Transition Elements Flashcards
Define transition element
a d-block element that form at least 1 stable ion w PARTIALLY-filled d subshell
Name exceptions electronic configuration of transition elements
- Cr & Cu, hv lone 4s e-
bcos, - extra stability fr symmetrical distribut n of charge ard nucleus for half-filled/fully filled 3d subshell for Cr & Cu
e- config of Cr: [Ar]3d5 4s1, NOT 3d4 4s2
e- config of Cu is [Ar]3d10 4s1, NOT 3d9 4s2
Why is Sc and Zn not classified as transition elements?
- all ions formed by Sc (Sc3+, *Sc2+ is unstable) & Zn (Zn2+) hv no partially-filled d subshell
electric config:
Sc: [Ar]3d1 4s2
Sc3+: [Ar] (no d e-)
Zn: [Ar]3d10 4s2
Zn2+: [Ar]3d10 (completely filled d subshell)
Describe chemical and physical properties of transition elements
transit n elements r:
- harder, hv higher densities
- hv higher mp, bp
- form cpd which show transit n elements’ variety of OS
- form cpd that show catalytic activity
- form coloured cpd, ion
- show great tendency form stable complexes
Describe how atomic and ionic radius of transition elements change across series
- across T.E. series, atomic/ionic radii relatively invariant (unchanged)
bcos,
across series, - nuclear charge increase
- e- added to inner 3d orbital, provide shielding for 4s e-
- increase in nuclear charge offset by increase shield effect
- eff nuclear charge vary oni slightly
- atomic/ionic radius remain relatively invariant
vs
across period 2, 3
- e- added to same outermost quantum shell
- nuclear charge increase but shield effect relatively const
- eff nuclear charge increase
- atomic/ionic radius decrease significantly
Describe how ionisation energy of transition element series change in general
across T.E. series,
i. 1st & 2nd IE relatively invariant
bcos
- both IE involve remove 4s valence e-
- inner 3d e- provide shielding for outer 4s e-
- increase in nuclear charge offset by increase shield effect
- eff nuclear charge oni slightly vary
- 1st, 2nd IE relatively invariant
ii. 3rd, 4th IE increase significantly
bcos
- involve remove valence e- fr inner 3d subshell
- across series, nuclear charge increase but shield effect remain approx const
- eff nuclear charge increase significantly
- significant increase in 3rd, 4th IE
Describe anomalies for ionisation energy of transition element series
3rd IE Fe lower than expected, 4th IE for Co lower than expected
*look at e- config, u see paired e-
bcos
- inter-electron repuls n present btw paired d e- in doubly-filled d orbital, so less energy needed remove valence e-
Explain why transition elements have a smaller atomic radii and a higher first ionisation energy than s block elements such as Ca
- transit n element hv more proton, so higher nuclear charge than Ca
- oni slight increase in shield effect as e- added to inner 3d orbital which provide shield for 4s e-
- there is greater eff nuclear charge, hence stronger e-static attract n btw nucleus & valence 4s e-
=> valence 4s e- more strongly attracted to nucleus
Explain hardness and density of transition elements
- transit n element harder, denser than s block element
bcos transit n element: - hv relatively smaller atomic radius, thus closer-packed structure
- hv higher relative atomic mass
=> thus, d-block element hv higher mass per unit volume, so higher density
Explain melting and boiling points of transition elements
- transit n element hv higher mp, bp than s block element
bcos - tho can both hv giant metallic (lattice) structure
- in transit n metal, both 3d, 4s e- involved in delocalisat n, so stronger e-static attract n btw cation & sea of delocalised e- (so stronger metallic bond)
- larger amt energy needed overcome stronger metallic bond to melt
- thus, higher mp,bp than s block metal
NOTE: in s block metals, oni s e- involved in delocalisat n in metallic bonding, so weaker metallic bond
Explain electrical and thermal conductivity of transition elements
- transit n element r better thermal, electrical conductor than s block element
bcos - both 3d, 4s e- available for delocalisat n
- higher no. of mobile e- act as charge carriers and to conduct heat
Explain why transition elements (Ti to Cu) have variable oxidation states
bcos
- 3d, 4s orbitals close in energies, so variable no. of 4s, 3d e- available for use in bond form n to form ion of similar stability
eg (some common OS)
Ti: +2,3,4
V: +2,3,4,5
Cr: +2,3,6
Mn: +2,4,6,7
Fe: +2,3
Co: +2,3
Ni:+2
Cu: +1,2
- BUT, s block element hv fixed OS
bcos,
once s e- removed, removal of p e- require too much energy, so unfavourable
Explain standard electrode potentials of transition elements
general trend
- -ve Eθ value for Ti, V, Cr show M3+ + e- –> M2+ is less feasible (eqm pos n tends twd oxidat n); hence M3+ more stable wrt M2+, so M2+ easily oxidised, making it good RA
eg Cr2+ oxidised by air to Cr3+
- +ve Eθ value for Mn to Cu show reduct n more feasible (eqm pos n tend twd reduct n); hence M2+ more stable wrt M3+, M3+ easily reduced, so it is good OA
eg Co3+ will b reduced by Cl- form Co2+
anomaly
- Eθ Fe3+/Fe2+ is less +ve than Eθ Mn3+/Mn2+
bcos
- easier to remove e- fr Fe2+ due to inter-electron repuls n btw paired d e- in doubly filled d-orbital (look at electron config)
- so, oxidat n more likely occur for Fe2+, so less +ve Eθ than expected
Define transition metal complex
complex containing central metal atom/ion attached to ligands thru dative bond
eg [Cu(H2O)6]2+
Define ligand
molecule or anion containing at least 1 lp e- available to form dative bond w central metal atom/ion
Define coordination number
no. of dative bonds each central metal atom/ion can form w its ligands
Explain why transition element ions can form complex ions
- transit n metal ion hv high charge density, can attract ligands containing at least 1 lp e-
=> high polarising pwr of transit n metal ion produce strong tendency twd covalent bond form n w ligand - transit n metal ion hv energetically accessible, vacant d orbitals to accommodate lp e- fr ligands via dative bond
Define monodentate ligand, bidentate ligand and polydentate ligand
- monodentate: form oni 1 dative bond per ligand
eg H2O, NH3 - bidentate: form 2 dative bond per ligand
eg ethane-1,2-diamine, ethanedioate - polydentate: form > 2 dative bond per ligand
eg EDTA 4- (hexadentate ligand)
Describe shapes of transition element complexes
i. coord no. 2
- linear
eg [Ag(NH3)2]+
ii. coord no. 4
- tetrahedral
eg [Cu(CN)4]2-
- square planar
eg [Ni(CN)4]2-
iii. coord no. 6
- octahedral
eg [Cu(EDTA)]2-
What to take note about drawing transition element complexes?
- identify correct donor atom (those w appropriate lp e-) of ligand
- show dative bond fr ligand to central atom/ion
- draw plane to represent 3D shapes if applicable
eg for octahedral, tetrahedral, square planar shapes - draw square bracket & charge for cationic/anionic complex
Explain, in terms of d orbital splitting, why transition element complexes are usually coloured
- a transit n metal ion hv partial fill d orbital; in presence of ligand, d orbital r split into 2 grp w energy gap. This effect aka d orbital splitting
- during d-d transit n, d e- fr lower energy d orbital absorb certain wavelength light fr visible spectrum, get promoted to higher energy d orbital
- colour observed is complementary to colour absorbed
Explain why d orbitals in an octahedral transition element complex split into two different energy levels
- in octahedral complex, central metal atom surrounded by 6 lp e- (on 6 ligand), along x,y,z axes
- all 5 d orbital experience e-static repuls n (of mag depending on orientat n of d orbital involved)
- fr shape, orientat n of d orbital, dx²-y² & dz² orbitals hv their lobe point at ligand along x,y,z axes respectively, so they experience greater repuls n fr ligand
- BUT, dxy, dxz, dyz orbital experience less repuls n as their lobe r in btw coordinate axes
=> 5 d orbitals r split into 2 energy lvl, w dx²-y² & dz² orbitals hv higher energy lvl, while dxy, dxz, dyz orbital hv lower energy lvl
NOTE:
for other geometries, d orbital r split diff
eg tetrahedral complex, dxy, dxz, dyz orbitals are at higher energy lvl than dx²-y² & dz² orbitals
Roughly explain colour wheel for transition element complex
NOTE: if sample absorb orange light (eg Cu2+), it appear blue or vice versa (complementary colour, based on colour wheel)
rough complementary colour pairs (+ wavelength)
- red (640-700nm) & green (450-560nm)
- orange (600-640nm) & blue (450 - 480nm)
- yellow (560nm - 600nm) & violet (400-450nm)
What affects colour of transition element complexes
- colour depend on energy gap E
related by E ∝ 1/λ (using photon energy formula) - E is affected by following factors:
1. e- config of metal atom/ion (associated w OS of metal)
eg
Fe2+: [Ar]3d6 vs Fe3+: [Ar]3d5 => Fe2+ is blue, Fe3+ is yellow
- Ligand field strength (associated w nature of ligand)
- diff ligand split energy lvl of d orbital to diff extent (amt energy E absorbed by d e- in d-d transit n differ)
- weak field ligand cause small E, long λ absorbed
- strong field ligand cause large E, short λ absorbed
*Ligand field strength not to confuse w ligand strength
-> ligand strength refer to ease of replace ligand in complex
Give formulae, colour and oxidation state of common transition complexes of V
- [V(H2O)6]2+, violet, +2
- [V(H2O)6]3+, green, +3
- [VO(H2O)5]2+, blue, +4
- [VO2(H2O)4]+, yellow +5
Give formulae, colour and oxidation state of common transition complexes/solutions/compounds of Cr
- [Cr(H2O)6]2+, blue, +2
- [Cr(H2O)6]3+, green, +3
- [Cr(OH)6]3-, deep/dark green, +3
- [Cr(NH3)6]3+, purple, +3
- CrO4 2-, yellow, +6
- Cr2O7 2-, orange, +6
Give formulae, colour and oxidation state of common transition complexes/solutions/compounds of Mn
- [Mn(H2O)6]2+, pale pink/colourless, +2
- [Mn(H2O)6]3+, red, +3
- MnO2, brown solid, +4
- MnO4 2-, green, +6
- MnO4-, purple, +7
Give formulae, colour and oxidation state of common transition complexes/solutions/compounds of Fe
- [Fe(H2O)6]2+, pale green, +2
- [Fe(CN)6]4-, yellow, +2
- [Fe(H2O)6]3+, yellow, 3+
- [Fe(CN)6]3-, orange red, +3
- [Fe(SCN)(H2O)5]2+, blood-red, 3+
Give formulae, colour and oxidation state of common transition complexes/solutions/compounds of Co
- [Co(H2O)6]2+, pink, +2
- [Co(NH3)6]2+, pale brown, +2
- [CoCl4]2-, blue, +2
- [Co(H2O)6]3+, blue, +3
- [Co(NH3)6]+3, yellow (may b dark brown due to mix of other Cr(III) complexes), +3
Give formulae, colour and oxidation state of common transition complexes/solutions/compounds of Ni
- [Ni(H2O)6]2+, green, +2
- [Ni(NH3)6]2+, blue, +2
- [Ni(CN)6]4-, yellow, +2
Give formulae, colour and oxidation state of common transition complexes/solutions/compounds of Cu
- Cu2O, reddish brown solid, +1
- CuCl, white solid, +1
- [CuCl2]-, colourless, +1
- [Cu(H2O)6]2+, blue, +2
- [Cu(NH3)4(H2O)2]2+, deep blue, +2
- [CuCl4]2-, yellow, +2
Explain ligand exchange
a stronger ligand can replace a weaker ligand fr cation complex in ligand exchange rxn
eg
when excess NH3(aq) added to Ni2+(aq), there is noticeable change in colour of resultant sol n fr green to blue
How do we know if a ligand exchange reaction has occurred?
predict n is usually based on given observ n (ie colour change, dissolut n of ppt, etc.) or given relative stability const Kstab (LCP concept) of complex ion formed
When dilute ammonia is gradually added to solution with Cu2+, a pale blue ppt forms, which dissolves on adding more dilute ammonia
Explain all the above transformation in terms of competing equilibria, writing equations for reactions which occur
- when small amt NH3(aq) (base, provide OH- ion) added gradually, pale blue ppt Cu(OH)2 forms
[Cu(H2O)6]2+ + 2OH- ⇌ Cu(OH)2 (s) + 6H2O —–(1) - in excess NH3, both NH3, OH- compete to combine w [Cu(H2O)6]2+
- NH3 ligand replace H2O ligand in ligand exchange rxn to form dark blue complex [Cu(NH3)4(H2O)2]2+
[Cu(H2O)6]2+ + 4NH3 ⇌ [Cu(NH3)4(H2O)2]2+ (aq) + 4H2O ——(2) - as conc of [Cu(H2O)6]2+ decrease in (2), eqm pos n in (1) shift left to increase [Cu(H2O)6]2+, so pale blue ppt Cu(OH)2 dissolves
Explain heterogeneous catalyst with respect to transition elements
- exist in diff phase fr rxt
- transit n metal & their cpd r good heterogeneous catalyst bcos of availability of 3d, 4s e- for temporary bond form n w rxt
- basic steps are:
1. adsorpt n (NOT absorp n)
2. activat n
3. desorpt n
Explain transition elements as catalyst, with example of production of ammonia via Haber Process using Fe catalyst
- N2, H2 molecules r adsorbed onto catalyst surface
- bonds break (activat n) btw N & N, H & H
- atoms re-arrange to form ammonia
- ammonia molecule leave catalyst surface (desorpt n); new N2, H2 molecule r adsorbed onto catalyst surface
Explain transition elements as catalyst, with example of hydrogenation of alkenes on nickel surface
eg ethene
1. C2H4 & H2 molecule adsorbed onto catalyst surface
2. bonds break in H-H
3. H atom re-arrange form C2H6 (activat n)
4. C2H6 molecule leave catalyst surface (desorpt n)
Explain transition elements as catalyst, with example of catalytic removal of oxides of nitrogen and unburnt hydrocarbons in exhaust gases from car engines
- catalytic converter in exhaust system of motor vehicles speeds up the convers n of pollutants eg CO, NOx and unburnt hydrocarbons (CxHy) into harmless pdt eg H2O, CO2, N2
-removal of pllutant
1. NOx reduced to N2 by excess Co present (oxidised to CO2) w rhodium as heterogeneous catalyst
2NO + 2CO —> 2CO2 + N2
- unburnt hydrocarbons, CO oxidised to CO2 & H2O (& O2 reduced to H2O), with Pt, Pd catalysts
CxHy + (x + y/4)O2 —> xCO2 + y/2 H2O
2CO + O2 —> 2CO2
Describe the mechanism for heterogeneous catalysis
Adsorpt n
- rxt molecule adsorbed onto catalyst surface thru form n of temporary bond (fr available 3d, 4s e-)
Activat n
- adsorpt n weaken covalent bond within rxt molecule, lowering Ea
- (kinetics) rxt molecules brought closer tgt, increasing surface conc of rxt, so rxn can occur btw rxt molecules more easily
desorpt n
- pdt formed diffuse away fr surface of catalyst
Define homogeneous catalyst with respect to transition element
- exist in same phase as rxt (usually aq)
- transit n metal & their cpd r gd homogeneous catalyst bcos of their ability to exist in various OS, so facilitate form n of rxn intermediate via alternative pathway of lower Ea
Describe mechanism for homogeneous catalysis, using example of S2O82-, I- reaction
- w/o catalyst,
redox rxn S2O8 2- + 2I- –> 2SO4 2- + I2
Ecell = Ered - Eox = 2.01-0.54 = +1.47V>0, so rxn feasible
BUT, rxn kinetically not feasible due to high Ea, fr e-static repuls n btw -ve charge ions
=> in presence of transit n metal ion eg Fe2+ or Fe3+ acting as homogeneous catalyst, rxn is accelerated - w catalyst (Fe3+)
- step 1: Fe3+ react w I-
2Fe3+ + 2I- —> 2Fe2+ + I2
Ecell = 0.77-0.54 = +0.23V > 0, so rxn spontaneous
- step 2: Fe2+ intermediate react w S2O8 2-
2Fe2+ + S2O8 2- —> 2SO4 2- + 2 Fe3+
Ecell = 2.01-0.77 = +1.24V>0, so rxn is spontaneous
Overall eqn: S2O8 2- + 2I- –> 2SO4 2- + I2 (still the same)
Both steps r spontaneous since opp charge ion involved, attract each other. Ea is lower, so rxn faster/kinetically feasible
Name common reactions undergone by transition metal ions
- ppt n rxn/soluble complex form n
- ligand exchange rxn
- redox rxn
- hydrolysis (for aqua, H2O, ligand, complex ion w high charge density)
