Chapter 23- Transition metals Flashcards
What is a transition metal?
A metal that can form one or more stable ions with a partially filled d sub-shell
What are some characteristic properties of transition metals?
-Variable oxidation states
-Can form complexes and complex ions
-Form coloured ions
-Act as catalysts
What are some trends in oxidation states of transition metals?
All can have oxidation state of 2+ caused by the loss of the 4s2 electrons
Highest actual oxidation state is 7+ in MnO4-
Highest oxidation state for a transition metal relates to loss of all electrons in 3d and 4s orbitals
What is a complex?
A central metal atom or ion surrounded by coordinately bonded ligands, can be charged (complex ion) or not (complex)
What is a ligand and what can they do?
A species with a lone pair of electrons to donate. They can form coordinate bonds with central metal ions by donating this pair of electrons.
What are some common examples of monodentate ligands?
H2O
Cl-
NH3
CN-
OH-
What is the coordination number?
The number of coordinate bonds that the transition metal ion can accept
What are some common coordination numbers?
Tends to be 6, silver’s is 2, platinum’s is commonly 4, if ligand is large eg Cl- can drop to 4
What commonly happens in reactions involving H2O and NH3 ligands and why?
Ligand substitution occurs because H2O and NH3 ligands are of similar size and are uncharged so ligand substitution can occur with no change in coordination number
What is ligand substitution?
A reaction in which a ligand is substituted by another ligand
What can happen during a ligand substitution involving H2O/NH3 and Cl-?
Coordination number of the complex/ complex ion can decrease as Cl- ligand is larger, eg from 6 to 4
What is the structure of a complex/ complex ion with just Cl- ligands?
Has a coordination number of 4 so is tetrahedral
What are monodentate/bidentate/multidentate ligands?
Ligands that can form 1,2 or many coordinate bonds with a metal ion
What are two examples of bidentate ligands?
- 1,2-diaminoethane, written as en when a ligand
- ethanedioate C2O4 2-
What is an example of multidentate ligand and what are they?
Ligands that can form up to 6 coordinate bonds as they have multiple lone pairs of electrons. A common example is EDTA 4-, which is hexadentate
What is haem?
A component of haemoglobin that is a tetradentate ligand bonded to a central Fe 2+ ion. Its shape and structure allows for transportation of oxygen and CO2 around the body
What does haem do and what happens with CO?
Haem’s shape and structure allows the central Fe2+ ion to bond with O2 and CO2 and transport it around the body.
CO is toxic as it replaces the O2 and CO2 in the haem complex stopping them from being transported around the body
What is the chelate effect/ chelation?
When monodentate ligands are substituted with bidentate or multidentate ligands
Why are reactions involving the chelate effect favoured?
-In ligand sub, a positive entropy change is favourable as it means a more stable complex is formed. There are more moles on the right than left.
-The more positive the entropy change, the more negative the Gibbs Free Energy so the more favourable the reaction
-Enthalpy change for ligand substitution reactions is very small as bonds broken and bonds formed are very similar so overall ΔH ≈ 0
What is a common structure of complexes?
Octahedral as with small ligands coordination number is often 6 so it is an octahedral structure with bond angle of 90°
What isomers can complexes form?
E-Z / cis-trans and optical
How do cis-trans isomers form in complexes?
Occur in octahedral complexes with different types of monodentate ligands. Ligands of the same type can be adjacent or opposite each other. Trans isomer has two opposite each other and cis isomer has two adjacent.
How do optical isomers form in complexes?
Occur in octahedral complexes with bidentate ligands where two non-superimposable mirror image complexes are formed
How can isomers form in tetrahedral complexes?
Can show optical isomerism if there are four different monodentate ligands
How can isomers form in square planar complexes?
Platinum and nickel form square planar complexes which consist of four coordinate bonds with a bond angle of 90°. These can cis-trans isomers depending on where two monodentate ligands are relative to each other.
What is cisplatin?
The cis isomer of a square planar complex with platinum and two Cl- and two NH3 ligands. It is a cancer therapy drug.
As cells are chiral, the correct orientation of drug is required to be effective so only cisplatin is effective as a chemo drug
Can cause side effects such as hair loss so is given in small amounts to reduce side effects
What is the shape of Ag+ complexes?
Has coordination number 2 so forms linear complexes. Tollens’ reagent has Ag+ linear complex with two NH3 ligands
How does colour arise?
Colour arises when some of the wavelengths of visible light are absorbed and the remaining wavelengths of light are transmitted or reflected.
Why do transition metals form coloured ions?
Requires a partially filled d-sub shell. On bonding with ligands, 3d sub-shell splits into a group of 3 and two pairs of electrons, group of 3 is an excited state where d orbitals can have different energies.
-Electrons move within 3d orbital to another by absorbing energy
-Colour seen corresponds to part of visible region on EM spectrum not absorbed
What causes a change in colour for a transition metal?
Change in oxidation state, coordination number and ligands change energy required for excitation and so colour
What is the equation for the change in energy required for an electron to move between ground and excited states?
ΔE = hf
What is colorimetry?
An analytical technique using absorption of visible light to determine the concentration of coloured ions by measuring absorbance
What is a calibration curve?
A calibration curve is used in colorimetry to plot the amount of absorbance vs concentrations of standard solutions. This is then used to measure the
concentration of an unknown solution of your transition metal ion
What are the oxidation states and colours of vanadium?
5+, VO2 + yellow
4+, VO 2+ blue
3+, V 3+ green
2+, V 2+ violet
How is vanadium oxidised and reduced?
Reduction- acidic conditions with Zn
Oxidation- alkaline conditions, ions react with water to produced OH- ions
How is Tollens’ reagent used?
Silver complex is reduced by aldehydes to form silver atoms, seen as a silver mirror. Distinguishes between aldehydes and ketones
What is the reaction for a redox titration with MnO4- ?
MnO4 - + 8H+ + 5e- –> Mn 2+ + 4H2O
Mn is reduced, colour change from purple to pale pink/ colourless
What is the reaction for a redox titration with Fe 2+ ?
Fe 2+ –> Fe 3+ + e-
Fe is oxidised, needs to be acidified not with HCl with sulfuric
What is the reaction for a redox reaction with C2O4 2-?
C2O4 2- –> CO2 +2e-
C is oxidised, needs to be acidified not with HCl with sulfuric
What is a catalyst?
A substance that speeds up the rate of reaction without being used up in the reaction. It provides an alternative reaction pathway with a lower activation energy
Why do transition metals make good catalysts?
Due to their variable oxidation states, electrons can be transferred to produce a reactive intermediate and speed up rate of reaction.
What is a heterogeneous catalyst?
A catalyst that is in a different phase from the reactants. The reaction takes place on the active sites on the surface of the heterogeneous catalyst.
Examples include iron in the Haber Process and vanadium(V) oxide in the Contact Process
How is iron used in the Haber process?
Solid iron or Fe2O3 is used as a catalyst. Cheap pea-sized pieces which last 5 years catalyse the reaction between nitrogen and hydrogen gases
How is vanadium oxide used in the Contact process?
V2O5 catalyses conversion of sulfur dioxide to sulfur trioxide
What are the two equations showing V2O5 catalysing conversion of sulfur dioxide to sulfur trioxide?
V2O5 + SO2 –> V2O4 + SO3
V2O4 + 0.5O2 –> V2O5 regenerated
What is adsorption?
The process of forming weak bonds by reactants to a surface of a solid catalyst.
What does adsorption do in catalysis?
Adsorbed molecules are on active site on solid catalyst. Active sites increase proximity of molecules and weakens covalent bonds.
Strength of adsorption depends on the catalyst, eg Fe, Co, Ni
What is catalyst poisoning?
When impurities in a reaction mixture bind to a heterogeneous catalyst’s surface and block reactants from being adsorbed, reducing the efficacy of the catalyst.
They increase cost of chemical production as catalysts have to be replaced or cleaned regularly, eg Fe in Haber process poisoned by sulfur
What are homogeneous catalysts?
Catalysts in the same phase as the reactants. They combine with reactants to form an intermediate
species which reacts to form the products and re-form the catalyst. eg Fe2+ ions for reaction between S2O8 2- and I - ions
What are the reactions for Fe 2+ ions being used in catalysis for the reaction between S2O8 2- and I - ions?
2Fe 2+ + S2O8 2- –> 2Fe 3+ + 2SO4 2-
2Fe 3+ + 2I- –> 2Fe 2+ I2
without catalyst, negative ions would not react as they have very high activation energy
What is autocatalysis?
When a product of a reaction acts as a catalyst for the reaction
What is an example of autocatalysis?
Redox reaction between MnO4 - and C2O4 2- ions which produces Mn 2+ which acts as a catalyst
What are the reactions for autocatalysis with Mn 2+?
4Mn 2+ + MnO4 - + 8H+ –> 5Mn 3+ + 4H2O
2Mn 3+ + C2O4 2- –> 2CO2 + 2Mn 2+,
requires initial heating to produce some Mn 2+ but then autocatalysed
