Transition metals Flashcards
what is a transition element
- element that forms at least one stable ion with an incomplete D sublevel
what is a ligand
- a molecule or ion that forms a co-ordinate bond with a transition metal atom or ion
what is a complex
- a central metal atom or ion surrounded by ligands joined by coordinate bonds
what is the coordination number
- the number of coordinate bonds to the central metal atom or ion
what complexes have a linear shape and describe their features
- Ag⁺ complexes only
- bond angle = 180
- coordination number of 2
what complexes have a square planar shape and describe their features
- only platinum (Pt²⁺) and nickel(Ni²⁺)
- bond angle = 90
- coordination number of 4
what complexes have a tetrahedral shape and describe their features
- when ligands are too big to fit 6 (Cl⁻)
- bond angle = 109.5
- coordination number of 4
what complexes have an octahedral shape and describe their features
- most complexes
- bond angle = 90
- coordination number of 6
what is a monodentate ligand
- a ligand that forms one coordinate bond to the transition metal ion
why do complexes containing Cl ligands form a different structure than those containing H₂O ligands
- Cl ligands are bigger than ligands and therefore can only fit 4 Cl ligands
what type of isomerism occurs in metal complexes + explain
- Cis-Trans Isomerism
- Cis = ligands are 90 degrees away
- Trans = ligands are 180 degrees away
what shape of metal complexes does Cis-Trans isomerism occur in?
- octahedral
- square planar
what is a bidentate ligand
- ligand which forms two co-ordinate bonds to a metal ion via two different atoms on the same ligand
what are the two bidentate ligands
- 1,2 diaminoethane (H₂NCH₂CH₂NH₂)
- ethanedioate ions (C₂O₄)²⁻
what type of isomerism occurs in metal complexes containing at least 2 bidentate ligands
- optical isomerism
what is a multidentate ligand
- a ligand that can form 2 or more coordinate bonds to a transition metal ion
what multidentate ligand do we need to know
- (EDTA)⁴⁻
why does EDTA most effective in alkaline conditions
- the OH⁻ reacts with the H⁺ causing equilibrium to shift to the right ensuring the EDTA can form 6 coordinate bonds
what are the uses of EDTA
- used to treat patients with lead poisoning by making the toxic ions present in the body harmless (chelation therapy)
what is Haemoglobin and what does it contain
- its an iron complex that transport oxygen around the body
- it contains the central iron ion and a multidentate ligand called a porphyrin ring which forms 4 coordinate bonds (haem unit)
describe the structure of haemoglobin in oxygen rich conditions
- each haem unit is bonded to a protein called globin via a lone pair on the nitrogen of the globin
- 4 coordinate bond from the porphyrin ring to the Fe leaving one more space for oxygen to bind and act as a monodentate ligand via coordinate bonding
why does carbon monoxide prevent the transport of oxygen
- when carbon monoxide is inhaled it forms a coordinate bond with the Fe²⁺ ion in the haem unit blocking the O₂ from binding
where do Cl⁻ ligands come from
- Concentrated HCl
what is the chelate effect
- ligand substitution reaction between multidentate ligand and monodentate ligand complexes to form more stable multidentate ligand complexes due to an increase in entropy
why does the chelate effect work
- increase in entropy as more moles of products are produced than moles of reactants therefore product is more stable
why would the ΔG value for a ligand substitution reaction between ligands that form the same type of coordinate bond always be negative
- increase in entropy as more moles of products are produced than moles of reactants therefore product is more stable
- enthalpy change will be 0 as the same bonds that are broken are made again
- since ΔG = ΔH - TΔS , ΔS is positive, ΔH=0, ΔG will be negative
describe how to reduce Vanadium (V) to Vanadium (II)
- dissolve NH₄VO₃ in sodium hydroxide solution to ensure VO₂⁺ is the main ion present
- transfer to a small conical flask and add small amount of zinc powder + HCl
- put cotton wool in the neck of the flask and swirl contents of the flask together slowly
why is cotton wool put in the neck of the conical flask when reducing vanadium ions
- to allow H₂ gas to escape but also minimise the oxidation of the vanadium ions
what are the colours of the vanadium ions in their variable oxidation states
- V⁵⁺ (VO₂⁺) - yellow
- V⁴⁺ (VO²⁺) - blue
- V³⁺ (V[H₂O]₆)³⁺- green
- V²⁺ (V[H₂O]₆)²⁺- violet
how are vanadium ions reoxidised
- let them stand in air due to the O₂
- add concentrated nitric acid
what causes colour changes of transition metals (3)
- change in oxidation state
- change in coordination number
- change in ligand/type of ligand
why are transition metals coloured
- d orbitals to split in energy level
- certain wavelengths of light are absorbed by sample causing electrons to be promoted from ground state to excited state
- wavelengths of light that aren’t absorbed are transmitted giving off a certain colour
equations to calculate energy gap when given frequency of light absorbed
- ΔE = hv
- energy gap = Planck’s constant x frequency of light absorbed
equations to calculate energy gap when not given frequency of light absorbed
ΔE = hc/λ
- energy gap = (Planck’s constant x speed of light)/wavelength of light absorbed
how would you determine the concentration of a transition metal ion
- add an appropriate ligand to intensify the colour
- set the colorimeter wavelength to λmax
- make up standard solutions of known concentrations of the metal ion, some higher and some lower
- measure absorbance of standard solutions and plot a graph of absorbance value against known concentrations (calibration graph)
- measure the absorbance of the unknown solution and determine its concentration from the graph
how to intensify the colour of a pale complex
- add a suitable ligand to intensify the colour
what are Heterogeneous catalysts
- catalysts that are in a different phase to the reactants
- usually a solid and the reaction takes place on the surface
what are some important heterogeneous catalytic processes
- the Haber process - industrial production of Ammonia
- the Contact process - production of sulfuric acid
how do heterogeneous catalysts work
- reactants are adsorbed onto active sites on catalysts surface
- reaction occurs with lower activation energy as bonds are weakened or new bonds are made between reactants that are being held close together
- the products are desorbed
how is the efficiency of a heterogeneous catalyst maximised
- by using a thin coating of the catalyst on a support medium in order to maximise surface area to save costs
what are the equations and catalyst involved in the production of Ammonia
- catalyst = Fe (s)
- N₂ (g)+ 3H₂ (g) ⇌ 2NH₃ (g)
what are the equations and catalysts involved in the contact process
- catalyst = V₂O₅ (s)
- overall eq = 2SO₂ (g) +O₂ (g) → 2SO₃
- actual eq = SO₂ + V₂O₅ → V₂O₄ + SO₃
½O₂ + V₂O₄ → V₂O₅
how can heterogeneous catalysts be poisoned
- impurities adsorb to the surface blocking the active site lowering its efficiency or making it totally ineffective
examples of poisoned heterogeneous catalysts
- lead poisoning of catalytic converters in cars
- the hydrogen in the Haber process is contaminated with sulfur leading to sulfur poisoning
what are homogenous catalysts
- catalysts that are in the same phase as the reactants
- usually in the aqueous state
what are some important homogeneous catalytic processes
- reaction between iodide ions (I⁻) and persulfate ions (S₂O₈²⁻) with Fe²⁺/Fe³⁺ as a catalyst
why does the reaction between iodide ions (I⁻) and persulfate ions (S₂O₈²⁻) have a high activation energy
- both reactants are negatively charged and therefore repel each other
- high energy is needed to overcome that repulsion
why does the reaction between iodide ions (I⁻) and persulfate ions (S₂O₈²⁻) occur faster with Fe²⁺/Fe³⁺ ions
- opposite charges on the ions attract , therefore lower the activation energy making the reaction faster
what is the overall equation involved in the reaction between iodide ions (I⁻) and persulfate ions (S₂O₈²⁻) with Fe²⁺/Fe³⁺ as a catalyst
overall : S₂O₈²⁻ + 2I⁻ → 2SO₄²⁻ + I₂
what are the actual equations involved in the reaction between iodide ions (I⁻) and persulfate ions (S₂O₈²⁻) with Fe²⁺/Fe³⁺ as a catalyst
actual : S₂O₈²⁻ + Fe²⁺ → 2SO₄²⁻ + 2Fe³⁺
2Fe³⁺ + 2I⁻ → 2Fe²⁺ + I₂
what is autocatalysis
- when one of the products from a reaction is a catalyst for the reaction
what is an example of autocatalysis
- the reaction between ethanedioate ions (C₂O₄²⁻) and manganate ions (MnO₄⁻) with Mn²⁺ catalyst
what is the source of ethanedioate ions
- ethanedioic acid
what is the overall equation involved in the reaction between ethanedioate ions (C₂O₄²⁻) and manganate ions (MnO₄⁻)
5C₂O₄²⁻ + 2MnO₄⁻ + 16H⁺ → 2Mn²⁺ + 10CO₂ +8H₂O
what are the actual equations involved in the reaction between ethanedioate ions (C₂O₄²⁻) and manganate ions (MnO₄⁻)
step 1: 8H⁺ + MnO₄⁻ + 4Mn²⁺ → 5Mn³⁺ + 4H₂O
step 2: 2Mn³⁺ + C₂O₄²⁻ → 2CO₂ + 2Mn²⁺
why does the rate of autocatalysed reactions change over time
- reaction is slow at first until some of the catalyst is formed
- after a substantial amount of catalyst is formed the rate of reaction increases until the reactant runs out
