g) + 2N2(G)
What are some industrial processes that involve heterogeneous catalysis
- Hydrogenation (Ni catalyst)
- Making oxygen (MnO2 catalyst) ( 2H2O2(aq) –> 2H2O(l0 + O2(g))
- Haber process (Fe catalyst) ( 3H2(g) + 2N2(g) ⇌ 2NH3(g) )
- Contact process (V2O5 catalyst) (2SO2(g) + O2(g) ⇌ 2SO3(g))
How does a heterogeneous catalyst work?
- At least one of the reactants is adsorbed onto the surface. This means the reactants form bonds to the atoms/ions in the solid surface. Partially filled d-orbitals in transition metals are good at accepting electron density
- Active sites: where reactants are adsorbed on the surface
- Adsorption of reactants onto the surface results in an increased rate
- The products are then desorbed from the surface of the catalyst, freeing up the active sites
What are the place where reactants are adsorbed called?
Active sites
Adsorption of reactants onto the surface can result in increased rate in a number of ways:
- Adsorption onto the surface effectively concentrates the reactants i.e. brings them closer together than in the gas phase, so increasing the likelihood of collision
- It may weaken some of the bonds in the molecule, making the reaction easier
- It may position the molecule in a favourable orientation for the reaction to occur
What needs to happen for the catalyst to work?
Molecules must be adsorbed onto the surface
What happens if adsorption is too weak?
Few molecules will be adsorbed so the catalyst will have very little effect
What happens if adsorption is too strong?
Molecules will not be able to move around the active sites, and so be less likely to meet another reactant and so be less likely to react
What is the nature of the catalyst?
Large surface area therefore less catalyst is needed to produce the same effect
- Maximising SA is important for cost savings
How is the surface area of a catalyst maximised?
By using a very thin coating of the catalyst on some type of support medium (often a honeycomb structure)
What’s the advantage of heterogeneous catalysts over homogeneous?
They can be easily separated from the reaction mixture by filtration
What are the two specific examples of heterogeneous catalysis?
V2O5 acts as a catalyst in the contact process
Catalytic converters decreases carbon monoxide and nitrogen monoxide
How does V2O5 act as a catalyst in the contact process?
SO2 (g) + V2O5 (s) –> SO3(g) + V2O4(s)
V2O4(s) + 1/2O2 (g) –> V2O5 (s)
Overall equation: SO2(g) + 1/2 O2 (g) –> SO3(g)
How does a catalytic converter decrease carbon monoxide and nitrogen monoxide emissions from internal combustion engines?
The catalyst in catalytic converters is made from various alloys of platinum, rhodium and palladium. CO and NO molecules are adsorbed onto the surface of the catalyst. This weakens the bonds in the molecules, making the subsequent reaction easier. The products, CO2 and N2 are then desorbed from the surface of the catalyst
What happens when catalysts are poisonous?
Blocks the active sites
- Lowers the efficiency of the catalyst, or could make it totally ineffective depending on the extent of the poisoning
- Poisons are difficult to remove
What is a homogeneous catalyst?
- A homogeneous catalyst is one that is in the same phase as the reactants
- Take place in solution
- Proceed via an intermediate species formed from a reactant and the catalyst, which then reacts further and regenerates the catalyst
How does a transition metal homogeneous catalyst work?
Transition metal ions have the ability to vary their oxidation states which allows them to act as catalysts
What are the two specific examples of a homogeneous catalyst?
The reaction between iodide ions and persulfate ions
Autocatalysis
Why is the reaction between iodide ions and persulfate ions slow?
2I- (aq) + S2O8 2- (aq) –> I2 (aq) + 2SO4 2- (aq)
Because it involves the reaction of two negative ions (they repel each other making collisions unlikely).
What is the reaction between iodide ions and persulfate ions catalysed by?
Fe2 + ions (aq), which act as a catalyst because they easily change between the oxidation states Fe (+2) and Fe (+3)
Give the equation for the reaction between iodide ions and persulfate ions:
Step 1: 2Fe 2+ (aq) + S2O8 2- (aq) –> 2Fe3+ (aq) + 2SO4 2- (aq)
Step 2: 2I- (aq) + 2Fe3+ (aq) –> I2 (aq) + 2Fe2+ (aq)
Overall equation: 2I- + S2O8 2- –> I2 + 2SO4 2-
The catalytic Fe2+ ions are therefore regenerated
What is autocatalysis?
Where one of the product catalyses the reaction
What is the reaction between manganate (VII) ions and ethanedioate ions?
Slow
- Rate can be increased by adding a source of Mn2+ ions
2MnO4- + 5C2O4 2- + 16H+ –> 2Mn2+ + 10CO2 + 8H2O
What is the intermediate in the reaction between manganate (VII) ions and ethanedioate ions?
Mn3+
