transition metals Flashcards
(70 cards)
1
Q
what is a transition metal?
A
a d block element that can form at least one stable ion w a partially filled (incomplete) d-subshells
2
Q
are scandium and zinc transition metals? why?
A
NO - do not form a stable ion with a partially filled d sub shell
Sc forms only 1 stable ion - Sc3+ - empty d sub shell!!
Zn only forms 1 stable ion of Xn2+ - full d sub shell!!
3
Q
which 2 elements behave differently when giving electron configurations?
A
- chromium + copper -> an electron from 4s orbital moves into 3d orbital to create more stable half full/ full 3d sub shell
4
Q
how do TM lose electrons
A
FROM 4S FIRST!
THEN 3D!
5
Q
what are the properties of TMs? 4
A
- variable oxidation states - bc electrons sitting in 4s and 3d energy levels are very close
- coloured ions in soln
- good catalysts
- form complex ions
6
Q
what is a complex ion
A
an ion where a central transition metal ion is surrounded by ligands bonded by dative covalent bonds
7
Q
what are ligands
A
an ion/atom/molecule that has at ;east one lone pair of electrons
8
Q
what are the dif types of ligand and give examples
A
- mono dentate/unidentate = 1 lone pair of e eg H20:, :NH3, :CL-
- bidentate = 2 atoms w a LP e- eg Ethanedioate, Ethane-1,2-diamine
- multidentate = more than 1 coordinate bond eg EDTA^4- -> can form 6 coordinate bonds w central metal ion
9
Q
what determines shape of complex ion
A
- size of ligands - larger = takes more space up
- coordination number = no. of coordinate bonds
10
Q
sizes of complex ion for dif CN; give bond angles too
A
coordination no = 6 -> OCTAHEDRAL - 90 deg bond angle - SMALL LIGANDS eg h20 + nh3
COORD no = 4 -> tetrahedral (109.5) - larger ligands eg Cl-
COORD no = 4 -> square planar (90deg) (cisplatin!)
CN = 2 -> linear shape - [Ag(NH3)2]+ } tollens reagent } Ag+ commonly forms linear
11
Q
what isomerism do complex ions show
A
- cis-trans isomerism (like E-Z - cis = same) -> SQUARE PLANAR + OCTAHEDRAL
- optical -> complex w 3 bidentate ligands
12
Q
what substitution reactions of ligands involve no change in CN? an anomaly?
A
- bw H20 and Nh3
- anonmaly = w Cu - reaction is incomplete} [Cu(h2o)6] + 4NH3 -> [Cu(nh3)4(h20)2]2+ + 4h2o
- in that reaction Cu becomes [Cu(nh3)4(h20)2]2+ = deep blue solo
13
Q
what abt substitution w Cl- ions
A
- adding high conc of Cl- (conc Hcl/saturated NaCl) to aqueous ion -> ligand substitution reaction
- but Cl- is larger than H20 + NH3 so change in CN!
- usually from CN6 -> CN4 for Cu and Co
14
Q
ligand substitution reaction for aq [Cu/Co(H20)6]2+ w conc Hcl; what is end colour
A
[Cu/Co(H20)6]2+ + 4Cl- -> [Cu/CoCl4] 2- + 6H20
- CuCl4]2- = yellow/green sol
- [CoCl4]2- = blue sol
15
Q
what complex is formed when a solid metal chloride is dissolved in water?
A
[M(H2O)6]2+ NOT chloride complex
16
Q
when does partial substitution of ethanedioate ions occur?
A
- a dilute aq soln containing ethanedivoate ions is added to a solo containing aq Cu ions
- 4 water molecules are replaced
- [Cu(H2O)6]2+ +2C2O42 -> [Cu(C2O4)2(H2O)2]2- +4H2O
17
Q
show an equation for formation of a multi dentate complex
A
[Cu(H2O)6]2+ + EDTA4- [Cu(EDTA)]2- + 6H2O
18
Q
what is haem
A
- an iron (II) complex w a multi dentate ligand
- O2 forms a coordinate bond to Fe (ii) in haemoglobin - enables O2 to be transported in blood
19
Q
why is CO toxic
A
CO can form a strong coordinate bond w haemoglobin - stronger bond than that made w O2 so it replaces O2
20
Q
what is the chelate effect?
A
Bidentate and multidentate ligands replace monodentate ligands from complexes and form even stronger complexes
21
Q
how can you explain the chelate effect?
A
- the positive entropy change as there are more moles of products than reactants } so more disorder - higher entropy
- enthalpy change is small as similar no bonds in both complexes
- so Gibbs free energy = neg as large ΔS, and small ΔH
22
Q
since EDTA complexes are really stable they have many applications, what are they?
A
- added to rivers to remove poisonous heavy metal ions as EDTA complexes aren’t toxic
- in many shampoos to remove CA2+ ions present in hard water - helps lathering
23
Q
what ratio is EDTA w metal ion
A
1:1
24
Q
what causes colour changes
A
- change energy split bw d orbitals
- that is caused by changes in:
1. oxidation state
2. CN
3. ligand
25
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.
- when light is absorbed some of it is used to move d electrons from the ground state to an excited state
- the rest is transmitted to give the substance colour
26
whats the eqn for energy difference between the ground state and the excited state of the d electrons
∆E = hν = hc /λ
∆E = energy dif in J
h = Plancks constant (Js)
v = freq of light (s^-1 OR Hz)
c = speed of light
λ = wavelength of light absorbed (m)
27
what compounds have no colour?
Sc3+/Zn2+/Cu+ -> d shell either empty or full so no space for electrons to transfer so no energy transfer equal to that of visible light
28
how to determine the concentration of coloured ions in solution.
use a simple colorimeter or spectrophotometry
29
what is spectrophotometry?
- use absorption of visible light to determine conc of coloured ions
- visible light of increasing freq is passed thru sample of coloured complex ion - some light is absorbed
- amount of light absorbed is prop to conc of absorbing species and to distance travelled thru soln
30
what happens if complex has only pale colours
- won't absorb light strongly
- add suitable ligand to intensify colour
31
how to choose suitable filter for spectrometer
- choose colour that would allow wavelengths of light through that would be most strongly absorbed by coloured soln
32
method for spectrometer
1. add an appropriate ligand to intensify colour
2. make up solns of known conc
3. measure absorption
4. plot graph abs vs conc
5. measure abs of unknown and compare
33
what are the general trends for variable oxidation states? 3
1. relative stability of +2 wrt +3 increases across period
2. high oxidation state = oxidising agents
3. low = reducing agents
34
what are vanadiums 4 main oxidation state - inc colours
VO2^+ = Oxidation state +5 ( a yellow solution)
VO^2+ = Oxidation state + 4 (a blue solution)
V3+ = Oxidation state + 3 (a green solution)
V2+ = Oxidation state + 2 (a violet solution)
your baby gone viral
34
what influences the redox potential for a transition metal ion changing from higher -> lower oxidation state
1. pH
2. ligand
35
how to reduce vanadium
add zinc to V in acidic soln
- Zn metal + acid = strong reducing agent
36
eqn showing reduction of Ag in tollens reagent w an aldehyde
Red 1⁄2 eq: [Ag(NH3)2]+ + e- -> Ag + 2NH3
Ox 1⁄2 eq: CH3CHO + H2O -> CH3CO2H + 2H+ + 2e-
37
what acid do u use in manganate titrations and why?
DILUTE SULFURIC ACID
other acids may set up alternative redox reactions - inaccurate titrate reading
38
what happens if insufficient volumes of sulphuric acid is used?
- this means soln is not acidic enough and MnO2 is formed instead of Mn2+ -> MnO2 = brown colour and will mask colour change -> greater inaccurate vol of Mn being used in titration
39
what happens if weak acid (ethanoic acid) used
- weak acid = cant supply large no. of H+ ions needed
40
what happens if conc HCl used?
Cl- ions would be oxidised to Cl2 by MnO4- so greater vol of Mn being used and poisonous Cl2 produced
41
if nitric acid used?
- its an oxidising agent so it would oxidise Fe2+ -> Fe3+ } smaller vol of Mn used
42
eqn for titration bw MnO4- and Fe2+
MnO4-(aq) PURPLE + 8H+ (aq) + 5Fe2+ (aq)Mn2+ (aq) + 4H2O (l) + 5Fe3+ (aq) COLOURLESS
43
why is the reaction between MnO4- and C2O4- slow to begin with? what can be done?
- they are both neg ions
- heat up conical flask to 60 deg to speed up initial reaction
44
eqn for titration bw MnO4- and C2O4-
Ox C2O42- -> 2CO2 + 2e-
Red MnO4-(aq) + 8H+(aq) + 5e- -> Mn2+ (aq) + 4H2O
Overall 2MnO4-(aq) + 16H+(aq) + 5C2O42 (aq) -> 10CO2(g) + 2Mn2+(aq) + 8H2O(l)
45
what happens when iron (II) ethanedivoate reacts w MnO4-
1 MnO4- reacts with 5 Fe2+ and 2 MnO4- reacts with 5C2O42-
MnO4-(aq) + 8H+(aq) + 5Fe2+ Mn2+ (aq) + 4H2O + 5Fe3+
2MnO4-(aq) + 16H+(aq) + 5C2O42- 10CO2 + 2Mn2+ (aq) + 8H2O
So overall
3MnO4-(aq) + 24H+(aq) + 5FeC2O4 10CO2 + 3Mn2+ (aq) + 5Fe3+ + 12H2O
So overall the ratio is 3 MnO4- to 5 FeC2O4
46
what is a heterogeneous catalyst
A heterogeneous catalyst is in a different phase from the reactants and the reaction occurs at active sites on the surface.
- catalyst usually solid; R = gaseous/ in sole
47
what are the steps in heterogeneous catalysis
1. Reactants form bonds with atoms at active sites on the surface of the catalyst (adsorbed onto the surface)
2. As a result bonds in the reactants are weakened and break
3. New bonds form between the reactants held close together on catalyst surface.
4. This in turn weakens bonds between product and catalyst and product leaves (desorbs).
48
what does the strength of adsorption help determine? give examples?
- effectiveness of catalytic activity
- some metals (W) have too strong adsorption and so products cant be released
- some (Ag) have too weak - R do not adsorb in high enough conc
- Ni + Pt = right strength - most useful as catalysts
49
give examples of heterogeneous catalysts
V2 O5 acts as a heterogeneous catalyst in the Contact process.
Fe is used as a heterogeneous catalyst in the Haber process.
50
equations for contact process
Overall equation : 2SO2 + O2 2SO3 step1SO2 +V2O5 SO3 + V2O4
step 2 2V2O4 + O2 2V2O5
51
eons for haber process
N2 + 3H2 2NH3
52
what happens if heterogeneous catalysts r poisoned?
- Heterogeneous catalysts can become poisoned by impurities that block the active sites and consequently have reduced efficiency; this has a cost implication.
53
what are homogeneous catalysts? what is req in this type of catalysis
A homogeneous catalyst is in the same phase as the reactants.
When catalysts and reactants are in the same phase, the reaction proceeds through an intermediate species.
54
why can transition metals act as homogeneous catalysts
they can form various oxidation states
55
what does Fe2+/Fe3+ catalyse?
reaction bw I- and S2O82-
S2O82- + 2I- 2SO42- + I2
stage 1 S2O82- + 2Fe2+ 2SO42- + 2Fe3+
stage2 2I- +2Fe3+2Fe2+ + I2
56
why is the uncatalysed reaction bw I- + S2O82- v slow so why use a catalyst
two neg ions - repulsion bw them hinders them from colliding successfully - high activation energy
catalyst is pos ion so lower Ea
57
For a substance to act as a homogenous catalyst where should its electrode potential lie
in bw the E cells of the 2 reactants
- first will reduce reactant w more pos E cell
- second step will oxidise reactant w more neg E cell
58
what does using E cell values to find catalyst show
- that catalysis is possible but doesn't guarantee increase in RoR
59
give an example of autocatalysis
autocatalysis by Mn2+ in titrations of C2O42- with MnO4-
2MnO4- + 5C2O42- + 16H+ -> 2Mn2+ + 10CO2 + 8H2O
Catalysed alternative route
Step1 4Mn2+ + MnO4- + 8H+ -> 5Mn3+ + 4H2O
Step 2 2Mn3+ + C2O42- -> 2Mn2+ + 2CO2
60
explain the graph of conc of [MnO4-] over time
1. initially reaction is V SLOW - uncatalysed reaction is slow as neg ions repel each other -> high Ea
2. Mn2+ ions produced act as an autocatalyst and so reaction speeds as lower Ea
3. eventually react slows as MnO4- conc drops
61
how to follow the reaction rate of C2O42- with MnO4-? 2 ways
1. removing samples at set times and titrating to work out conc of MnO4-
2. use a spectrometer to measure intensity of purple colour -> DOESNT DISRUPT REACTION MIXTURE! - much quicker determination of conc
62
how to construct a catalysed mechanism for a reaction?
1. split overall eqn into its 2 half eqns
2. write out the catalysts redox eqns
3. combine the 2 and check the 2 eqns add up to OG eqn
63
how is Ag unlike TMs? 2
- doesn't form variable oxidation states - only 1+
- doesn't form coloured compounds - all colourless
64
what complexes does Ag+ form
linear
65
how is Ag like TMs?
- can form complexes
- can show catalytic behaviour although it adsorbs too weakly for many examples
66
why is Ag not a TM by definition?
it is 4d10 in both atom + ion so doesn't have a partially filled d sub shell so cant do e- traditions bw d orbitals that enable coloured compounds
67
uses of Ag
- test for halifes
- test for aldehydes
68
how to use silver nitrate to work our formulae of Cl- containing complexes
- Co(NH3)6Cl3 reacts on a 1:3 mole ratio with silver nitrate as there are three free Cl- ions. So all 3 Cl- ions are outside the complex.
- Cr(NH3)5Cl3 reacts on a 1:2 mole ratio with silver nitrate as there are two free Cl- ions. So 1 Cl- is a ligand and 2 are outside the complex.
- Cr(NH3)4Cl3 reacts on a 1:1 mole ratio with silver nitrate as there is one free Cl- ion. So 2 Cl’s are ligands and 1 is outside the complex.
69
Why do TMz have variable oxidation states
Bc electrons in 4s and 3D r very close
