A - Transition Metals Flashcards
1
Q
Where are transition metals found on the periodic table?
A
D block
2
Q
Give the definition of a transition metal.
A
A metal that can form one or more stable ions with a partially filled d sub-level.
3
Q
Where do the transition metal characteristics of elements Ti-Cu arise from?
A
An incomplete d sub-level in atoms or ions.
4
Q
What are the characteristic properties of the transition metals that I need to know about?
A
- Complex formation
- Formation of coloured ions
- Variable oxidation state
- Catalytic activity
5
Q
What is a ligand?
A
A molecule or ion that forms a co-ordinate bond with a transition metal by donating a pair of electrons.
6
Q
What is a complex?
A
A central metal atom or ion surrounded by ligands.
7
Q
What is co-ordination number?
A
The number of co-ordinate bonds to the central metal atom or ion.
8
Q
Give the 4s and 3d electron configurations of Cr and Cu.
Why do these differ from those of the other transition metals’ electron configurations and why?
A
Cr - 4s1 3d5
Cu - 4s1 3d10
The 4s sub-level isn’t full before the 3d sub-level is filled as the atoms are more stable this way.
9
Q
Why are scandium and zinc not transition metals?
A
Sc - only forms one ion, Sc3+, which has an EMPTY d sub-level.
Zn - only forms one ion, Zn2+, which has a FULL d sub-level.
10
Q
Do transition metals form positive or negative ions?
A
Positive ions.
11
Q
Name some of the physical properties of transition metals.
A
- Good conductora of heat and electricity
- Hard
- Shiny
- Strong
- High melting and boiling points
- High density
12
Q
What electrons are lost first from all transition metals when ions are formed?
A
4s electrons
13
Q
What is another name for a co-ordinate bond?
A
Dative covalent bond
14
Q
What is a co-ordinate bond?
A
A covalent bond in which both electrons in the shared pair come from the same atom.
15
Q
What is the resulting species called when ligands bond to a single transition metal?
A
Complex ion.
16
Q
What shapes do the following complex ions form?
- Complex ion with co-ordination number 6
- Complex ion with co-ordination number 4
- Complex ion with co-ordination number 2
A
- Octahedral
- Tetrahedral or square planar
- Linear
17
Q
What are the bond angles in an octahedral complex ion with co-ordination number 6?
A
All bond angles are 90 degrees
18
Q
What are the bond angles in a tetrahedral complex ion with co-ordination number 4?
A
Bond angles are 109.5 degrees.
19
Q
What are the bond angles in a square planar complex ion with co-ordination number 4?
Give an example of one of these complex ions.
A
Bond angles are 90 degrees.
Cisplatin (anti-cancer drug)
20
Q
What are the bond angles in a linear complex ion with co-ordination number 2?
A
Bond angles are 180 degrees.
21
Q
Describe the different arrows used to draw 3D complex ions.
A
Wedge shaped arrows represent bonds coming toward you.
Dashed arrows represent bonds sticking out behind the molecule.
22
Q
What is the total oxidation state of a complex ion?
A
The overall charge of the ion (found outside the square bracket).
23
Q
How do you calculate the oxidation state of the metal ion?
A
Total oxidation state - the sum of the oxidation states of the ligands
24
Q
What are monodentate ligands?
Give some examples.
A
Ligands that can only form one co-ordinate bond.
H2O, NH3, Cl-
25
What is similar about the ligands NH3 and H2O?
They are similar in size and both are uncharged.
26
What remains the same when the ligands NH3 and H2O are exchanged?
Co-ordination number.
27
What is a multidentate ligand? Give an example.
A ligand that can form more than one co-ordinate bond.
Eg - EDTA4- which has six lone pairs of electrons.
28
What are bidentate ligands? Give two examples.
Multidentate ligands which can form two co-ordinate bonds.
Eg - ethane-1,2-diamine and ethanedioate.
29
One ligand can be swapped for another. What is this process called?
What does this process usually result in?
Ligand substitution or exchange.
Colour change.
30
What happens when ligands of similar size and the same charge exchange?
There will usually be a colour change but co-ordination number and shape of the complex ion remains the same.
31
What happens when ligands of different sizes are exchanged?
There’s a change of co-ordination number and shape of the complex ion.
32
How does Cl- differ in size from NH3 and H2O?
Cl- is larger, H2O and NH3 are similar in size.
33
Ligand substitution reactions can be easily reversed unless what?
Unless the new complex ion is much more stable than the old one.
34
Do multidentate or monodentate ligands form more stable complexes?
Multidentate ligands
35
What is haemoglobin?
A protein that is found in the blood and helps to transport oxygen around the body.
36
Describe the structure of haemoglobin as a complex ion.
Fe2+ ions are hexa-coordinated (6 lone pairs are donated to Fe2+ to form 6 co-ordinate bonds in an octahedral structure).
Four of the co-ordinate bonds come from one multidentate ligand (4 N atoms co-ordinate around Fe2+ forming a circle) - this part of the molecule is called haem.
One of the other co-ordinate bonds comes from a protein called globin, and the other from either water or oxygen (this is interchangeable which allows the molecule to transport oxygen to where it is needed).
37
Describe how the complex ion, haemoglobin, transports oxygen around the body.
In the lungs, where O2 concentration is high, an O2 molecule substitutes the water ligand to form oxyhaemoglobin. This is carried around the body in the blood.
The O2 molecule is exchanged for an H2O molecule when oxyhaemoglobin reaches a place where O2 is needed. The haemoglobin then returns to the lungs where the whole process repeats.
38
What happens to the complex ion, haemoglobin, if carbon monoxide is inhaled?
What can this result in and why?
Carbon monoxide is exchanged for the oxygen co-ordinately bonded to the Fe2+ ion. CO is a strong ligand and doesn’t readily exchange for oxygen or water ligands, meaning haemoglobin is unable to transport oxygen any longer.
CO is toxic and can cause CO poisoning. It starves the organs of oxygen and can cause headaches, dizziness, unconsciousness and death if left untreated.
39
What are aqua ions?
Occurs when the salt of a transition metal is dissolved in water. The positively charged metal ion becomes surrounded by (usually) 6 water molecules acting as ligands in an octahedral arrangement.
40
What is the chelate effect?
Bidentate and multidentate ligands replace monodentate ligands from complexes.
41
Why do multidentate ligands always form more stable complexes than monodentate ligands?
When monodentate ligands are substituted with bidentate or multidentate ligands, the number of particles in solution increases. The more particles, the greater the entropy. Reactions that result in an increase in entropy are more likely to occur. Enthalpy change for a ligand reaction is often very small.
The chelate effect accounts for the balance between entropy and enthalpy change in these reactions.
42
Why is it difficult to reverse a reaction where a monodentate ligand has been substituted for a multidentate ligand?
Because reversing the reaction would cause a decrease in entropy and a decrease in entropy means a reaction is less likely to occur.
43
What type of isomerism do octahedral complexes with monodentate ligands display?
Cis-trans isomerism (a special case of E-Z isomerism).
44
What type of isomerism do octahedral complexes with bidentate ligands display?
Optical isomerism.
45
What type of isomerism do square planar complexes display?
Cos-trans isomerism.
46
What is the cis isomer of the complex of platinum(II) with two Cl- ions and two ammonia molecules called and used for?
Where are the two Cl- ions in the cis isomer?
What is the shape of this complex?
Cisplatin - anti-cancer drug.
Cl- ions are next to each other (the same groups are on the same sides).
Square planar.
47
When will octahedral complexes form optical isomers?
When bonded to two or more bidentate ligands.
48
Ag+ forms a linear complex used in Tollen’s reagent. What is the formula for this complex ion?
[Ag(NH3)2]+
49
What is the silver mirror test used to distinguish? How is the test carried out?
Aldehydes from ketones.
A solution containing [Ag(NH3)2]+ (Tollen’s reagent) is added to the solution being tested.
Aldehydes reduce the complex ion to Ag (metallic silver) while ketones do not.
The silver forms a mirror on the surface of the test tube signifying that an aldehyde is present.
50
How may transition metal ions be identified?
By their colour.
51
How does colour arise?
When some of the wavelengths of visible light are absorbed and the remaining wavelengths of light are transmitted or reflected.
52
How do ligands affect the electron configuration of transition metal ions?
When ligands bond to the metal ions, some of the orbitals gain energy. This splits the 3d orbitals into two different energy levels.
53
What two energy levels do the 3d orbitals split into when bonded to ligands?
Ground state and excited state.
54
How can d electrons move from the ground state to an excited state?
When light is absorbed. This must give energy that is equal to the energy gap.
55
How do you see colour from transition metal compounds?
When visible light hits a transition metal ion, some frequencies are absorbed when electrons jump to higher orbitals.
The rest of the frequencies are transmitted or reflected. These frequencies combine to make the complement of the colour of the absorbed frequencies which is the colour that you see.
56
Give the equation which is used to show the energy difference between the ground state and the excited state of the d electrons.
DeltaE = hv = hc/Lambda
```
V = frequency of light absorbed (Hz)
h = Planck’s constant (6.63 x 10-34 J s)
c = the speed of light (3.00 x 10^8 m s-1)
Lambda = wavelength of light absorbed (m)
```
57
What factors can alter the energy difference between ground and excited state electrons?
What does this lead to?
Changes in oxidation state, co-ordination number and ligand.
Leads to a change in colour.
58
What can a spectroscopy reveal and how?
Determines the concentration of a solution by measuring how much visible light it absorbs.
59
What piece of equipment can be used to determine the concentration of coloured ions in solution?
A simple colorimeter.
60
How is a colorimeter used to determine the concentration of a solution of transition metal ions?
1. White light is shone through a filter, which is chosen to only let through the colour of light that is absorbed by the sample.
2. The light passes through the sample to a colorimeter, which calculates how much light was absorbed by the sample.
3. The more concentrated a coloured solution is, the more light it will absorb. So you can use this measurement to work out the concentration of a solution of transition metal ions.
You will need a calibration curve to do this.
61
What is a calibration curve?
A graph that shows the absorbance of known concentrations of solutions.
Y axis - relative absorbance.
X axis - concentration of ion.
62
Transition metals can exist in variable oxidation states. What type of reaction involves a change in oxidation state?
Redox reaction.
63
How many oxidation states can vanadium exist in in solution? What are these?
4.
+2, +3, +4 and +5.
64
What is the colour and oxidation state of the vanadate (V) ion?
+5, yellow.
65
How can vanadate (V) ions be reduced?
By adding them to zinc metal in an acidic solution.
66
What are the oxidation states and colours for the following ions?
1. VO^2+
2. V^3+
3. V^2+
1. +4 and blue.
2. +3 and green.
3. +2 and violet.
67
What is the redox potential for a transition metal ion changing from a higher to a lower oxidation state influenced by?
pH and by the ligand.
68
What does the redox potential of an ion or atom tell you?
What is another name for redox potential?
How easily it is reduced to a lower oxidation state.
Electrode potential.
69
What does a larger redox potential mean?
The larger the redox potential, the less stable the ion will be and so the more likely it is to be reduced.
70
How do ligands affect redox potentials?
Standard electrode potentials are measured in aqueous solution, so any aqueous ions will be surrounded by water ligands.
Different ligands may make the redox potential larger or smaller depending on how well they bind to the metal ion in a particular oxidation state.
71
How is Tollen’s reagent prepared?
By adding just enough ammonia solution to silver nitrate solution to form a colourless solution containing the complex ion [Ag(NH3)2]+.
72
What redox reactions take place when Tollen’s reagent reacts with an aldehyde?
The aldehyde is oxidised to a carboxylic acid and the Ag+ ions are reduced to silver metal.
73
How could you measure the concentration of an oxidising or reducing agent?
Doing a redox titration.
74
What are the two groups of catalysts called?
1. Heterogeneous.
| 2. Homogeneous.
75
What is a heterogeneous catalyst and how does this work?
A catalyst that is in a different phase (physical state) to the reactants.
The reaction occurs at active sites on the surface of the catalyst.
76
Give two examples of heterogeneous catalysts and the reactions they are used for.
1. Iron in the Haber process for making ammonia.
| 2. Vanadium (V) oxide in the Contact process for making sulfuric acid. (V2O5)
77
How is a support medium used for a heterogeneous catalyst to minimise cost?
Support mediums are used to maximise the surface area of the heterogeneous catalyst. This minimises the cost of the reaction because only a small coating of catalyst is needed with a support medium to provide a large surface area.
78
Use equations to explain what happens during the Contact Process.
Vanadium (V) oxide (V2O5) oxidises SO2 to SO3 and is reduced itself:
V2O5 + SO2 —> V2O4 + SO3 where vanadium (V) is reduced to vanadium (IV)
Then the reduced catalyst is oxidised by oxygen gas back to its original state:
V2O4 + 1/2 O2 —> V2O5 where vanadium (IV) is oxidised to vanadium (V)
79
What can poison heterogeneous catalysts and how?
Impurities in the reaction mixture can poison heterogeneous catalysts as they can bind to the catalyst’s surface and block reactants from being absorbed.
This reduces the surface area of the catalyst available to the reactants, slowing down the reaction.
80
What is the cost implication of catalyst poisoning?
It increases the cost of a chemical process because less product can be made in a certain time or with a certain amount of energy.
The catalyst may even need replacing or regenerating which also costs money.
81
What is a homogeneous catalyst and how does it work?
A catalyst which is in the same physical state as the reactants.
They work by combining with the reactants to form an intermediate species which then reacts to form the products and re-form the catalyst.
82
In what phase is a homogeneous catalyst usually found?
In aqueous solution for a reaction between two other aqueous solutions.
83
Why does an enthalpy profile for a homogeneously catalysed reaction have two humps?
The two humps correspond to the two steps in the reaction.
1. Forming the intermediate species
2. Forming the products and re-forming the catalyst.
84
Why do transition metals make good catalysts?
Because they can change their oxidation states by gaining or losing electrons wishing their d orbitals. This means they can transfer electrons to speed up reactions.
85
What is the name for S2O8^2-?
Peroxodisulfate ions.
86
Why does the reaction between iodide ions and peroxodisulfate ions take place so slowly?
Because both ions are negatively charged so repel each other making it unlikely for them to collide and react.
87
How does the addition of the catalyst, Fe^2+ speed up the reaction between iodide ions and peroxodisulfate ions?
Use equations to explain the answer.
The addition of the positive ions means there’s now no repulsion between the other two negatively charged ions.
First, the Fe^2+ ions are oxidised to Fe^3+ ions by the S2O8- ions:
S2O8- + 2Fe^2+ —> 2Fe^3+ + 2SO4^2-
The newly formed intermediate, Fe^3+ ions now easily oxidise the I- ions to iodine, and the catalyst is regenerated:
2Fe^3+ + 2I- —> I2 + 2Fe^2+
88
The reaction between C2O4^2- and MnO4- is an autocatalysis reaction. What does this mean?
The homogeneous catalyst Mn^2+ is used in this reaction. It is a product of the reaction as well as the catalyst which means that as the reaction progresses and the amount of product increases, the reaction speeds up.
89
Give the equations of the two step reaction between C2O4- and MnO4- with the catalyst Mn^2+.
1. Mn^2+ catalyses the reaction by first reacting with MnO4- to form Mn^3+ ions:
MnO4- + 4Mn^2+ + 8H+ —> 5Mn^3+ + 4H2O
2. The newly formed Mn^3+ ions then react with C2O4^2- ions to form CO2 and re-form Mn^2+ ions:
2Mn^3+ + C2O4^2- —> 2Mn^2+ + 2CO2
90
What are the names of the following:
1. C2O4^2-
2. MnO4-
1. Ethanedioate ions
| 2. Manganate (VII) ions.
