Transition Metals Flashcards
What are transition metals?
Elements with an incomplete d-sub shell that can form at least 1 stable ion with an incomplete d-sub shell
2 exceptions to the Aufbau principle?
Chromium
Copper
- 1 electron promoted from 4s to the 3d
- more energetically stable in this form
Properties of transition metals?
Variable oxidation states
Form complex ions
Form coloured compounds
Behave as catalysts
How are variable oxidation numbers achieved in transition metals ? And why?
When transition elements form ions , they lose electrons from the 4s sub shell first
Becuase :
when orbitals occupied, repulsion between electrons pushes the 4s into a higher energy state so slightly higher in energy than 3d subshell
The 4s is the outer shell/loses electrons first
Roman numeral indicate ox. States of the ion
What is a ligand?
An atom,ion or molecule that donates a pair of electrons to a central metal atom/ion
- must have at least 1 lone pair to form DATIVE BOND
Which metals are not transition metals?
Scandium - Sc - only form 1 ion (Sc3+) which has empty d subshell
Zinc - Zn - only forms 1 ion (Zn2+) which has a full d subshell
What is a complex ion?
Metal ion surrounded by datively covalently (coordinately) bonded ligands
What are monodentate, bidentate and multidentate ligands?
MONODENTATE: ligand with one lone pair (H2O, NH3,CL-,OH-)
BIDENTATE: ligands with 2 lone pairs (1,2 diaminoethane,ethanedioate ions(C2O4 2-)) - each form 2 dative covalent bonds with metal ion
MULTIDENTATE* : ligands with more the 2 lone pairs ( EDTA4-has 6 lone pairs (hexadentate) - form 6 dative bonds its metal ion)
What is coordination number?
Number of coordinate bonds to central atom/ion
How to name complexes?
If overall ion is cation (+) then:
Prefix of no. Ligands , ligand name , element, ox number
Prefixes include: Di (2), tetra (4), hexa (6)
If overall ion is anion (-) then:
Name of element ENDS IN -ATE + sometimes Latin word stems are used
E.g (tetrachlorcuprate (II) ) cuprate - copper
How to find charge of complex ion?
Sum of ox states of all species present
Why are transition metal compounds colored?
Absorb energy corresponding to certain parts of visible electromagnetic spectrum
- colours absorbed are complementary to colour observed
What are complementary colours?
Any 2 colours which are directly opposite each other in colour wheel ( if green is observed, red will be absorbed )
Why are Zn2+m Sc3+ ions not coloured?
Have completely filled/empty 3d energy levels
What happens to 3d subshell when ligand bond to metal ions?
3d orbitals split into 2 different energy levels
If electrons in lower energy level absorb energy from visible light spectrum —> MOVE TO HIGHER ENERGY LEVEL (PROMOTION/EXCITATION)
- to jump move to higher energy level ,they
need energy = energy gap (ΔE)
- larger the energy gap , higher the frequency of light absorbed, lower the wavelengths
What does the amount of energy (and frequency of light) needed to move to higher energy level depend on?
Central metal ion/ its nuclear charge
Oxidation number of metal
Ligands (size/type)
Coordination number/shape of complex
THEY ALL AFFECT SIZE OF ENERGY GAP
How does size/type of ligand affect ΔE/colour observed?
Ligands have different CHARGE DENSITIES
Greater charge density—> the stronger the ligand interacts with metal ion —> greater splitting of d-orbitals —> more energy needed to promote electrons
- therefore shifted to region of visible light spectrum with HIGHER FREQUENCY/lower wavelengths
- different colour of light absorbed/ complementary colour observed (same metal but different ligands can have different colour)
How does oxidation number affect ΔE/colour?
Higher ox number of metal, stronger interaction with ligand —> —> greater splitting of d-orbitals —> greater ΔE —> higher frequency of light absorbed —> different colour observed/absorbed
E.g Fe(II): [Fe(H2O)6]2+ absorbs in the red region and appears green
Fe(III): [Fe(H2O)6]3+ absorbs in blue region and appears orange
How does coordination number affect ΔE/colour?
Change in coordination no./geometry of complex ion/change in ligand changes colour as they alter strength of interactions between metal ion/ligand
Splitting energy (ΔE) of d orbitals affected by orientation of ligands/d orbitals
Octahedral complex - coordination no.,bond angle?
COORDINATION NO. : 6
BOND ANGLE : 90 degrees
Tetrahedral complex: coordination no. And bond angle?
Coordination no. : 4
Bond angle : 109.5 degrees
(Cl- common ligand for this shape which are quite large)
Structure of cis platin?
Bond: 4 coordinate bonds - square planar (not tetrahedral)
Complex of platinum (II) with 2 Cl atoms, 2 NH3 molecules (same group on same side)
Bond angle: 90 degrees
Why is cis platin used for cancer treatment supplied as a cis isomer instead of its trans form?
Trans platin is toxic (same groups on opposite sides)
Structure of haemoglobin?
Contains Multidentate ligand made up of 4 haem groups ,
- iron (II) at its centre
Nitrogen atom from each haem group form dative covalent bond to Fe2+ ion in a square planar complex
5th dative bond from protein (globin) to Fe2+ ion
Ligand exchange of haemoglobin example?
Normally,Oxygen atoms form dative bond with iron(II) , but bnd weakly (to break off/be transported into cells)
CO is better ligand and bind strongly, irreversibly to iron(II) - prevent O2 binding
Why are transition metals used as catalysts?
Can form more than 1 stable ox state so can accept/lose electrons easily means they can catalyse some redox reactions
( can be reduced/oxidised again or vice versa)
2 types of catalyst?
Heterogeneous: catalyst in different physical state from reactants
- reaction occurs at active sites on surface of catalyst
E.g using IRON in haber process ( N2 + 3H2–> 2NH3)
homogeneous : catalyst in same physical state as reactants
How does heterogenous catalyst work?
Usually solid/reactnats are gaseous/in solution
SURFACE ADSORPTION THEORY
1. ADSORPTION: one or more reactnats attached to surface of catalyst - can lead to catalytic action
2. bonds between reactant molecules weakened or molecules held in a more reactive configuration
3. DESORPTION: reaction product becomes detached from surface of catalyst
What happens if a metal has too strong/too weak adsorption to catalyst?
Products cannot be released (metal W)/ cant adsorb in high enough concentration (Ag)
(Ni/Pt have the right strength so most useful as catalyst)
How is the surface area of heterogenous catalyst maximised?
A support medium is used
- minimises cost as well as
Advantages of heterogenous catalysts?
- heterogenous catalyts can be filtered off/easy to separate from liquid or gaseous products
- good for continuous processes (rather than batch processes)
Eg of heterogenous catalyst in contact process? What does this show?
*V2O5 - catalyst in manufacturing sulfuric acid
overall: 2SO2 + O2 —> 2SO3
Step 1: SO2 + V2O5 —> SO3 + V2O4
(Ox no. Of vanadium decreases from +5 TO -4)
Step 2 : 2V2O4 + O2 —> 2V2O5
(Vanadium(V) oxide regenerated by reacting with oxygen) V goes from +4 to +5
- shows that variable oxidation state can be utilised in heterogenous catalysis
What does catalytic converters do ?
Remove CO, NO and unburned hydrocarbons from exhaust gases , Turing them into less harmful CO2 , N2, H2O
2No + 2CO —> N2 + 2CO2
CH3CH2CH3 + 5O2 —> 3CO2 + 4H2O
Method of action of catalytic converters?
Uses SURFACE ADSORPTION THEORY
1. CO and NO form bonds with active sites on surface of catalyst - adsorbed
2. Results in bonds in reactants to weaken/break
3. New bonds form between the reactants held close tgt on catalyst surface
4. Desorption of CO2 AND N2 product molecules
How to minimise cost and efficiency of catalyst ?
- Increase SA of catalyst
Achieved by coating inert surface medium with catalyst (platinum, palladium,rhodium) - reduce amounts of catalyst used
Done by spreading catalyst over honeycomb support medium
What happens in homogeneous catalysis ? What does this reaction show?
- all reactants in same phase (all in gaseous/in solution)
Homogenous catalyst react with reactants to form an intermediate species, which then reacts to form products and reform the catalyst
Ea needed to form intermediates and form the products from intermediates is LOWER than Ea needed to form prodcuts straight from reactants - intermediate has different ox state to original transition metal catalyst/ at the end of reaction , original ox state will reoccur
SHOWS IMPORTANCE OF VARIABLE OX STATES of transition metals in catalysis
Why can transition element ions act as homogenous catalysts?
Have more than 1 stable ox state
- so they can accept/lose electrons easily to go from 1 ox state to another
( so catalyse redox reactions by acting as reducing/oxidising agents)
Iron often used as can form Fe(II) and Fe(III)
E.g of homogeneous catalyst?
Reaction between iodide and persulfate ions
Overall : S2O8 2- + 2I- —> 2SO4 2- + I2 (catalysed by Fe2+ - slow bc repulsion of 2 negative ions so high Ea)
Fe3+ + e- —> Fe2+ (Fe(II) acts as reducing agent/gets oxidised itself)
If iron (II) added , rate is quicker - LOWERS Ea
S2O82- + 2Fe2+ → 2SO42- + 2Fe3+ (Fe3+ is the intermediate species)
-peroxodisulfate reduced to sulfate ions /iron (III) produced**
2I- + 2Fe3+ → I2 + 2Fe2+
Iron (III) oxidise iodide ions / reduced themselves to Fe(II)
Both stages involve collisions between + and - ions so lower Ea
How does homogeneous catalysts work in terms of electrode potentials?
Homogeneous catalyst’s electrode potential must lie between electrode potentials of the 2 reactants
- so can reduce reactant with more + electrode potential
- and oxidise reactant with more negative electrode potential
What does the E value show us in homogeneous catalysis?
Shows if catalysis is possible
- doesn’t guarantee rate of reaction will be increased
Autocatalysis example? Reaction between ethanedioate and manganate ions?
Overall: 2MnO4- + 5C2O4 2- + 16H+ —> 2Mn2+ + 10CO2 + 8H2O
Reaction is slow as it is a collision between 2 negative ions - repel - INCREASED Ea
Mn2+ ion product is AUTOCATALYST + speeds up reaction as brings alternative route with lower Ea
Catalysed route:
Step 1 : 4Mn2+ + MnO4– + 8H+ → 5Mn3+ + 4H2O
2Mn3+ + C2O42- → 2CO2 + 2Mn2+
Mn2+ regenerated here
Reaction starts to slow as MnO4 - conc drops
How is the rate of reaction between ethanedioate and manganate ions measured? Advantages of this method?
Remove samples at set times+ titrate to work out conc of MnO4-
Colorimeter - rate at which purple Mn(VII) consumed accelerates with time
- doesn’t disrupt reaction mixture
- quicker determination of conc
How do impurities reduce efficiency of heterogenous catalyst?
The impurities can:
Adsorb onto catalyst surface and occupy active sites
Prevent bond weakening in the reactants
‘Take up’ the surface area of catalyst by forming strong bonds to surface of catalyst so are less likely to desorb from surface of catalyst
