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A Level Chemistry > Topic 15: Transition Metals > Flashcards

Topic 15: Transition Metals Flashcards

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1
Q

Where are the transition metals found on the periodic table?

A
  • most of the elements n the d-block are transition metals
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2
Q

What is a transition metal?

A
  • an element that has one or more stable ions with incompletely filled d orbitals
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3
Q

When ions are formed where are electrons lost first?

A
  • when transition metals form positive ions, outer s electrons are removed first, then d electrons
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4
Q

Why aren’t scandium and zinc transition metals?

A
  • Scandium only forms one ion, Sc3+ , which as an empty d subshell
  • it has the electronic configuration [Ar] 3d1 4d2, so when it loses three electrons, it ends up with the configuration [Ar]
  • Zinc only forms one ion, Zn2+, which has a full d subshell
  • as it has the electronic configuration [Ar] 3d10 4s2, when it loses 2 electrons, it keeps its full 3d subshell
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5
Q

Why can transition metals have different oxidation numbers?

A
  • most transition metals can form multiple stable ions
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6
Q

How can compounds or complexes containing an ion with a certain oxidation number occur?

A
  • the energy given out when the ion forms a compound or a complex needs to be greater than the energy taken to remove the outer electrons and form the ion (the ionisation energy)
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7
Q

Why is it possible for transition metals to form ions from both 4s and 3d subshells?

A
  • 4s and 3d subshells are at similar energy levels
  • a similar amount of energy is used to remove an electron from 4s subshell as it does to remove one from 3d
  • there is not a large increase between ionisation energies of removing successive electrons either so multiple electrons can be removed from these subshells to form ions with different oxidation numbers
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8
Q

What affects the energy released when ions form a complex or compound?

A
  • the energy released when ions from a complex or compound increase with an ionic charge
  • therefore, the increase in energy required to remove outer electrons to form transition metals ions with higher oxidation number is usually counteracted by the increase in the energy instead
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9
Q

What is a complex ion?

A
  • a complex ion is a metal ion surrounded by dative covalently (coordinately) bonded ligands
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10
Q

What are ligands?

A
  • a ligand is an atom, ion or molecule that donates a pair of electrons to a central metal atom or ion
  • a ligand must have at least one lone pair of electrons, otherwise, it does not have anything to form dative covalent bonds with
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11
Q

What are ligands with one lone pair called?

A
  • monodentate
  • e.g. H2O: , :NH3 etc
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12
Q

What are ligands with two lone pairs called?

A
  • bidentate
  • they can form two dative covalent bonds with a metal ion
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13
Q

What are ligands with more than two lone pairs called?

A
  • multidentate
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14
Q

Describe haemoglobin as a ligand

A
  • it is an iron(II) complex containing a multidentate ligand called a haem group
  • the haem group is made up of a ring containing 4 nitrogen atoms
  • this means it is able to form 4 dative covalent bonds to the iron(II) ion
  • there are two other ligands bonded to the iron(II) ion: a protein called globin and either oxygen or water
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15
Q

What is the overall charge on a complex ion?

A
  • its overall charge is its oxidation number
  • it is put outside the square brackets
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16
Q

How do you work out the oxidation number of the complex metal ion?

A
  • oxidation number of the metal ion = total oxidation number - sum of the charges of the ligands
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17
Q

What is the coordination number?

A
  • the coordination number is the number of dative covalent (coordinate) bonds formed with the central metal ion
  • usual coordination numbers are 6 and 4
  • if ligands are small, 6 can fit around the central metal ion
  • if ligands are larger, only 4 can fit
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18
Q

Why do complex ions with different coordination numbers have distinct shapes?

A
  • the bonding electrons in the dative covalent bonds of a complex repel each other
  • therefore, in general, the ligands are positioned as far away from each other as possible
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19
Q

What shape do complexes with six-fold coordination have? What are their bond angles?

A
  • an octahedral shape
  • all bond angles are 90°
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20
Q

What shapes do complexes with four-fold coordination have? What bond angles?

A
  • usually a tetrahedral shape
  • Bond angles are 109.5°
  • some have a square planar shape
  • Bond angles are 90°
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21
Q

What type of E/Z isomerism can complex ions show?

A
  • cis/trans isomerism
  • square planar and octahedral complex ions that have at least two pairs of identical ligands show cis/trans isomerism
  • cis isomers have the same groups on the same side
  • trans have the same groups opposite each other
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22
Q

What is cis-platin? How may it treat cancer?

A
  • a complex of platinum(II) with two chloride ions and two ammonia molecules in a square planar shape
  • it is used as an anti-cancer drug:
  • bonds to DNA molecule by dative covalent bonding and H bonding
  • stops the cancer cells from dividing (and also healthy cells, so there are side effects)
  • the two chloride ions are next to each other, so this complex is cis-platin up
23
Q

How do ligands split the 3d subshell into two energy levels?

A
  • normally the 3d orbitals of transition metal ions are all have the same energy
  • but when ligand bond to the ions, the 3d orbitals split into two different energy levels
  • electrons tend to occupy the lower level orbitals
  • to jump up to the higher orbitals (excited states) they need energy equal to the energy gap, deltaE
  • they get this energy from the visible light
  • the larger the energy gap, the higher the frequency of light that is absorbed
  • the amount of energy (and so the frequency of light) need to make electrons jump depends on the:
  • Central metal ion
  • its oxidation number
  • the ligands
  • coordination
  • these affect the size of the energy gap (deltaE)
24
Q

What do the colours of the complex ions show?

A
  • due to the splitting of the d-orbitals, some frequencies of light are absorbed by the complexes
  • the rest of the frequencies of light or transmitted (or reflected)
  • these transmitted or reflected frequencies combine to make the complement the colour of the absorbed frequencies
  • this is the colour you see
  • a Colour wheel shows complimentary colours
  • if there are no 3d electrons or the 3d subshell is full, then no electrons will be absorbed, so no energy will be absorbed
  • if there is no energy absorbed, the compound will look white or colourless
25
Q

What can help identify transition metal ions?

A
  • the colour of the transition metal ion dissolved in water
26
Q

What reaction occurs when you switch between oxidation numbers?

A
  • a redox reaction
27
Q

How can you work out whether a redox reactions involving transition metals are likely to happen?

A
  • you can use reduction potentials
  • e.g. vanadium
28
Q

What oxidations numbers do chromium tend to exist in?

A
  • +3 ( most stable)
  • +6
  • +2 (least stable)
  • chromium forms two ions with oxygen in the +6 oxidation number:
  • chromate(VI) ions, CrO42-
  • dichromate(VI) ions, Cr2O72-
  • these are good oxidising agents because they are easily reduced to Cr3+
  • when Cr3+ ions are surrounded by 6 water ligands, they’re violet
  • but the water ligands are usually substituted with impurities making them look green
29
Q

What are some reactions that chromium ions can take part in?

A
  • dichromate(VI) ions can be reduced using a reducing agent such as zinc and dilute acid
  • zinc will reduce Cr3+ further to Cr2+
  • you’ll need to use an inert atmosphere
  • Cr2+ is so unstable that it oxidises straight back to Cr3+ in air
  • Cr3+ can be oxidised to chromate(VI) ions with hydrogen peroxide in an alkaline solution
  • if you add some acid to this yellow solution, you form an orange solution that contains dichromate(VI) ions
  • this is a reversible reaction
  • so an equilibrium exists between chromate(VI) ions and dichromate(VI) ions
30
Q

How is chromium hydroxide made?

A
  • when you mix an aqueous solution of chromium(III) ions with aqueous sodium hydroxide or aqueous ammonia (NH3)
  • you get a chromium hydroxide precipitate
  • Cr(OH)3(H2O)3 (s)
31
Q

What does ‘chromium is amphoteric’ mean?

A
  • the compound can react with both acids and bases
32
Q

What happens when you add excess sodium hydroxide to a chromium hydroxide precipitate?

A
  • the H2O ligands deprotonate, and a solution containing [Cr(OH)6]3- (aq) forms
33
Q

What happens if you add acid to the chromium hydroxide precipitate?

A
  • the OH- ligands protonate and a solution containing [Cr(H2O)6]3+ (aq) forms
34
Q

What happens if you add excess ammonia to the chromium hydroxide precipitate?

A
  • a ligand exchange reaction occurs
35
Q

Describe the reaction of making chromium(II) ethanoate, Cr2(CH3COO)4(H2O)2

A
  • orange sodium dichromate(VI) is reduced with zinc in acid solution to first form a green solution containing Cr3+ ions and then to give a blue solution of Cr2+ ions
  • sodium ethanoate is mixed with this solution and a red precipitate of chromium(II) ethanoate forms
  • however, Cr2+ is easily oxidised
  • the whole experiment has to be done in an inert atmosphere, such as nitrogen, to keep the air out and remove the oxygen from all the liquids in your experiment before them
36
Q

Describe the experiment for how to make chromium ethanoate

A
37
Q

What is ligand exchange?

A
  • one ligand can be swapped for another ligand
  • it usually causes a colour change
38
Q

What happens if ligands of similar size are exchanged?

A
  • e.g H2O, NH3, CN-, OH-
  • the coordination number of the complex ion doesn’t change
  • neither does the shape
39
Q

What happens if a small, uncharged ligand (e.g. H2O), is substituted for a large, charged ligand (e.g. Cl-), or vice versa?

A
  • there is a change of coordination number
  • and a change of shape
  • sometimes, the substitution is only partial
40
Q

How does carbon monoxide poisoning occur?

A
  • the oxygen or water molecule in haemoglobin can be replaced in a ligand exchange reaction by carbon monoxide
  • this forms carboxyhaemoglobin
  • carbon monoxide forms strong dative covalent bonds with the iron ion and doesn’t readily exchange with oxygen or water ligands, meaning the haemoglobin can’t transport oxygen any more
  • this leads to carbon monoxide poisoning
41
Q

How large the enthalpy change for a ligand exchange reaction?

A
  • when a ligand exchange reaction occurs, dated bonds are broken and formed
  • the strength of the bonds being broken it is often very similar to the strength of the new bonds being made
  • so the enthalpy change for a ligand exchange reaction is usually very small
  • for example, the reaction substituting ammonia with thane-1,2-diamine in a nickel complex has a very small enthalpy change of reaction
  • this is actually a reversible reaction but the equilibriums lies so far to the right that it is thought of as being irreversible
  • the reactant is much more stable than the product
  • this isn’t accounted for by an enthalpy change, but by the entropy change
42
Q

How does entropy affect ligand exchange reactions?

A
  • 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
43
Q

Describe the stability when the hexadentate ligand EDTA4- replaces monodentate or bidentate ligands

A
  • the complex becomes more stable
44
Q

What happens when you mix an aqueous solution of transition elements ions with aqueous sodium hydroxide or aqueous ammonia?

A
  • the water ligands are deprotonated needed in an acid-base reaction and you get a coloured hydroxide precipitate
  • these reactions can be reversed by adding an acid to the hydroxide precipitate
  • the hydroxide ligands will protonate and the precipitate will dissolve as the soluble metal-aqua ions are reformed
45
Q

What are the equations for copper(II) hydroxide reactions?

A
46
Q

What are the equations for iron(II) hydroxide reactions?

A
47
Q

What are the equations for iron(III) hydroxides?

A
48
Q

What are the equations for the cobalt(II) hydroxide reactions?

A
49
Q

Why are transition metals and their compounds good catalysts?

A
  • they can change their oxidation number by gaining or losing electrons within their d-orbitals
  • this means they can transfer electrons to speed up reactions
50
Q

How are transition metal compounds good homogeneous catalysts?

A
  • homogeneous catalysts are in the same physical state as the reactants
  • usually they are an aqueous catalyst for a reaction between two aqueous solutions
  • they work by combining with the reactants to form an intermediate species which then reacts to form the products and reform the catalyst
  • the activation energy needed to form the intermediates is lower than that needed to make the products directly from the reactants
  • the catalyst is always reformed so it can carry on catalysing the reaction
51
Q

What is autocatalysis?

A
  • this is when a molecule or ion is a product of the reaction and acts as a catalyst for the reaction
  • this means that as the reaction progresses and the amount of the product increases, the reaction speeds up
52
Q

Give an example of an autocatalysis reaction

A
53
Q

How can transition metals and their compounds be good heterogeneous catalysts?

A
  • this is when the catalyst is in a different phase from the reactants
  • usually there reactants are gases or in solution and the catalyst is solid
  • they make good heterogeneous catalysts because they can use the partially filled d-orbitals to make weak bonds with the reactant molecules
54
Q

How does a heterogeneous catalyst work?

A
  • 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
  • in the reaction between nitrogen monoxide and CO2, the bonds between the reactants’ atoms are weakened making them easier to break and reform as the products
  • the [rpduct molecules leave the surface of the catalyst making way for fresh reactants to take their place
  • desorption

A Level Chemistry (19 decks)

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