Transition Metals Flashcards

1
Q

What are transition metals?

A

A d-block element forming one or more stable ions with partially filled (incomplete) sub-shell of d-electrons.

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2
Q

What are the exceptions of d-block elements that aren’t described as transition metals?

A

By definition, a transition metal is one that forms at least one stable ion with a partially full d-shell of electrons.
-Scandium only forms Sc3+, which is 3d0 in all its compounds.
-Zinc only forms Zn2+ (3d10).
As scandium has an empty 3d-orbital and zinc has a full 3d-orbital, they are not considered transition metals and cannot form complex or coloured ions.

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3
Q

What are the chemical properties of transition metals?

A
  • Variable oxidation states
  • Colour
  • Catalysis
  • Complex formation
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4
Q

How is variable oxidation states a feature of transition metals?

A

Transition metals have more than one oxidation state in their compounds, e.g. Cu(I) and Cu(II). This arises from the similar energies required for removal of 4s-and 3d-electrons.

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5
Q

How is colour a feature of transition metals?

A

The majority of transition metal ions are coloured, Cu2+(aq) is blue. The electrons in the d-subshell determine this.

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6
Q

How is catalysis a feature of transition metals?

A

Catalysts affect the rate of reaction without being used up or chemically changed themselves. Many transition metals and their compounds show catalytic activity.
-Iron is the catalyst in the Haber process, vanadium (V) oxide in the contact process and manganese (IV) oxide in the decomposition of hydrogen peroxide.

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7
Q

How is complex formation a feature of transition metals?

A

Transition elements form complex ions. A complex ion is formed when a transition metal ion is surrounded by ions or other molecules, collectively called ligands, which are bonded to it by co-ordinate bonds.
-E.g. [Cu(H2O)6]2+ is a complex ion that is formed when copper sulfate dissolves in water.

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8
Q

What are the physical properties of transition metals?

A

-High density
-High melting and boiling points.
-Ionic radii are more or less the same.
The properties don’t gradually change across the period as might be expected.

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9
Q

What is a complex ion?

A

A metal ion surrounded by coordinately bonded ligands. All transition metal ions can form coordinate bonds by accepting electron pairs from other ions or molecules.

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10
Q

What is a ligand?

A

An atom, ion or molecule that donates a pair of electrons to a central metal ion. A ligand must have at least one lone pair of electrons to use to form the coordinate bond, and can be neutral or negatively charged. All ligands function as Lewis bases.

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11
Q

What are unidentate ligands?

A

Ligands that can only form one coordinate bond.

  • Ammonia, chloride ions, water
  • Although water has two lone pairs, they are so close together that it can only form one coordinate bond.
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12
Q

What are bidentate ligands?

A

Ligands that can form two coordinate bonds.

-Etanedioate (C2O42-), Ethane-1,2diamine, benzene-1,2-diol.

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13
Q

What are multidentate ligands?

A

Ligands that can form more than one coordinate bonds (inc. bidentate ligands).
-EDTA (ethylendiaminetetra-acetic acid). EDTA acts as a hexadentate ligand, using long pairs on four oxygen and two nitrogen atoms.

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14
Q

What is chelation?

A

Complex ions with polydentate ligands are called chelates. Chelates can be used to remove d-block metal ions from solution.
-This is due to the chelate effect.

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15
Q

What is the chelate effect?

A

The enhanced affinity of chelating ligands for a metal ion compared to the affinity of a similar non-chelating (unidentate) ligands for the same metal.

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16
Q

What is coordination number?

A

The number of coordinate bonds to ligands that surround the metal ion.

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17
Q

What is the shape of complex ions governed by?

A

Shape is governed by the number of ligands around the central ion.

  • Transition metal ions commonly form octahedral complexes with small ligands (e.g. H2O and NH3).
  • They also commonly form tetrahedral complexes with larger ligands (e.g. Cl-).
  • Some complex ions also form square planar complexes, such as cisplatin.
  • It is common for Ag+ to form linear complexes, e.g. [Ag(NH3)2]+, present in Tollen’s reagent.
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18
Q

Do complex ions form optical isomers?

A

Organically, optical isomerism is displayed in chiral compounds, but it can also be seen I complex ions due to the planes causing the ions to be non-super imposable.
-This is most likely to occur in octahedral complexes, containing bidentate ligands, like [Ni(NH2CH2CH2NH2)3]3+.

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19
Q

What factors determine the colours of complex ions?

A
  • Oxidation state
  • Coordination number and shape
  • Type of ligands.
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20
Q

Why are transition metal complexes coloured?

A
  • They are coloured because they have partially filled d-orbitals, making it possible for electrons to move from one d-orbital to another.
  • In an isolated transition metal atom, all the d-orbitals are of exactly the same energy, but in a compound, the presence of other atoms nearby makes the d-orbitals have slightly different energies.
  • When the electrons move from one d-orbital to another of a higher energy level, they often absorb energy in the visible region of the spectrum, equal to the difference in energy between levels.
  • The colour is missing from the spectrum, we see colours that aren’t absorbed.
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21
Q

What is the formula to calculate the energy absorbed by complex ions?

A
∆E = hν
ΔE = energy absorbed
h = Planck’s constant (6.63x10^-34Js)
ν = frequency of light absorbed (hertz/Hz)
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22
Q

What colours do complex ions appear?

A

When visible light hits a transition metal, some frequencies are absorbed as electrons jump to the higher orbitals. The frequency absorbed depends on the size of the energy gap.
The rest of the frequencies are reflected. These reflected frequencies combine to make the complement of the colour of the absorbed frequencies.
-Red + green = yellow
-Red + blue = purple
-Blue + green = turquoise.

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23
Q

How do transition metals have variable oxidation states?

A

A typical transition metal can use its 3d as well as its 4s electrons in bonding, so it can have a greater variety of oxidation states in different compounds.

24
Q

How can dichromate (VI) ions (Cr2O72-) be reduced?

A

Dichromate (VI) ions can be reduced using a good reducing agent, such as zinc and dilute acid.
Cr2O72-(aq) + 14H+(aq) + 3Zn(s) –> 3Zn2+(aq) + 2Cr3+(aq) + 7H2O(l)
Orange to green
-Zinc will reduce Cr3+ further to Cr2+, but this must be in an inert atmosphere as Cr2+ oxidises straight back to Cr3+ in air.
2Cr3+(aq) + Zn(s) –> Zn2+(aq) + 2Cr2+(aq)
Green to blue
-With iron

25
Q

How can dichromate (VI) ions be reduced with iron?

A

Acidified dichromate (VI) ions are reduced to chromium (III) ions
Cr2O72+(aq) + 14H+(aq) + 6e- –> 2Cr3+(aq) + 7H2O(l)
Orange to green
Iron (II) ions are oxidised to iron (III) ions.
Fe2+(aq) –> Fe3+(aq) + e-
Green to pale violet
Overall: Cr2O72+(aq) + 14H+(aq) + 6Fe2+(aq) –> 2Cr3+(aq) + 7H2O(l) + 6Fe2+(aq)
(-It is not possible to see the colour change during this reaction so sodium diphenylaminesulfonate (colourless to purple) must be used.

26
Q

How can Cr3+ be oxidised?

A

Cr3+ can be oxidised to chromate (VI) ions using H2O2 in an alkaline solution.
2Cr3+(aq) + 10OH-(aq) + 3H2O(aq) –> 2CrO42-(aq) + 8H2O(l)
Green to yellow.

27
Q

How do chromate (VI) and dichromate (VI) ions exist in equilibrium?

A

Cr2O72-(aq) + H2O(l) 2CrO42-(aq) + 2H+(aq)

  • If acid (H+) is added, the equilibrium shifts to the left to form orange dichromate (VI) ions.
  • If alkali (OH) is added, it combines with H+ to form water, the equilibrium shifts to the right to form yellow chromate (VI) ions.
28
Q

How can Co2+ be oxidised?

A

-Co3+ can made by oxidising Co2+(aq) with hydrogen peroxide in alkaline conditions.
2Co2+(aq) + H2O2(aq) –> 2CO3+(aq) + 2OH-(aq)
Co2+ –> Co3+ + e-.
-In air in an ammoniacal solution.

29
Q

How can manganate (VII) ions be reduced?

A

-With Iron
Acidified manganate (VII) ions are reduced by Fe2+ to manganese (II) ions.
MnO4-(aq) + 8H+(aq) + 5e- –> Mn2+ + 4H2O(l)
Purple to pink
Iron (II) ions are oxidised to iron (III) ions
Fe2+(aq) –> Fe3+(aq) + e-
Green –> pale violet
Overall: MnO4-(aq) + 8H+(aq) + 5Fe2+(aq) –> Mn2+(aq) + 4H2O(l) + 5Fe3+(aq).
(-this is a self-indication reaction because when MnO4- is added to Fe2+, the solution turns virtually colourless. One drop too many = purple.)

30
Q

What is a catalyst?

A

A catalyst is something that speeds up the rate of a reaction by providing an alternate reaction pathways with a lower energy, whilst remaining chemically unchanged/regenerated.

31
Q

Why do transition metals make good compounds?

A

Transition metals and their compounds make good catalysts because they can change oxidation states by gaining or losing electron in their d orbitals.
-This means they can transfer electrons to speed up reactions.

32
Q

What are the two types of catalysts?

A
  • Heterogeneous catalysts

- Homogeneous catalysts

33
Q

What are heterogeneous catalysts?

A

One that is in a different phase from the reactant, i.e. in a different physical state.

  • They are usually present as solids, while the reactants may be liquids or gases, their catalytic action occurs on the solid surface.
  • The Haber process, the contact process, the manufacture of methanol.
34
Q

What are the steps in heterogeneous catalysis?

A
  • Reactants form bonds with atoms at active sites on the surface of the catalyst (adsorbed onto the surface).
  • As a result, bonds in the reactants are weakened and break.
  • New bonds form between the reactants held close together on catalyst surface.
  • This in turn, weakens bonds between produced and catalyst and product leaves (desorbs).
35
Q

What catalyst is used in the Haber Process?

A

Iron catalyst
N2(g) + 3H2(g) 2NH3(g)
-The iron catalyst is present in pea-sized lumps to increase the surface area.
-The catalyst last about five years before it becomes poisoned by impurities in the gas stream, such as sulfur compounds.

36
Q

What catalyst is used in the Contact Process?

A

Vanadium (V) oxide (V2O5)
2SO2(g) + O2(g) SO3(g) – this step is catalysed by V2O5 in two sub-steps:
-The vanadium (V) oxide oxides sulfur dioxide to sulfur trioxide and it itself reduced to vanadium (IV) oxide: SO2 + V2O5 –> SO3 + V2O4.
-The vanadium (IV) oxide is then oxidised back to vanadium (V) oxide by oxygen: 2V2O4 + O2 –> 2V2O5
The vanadium (V) oxide is regenerated unchanged, each of the two steps have a lower activation energy than the uncatalysed single step and therefore the reaction goes faster.

37
Q

What catalyst is used to manufacture methanol?

A

Chromium oxide (Cr2O3)

  • Synthesis gas is first made from methane: CH4(g) + H2O(g) –> CO(g) + 3H2(g).
  • The mixture of carbon monoxide and hydrogen is then used to make methanol. This reaction is catalysed by Cr2O3: CO(g) + 2H2(g) –> CH3OH(g).
38
Q

Why does a catalysts activity need to be as efficient as possible?

A

Catalysts are often expensive, so the more efficiently they work, the more the costs can be minimised. As heterogenous catalyst activity takes place on the surface, we can increase their surface area by using a support medium.

39
Q

How can catalyst efficiency be improved?

A

By spreading the catalyst onto an inert support medium or even impregnating it into one, the surface-are-to-volume rate is increased.

40
Q

How do catalytic converters work?

A

Catalytic converters are used to ‘clean up’ emissions from car engines and contain a ceramic lattice coated with a thin layer of rhodium. The rhodium acts as a catalyst to help convert the waste gases to less harmful products, e.g. 2CO(g) + 2NO(g) –Rh(s) catalyst-> 2CO2(g) + N2(g).
-The lattice structure maximises the surface area of the catalyst, making it more effective. It also minimises costs because only a thin coating is needed.

41
Q

What is catalyst poisoning?

A

During a reaction, reactants are adsorbed onto the active sites on the surface of heterogeneous catalysts. Impurities in the reaction mixture may also bind to the catalysts surface and block reactants from being adsorbed. This process is called catalyst poisoning, which reduces the surface area available to the reactants, slowing down the reaction.

  • It It also increases the cost because less product can be made in a certain time or with a certain amount of energy.
  • The catalyst may also need replacing or regenerating, which is also costly.
  • Catalyst poisoning can be reduced by purifying the reactants, to remove any impurities that could poison the catalyst.
42
Q

What catalyst poisoning occurs in catalytic converters?

A

Lead can coat the surface of the catalyst in catalytic converted so vehicles that have them fitted must only be run on unleaded petrol.

43
Q

What catalyst poisoning occurs in the Haber Process?

A

The hydrogen in the Haber Process is produced from methane, which is obtained from natural gas, which contains impurities including sulphur compounds. Any sulphur that is no removes is adsorbed onto the iron catalyst, forming iron sulphide, and stopping the iron from catalysing the reaction efficiently.

44
Q

What are homogeneous catalysts?

A

One that is in the same phase as the reactants, an intermediate is formed. This is usually an aqueous catalyst with aqueous reactants.

45
Q

How do homogenous catalysts work?

A

A homogenous catalyst works by forming an intermediate species, which then reacts to form the products and reform the catalyst.
-The activation energy needed to form the intermediate (and to form the products from the intermediates) is lower than that needed to make the products directly.
(The intermediate will often have a different oxidation state to the original transition metal. At the end of the reaction the original oxidation state will reoccur.

46
Q

Why is a catalyst needed for the reaction between iodide ions and persulphate ions?

A

S2O8 2-(aq) + 2I -(aq) –> I2(aq) + 2SO4 2-(aq)

  • This reaction takes place very slowly because both ions are negatively charged. The ions repel each other, so its unlikely they’’ collide, giving a high activation energy.
  • If Fe2+ is added, each reaction involves a positive and negative ion so there’s no repulsion, so rate of reaction increases.
47
Q

How does Fe2+ catalyse the reaction between iodide ions and persulphate ions?

A

First, Fe2+ ions are oxidised to Fe3+ by S2O8 2- ions:
-S2O8 2-(aq) + 2Fe2+(aq) –> 2Fe3+(aq) + 2SO4 2-(aq)
The newly-formed intermediate Fe3+ ions now easily oxidise the I- ions to iodine, Fe3+ ions are reduced back to Fe2+ ions.
-2Fe3+(aq) + 2I-(aq) –> I2(g) + 2Fe2+(aq).

48
Q

What test can we carry out to see if iodine is present?

A

Start solution can be added to see if iodine is present, if it is it will turn blue-black.

49
Q

What is autocatalysis?

A

Autocatalysis is when a reaction is catalysed by one of its products.

50
Q

How does Mn2+ autocatalyse the reaction between MnO4 – and C2O4 2-?

A

Mn2+ is a homogeneous catalyst and a product of the reaction, As the reaction progresses and the amount of product increases, the reaction speeds up: 2MnO4-(aq) + 16H+(aq) + 5C2O4 2-(aq) –> 2Mn2+ (aq) + 8H2O(l) + 10CO2(g).

  • At the start of the reaction, no Mn2+ is present, so rate is slow, Ea is very high due to collision of negative ions. Once Mn2+ is made: 4Mn2+(aq) + MnO4-(aq) + 8H+(aq) –> 5Mn3+(aq) + 4H2O(l)
  • The Mn3+ ions are the intermediate: 2Mn3+(aq) + C2O4 2-(aq) –> 2Mn2+(aq) + 2CO2(g)
51
Q

How does haemoglobin act as a complex ion?

A

Haemoglobin is responsible for carrying oxygen from the lungs to body cells. The molecule consists of an Fe2+ ion, with a coordination number of 6. Four are taken up by polyphyrin, a tetradentate ligand, this complex is called haem.

52
Q

How does the transition metal in haemoglobin help its function?

A
  • In the lungs, where oxygen concentration is high, water ligands are substituted for oxygen molecules, which is carried around the body in the blood as oxyhaemoglobin, which makes them red.
  • When the oxyhaemoglobin gets to a place where oxygen is needed, the oxygen molecules are exchanged for water ligands. The haemoglobin then returns to the lungs.
53
Q

How can and individual suffer from carbon monoxide poisoning?

A

The process of oxygen transport can be disrupted if carbon monoxide is inhaled. CO is a colourless, odourless and tasteless gas which is produced by incomplete combustion. When CO is inhaled, the haemoglobin can swap its water ligands for carbon monoxide ligands, forming carboxyhaemoglobin. Carbon monoxide is a strong ligand and doesn’t readily exchange with oxygen or water ligands, so haemoglobin cannot transport oxygen, causing headaches, dizziness, unconsciousness and even death if left untreated.

54
Q

What is cisplatin?

A

Cisplatin is a complex of platinum (II) and two chloride ions and two ammonia molecules in a square planar shape.

  • Cisplatin can be used as an anticancer drug as it prevents cancer cells from reproducing.
  • Before a cell can divide it has to replicate its DNA, which involves unwinding two strands of the DNA double helix so that they can be copied. Cisplatin forms coordinate bonds with nitrogen atoms in the DNA molecule, preventing the two strands from unwinding. This means the cell can no longer replicate its DNA so it can’t divide.
    • However this causes problems because it also prevents normal body cells from replicating too.
  • Its isomer, transplatin has no effect on cell division.
55
Q

What is Tollens’ reagent?

A

Tollens’ reagent is prepared by adding just enough ammonia solution to silver nitrate solution to form a colourless solution containing the complex ion [Ag(NH3)2]+. Tollens’ reagent is used to distinguish between aldehydes and ketones.
-Aldehydes react to give a silver mirror on the inside of the test tube; RCHO + 2[Ag(NH3)2]+ + 3OH- –> RCOO- + 2Ag + 4NH3 + 2H2O.