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Chemistry A2 > Transition Metals > Flashcards

Transition Metals Flashcards

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1
Q

Transition Metal

A
  • any d-block element that forms at least one ion with a partially filled d SUB-shell of electrons
  • found between Sc and Zn
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2
Q

Which subshell are electrons removed from first to form transition metal ions

A

4s then 3d

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3
Q

State chemical properties of transition metals

A
  • variable oxidation states
  • characteristic colour of complexes
  • catalysts
  • complex formation
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4
Q

Suggest why transition metals have a variable oxidation state

A

partially filled d shell is able to gain or lose electrons

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5
Q

Give reasons why transition metals act as good catalysts

A
  • able to donate and accept electrons
  • variable oxidation states
  • form complexes
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6
Q

Ligand

A
  • electron pair donor

- forms coordinate bond with central metal atom/ion

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7
Q

Complex

A

central metal atom or ion surrounded by ligands

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8
Q

Coordination Number

A

number of co-ordinate bonds to central metal atom or ion

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9
Q

Monodentate ligand

A

donate a single pair of electrons from a single atom to central metal atom or ion

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10
Q

Multidentate ligand

A

donate two or more electron pairs from two or more atoms to central metal atom or ion per ligand

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11
Q

Examples of ionic and neutral monodentate ligands

A

ionic - Cl-, F-, CN-, H-

neutral - :CO, :NH3, H2O:

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12
Q

Examples of ionic and neutral bidentate ligands

A

ionic - ethanedioate (C2O42-)
neutral - 1,2-diaminoethane (H2NCH2CH2NH2),
benzene-1,2-diol (C6H4(OH)2)

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13
Q

Example of hexadentate ligand

A

EDTA4-

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14
Q

How to determine the shape of a complex based on its ligands

A
  • small ligands (e.g. H2O/NH3) = octahedral
  • large ligands (e.g. Cl-) = tetrahedral
  • Ag+ forms linear e.g. [Ag(NH3)2]+ (Tollen’s reagent)
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15
Q

How to determine the shape of a complex based on coordination number of metal

A 
6 = octahedral 
4 = tetrahedral (or square planar for Ni2+ and Pt2+)
2 = linear
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16
Q

Ligand Substitution

A

ligands in a complex are exchanged for ligands which bind more strongly to central metal atom or ion

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17
Q

Types of ligand substitution reactions

A
  • similar size ligands like NH3 and H2O occurs without
    change of co-ordination number
  • different size ligands like H2O by Cl– can involve a change of co-ordination number
  • ligand substitution may be incomplete
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18
Q

Give an example of incomplete ligand substitution

A
  • replacing water in hexaaqua copper(II) ion
  • only four of six water molecules replaced by NH3
  • [Cu(NH3)4(H2O)2]2+
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19
Q

Explain how haem is a transition metal complex and how ligand substitution occurs in carbon monoxide poisoning

A
  • (iron(II) complex with multidentate ligand)
  • oxygen forms a co-ordinate bond to Fe2+ in haemoglobin
  • carbon monoxide has greater affinity for Fe2+/ bonds more strongly than oxygen so displaces it
  • toxic since oxygen no longer transported in blood
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20
Q

Chelation

A

multidentate ligands act as chelating agents by replacing several monodentate ligands from complexes

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21
Q

Sequestering Agent

A
  • (chelating agent)
  • remove (unwanted) metal ions form a solution
  • monodentate (water ligands replaced in aqua ions) by multidentate ligand so a precipitate forms
  • forms complex ion that does not have the chemical reactions of the ion that is removed
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22
Q

Chelate Effect

A
  • chelate complex more stable than monodentate
  • due to increase in entropy during chelation
  • few species replaced by several species
  • S = positive, G = negative so reaction favourable
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23
Q

Type of isomerism in monodentate ligand complexes

A
  • octahedral / square planar / tetrahedral

- cis-trans isomerism

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24
Q

Type of isomerism in bidentate/multidentate ligand complexes

A
  • octahedral

- optical isomerism

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25
Q

Describe method for redox titration with Fe2+ or C2O42- with MnO4-

A
  • rinse and then fill burette with MnO4- using funnel
  • rinse with then transfer standard volume of Fe2+ or C2O42- to conical flask using volumetric pipette
  • add excess sulfuric acid to conical flask
  • add MnO4- to solution in conical flask and swirl
  • dropwise near end point
  • end point indicated by permanent colour change to pink observed against white tile
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26
Q

Explain why Fe2+ and C2O42- are acidified during redox titration of MnO4-

A

provides H+ to reduce MnO4-

27
Q

Explain why the colour of KMnO4 purple before the titration and after the end point it is pink

A
  • burette contains MnO4- (Mn7+) = purple
  • when added to Fe2+/C2O42- redox reaction occurs so MnO4- is reduced to Mn2+ = colourless
  • once fully reacted at end point, additional MnO4- remains in oxidation state +7 but concentration is only small so appears pink
28
Q

Explain how the redox potential for a transition metal ion changing from a higher to a lower oxidation state is influenced by pH

A
  • metal ions often exist in an equilibrium with H+

- so their tendency to undergo oxidation or reduction depends on pH

29
Q

Explain how the redox potential for a transition metal ion changing from a higher to a lower oxidation state is influenced by ligands it bonds with

A
  • some ligands stabilise higher oxidation states more than lower ones and other ligands, visa versa
  • different ligands change the ability of the transition metal ion to alter its oxidation state (redox potential)
30
Q

Explain how a yellow solution of vanadium (V) can be reduced to form a purple solution

A
  • reducing agent zinc in acidic conditions
  • VO2^+ + 4H+ + 3e- -> V2^+ + 2H2O
  • Zn -> Zn2+ + 2e-
  • yellow (VO2^+) to blue (VO^2+) to green (V^3+) to purple (V2^+ )
31
Q

Explain in terms of electrons why transition metal complexes are coloured

A
  • certain wavelengths of visible light are absorbed
  • electrons in d orbitals are excited to higher energy levels
  • remaining wavelengths transmitted are within the visible range
32
Q

Explain why the complex ion formed after chelation is more stable

A
  • entropy increases

- several species formed from a few

33
Q

Suggest factors which cause transition metal complexes to change colour

A
  • change in oxidation state
  • change in coordination number
  • change in TYPE of ligand
34
Q

Explain why ligand substitution reactions cause colour changes in transition metal complexes

A
  • in different complexes d orbital splitting will be different / d orbital electrons have different energies
  • energy absorbed causes electrons in d orbitals to excite
  • different wavelengths/frequencies of absorbed and transmitted depending on complex
35
Q

Equation for energy difference between ground and excited state of d electrons

A

E = hv = hc/wavelength

36
Q

Colorimeter

A

measures intensity of light of a particular wavelength passing through a coloured solution

37
Q

Describe how colorimetry is used to determine the transition metal ion concentration in an aqueous sample

A
  • make up standard solutions of known metal ion concentrations by serial dilution (lighter and darker than unknown sample)
  • choose a suitable filter (light most absorbed by coloured solution)
  • calibrate colorimeter using distilled water in cuvette to zero
  • measure absorbable reading of different known concentrations
  • plot calibration curve of absorbance against concentrations
  • measure absorbance reading of unknown concentration
  • interpolate concentration from calibration curve
38
Q

Describe how to determine the formula of a transition metal complex using colorimetry

A
  • make upon standard solutions of different ratios of metal ions and ligand solutions
  • plot graph of absorbance against ratios of metal ion to ligand concentrations
  • mixture with maximum absorbance has same ratio of ligands to metal ions as in complex
39
Q

Homogenous Catalyst

A

catalyst in same phase as reactant and intermediate species formed

40
Q

Heterogenous Catalyst

A

catalyst in different phase to reactants and provides a surface for reaction

41
Q

Suggest advantages of heterogenous catalyst over homogenous catalyst

A
  • catalyst not lost

- catalyst does not need to be separated from products

42
Q

Describe how to increase efficiency of heterogenous catalyst

A
  • increase surface area

- impregnate onto inert support medium to increase surface area to mass ratio

43
Q

Explain how heterogenous catalysts work

A
  • transition metal ions have partially filled d sub-shell so from weak chemical bonds with reactants when adsorbed
  • weaken bonds so new bonds from
    or
  • bring reactants in closer proximity so increase concentration in particular area
    or
  • absorbed in correct orientation for reaction
44
Q

Explain why d orbital splitting occurs in transition metal complexes

A
  • ligand forms dative covalent bonds with metal ion
  • electrons in some of the orbitals will experience a greater repulsion than electrons in other orbitals
  • d sub-shell splits to higher and lower energy levels
45
Q

Catalyst Poisoning (with examples)

A
  • substances adsorbed too strongly to active sites of catalyst
  • so active sites blocked
  • lead poisoning to catalytic converters
  • sulfur poisoning of Fe in Haber process
46
Q

Give equations to show how iron(II) acts as a homogenous catalyst in reaction of iodide ions with peroxodisulfate (S2O82-) and explain specific need for catalyst

A

S2O82- + 2Fe2+ -> 2SO42- + 2Fe3+
2Fe3+ + 2I- -> 2Fe2+ + I2

opposite charges repel so reaction between S2O82- and I- has high activation energy

47
Q

Give equations to show how vanadium pentoxide acts as a heterogenous catalyst in contact process and explain specific need for catalyst

A

SO2 + V2O5 -> SO3 + V2O4
2V2O4 + O2 -> 2V2O5

gases are far part so vanadium pentoxide provides surface for reaction by bringing reactants in close proximity and in correct orientation

48
Q

Autocatalysis

A

product of reaction is catalyst for reaction

49
Q

Give the half equations and overall equation for oxidation of ethanedioate ions by manganate (VII) ions to form carbon dioxide

A

MnO4- + 8H+ + 5e- -> Mn2+ + 4H2O
C2O42- -> 2CO2 + 2e-

5C2O42- + 2MnO4- + 16H+ -> 2Mn2+ + 8H2O + 10CO2

50
Q

Give equations to show how Mn2+ behaves as an autocatalyst in oxidation of ethanedioate ions by manganate (VII) ions to form carbon dioxide

A

MnO4- + 4Mn2+ + 8H+ -> 5Mn3+ + 4H2O
2Mn3+ + C2O42- -> 2Mn2+ + 2CO2
(no electrons so balance charges using Mn3+ / Mn2+)

  • construct ionic equation from MnO4- + Mn2+ -> Mn3+
  • MnO4- + Mn2+ + 8H+ -> 2Mn3+ + 4H2O (charge LHS = 9+, RHS=6+)
  • 3Mn2+ -> 3Mn2+ (charge LHS = 6+, RHS=9+)
  • combine equations so charges balance
51
Q

Examples of square planner complex

A

Pt(NH3)2Cl2 or PtCl42- or NiCl42-

52
Q

When is H positive for ligand substitution

A

more coordinate bonds are broken than made

53
Q

When is H negative for ligand substitution

A

more coordinate bonds are made than broken

54
Q

Suggest why NH3 is a better ligand compared to H2O

A
  • N less electronegative than O

- more willing to donate electron pair to central metal ion

55
Q

Suggest methods by which concentrations of transition metal ion solutions can be determined

A
  • titration

- colorimetry

56
Q

Suggest how filters can be tested to see which is appropriate complementary colour to use in colorimeter when determining metal ion concentration

A
  • place solution in cuvette and measure absorbance reading using each filter
  • filter which gives highest absorbance is complementary colour
57
Q

Explain why mixture with same ratio of ligands to metal ions as transition metal complex has maximum absorbance

A

maximum amount of complex precipitate formed

58
Q

Explain why s block elements are not good catalysts

A

exist only in one oxidation state/ do not have a variable oxidation state

59
Q

Explain why Fe3+ are as effective as Fe2+ at catalysing reaction between I- and S2O82-

A

reactions can occur in any order

60
Q

Explain graph for autocatalysis reaction

A
  • rate of reaction increases as catalyst formed

- rate of reaction decreases as reactants are used up

61
Q

Suggest a property of an impurity that could cause calculated mass of FeSO4.7H2O to be greater than actual mass in redox titration of FeSO4.7H2O solution against K2Cr2O7

A
  • impurity reduces Cr2O72-

- impurity reacts with more Cr2O72- for equal masses of impurity and FeSO4.7H2O

62
Q

Outline a plan of an experiment to determine the percentage of iron present as iron(III) in a solution containing both Fe2+ (aq) and Fe3+ (aq) ions - provided with Zn (reduces Fe3+) and K2Cr2O7/H+

A

6Fe2+ + Cr2O72- + 14H+ -> Fe3+ + 2Cr3+ + 7H2O
Zn + 2Fe3+ -> Zn2+ + 2Fe2+
- titrate measured volume of solution against K2Cr2O7
- reduce same volume with excess zinc
- filter off excess zinc
- titrate remaining solution using K2Cr2O7
- % Fe3+ = (titre 2 - titre 1)/titre 2

63
Q

Suggest how selecting the wrong filter will reduce accuracy of absorbance reading of colorimeter

A

absorbance reading will be lower than actual absorbance since not all light absorbed by the sample regardless of concentration

Chemistry A2 (12 decks)

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