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AQA Chemistry A-Level > 3.2.5 Transition Metals > Flashcards

3.2.5 Transition Metals Flashcards

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1
Q

Define a transition element.

A

An element which forms at least one stable ion with a partially full d-shell of electrons

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2
Q

Where the transition metals located in the periodic table?

A

In the middle (Ti to Cu) part of the d-block

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3
Q

What are some characteristic physical properties of transition metals?

A

Metallic, good conductors of heat and electricity, hard, strong, shiny, high melting point, high boiling point, low reactivity.

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4
Q

Some uses of iron?

A

Vehicle bodies, to reinforce concrete

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5
Q

Some uses of titanium?

A

Jet engine parts

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6
Q

Some uses of copper?

A

Water pipes

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7
Q

What are the characteristic chemical properties of transition metals (4)?

A
  • Variable oxidation states -> take part in many redox reactions
  • Coloured compounds/ions in solution
  • Good catalysts
  • Form complex ions
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8
Q

Define the term complex ion.

A

Central transition metal ion surrounded by ligands (other ions/molecules) that are co-ordinate bonded to it.

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9
Q

Give some examples of transition metals catalysts and the processes/reactions they catalyse (3)?

A

Iron - Haber process
Vanadium (V) oxide - Contact process
MnO₂ - decomposition of H₂O₂

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10
Q

Which electrons do transition metals lose first when forming ions?

A

4s

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11
Q

Define the term ligand.

A

An ion or molecule with at least one lone pair of electrons, that donates them to a transition metal ion to form a co-ordinate bond and thus a complex ion.

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12
Q

Define the term mono/unidentate ligands.

A

A ligand that forms one co-ordinate bond to the central metal ion (one lone pair to donate).

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13
Q

Define the term bidentate ligand.

A

A ligand that forms two co-ordinate bonds to the central metal ion (2 lone pairs to donate)

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14
Q

Define the term multidentate ligand.

A

A ligand that forms three or more co-ordinate bonds to the central metal ion

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15
Q

Give some examples of common monodentate ligands (4).

A

Cl⁻, H₂O, NH₃, CN⁻

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16
Q

Draw ethanedioate. How many co-ordinate bonds can it form to a transition metal ion?

A

2 co-ordinate bonds

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17
Q

Draw benzene-1, 2-diol. How many co-ordinate bonds can it form to a transition metal ion?

A

2 co-ordinate bonds

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18
Q

Draw ethane-1, 2-diamine. How many co-ordinate bonds does it form?

A

2 co-ordinate bonds

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19
Q

How many co-ordinate bonds does EDTA⁴⁻ form?

A

six

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20
Q

Define the term coordination number.

A

The number of co-ordinate bonds the metal ion has formed to surrounding ligands.

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21
Q

What is the Chelate effect?

A

Chelate complexes with multidentate ligands are favoured over monodentate ligands or ligands that form fewer co-ordinate bonds per molecule.

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22
Q

Explain the Chelate effect in terms of entropy and the reaction that is occurring.

A

Number of molecules increases when multidentate ligands displace ligands that form fewer co-ordinate bonds per molecule
Significant increase in entropy -> Gibbs’ free energy change < 0 -> feasible reaction
A more stable complex ion is formed

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23
Q

What ion is usually formed when a transition metal compound is dissolved in water? What shape is it? Draw an example.

A

Aqua ion, 6 H₂O ligands around the central metal ion. Octahedral complex ion is formed.

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24
Q

If a transition metal ion has 2 ligands, what shape is it usually?

A

Linear

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25
If a transition metal ion has 4 ligands, what shape is it usually?
Tetrahedral
26
Name an exception to the general rule that ions with 4 ligands is generally tetrahedral. What shape is it?
Platin is square planar -> forms cisplatin
27
What shape is a complex ion if it has 6 ligands?
Octahedral
28
How can complex ions display E-Z isomerism? What shapes of ion does this apply to?
- Ligands differ in the way in which they are arranged in space. - 2 ligands of the same type can be on the same side of the metal ion (next to each other), which forms the E isomer - 2 ligands of the same type can be on opposite sides of the metal ion (not next to each other), which forms the Z isomer. - Applies to square planar and octahedral complex ions
29
What conditions are needed for a complex ion to display optical isomerism?
Usually applies to octahedral molecules with 2 or more bidentate ligands, so that the mirror images and non- superimposable
30
What happens to Co²⁺, Cu²⁺ and Fe³⁺ 's coordination numbers when Cl⁻ ligands replace NH₃ or H₂O ligands?
Decreases from 6 to 4 as Cl⁻ is a much larger ligand than H₂O and NH₃
31
What is haem - its metal ion, coordination number and ligands?
A molecule which makes up protein chains, with an Fe²⁺ central metal ion, which has a coordination number of 6. 4 of these bonds are to a ring system called porphyrin. 1 is to the nitrogen of a globin (protein) molecule and one is to an oxygen in an O₂ molecule
32
How does haemoglobin transport oxygen?
O₂ forms a weak coordinate bond to the metal ion, then is transported around the body. The bond breaks when haemoglobin reaches cells and oxygen is released.
33
Why is CO toxic?
CO also co-ordinately bonds to the Fe²⁺, and is a better ligand, so bonds more strongly than O₂. Stops O₂ from bonding to haemoglobin, so O₂ cannot be transported around the body.
34
Why are transition metal compounds coloured?
They have partially filled d-orbitals and electrons are able to move between d-orbitals. - In compounds (when ligands coordinately bond to the ion), the d-orbitals split into different energy levels. - Electrons can absorb energy in the form of photons to become excited and move to a higher energy level (excited state). Energy of photon = energy difference between levels - Energy of photon is related to frequency of light by E = hf - The colour corresponding to the frequency of the energy change is missing from the spectrum, so we see a combination of all the colours that aren't absorbed
35
How do you calculate ΔE from f and/or λ?
ΔE=hf=hc/λ
36
What affects the colour of a transition metal compound?
ΔE affects the frequency of absorbed photons, so determines the colour. ΔE is changed by oxidation state of the metal, number and type of ligands, shape, co-ordination number
37
Give the oxidation number and colour of the following species: a) VO₂⁺ b) VO²⁺ c) V³⁺ d) V²⁺
a) 5+ and Yellow b) 4+ and Blue c) 3+ and Green d) 2+ and Violet
38
What can you use to reduce Vanadium?
Zinc
39
What colour is Fe²⁺'s aqua ion?
Green
40
What colour is Fe³⁺'s aqua ion?
Pale brown
41
What colour is Cr²⁺'s aqua ion?
Blue
42
What colour is Cr³⁺'s aqua ion?
red/violet
43
What colour is Co²⁺'s aqua ion?
Brown
44
What colour is Cr³⁺'s aqua ion?
Yellow
45
What does a colorimeter do?
Measures the absorbance of a particular wavelength of light by a solution.
46
How would you use colorimetry experimentally?
Use solutions of known concentration to create a calibration graph and use the graph to find unknown concentration
47
What information can a colorimeter give you?
The concentration of a certain ion in the solution
48
Why can transition metals have variable oxidation states?
They have partially filled d-orbitals, so can lose 4s and 3d electrons.
49
Which oxidation states do all transition metals have? (except Sc). Why?
+2 due to loss of electrons from 4s orbital
50
When oxidation state is high, do the transition metals exist as simple ions?
No, after oxidation state of about III, metal ions covalently bond to other species.
51
What is the use of the complex [Ag(NH₃)₂)]⁺ ion?
Tollens' reagent to test for aldehydes/ketones (silver mirror formed aldehyde, no visible change with ketone)
52
What colour is MnO₄⁻?
Deep purple
53
What colour is Mn²⁺?
Pink
54
Write a half equation for the reduction of MnO₄⁻ to Mn²⁺.
MnO₄⁻ + 8H⁺ + 5e⁻ -> Mn²⁺ + 4H₂O
55
Why are redox titrations with transition metal compounds said to be self-indicating?
They usually involve a colour change as the metal changing oxidation state but sometimes an indicator is still needed/useful
56
What colour is Cr₂O₇²⁻?
Orange
57
What colour is Cr³⁺?
Green
58
Write a half equation for the reaction of Cr₂O₇²⁻ to Cr³⁺.
Cr₂O₇²⁻ + 14H⁺ + 6e⁻ -> 2Cr³⁺ + 7H₂O
59
What happens to aqua metal ions in acidic conditions?
They get reduced
60
What happens to aqua metal ions in alkaline conditions?
They get oxidised
61
What happens to aqua metal ions in neutral conditions?
No change
62
What does whether reduction/oxidation occurs and the readiness of the reaction depend on?
E° values
63
What can change these E° values?
pH, ligands involved
64
Define a catalyst
A substance that increases the rate of reaction without being chemically changed at the end of the reaction
65
How do catalysts usually work?
Provide an alternative reaction pathway with a lower activation energy
66
Why are transition metals good catalysts?
They can exist in variable oxidation states, so can provide alternative pathways easily
67
Why are group 1, 2 and 3 metals not as good catalysts?
Only exist in one oxidation state
68
What are advantages of using a catalyst for a reaction?
Allows reactions to proceed at lower temperatures and pressures -> saves valuable energy and resources
69
What metals are used in a catalytic converter and which reactions do they catalyse?
Pt, Rd, Pd Catalyse CO, NO -> CO₂, N₂ and alkanes -> H₂O, CO₂
70
Define a heterogeneous catalyst
A catalyst that is present in the reaction in a different phase to the reactants (usually a solid, with gas/liquid reactants). Catalytic activity occurs on the solid surface as the reactants pass over it.
71
What is an advantage of using a heterogeneous catalyst?
No need for separation of products from catalyst.
72
How do heterogeneous catalysts work?
Reactants adsorb to the catalyst's surface at active sites. This weakens bonds within the reactants, holds reactants close together on the surface and/or in the correct orientation to react. Once the reaction has occurred, products desorb from the active sites.
73
What properties does the catalyst need to have to make it a good catalyst?
Can't adsorb too strongly, otherwise the products will not desorb. Can't adsorb too weakly as reactant would not be held in place for long enough and bonds would not be sufficiently weakened, Need a good balance between desorption and adsorption.
74
How can you increase the efficiency of heterogeneous catalysts?
Increase the surface area to increase the number of active sites that are present. Also spread onto an inert support medium (e.g. ceramic) to increase the surface/mass ratio. Use ceramic honeycomb matrix/mesh/sponge.
75
What is catalyst poisoning?
Unwanted impurities adsorb to the catalyst's active sites and do not desorb. This blocks the active sites on the catalyst's surface.
76
What effect does catalyst poisoning have on catalytic activity?
Decreases the effectiveness of the catalyst over time.
77
How else can a catalyst be degraded?
Finely divided catalysts can be gradually lost from their support medium
78
What is the Haber process? What catalyst is used?
N₂ (g) + 3H₂ (g) -> 2NH₃ (g) Makes ammonia, uses iron (Fe) catalyst.
79
What size/shape is the catalyst from the Haber process?
Pea sized lumps to increase surface area
80
How long does the catalyst last for the Haber process? What is it poisoned by?
About 5 years. Poisoned by sulfur impurities in the gas streams.
81
What is the Contact Process? What is the catalyst?
2SO₂ (g) + O₂ (g) -> 2SO₃ (g) Makes H₂SO₄. Catalysed by vanadium (V) oxide (V₂O₅)
82
What are the two reactions that are involved in the contact process?
SO₂ + V₂O₅ -> SO₃ + V₂O₄ V₂O₄ + 1/2 O₂ -> V₂O₅
83
Why is V a good catalyst in the case of the contact process?
Can change oxidation state from 5+ to 4+ and back to 5+ (so can be used again)
84
Define homogenous catalyst.
A catalyst that is in the same phase as the reactants.
85
How do homogeneous catalysts work?
Form intermediates to give a different reaction pathway with lower Eₐ
86
What is the reaction between S₂O₈²⁻ ions and I⁻ ions?
S₂O₈²⁻ + 2I⁻ -> 2SO₄²⁻ + I₂ (all aq)
87
Why does the reaction between S₂O₈²⁻ ions and I⁻ ions have a high Eₐ in normal conditions?
Two negative ions are reacting. They repel each other so Eₐ is high.
88
Which transition metal ions catalyse the reaction between S₂O₈²⁻ ions and I⁻ ions? Write two equations to show how.
89
Define the term autocatalysis.
When the product of a reaction is also a catalyst for that reaction.
90
Draw a concentration of reactant against time graph for an autocatalysed reaction. Explain each stage.
Initially slow, uncatalysed as not much of catalyst has been formed. Rate increases as catalyst is made so catalysed reaction is faster. Slows down as reactants are used up.
91
What an equation for the reaction between C₂O₄²⁻ ions and MnO₄⁻ ions. How does Mn²⁺ autocatalyse this reaction?
92
How can you monitor the concentration of MnO₄⁻ ions?
Using a colorimeter

AQA Chemistry A-Level (34 decks)

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