Topic 15 - Transition Metals Flashcards
1
Q
Where in the periodic table can you find transition metals?
A
In the d-block (middle, bottom block).
2
Q
Are all d-block elements transition metals?
A
No, but most are.
3
Q
Which transition metals do you need to know about mostly?
A
The ones in the first row (titanium to copper).
4
Q
Define transition metals.
A
d-block elements that can form one or more stable ions with incompletely filled d-orbitals.
5
Q
Which elements in the first row of the d-block are not transition metals?
A
Scandium and zinc
6
Q
Why are scandium and zinc not transition metals?
A
- Transition metals are those that form one or more stable ions with INCOMPLETELY FILLED d-orbitals.
- Scandium -> Forms only Sc³⁺, which has no d electrons
- Zinc -> Forms only Zn²⁺, which has a full d subshell
7
Q
Why does zinc keep a full d subshell when it forms its ion?
A
It loses the 2 electrons from the 4s subshell, so its d subshell remains full.
8
Q
How many orbitals does the d subshell have?
A
5 orbitals (so it can hold 10 electrons)
9
Q
Write down the electronic configuration of titanium.
A
1s² 2s² 2p⁶ 3s² 3p⁶ 3d² 4s²
10
Q
Which transition metals have unusual electronic configurations?
A
Chromium and copper
11
Q
Write down the electronic configuration of chromium.
A
1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁵ 4s¹
12
Q
Write down the electronic configuration of copper.
A
1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s¹
13
Q
What must you remember about the 3d and 4s subshells?
A
The 4s fills up before the 3d.
14
Q
Explain the unusual electronic configuration of chromium.
A
- 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁵ 4s¹
- The second electron from the 4s subshell is donated to the 3d subshell to make it half-full
- This gives extra stability
15
Q
Explain the unusual electronic configuration of copper.
A
- 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s¹
- The second electron from the 4s subshell is donated to the 3d subshell to make it full
- This gives extra stability
16
Q
Write down the electronic configuration of Cu²⁺.
A
1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁹
17
Q
When transition metals form positive ions, which electrons are removed first?
A
s electrons (them d electrons)
18
Q
Titanium is 1s² 2s² 2p⁶ 3s² 3p⁶ 3d² 4s². Give the electronic configuration of Ti²⁺.
A
1s² 2s² 2p⁶ 3s² 3p⁶ 3d²
19
Q
Titanium is 1s² 2s² 2p⁶ 3s² 3p⁶ 3d² 4s². Give the electronic configuration of Ti³⁺.
A
1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹
20
Q
What must you remember about the 3d and 4s subshells in transition metals?
A
1) The 4s subshell fills up before the 3d subshell.
2) When forming ions, electrons are lost first from the 4s subshell, then the 3d subshell.
21
Q
Remember to practise writing out the electronic configuration (and electron diagrams) for all of the transition metals.
A
See diagram pg 168 of revision guide
22
Q
Can transition metals form only one stable ion?
A
No, most can form many.
23
Q
How can you express the idea that transition metals can form multiple stable ions?
A
Transition metals have variable oxidation numbers.
24
Q
What condition must be met in order to form a compound or complex containing an ion with a certain oxidation number?
A
The energy given out when the ion forms the compound or complex needs to be greater than the energy taken to remove the outer electrons and form the ion (ionisation energy).
25
Why can transition metals form multiple ions?
* Ions are formed by losing both 4s and 3d electrons
* These two subshells are at similar energy levels, so it takes similar amounts of energy to remove electrons from each one
* There is also little increase in the ionisation energies of successive electrons in each subshell
* The energy released when ions form a complex or compound increases with ionic charge, so the increase in ionisation energy required to remove electrons and form transition metal ions with high oxidation numbers is counteracted by the increase in energy released.
* This means ions can be formed with different oxidation numbers (since it is energetically viable).
26
The ionisation energy required to form ions of transition metals with high oxidation numbers is high. Why is the formation of these ions energetically viable?
* The energy released when ions form a complex or compound increases with ionic charge
* So the increase in ionisation energy required to remove electrons and form transition metal ions with high oxidation numbers is counteracted by the increase in energy released.
27
Describe and explain the graph of ionisation energy against ionisation number for vanadium [Ar] 4s² 3d³.
* Starts at an ionisation number of 1, with a very low ionisation energy.
* Steady small increases between ionisation numbers 1 and 5.
* Sharp increase between ionisation numbers 5 and 6.
* The gradual increase between 1 and 5 is due to the fact that the electrons are being removed from the 4s and 3d subshells, which have similar energies.
* The sharp increase between 5 and 6 is due to the next electrons being removed from the 3p subshell, which is more inner.
28
Describe and explain the graph of ionisation energy against ionisation number for calcium [Ar] 4s².
* Starts at an ionisation number of 1, with a very low ionisation energy.
* Small increase between ionisation numbers 1 and 2.
* Sharp increase between ionisation numbers 2 and 3.
* Steady small increases between ionisation numbers 3 and 6.
• The sharp increase between 2 and 3 is due to the next electrons being removed from the 3p subshell, which is more inner.
29
Remember to practise drawing out the graph of ionisation energy against ionisation number for calcium and vanadium.
Pg 169 of revision guide.
30
Why don’t V⁺ ion complexes form?
V⁺ is not as stable as other possibilities.
31
What are complex ions of transition metals?
A metal ion surrounded by dative covalently bonded ligands.
32
What is another name for ligands being dative covalently bonded to the central metal?
Coordinately
33
What is a ligand?
An atom, ion or molecule that donates a pair of electrons to a central metal atom or ion.
34
What are the requirements for a ligand?
It must have at least one pair of electrons (or it can’t form dative covalent bonds).
35
What are the different types of ligand?
* Monodentate
* Bidentate
* Multidentate
36
What are monodentate ligands?
Ligands with one lone pair.
37
What are bidentate ligands?
Ligands with two lone pairs.
38
What are multidentate ligands?
Ligands with more than two lone pairs.
39
What is the name for a ligand that can form 6 dative covalent bonds with a metal ion?
Hexadentate
40
Describe the structure of haemoglobin.
Fe(II) ion surrounded by 6 ligands:
• O₂ or H₂O
• Ring of 4 N atoms
• Globin protein
41
What is a haem group?
Ring of 4 N atoms (that can form 4 dative covalent bonds with a central metal ion).
42
Remember to practise drawing out the structure of haemoglobin.
Pg 170 of revision guide
43
What is the equation for the oxidation number of a metal ion in a complex ion?
Oxidation number of metal ion = Total oxidation number - Sum of charges of the ligands
44
What is the oxidation number of the iron in [Fe(CN)₆]⁴⁻?
-4 - (6 x -1) = +2
45
What is the coordination number of a complex ion?
The number of dative covalent (coordinate) bonds formed with the central metal ion.
46
What are the usual coordination numbers?
6 and 4
47
When is the coordination number 4 and when is it 6?
* When the ligands are small, like H₂O, 6 can fit around the central metal ion
* When the ligands are large, like Cl⁻, only 4 can fit around the central metal ion
48
What is the term for 6 ligands surrounding a central metal ion?
Six-fold coordination
49
What shape and bond angles does six-fold coordination result in?
* Octahedral
| * 90° bond angles
50
What shape and bond angles does four-fold coordination result in?
```
USUALLY:
• Tetrahedral
• 109.5° bond angles
OCCASIONALLY:
• Square planar
• 90° bond angles
```
51
What shape is [Fe(H₂O)₆]²⁺?
Octahedral
52
What shape is [CuCl₄]²⁻?
Tetrahedral
53
What shape is [CoCl₄]²⁻?
Tetrahedral
54
What shape is cis-platin?
Square planar
55
What notable exception to four-fold coordination complexes do you need to know about?
* Cis-platin
| * It is square planar instead of tetrahedral
56
What sort of complex ions can show cis/trans isomerism?
* Square planar
| * Octahedral
57
How can you tell if an octahedral complex ion is cis or trans?
* Cis -> Have the same groups on the same side
| * Trans -> Have the same groups opposite each other
58
Remember to practise identifying cis and trans isomers of complex ions.
Pg 171 of revision guide
59
Describe the structure of cis-platin.
* Central platinum(II) ion
* 2 Cl groups next to each other on one side
* 2 NH₃ groups next to each other on the other side
60
Remember to practise drawing out he structure of cis-platin.
Pg 171 of revision guide
61
What is the use of cis-platin?
Anti-cancer drug
62
What is the problem with using cis-plain as an anti-cancer drug?
It can not contain any trans-platin, because it is toxic.
63
What happens in terms of energy levels when ligands join onto a transition metal?
The 3d orbitals in the transition metal split into two different energy levels.
64
What gives transition metal complex ions colour?
* The ligands cause the 3d orbitals in the transition metal to split into two different energy levels.
* Most electrons tend to occupy the lower orbitals (ground state) and some can jump up to the higher orbitals (excited state) when provided with energy.
* This energy comes from visible light.
* The lather the energy gap, the higher the frequency of light that is absorbed.
* The colour of the complex is the complement of the colours absorbed.
(See diagram pg 172 of revision guide)
65
What is the symbol for the energy required to excite electrons from the ground state to the excited state?
ΔE
66
Where does the energy for the excitation of electrons in transition metal complex ions come from?
Visible light
67
What determines the amount of energy needed to make electrons jump from the ground state to the excited state in transition metal complex ions? What is the effect of this?
* Central metal ion
* Oxidation number of the metal ion
* Ligands
* Coordination number
These are the factors that determine the colour of the complex ion.
68
[Cu(H₂O)₆]²⁺ absorbs red light. What colour does it appear and why?
* Bright blue
| * Because the remaining colours that are transmitted or reflected combine to give this colour.
69
When will a transition metal complex ion appear white or colourless?
When there are no 3d electrons or the 3d subshell is full (since there are no electrons that can jump).
70
What happens when a solid containing a transition metal ion is dissolved in water?
The metal ion is surrounded by water ligands (forming an aqueous complex).
71
Which transition metals do you need to know the colours of the aqueous complex for?
* Vanadium
* Chromium
* Iron
* Cobalt
* Copper
72
What is the ion in which vanadium has a +2 oxidation number and what is the colour of its aqueous complex?
* V²⁺
| * Violet
73
What is the ion in which vanadium has a +3 oxidation number and what is the colour of its aqueous complex?
* V³⁺
| * Green
74
What is the ion in which vanadium has a +4 oxidation number and what is the colour of its aqueous complex?
* VO²⁺
| * Blue
75
What is the ion in which vanadium has a +5 oxidation number and what is the colour of its aqueous complex?
* VO₂⁺
| * Yellow
76
What is the ion in which chromium has a +2 oxidation number and what is the colour of its aqueous complex?
* Cr²⁺
| * Blue
77
What is the ion in which chromium has a +3 oxidation number and what is the colour of its aqueous complex?
* Cr³⁺
| * Green
78
What is the ion in which chromium has a +6 oxidation number and what is the colour of its aqueous complex?
* Cr₂O₇²⁻
* Orange
OR
* CrO₄²⁻
* Yellow
79
What is the ion in which iron has a +2 oxidation number and what is the colour of its aqueous complex?
* Fe²⁺
| * Pale green
80
What is the ion in which iron has a +3 oxidation number and what is the colour of its aqueous complex?
* Fe³⁺
| * Yellow
81
What is the ion in which cobalt has a +2 oxidation number and what is the colour of its aqueous complex?
* Co²⁺
| * Pink
82
What is the ion in which copper has a +2 oxidation number and what is the colour of its aqueous complex?
* Cu²⁺
| * Pale blue
83
Remember to practise drawing out the complex ion colour table.
See diagram pg 172 (except titanium, manganese and nickel)
84
What ions does vanadium exist in and what is the colour of each aqueous complex?
* V²⁺ -> Violet
* V³⁺ -> Green
* VO²⁺ -> Blue
* VO₂⁺ -> Yellow
85
What ions does chromium exist in and what is the colour of each aqueous complex?
* Cr²⁺ -> Blue
* Cr³⁺ -> Green
* Cr₂O₇²⁻ -> Orange
* CrO₄²⁻ -> Yellow
86
What ions does iron exist in and what is the colour of each aqueous complex?
* Fe²⁺ -> Pale green
| * Fe³⁺ -> Yellow
87
What ions does cobalt exist in and what is the colour of each aqueous complex?
• Co²⁺ -> Pink
88
What ions does cobalt exist in and what is the colour of each aqueous complex?
• Cu²⁺ -> Pale blue
89
When a transition metal in a complex ion changes oxidation number, what type of reaction is it?
Redox
90
What is a reduction potential?
The electrode potential when a transition metal is reduced.
91
What is the half equation for VO₂⁺ being reduced?
VO₂⁺ + 2H⁺ + e⁻ -> VO²⁺ + H₂O
| Note: This is reversible!
92
What is the half equation for VO²⁺ being reduced?
VO²⁺ + 2H⁺ + e⁻ -> V³⁺ + H₂O
| Note: This is reversible!
93
What is the half equation for V³⁺ being reduced?
V³⁺ + e⁻ -> V²⁺
| Note: This is reversible!
94
What is the half equation for V²⁺ being reduced?
V²⁺ + 2e⁻ -> V
95
How does the reduction potential of vanadium ion complexes change as vanadium’s oxidation number decreases?
It goes from positive to negative.
96
What are the chromium ion oxidation states in order of stability?
Most stable: +3
+6
Least stable: +2
97
What is the name of CrO₄²⁻?
Chromate(VI) ion
98
What is the name of Cr₂O₇²⁻?
Dichromate(VI) ion
99
Why are CrO₄²⁻ and Cr₂O₇²⁻ good oxidising agents?
They are easily reduced to Cr³⁺.
100
What is confusing about the colour of Cr³⁺ ions in water?
They should be violet, but the water ligands are usually substituted with impurities in the water (e.g. Cl⁻), which makes the solution look green.
101
What is the equation for dichromate(VI) ions being reduced by zinc and dilute acid?
Cr₂O₇²⁻ + 14H⁺ + 3Zn -> 3Zn²⁺ + 2Cr³⁺ + 7H₂O
102
Is the reduction of Cr³⁺ to Cr²⁺ energetically viable?
Yes, the E° value is positive.
103
In what conditions will Cr³⁺ be reduced to Cr²⁺ and why?
* In an inert atmosphere
| * Or the Cr²⁺ will oxidise straight back to Cr³⁺
104
What is Cr³⁺ reduced to?
Cr²⁺
105
What is the equation for Cr³⁺ being reduced?
2Cr³⁺ + Zn -> Zn²⁺ + 2Cr²⁺
106
Remember to practise doing the electrode potential question on pg 173 of revision guide. Important!
Do it!
107
How can Cr³⁺ be oxidised?
Reacting with hydrogen peroxide in an alkaline solution.
108
What is the equation for Cr³⁺ being oxidised?
2Cr³⁺ + 10OH⁻ + 3H₂O₂ -> 2CrO₄²⁻ + 8H₂O
109
How can you convert from CrO₄²⁻ to Cr₂O₇²⁻?
* Add acid.
| * This is reversible, so adding alkali causes the reverse.
110
What is the equation for converting from chromate(VI) ions to dichromate(VI) ions?
2CrO₄²⁻ + 2H⁺ -> Cr₂O₇²⁻ + H₂O
| NOTE: This is reversible!
111
How can you convert from Cr₂O₇²⁻ to CrO₄²⁻?
* Add alkali.
| * This is reversible, so adding acid causes the reverse.
112
What is produced when you mix an aqueous solution of chromium(III) ions with aqueous sodium hydroxide or aqueous ammonia?
Chromium hydroxide precipitate - Cr(OH)₃(H₂O)₃
Plus water or ammonium ions.
113
What is the colour and state of [Cr(H₂O)₆]³⁺?
Green solution
114
What is the colour and state of Cr(OH)₃(H₂O)₃?
Grey-green precipitate
115
What is the colour of [Cr(OH)₆]³⁻?
Dark green solution
116
What is the colour and state of [Cr(NH₃)₆]³⁺?
Purple solution
117
What is the chemical formula for chromium hydroxide?
Cr(OH)₃(H₂O)₃
| When in aqueous solution, at least
118
Name this: Cr(OH)₃(H₂O)₃
Chromium(III) hydroxide
119
Give the chemical equation for an aqueous solution of chromium(III) ions reacting with aqueous sodium hydroxide.
[Cr(H₂O)₆]³⁺ (aq) + 3OH⁻ (aq) -> [Cr(OH)₃(H₂O)₃] (s) + 3H₂O (l)
120
Give the chemical equation for an aqueous solution of chromium(III) ions reacting with aqueous ammonia.
[Cr(H₂O)₆]³⁺ (aq) + 3NH₃ (aq) -> [Cr(OH)₃(H₂O)₃] (s) + 3NH₄⁺ (l)
121
Describe the changes observed when an aqueous solution of chromium(III) ions reacts with aqueous sodium hydroxide or aqueous ammonia.
Grey-green precipitate appears in the green solution.
122
What does amphoteric mean?
When a species can react with both acids and bases.
123
Is chromium hydroxide amphoteric?
Yes, it can react with both acids and bases.
124
What is produced when you add excess sodium
| hydroxide to chromium hydroxide precipitate?
[Cr(OH)₆]³⁻ and H₂O
125
Give the chemical equation for adding excess sodium
| hydroxide to chromium hydroxide precipitate.
[Cr(OH)₃(H₂O)₃] (s) + 3OH⁻ (aq) -> [Cr(OH)₆]³⁻ (aq) + 3H₂O (l)
126
Describe the changes observed when you add excess sodium
| hydroxide to chromium hydroxide precipitate.
Grey-green precipitate disappears and the solution becomes dark green.
127
What is produced when you add acid to chromium hydroxide precipitate?
[Cr(H₂O)₆]³⁺
128
Give the chemical equation for adding acid to chromium hydroxide precipitate.
[Cr(OH)₃(H₂O)₃] (s) + 3H⁺ (aq) -> [Cr(H₂O)₆]³⁺ (aq)
129
Describe the changes observed when you add acid to chromium hydroxide precipitate.
Grey-green precipitate disappears and the solution becomes green.
130
Are the reactions of chromium hydroxide ligand substitutions?
* No, they are acid-base reactions - the ligands are chemically modified by either gaining or losing a H⁺ ion.
* The only exception is adding excess ammonia.
131
What sort of reaction is adding excess sodium hydroxide to chromium hydroxide?
* [Cr(OH)₃(H₂O)₃] (s) + 3OH⁻ (aq) -> [Cr(OH)₆]³⁻ (aq) + 3H₂O (l)
* This is deprotonation, because each H₂O ligand loses a H.
132
What sort of reaction is adding acid to chromium hydroxide?
* [Cr(OH)₃(H₂O)₃] (s) + 3H⁺ (aq) -> [Cr(H₂O)₆]³⁺ (aq)
| * This is protonation, because each OH group gains a H.
133
Give the chemical equation for adding excess aqueous ammonia to chromium hydroxide precipitate.
[Cr(OH)₃(H₂O)₃] (s) + 6NH₃ (aq) -> [Cr(NH₃)₆]³⁺ (aq) + 3OH⁻ (aq) + 3H₂O (l)
134
Describe the changes observed when you add excess aqueous ammonia to chromium hydroxide precipitate.
Grey-green precipitate disappears and the solution turns purple.
135
What type of reaction is adding excess aqueous ammonia to chromium hydroxide?
Ligand exchange
136
Describe all of the reactions to form chromium hydroxide and all of its reactions.
TO FORM:
• [Cr(H₂O)₆]³⁺ + 3OH⁻ -> [Cr(OH₃)(H₂O)₃] + 3H₂O
• [Cr(OH)₆]³⁻ + 3NH₃ -> [Cr(OH₃)(H₂O)₃] + 3NH₄⁺
WITH EXCESS SODIUM HYDROXIDE:
• [Cr(OH₃)(H₂O)₃] + 3OH⁻ -> [Cr(OH)₆]³⁻ + H₂O
WITH EXCESS ACID:
• [Cr(OH₃)(H₂O)₃] + 3H⁺ -> [Cr(H₂O)₆]³⁺
WITH EXCESS AMMONIA:
• [Cr(OH₃)(H₂O)₃] + 6NH₃ -> [Cr(NH₃)₆]³⁻ + 3OH⁻ + 3H₂O
137
What is produced when excess aqueous ammonia is added to chromium hydroxide?
[Cr(NH₃)₆]³⁺ and OH⁻ and H₂O⁺
138
```
What is the colour and state of:
• [Cr(H₂O)₆]³⁺
• [Cr(OH)₃(H₂O)₃]
• [Cr(OH)₃]³⁻
• [Cr(NH₃)₆]³⁺
```
* [Cr(H₂O)₆]³⁺ -> Green solution
* [Cr(OH)₃(H₂O)₃] -> Grey-green precipitate
* [Cr(OH)₃]³⁻ -> Dark green solution
* [Cr(NH₃)₆]³⁺ -> Purple solution
139
How can most transition metal complexes be made?
Adding a solution or solid containing the transition metal ion to the solution containing your ligand.
140
What is the formula for chromium(II) ethanoate?
Cr₂(CH₃COO)₄(H₂O)₂
141
Describe the colour changes in each step of the preparation of chromium(II) ethanoate from sodium dichromate(VI).
* Orange sodium dichromate(VI) is reduced to form a green solution of Cr³⁺ ions
* This is then reduced further to give a blue solution of Cr²⁺
* When mixed with sodium ethanoate, this produces the red precipitate of chromium(II) ethanoate
142
What makes the preparation of chromium(II) ethanoate unusual?
It must be done in an inert atmosphere.
143
Why is the preparation of chromium(II) ethanoate dome in an inert atmosphere?
The Cr²⁺ ions involved in the production are very easily oxidised.
144
How can an inert atmosphere be created?
* Use containers containing nitrogen gas
| * Bubble all liquids through with nitrogen in order to remove oxygen
145
Describe the preparation of chromium(II) ethanoate from sodium dichromate(VI).
* Slowly add HCl to a flask containing sodium dichromate(VI) and zinc mesh. The zinc reduces the dichromate(VI) ions and reacts with the acid to produce hydrogen gas, which can escape through a rubber tube into a beaker of water
* As soon as the solution turns blue, pinch the rubber tube shut so hydrogen can no longer escape
* The build up of pressure in the flask forces the Cr²⁺ solution up a tube and into a flask of sodium ethanoate.
* These two react to make a red precipitate of chromium(II) ethanoate.
* Filter off the precipitate and wash it using water, then ethanol, then ether.
(See diagram pg 175)
146
Do you need to memorise all the details of the preparation of chromium(II) ethanoate?
No, probably not. Check pg 175 though.
147
What is a ligand change?
When one ligand is swapped for another.
148
How is a ligand exchange usually observed?
There is a colour change.
149
What are the equations involved in making chromium(II) ethanoate from sodium dichromate(VI)?
* Cr₂O₇²⁻ + 14H⁺ + 3Zn -> 3Zn²⁺ + 2Cr³⁺ + 7H₂O
* 2Cr³⁺ + Zn -> 2Cr²⁺ + Zn²⁺
* 2Cr²⁺ + 4CH₃COO⁻ + 2H₂O -> [Cr(CH₃COO)₄(H₂O)₂]
150
In ligand exchange, when does the complex ion change shape?
When the ligands are of different size, a change of coordination number and shape will occur.
151
Are all of the ligands always substituted in ligand exchange?
No, sometimes it is only partial.
152
What is meant by an aqueous complex of a transition metal?
When a solid of the transition metal is dissolved so that the metal ion is surrounded by water.
153
What will the colour be when there is a transition metal in this form: [X(H₂O)₆]
It will simply be the colour of the transition metal’s aqueous complex, since this is the definition.
154
What is the colour of [Cr(H₂O)₆]³⁺?
Dark green
155
What is the colour of [Cr(NH₃)₆]³⁺?
Purple
156
What is the colour of [Cu(H₂O)₆]²⁺?
Pale blue
157
What is the colour of [CuCl₄]²⁻?
Yellow
158
What is the colour of [Co(H₂O)₆]²⁺?
Pale pink
159
What is the colour of [CoCl₄]²⁻?
Blue
160
What is the colour of [Cu(NH₃)₄(H₂O)₂]²⁺?
Deep blue
161
What are the small ligands?
* H₂O
* NH₃
* CN⁻
* OH⁻
162
What are the large ligands?
* Cl⁻
| * Large molecules
163
What is the equation for [Cr(H₂O)₆]³⁺ reacting with excess ammonia?
What is the associated colour change?
[Cr(H₂O)₆]³⁺ + 6NH₃ -> [Cr(NH₃)₆]³⁺ + 6H₂O
Dark green to purple
164
What is the equation for [Cu(H₂O)₆]²⁺ reacting with chloride ions?
What is the associated colour change?
[Cu(H₂O)₆]²⁺ + 4Cl⁻ -> [CuCl₄]²⁻ + 6H₂O
Pale blue to yellow
165
What is the equation for [Co(H₂O)₆]²⁺ reacting with chloride ions?
What is the associated colour change?
[Co(H₂O)₆]²⁺ + 4Cl⁻ -> [CoCl₄]²⁻ + 6H₂O
Pale pink to blue
166
What is the equation for [Cu(H₂O)₆]²⁺ reacting with excess ammonia?
What is the associated colour change?
[Cu(H₂O)₆]²⁺ + 4NH₃ -> [Cu(NH₃)₄(H₂O)₂]²⁺ + 4H₂O
Pale blue to deep blue
167
What is the equation for [Cu(H₂O)₆]²⁺ reacting with insufficient ammonia?
What is the associated colour change?
[Cu(H₂O)₆]²⁺ + 2NH₃ -> [Cu(OH)₂(H₂O)₄] + 2NH₄⁺
Pale blue to blue
168
As a general rule, when does NH₃ ligand substitution into a complex ion occur?
When the NH₃ is in excess - otherwise it is likely to just be deprotonation.
169
IMPORTANT: As a general rule, with transition metal complexes, what is likely to be solid and what is likely to be a solution?
* Solid -> Hydroxides
| * Solution -> Everything else
170
When do you not need to include the (H₂O) ligands in a complex ion formula?
When the complex is a precipitate containing these ligands.
171
Why does carbon monoxide poisoning occur?
* The oxygen or water molecule in haemoglobin is replaced by CO in a ligand exchange reaction
* This forms carboxyhaemoglobin
* This is a strong dative covalent bond, so the haemoglobin can no longer exchange the CO for oxygen to transport around the body
172
What type of reaction is carbon monoxide poisoning?
Ligand exchange
173
Describe and explain the enthalpy change in a ligand exchange reaction.
* It is usually very small
| * This is because the strength of the bonds being broken is often very similar to the strength of the bonds being made
174
Ligand exchange frequently involves a reversible reaction. What determines the side that the equilibrium lies on?
Entropy, because the enthalpy is usually very small and therefore insignificant.
175
How can you tell which side of a reversible ligand exchange has the more stable complex ion?
Look at entropy:
• The side with more particles has higher entropy
• Therefore, this side is more stable and the equilibrium lies to this side
176
What type of ligands are substitutes by what to give more stable complex ions?
* Monodentate ligands are substituted by bidentate or multidentate ligands
* This increases the number of particles in the solution, which means the entropy is greater
* Since, ΔS(System) is greater, this is most likely to occur
177
What is EDTA⁴⁻?
Hexadentate ligand
178
What type of ligand is EDTA⁴⁻?
Hexadentate
179
Are complex ions with lots of ligands or few ligands more stable?
Few ligands
180
Describe what happens when you mix an aqueous solution of transition element ions with aqueous sodium hydroxide (NaOH) or (non-excess) aqueous ammonia (NH₃).
The water ligands are deprotonated (in an acid-base reaction) and you get a coloured hydroxide precipitate.
181
How can you reverse the reaction of sodium hydroxide or (non-excess) aqueous ammonia reacting with an aqueous solution of transition element ions with aqueous sodium hydroxide?
Add acid - this reverses the deprotonation.
182
Give the general formula of transition metal elements in water.
[M(H₂O)₆]^n+
183
What are the two ways of writing the chemical formula for a transition element ion in water?
* [M(H₂O)₆]^n+
* M^n+ (As long as the metal ion is only bonded to water. If it is bound to anything else, you need to write out the whole formula.)
184
What is a metal-aqua ion?
A metal ion complex that only contains water ligands (e.g. [Cu(H₂O)₆]²⁺)
185
For which aqueous solutions of transition metal elements do you need to know the reactions with sodium hydroxide and ammonia for?
* Copper(II)
* Iron(II)
* Iron(III)
* Cobalt(II)
* Chromium(III)
186
For which aqueous solutions of transition metal elements do you need to know the reactions with excess ammonia for?
* Copper(II)
* Cobalt(II)
* Chromium(III)
187
Give the equations for the reaction of copper(II) ions in water with:
• Sodium hydroxide
• Ammonia
• Excess ammonia
SODIUM HYDROXIDE:
• [Cu(H₂O)₆]²⁺(aq) + 2OH⁻(aq) -> [Cu(OH)₂(H₂O)₄](s) + 2H₂O(l)
AMMONIA:
• [Cu(H₂O)₆]²⁺(aq) + NH₃(aq) -> [Cu(OH)₂(H₂O)₄](s) + 2NH₄⁺(aq)
EXCESS AMMONIA:
• [Cu(OH)₂(H₂O₄)](s) + 4NH₃(aq) -> [Cu(NH₃)₄(H₂O)₂]²⁺(aq) + 2OH⁻(aq) + 2H₂O(l)
188
Give the equations for the reaction of iron(II) ions in water with:
• Sodium hydroxide
• Ammonia
SODIUM HYDROXIDE:
• [Fe(H₂O)₆]²⁺(aq) + 2OH⁻(aq) -> [Fe(OH)₂(H₂O)₄](s) + 2H₂O(l)
AMMONIA:
• [Fe(H₂O)₆]²⁺(aq) + 2NH₃(aq) -> [Fe(OH)₂(H₂O)₄](s) + 2NH₄⁺(aq)
189
Give the equations for the reaction of iron(III) ions in water with:
• Sodium hydroxide
• Ammonia
SODIUM HYDROXIDE:
• [Fe(H₂O)₆]³⁺(aq) + 3OH⁻(aq) -> [Fe(OH)₃(H₂O)₃](s) + 3H₂O(l)
AMMONIA:
• [Fe(H₂O)₆]³⁺(aq) + 3NH₃(aq) -> [Fe(OH)₃(H₂O)₃](s) + 3NH₄⁺(aq)
190
Give the equations for the reaction of cobalt(II) ions in water with:
• Sodium hydroxide
• Ammonia
• Excess ammonia
SODIUM HYDROXIDE:
• [Co(H₂O)₆]²⁺(aq) + 2OH⁻(aq) -> [Co(OH)₂(H₂O)₄](s) + 2H₂O(l)
AMMONIA:
• [Co(H₂O)₆]²⁺(aq) + NH₃(aq) -> [Co(OH)₂(H₂O)₄](s) + 2NH₄⁺(aq)
EXCESS AMMONIA:
• [Co(OH)₂(H₂O₄)](s) + 6NH₃(aq) -> [Cu(NH₃)₆]²⁺(aq) + 2OH⁻(aq) + 4H₂O(l)
• On standing, this is oxidised to form a brown solution containing [Co(NH₃)₆]³⁺ ions
191
Give the colour changes for the reaction of copper(II) ions in water with:
• Sodium hydroxide
• Ammonia
• Excess ammonia
* Sodium hydroxide / Ammonia -> Pale blue solution to blue precipitate
* Excess ammonia -> Blue precipitate to a deep blue solution
192
Give the colour changes for the reaction of iron(II) ions in water with:
• Sodium hydroxide
• Ammonia
• Sodium hydroxide / Ammonia -> Pale green solution to green precipitate, which darkens on standing (as the precipitate is oxidised by water and oxygen in the air to form iron(III) hydroxide)
193
Give the colour changes for the reaction of iron(III) ions in water with:
• Sodium hydroxide
• Ammonia
• Sodium hydroxide / Ammonia -> Yellow solution to an orange precipitate, which darkens on standing
194
Give the colour changes for the reaction of cobalt(II) ions in water with:
• Sodium hydroxide
• Ammonia
• Excess ammonia
* Sodium hydroxide / Ammonia -> Pale pink solution to a blue precipitate, which turns brown on standing
* Excess ammonia -> Blue (or pink) precipitate dissolves to form a yellow-brown solution
195
What happens to the [Fe(OH)₂(H₂O)₄] precipitate when it is left to stand?
It is oxidised by water and oxygen in the air to form iron(III) hydroxide.
196
What happens to the [Fe(OH)₃(H₂O)₃] precipitate when it is left to stand?
It darkens to a darker orange.
197
What happens to the [Co(OH)₂(H₂O)₄] precipitate when it is left to stand?
It turns brown on standing.
198
What happens to the [Co(NH₃)₆]²⁺ precipitate when it is left to stand?
Dissolves to form a yellow-brown solution.
199
Remember to practise writing out all of the equations for the reactions of aqueous transition metal ions with sodium hydroxide, ammonia and excess ammonia.
Pg 177 of revision guide
200
What is the main use of transition metals?
Catalysts
201
Why do transition metals make good catalysts?
* They can easily change oxidation number by gaining or losing electrons within their d-orbitals
* This means they can transfer electrons to speed up reactions
202
What is the equation for the part of the Contact Process?
SO₂ + 1/2O₂ -> SO₃
203
What is the catalyst for part of the Contact Process?
Vanadium(V) oxide
204
Describe how vanadium(V) oxide acts as a catalyst for part of the Contact Process.
Vanadium oxidised SO₂ to SO₃:
• V₂O₅ + SO₂ -> V₂O₄ + SO₃
The reduced vanadium is then oxidised by O₂ back to its original state:
• V₂O₄ + 1/2O₂ -> V₂O₅
Overall reaction: SO₂ + 1/2O₂ -> SO₃
205
What are homogeneous catalysts?
Those in the same physical state as the reactants.
206
What state are homogeneous catalysts usually in?
They are usually aqueous catalysts for a reaction between two aqueous solutions.
207
How do homogeneous catalysts work?
* Combine with the reactants to form an intermediate species
* This species then reacts to form the products and reform the catalyst
* The energy needed for this in total is lower than the energy needed for the direct reaction
208
Give an example of transition metals acting as homogeneous catalysts.
* Iron(II) ions catalysing the reaction of peroxodisulfate (S₂O₈²⁻) ions oxidising iodide ions
* Autocatalysis of the reaction between C₂O₄²⁻ and MnO₄⁻ by Mn²⁺
209
Give the equation for peroxodisulfate (S₂O₈²⁻) ions reacting with iodide ions.
S₂O₈²⁻(aq) + 2I⁻(aq) -> I₂(aq) + 2SO₄²⁻(aq)
210
Why is the rate of the uncatalysed reaction between iodide and peroxodisulfate ions so slow?
Both of the ions are negatively charged, so the ions repel.
211
What can catalyse the reaction between peroxodisulfate (S₂O₈²⁻) ions and iodide ions?
Iron(II) ions
212
Describe how iron(II) ions can catalyse the reaction between peroxodisulfate (S₂O₈²⁻) ions and iodide ions.
* S₂O₈²⁻(aq) + 2Fe²⁺(aq) -> 2Fe³⁺(aq) + 2SO₄²⁻(aq)
* 2Fe³⁺(aq) + 2I⁻(aq) -> I₂(aq) + 2Fe²⁺
Overall:
S₂O₈²⁻(aq) + 2I⁻(aq) -> I₂(aq) + 2SO₄²⁻(aq)
213
What is the name for the product of a reaction catalysing the same reaction?
Autocatalysis
214
Give an example of autocatalysis.
Autocatalysis of the reaction between C₂O₄²⁻ and MnO₄⁻ by the Mn²⁺ product.
215
What might cause the rate of a reaction to speed up as the reaction progresses?
Autocatalysis
216
Give the ionic equation for the reaction between C₂O₄²⁻ and MnO₄⁻.
2MnO₄⁻(aq) + 16H⁺(aq) + 5C₂O₄²⁻(aq) -> 2Mn²⁺(aq) + 8H₂O(l) + 10CO₂(g)
217
What type of catalyst is Mn²⁺ in the autocatalysis of the reaction between C₂O₄²⁻ and MnO₄⁻ by Mn²⁺?
Homogeneous catalyst
218
Describe how the autocatalysis of the reaction between C₂O₄²⁻ and MnO₄⁻ by Mn²⁺ works.
• 2MnO₄⁻(aq) + 16H⁺(aq) + 5C₂O₄²⁻(aq) -> 2Mn²⁺(aq) + 8H₂O(l) + 10CO₂(g)
This produces Mn²⁺, which catalyses the reaction by:
• MnO₄⁻(aq) + 4Mn²⁺(aq) + 8H⁺(aq) -> 5Mn³⁺(aq) + 4H₂O(l)
• 2Mn³⁺(aq) + C₂O₄⁻(aq) -> 2Mn²⁺(aq) + 2CO₂(g)
219
What are heterogeneous catalysts?
Those in a different physical state from the reactants.
220
What state are heterogeneous catalysts usually in?
Solids catalysing a reaction between gases or solutions.
221
Why do transition metals make good heterogeneous catalysts?
They can use their partially-filled d-orbitals to make weak bonds with the reactant molecules.
222
Give an example of transition metals acting as heterogeneous catalysts.
Catalytic converters in cars
223
What are catalytic converters used for?
Reducing emissions of nitrogen monoxide and carbon monoxide produced in internal combustion engines.
224
What catalysts are used in catalytic converters?
Platinum or rhodium
225
What is the equation for the reaction in a catalytic converter?
2NO(g) + 2CO(g) -> N₂(g) + 2CO₂(g)
226
How does a heterogeneous catalyst work?
* The reactant molecules are attracted to the surface of the solid catalyst and stick to it (adsorption)
* The surface of the catalyst activates the molecules so they react more easily
* The product molecules leave the surface of the catalyst making way for new reactants (desorption)
