Transition Metals Flashcards
1
Q
Where are the transition metal elements found in the periodic table?
A
The central d block of the periodic table
2
Q
Where are the transition metal elements found in the periodic table?
A
The central d block of the periodic table
3
Q
What are the common properties of period 4 elements?
A
- they form complex ions
- their ions are coloured
- they show catalytic activity
- their oxidation states are variable
4
Q
What is a transition metal defined as?
A
An element that has an incomplete d subshell either in the element or in one of its ions
5
Q
What are the properties of transition elements directly related to?
A
The electronic structures of their atoms
6
Q
The 4s subshell usually fills before the 3d subshell. What can chromium and copper do that leads to a more stabilised d subshell?
A
They can promote a 4s electron to the d subshell and this gives rise to a half-filled and fully filled d shell which are stabilised.
7
Q
Why do transition metal atoms lose 4s electrons before 3d electrons?
A
Because if electrons occupy the 3d subshell the 4s subshell increases to a higher energy level
8
Q
Why are zinc and scandium not considered transition metals?
A
- the normal ion formed in scandium compounds is Sc(III). This has no electrons in the d orbital - its electron configuration is [Ar] 3d04s0 so it isn’t considered a transition metal
- zinc forms Zn(II) compounds with the configuration [Ar]3d104s0. It is not considered a transition metal because the d subshell is complete
9
Q
What do transition metals in water solutions do?
A
Form coordination complexes with water
10
Q
What are complexes with water ligands called and how may they be represented?
A
They’re called metal-aqua ions and may be representer as Co2+(aq) ect..
11
Q
In a complex what is the number of bonds formed between the metal ion and lone pair from the ligands called?
A
The coordination number
12
Q
What are ligands?
A
Molecules or ions which share an electron pair with a metal ion to form a coordination complex
13
Q
What is the reaction of the replacement of one or more ligands called?
A
Ligand substitution
14
Q
What are divalent meral-aqua ions?
A
They have a 2+ charge
15
Q
What is the trend in the reactivity of divalent metal aqua ions in the fourth period?
A
It increases from Mn2+ to Cu2+
16
Q
What happens to the coordination mumber in ligand substitution between water and ammonia?
A
It doesn’t change as they are neutral molecules of similar size
17
Q
What is complete and incomplete ligand substitution?
A
- complete substitution occurs when all ligands are replaced
* incomplete substitution occurs when only some of the ligands are replaced
18
Q
What is the overall reaction of Cu2+(aq) with NH3 made up of?
A
Four seperate equilibrium reactions. And each of these ligand substitution reactions has it’s own equilibrium constant
19
Q
What can the equations for all the ligand substitution reactions be simplified to give?
A
A dissociation constant of the complex Kd
20
Q
What is the formation constant, Kf?
A
The reciprocal of the complex dissociation constant
Kf=1/Kd
21
Q
Why does the ligand substitution of water with Cl- from H involve a change in coordination number from 6 to 4?
[Cu(H2O)6]2+ + 4Cl- [CuCl4]2- + 6H2O
A
Becayse halide ions are negatively charged ligands that are much larger than water or ammonia and metal ions of the fourth period are too small to coordinate six halide ions
22
Q
What is the difference between bidentate ligands and monodentate ligands?
A
Bidentate ligands have two electron pairs each (are said to have two teeth) whereas monodentate ligands have one electron pair (one tooth)
23
Q
What are multidentate ligands and what do they form with transition metals?
A
They have more than two electron pairs. They can form very stable complexes with transition metal ions
24
Q
What are all chemical reactions governed by?
A
The second law of thermodynamics?
25
What reaction is Kf proportional to the inverse of?
The free energy of the reaction:
DeltaG° = deltaH° -TdeltaS°
```
DeltaG° = gibbs free energy of the reaction
DeltaH° = enthalpy change
T = absolute temperature in K
DeltaS° = entropy change
```
26
What does the feasibility of a reaction happening depend on?
The magnitudes of the two energy terms deltaH° and deltaS°
27
What does a high or low value of Kf indicate?
A high value of Kf indicates the forward reaction is thermodynamically favoured and a low value indicates the reverse reaction is favoured
28
The enthalpy content when Co2+ is bonded to a molecule of sarcophagine is similar to when it is bonded to six molecules of ammonia. What does this mean for the enthalpy change and the entropy change?
* the enthalpy change for the reaction is very small
* the entropy change has a high positive value. Thisbis because the forward reaction is between one ion and one molecule producing one ion and six molecules, the system has become more disordered. Therefore it is an entropy driven reaction
29
How does haemoglobin pick up oxygen?
Each haemoglobin molecule contains four chelating agents called haem groups, each coordinated to an Fe2+ ion. Fe2+ is also coordinated by one histidine amino acid of the protein and one molecule of water. This molecule of water is replaced by O2 in the pulmonary alveoli by ligand substitution:
Hb(H2O) + O2 Hb(O2) + H2O
30
What are transition metals arranged in if the coordination number is 6?
Octahedral symmetry
31
What is the shape of transition metal complexes in the coordination number is 4?
Tetrahedral or square planar
32
What is the shape of the transition metal complex if the coordination number is 2?
Linear
33
What does the size of the ligand have an effect on?
The coordination number and shape (symmetry) of the complex
34
What ligands form an octahedral complex?
Water, ammonia and cyanide
35
What can octahedral complexes with two or three different ligands exist as?
Cis or trans isomers, depending on the position of the coordinating ligands around the metal. (Same side = cis) (opposite side = trans)
36
Which octahedral complex exists as a pair of enantiomers?
An octahedral complex with bidentate ligands
37
Which ions have a preffered geomtery of a tetrahedral complex?
Larger ions such as chloride ions
38
What prefers a square planar geometry?
Complexes of large metal ions with coordination number 4
39
In a square planar complex in what configuration can the ligands be arranged in?
In a cis or trans configuration. In the cis configuration the A ligands are adjacent and in the trans configuration the A ligands are opposite eachother with respect to the metal ion.
40
What property is used in the silver mirror test to distinguish between ketones and aldehydes?
The complex formed in Tollens reagent will react with compounds that are easily oxidised to form metallic silver.
Dilute ammonia is added to silver nitrate solution until the precipitate of Ag2O which is formed initially redussolves. The solution formed is called Tollens reagent and contains the soluble [Ag(NH3)2]+ ion. When Tollens reagent is warmed with an aldehyde the aldehyde is oxidised to an acid and the silver ion is reduced to silver, which forms a silver mirror on the surface of the container
41
What is a typical property of the transition metal compounds?
That they're coloured
42
How is colour produced?
When some parts of the visible spectrum are absorbed and others are reflected
43
What do the colours of transition metal compounds depend on?
The electronic configuration of the metal ion
44
What happens when light falls on a coloured solid?
Some wavelengths are absorbed and others are reflected. The reflected wavelengths are what we see
45
What happens when light falls on a coloured solution?
Some wave lengths are absorbed and others pass through. The wavelengths that pass through are what we see
46
What has the shortest wavelength (highest energy) and what has the longest wavelength (lowest energy) of the visible light spectrum?
Violet has the shortest wavelength and red the longest
47
What is ultraviolet?
The portion of the electromagnetic spectrum with wavelengths shorter than 400 nm
48
What is infrared?
The portion of the electromagnetic spectrum with wavelengths longer than 800 nm
49
In 1900 what dir Max Planck propose?
That energy can only be transferred in definite amounts called quanta
50
What does each amount of energy correspond to?
A particular frequency of radiation
51
What is frequency and what is the equation for thisM
The frequency is the number of wavelengths of light passing a particular point in a second
The equation for this relationship is
DeltaE=hv
```
DeltaE= energy difference
v= frequency of radiation
h= plancks constant
```
Or Delta E= hc/landa
Where c=wave speed and landa= wavelength
52
What happens if an electron in an atom, ion or molecule absorbe an amount of energy equal to the energy difference DeltaE between two energy levels?
It is promoted from a lower to a higher energy level
53
In a first-row transition metal (Ti-Cu), the electrons in the five d orbitals have the same energy. However what happens when the atom forms a complex ion?
The ligands split the d orbitals into two levels, one of higher and one of lower energy. This is due to electrons in the ligand repelling electrons in 3d orbitals which are in close proximity.
54
What is the colour of transition metal complex ions due to?
The absorption of light with energy that can excite an electron from a 3d orbital of lower energy to one of higher energy.
55
What happens when cobalt chloride is dissolved in water and when cobalt chloride is dissolved in concentrated HCl?
* cobalt chloride dissolved in water is pink the ions present are [Co(H2O)6]2+. They are octahedral. The pink colour sjows that radiation from the blue region of the visible spectrum has been absorbed
* cobalt chloride dissolved in concentrated HCl acid is blue. The ions present are [CoCl4]2-. They are tetrahedral. The blue colour shows that radiation from the red region of the visible spectrum has been absorbed
56
What are the factors that affect that value of DeltaE?
* the ligand (its size and the strength of the ligand-metal bond in the complex)
* the shape of the complex
* the coordination number
* the oxidation state of the metal in the central ion
57
What can ligands be classified as?
* strong-field ligands - those that induce a large deltaE, e.g. CN-, CO and P(C6H5)3
* weak-field ligands - those that induce a small deltaE, e.g. NH3, H2O and Cl-
58
What will change the value of deltaE for a particular transition metal?
* changing the ligand
| * changing the oxidation state
59
How does cobalt chloride paper work to detect the presence of water?
Water and chloride ions cause the ligand and the coordination number to change but with Co2+ ions there is no change in oxidation state. With cobalt chloride paper, this colour change is used to detect the presence of water
60
What will the amount of light absorbed by a coloured solution be affected by?
* the nature of the ion (which relates to deltaE)
* the concentration of the solution
* the distance the light has to travel through the solution (the path length)
61
What happens if a solution is more concentrated in regards to light absorbed?
More ions are present in a given volume and more will interact, so the colour will be more intense
62
What happens if the light has to travel a greater distance in regard to colour?
There will be more ions to interact with the light so the colour will be more intense
63
What is the instrument used to investigate the colour of solutions?
A spectrophometer
64
How does a spectrophotometer work?
It uses visible or ultraviolet radiation, which shines through the sample. The detector measures the radiation that passes through the solution. From this the instrument calculates the absorbance and displays this on a meter or as a graph.
If the nature of the coloured ion in solution and the path length are fixed, the concentration in a sample can be determined by comparing it with standard solutions
65
Why does a typical spectrophotometer have two radiation sources?
Two provide frequencies in both the ultraviolet and the visible regions.
66
What are the various frequencies separated by and why in a spectrometer?
A diffraction grating. This produces a spectrum. The grating rotates very slowly, allowing narrow bands of radiation to pass one after another through a narrow slit. This enables the instrument to measure the absorbance for radiation within each band. The narrower the range of wavelengths in a band, the more precise the measurement by the instrument.
67
What does a spectrophometer plot?
The absorbance for each band against the average eavelength of the band and displays this on a computer screen
68
How can spectrophometers be used to determine the concentration of transition metal ions in aqueous solution?
The wavelength at which the absorbance is greatest is used. A sample is placed in the spectrophometer and the absorbance is measured at the wavelength of maximum absorption. Its concentration is calculated from a calibration graph
69
What happens if the solution is too dilute to give a sufficiently intense colour for the spectrophometer to measure?
A suitable ligand is first added to the ion to give an intense colour. The visible range is scanned to find the most jntense absorption frequency, and then a suitable range of frequencies is passed through a solution of fixed path length
70
What is a simpler instrument that can be used to measure the absorption of radiation?
The colourimeter
71
How does a colourimeter work?
It uses a lamp as a source of white light, which passes through a filter to produce light of one colour. This colour will be the one that the sample will absorb the most and is called its complementary colour. Usually a set of standard solutions is prepared to give a calibration graph for the instrument, and then the unknown concentration can be determined from the calibration graph, using the colourimeter reading for the unknown
72
Why can transition metals have variable oxidation states?
When metals react they transfer electrons, with those if higher energy level being transferred first. For the transition metals, these are the 4s electrons which gives a commonly occuring oxidation state of 2+ for these metals. However transition metals can form compounds in which the transition metal has other oxidation states; for example iron can form Fe2+ or Fe3+ compounds
73
What happens to the oxidation state when an atom is oxidised?
It forms a higher oxidation state and the other reactant is reduced and changes to a lower oxidation state. This is a redox reaction
74
How can you form lower oxidation states of chromium, Cr(VI) if you take the highest oxidation state?
Using zinc and HCl acid as a reducing agent air must be exclued from the Vr2+ species otherwise oxidation will occur giving Cr3+. The oxidation states of chromium change in the presence of HCl when it is used as an acid. The colour changes from green to blue
When HCl is used with zinc for the reduction, the Cl- ion acts as a ligand, donating to the Cr3+ ion.
75
How us the oxidation if [Co(H2O)6]2+ to [Co(H2O)6]3+ possible?
* In alkaline solution a precipitate of Co(OH)2 forms initially. This is readily oxidised to Co(OH)3 by air, at the surface or dissolved in solution. An oxidising agent such as H2O2 (hydrogen peroxide) will also do this
* similarly if an excess of concentrated ammonia solution is added the initial precipitate of Co(OH)2 dissolves to form the pale brown complex [CoNH3)6]+2. This is readily oxidised by air or H2O2. The reaction produces the yelliw complex hexaammine cobalt (III)
76
How is Cr(III) oxidised?
First the Cr(III) ions are reacted with sodium hydroxide 5o give Cr(OH)3(aq). This gives a green precipitate which dissolves in an excess of sodium hydroxide giving a deep green solution of [Cr(OH)6]3-. When H2O2 is added [Cr(OH)6]3- is oxidised to CrO42-. The yellow chromate (VI) ion is formed. This can be converted to dichromate (VI) by simply adding acid
77
What does potassium permanganate contain and what can this be used for?
It contains the manganate (VII) ion, which is a strong oxidising agent and can be used in titrations to estimate the amount of iron(II) ions in solution
78
What happens in terms of redox reactions in the manganate titrations?
The manganate (VII) us reduced to Mn2+(aq). The iron is oxidised to Fe3+
79
Why is excess acid added to the iron(II) ions before the titration reaction begins?
Because if insufficient acid is present tbe brown solid MnO2 will be formed and MnO4- to Mn4+ is only a 3e- change and has a different stoichometry
80
Why do we use dilute sulfuric acid in the manganate titrations?
The acid must:
• be a strong one because a high concentration of hydrogen ions are needed
• not be an oxidising agent because they may react with the reductant and affect the titration results.
• not be a reducing agent, because it may be oxidised by the manganate (VII) ions and affect the titration results
Sulfuric acid fits these conditions
81
Why don't we use HCl, HNO3, concentrated H2SO4 or CH3COOH in manganate titrations?
* HCl can be oxidised to chlorine by the manganate ions
* nitric acid is an oxidising agent and may oxidise the substance being analysed
* concentranted sulfuric acid may oxidise the sunstance being analysed
* ethanouc acid is a weak acid so the concentration of hydrogen ions will be insufficient
82
How does the potassium permanganate act as it's own indicator in the titration?
As the potassium permanganate solution is added to the titration flask from the burette is reacts rapidly with the Fe2+(aq). The manganese(II) ions have a very pale pink tinge but they are present in such a low concentration the solution looks colourless. As soon as all the iron(II( ions have reacted with the added manganate (VII) ions a pink tinge appears in the flask due to an excess of manganate (VII)
83
Apart from potassium permanganate what elese can be used to find out the amount of iron in a titration and what is the colour change?
Potassium dichromate. The dichromate (VI) ion is a very powerful oxidising agent when it changes from the +6 oxidation state in the dichromate ion to the +3 oxidation state on the Cr3+ ion. The colour change for this titration is orange to bluish green. To give a mire visible endpoint the indicator sodium diphenylaminesulfonate is used. This turns from colourless to purple at the endpoint
84
What is the general sequence for redox titration calculations?
1) write the half equations for oxidant and reductant
2) deduce the equation for the overall reaction
3) calculate the number of moles of manganate(VII) or dichromate(VI) used
4) calculate the ratio of moles of oxidant to moles of reductant from the redox equation
5) calculate the number of moles in the sample solution of reduction
6) determine either the concentration of the original solution or the percentage of reductant in a known quantity of sample
85
What is an inhibitor/negative catalyst?
A catalyst that slows down chemical reactions
86
What type of catalysts are transition metals examples of?
Heterogeneous catalysts. Reactions happen of the surface of the metal, typically involving reactants in the gas phase, though they may also be in the liquid phase
87
What is adsorption?
When in a heterogenous catalyst at least one of the metals binds to the metal's surface using available orbitals by a process similar to ligand donation. It is in this state that the reactant is more likely to undergo a given reaction
88
Why can adsorption onto a metal surface speed up a reaction?
* it can weaken bonds within the reactant molecule, reducing the activation energy
* it can cause a reactant molecule to break up into more reactive fragments, again reducing the activation energy
* it can hold a reactant in a favourable position, increasing the chance of a favourable collision
* it can give a higher concentration of one reactant on the catalyst surface, increasing the chance of a favourable collision with another reactant
89
Why is the strength of the bond between the metal and the catalyst important in determining the suitability of a catalyst for a particular reaction?
Metal atoms on the surface of the catalyst need to form bonds with reactant gas molecules that are strong enough to hold the reactant in position while it reacts with other molecules. However if the bond is too strong the product molecule will not be released by a metal catalyst amd the reaction will not proceed
90
Why is tungsten not often used as a catalyst?
It forms very strong bonds with some molecules
91
What is the opposite of absorption?
Desorption
92
What is the opposite of absorption?
Desorption
93
What are some examples of good catalysts but what are the problem with these?
Good catalysts include palladium, rhodium and platinum but they can be expensive
94
What helps to minimise costs and maximise the surface area of a catalyst?
* A cheap support medium (called a substrate) is often coated with a thin layer of the expensive metal catalyst. The support material is often a ceramic
* use a fluidised bed. When gases are blown through a very fine powder, the powder 'floats' on the gas; seperating the catalyst partciles so that all of the surface of the catalyst is available for reactions
95
Why is a larger catalytic surface area more effective?
There are more active sites
96
What is catalyst poisoning?
When unwanted gases adsorb strongly on a catalyst, 'blocking' the active sites and reducing the catalyst's effectiveness
97
What poison an iron catalyst?
Sulfur and carbon monoxide
98
What can catalytic converters on cars be poisoned by?
They are made up of a mixture of the metals palladium, platinum and rhodium, coated onto cerium oxide. If the exhuast emissions contain sulfur dioxide it will adsorb very strongly onto the catalyst and block it's active sites
99
How do manufacturers of catalytic converters help to overcome sulfur dioxide poisoning?
They mix in aluminium oxide with the catalyst. This 'stores' the sulfur oxide under oxidising conditions (normal running conditions) and converts them to hydrogen sulfide under reducing (accelerating) conditions. This removes the sulfur periodically so that it does not permanently poison the catalyst
100
What is the haber process?
An industrial method for the manufacture of ammonia (NH3).
101
What catalyst is used in the haber process?
An iron catalyst
102
How is the catalyst made in the haber process?
The iron is mixed with aluminium oxide and potassium oxide which act as promoters. The promoters are present in smaller amounts than the iron and improve the efficiency of the catalyst.
103
How is the hydrogen for the haber process produced?
N2(g) + 3H2(g) -> 2NH3(g)
The hydrogen is obtained from methane so trace amounts of steam and CO are found as impurities in the hydrogen. If the CO was allowed to remain in the mixture it would poison the catalyst by forming an iron carbonyl compound, so it must be removed. The carbon monoxide is removed by heating with an excess of steam in a two-stage process, first with an iron oxide catalyst and then with a zinc/copper catalyst. The products are carbon dioxide, which is easier to remove from the mixture and hydrogen
104
What is the contact process?
An industrial methos dor the manufacture of sulfuric acid, H2SO4. A key step in the manufacture is the oxidation of sulfur dioxide to sulfur trioxide:
2SO2(g) + O2(g) -> SO3(g)
105
What catalyst is used in the contact process?
A vanadium (V) pentoxide catalyst V2O5
106
Why is a vanadium pentoxide catalyst used over a platinum catalyst in the contact process?
Because it is cheaper andess prone to poisoning by impurities
107
What are vanadium pentoxides catalytic properties linked to? And how does this show itself in the contact process?
Its ability to exist in more than one oxidation state. In V2O5, the oxidation state of vanadium is +5. V2O5 reacts with SO2 and is reduced to VO2 wherebthe vanadium is in the +4 oxidation state. Sulfur is oxidised from the 4+ state in SO2 to +6 in SO3. VO2 us oxidised back to VO5 by oxygen
108
What is the ability of transition metal ions to act as homogenous catalysts linked to?
A transition metals ability to form ions in different oxidation states
109
How does the hexaaquacobalt (II) ion act as a homogenous catalyst?
Under suitable reaction condituons the Co2+ ion is easily oxidised to Vo3+ which can form temporary intermdeiates in for wxample the reaction between hydrogen peroxide and sodium potassiym tartrate
110
What happens in the reaction between sodium potassium tartrate and hydrogen peroxide?
If cobalt (II) ions are added to ansolution of sodium potassium tartrate and hydrogen peroxide, the cobalt ions form an intermediete complex with the tartrate. In this intermediete the cobalt (II) ions are more easily oxidised to cobalt (III) by the hydrogen peroxide (changing the ligands changes the electrode pitentaul for oxidarion of Co(II) to Co(III).) A green cobalt (III) tartrate complex forms. The cobalt (III) ions in the complex are then ablento oxidise the tartrate to produce CO2 and a carbonyl compound, and Co(III) is reduced back to the pink Co(II) complex [Co(H2O)6]2+
111
Is it easier for cobalt (III) to be reduced to cobalt (II) or cobalt (II) oxidised to cobalt (III)
The reduction can occur spontaneously with H2O as a ligand so is easier. A powerful oxidising agent is needed to overcome the reduction potential so it is hard for the oxidation to happen. However some ligands can lower the reduction potential significantly enough for a mild oxidising agent to allow the cobalt to be oxidised
112
What is the catalysis of a reaction by one of it's products called?
Autocatalysis
113
What happens when manganate (VII) is titrated with a warmed acidified solution of ethaneduoate ions C2O42-?
The reaction is initially quite slow and during the first addition of manganate (VII) the purple colour is slow to decolourise when more manganate (VII) is added, however, the solution immedietly turns colourless and continues to do so until the endpount is reached. The reaction is catalysed by the Mn2+ ion which is formed during the reaction. The reaction is initially slow because both the oxidant and the reductant are negatively charged, so they are unlikely to make fruitful collisions. However, the positive Mn2+ ions can react with MbO4- to form Mn3+ ions. These ions can then reacr with C2O42- ions to liberate CO2 and reform Mn2+ ions, which can then continue the autocatalysis
