Transition metals Flashcards
What is the formal definition of a transition element
A transition element forms at least one stable ion with a part-full d shell of electrons.
List the four main chemical properties of all transition metals
- Variable oxidation states: Transition metals have more than one oxidation state in their compounds eg. Cu(I) and Cu(II)
- Colour: The majority of transition metal ions are coloured
- Catalysis: Catalysts affect the rate of reaction without being used up or chemically changed themselves. Many transition metals and their compounds show catalytic activity.
- Complex formation: Transition metals form complex ions. A complex ion is formed when a transition metal ion is surrounded by ions or other molecules, called ligands, which are bonded to it by coordinate bonds.
Why are some key example of transition metal catalysts and what they catalyse
- Iron is the catalyst in the haber process
- Vanadium (V) oxide is the catalyst for the contact process.
- Manganese (IV) oxide is the catalyst in the decomposition of hydrogen peroxide.
What is a ligand
An ion or molecule with a lone pair of electrons that forms a co-ordinate bond with a transition metal
What are some key examples of ligands
H2O
NH3
Cl-
CN-
What is the coordination number
The number of coordinate bonds to ligands that surround the d-block metal ion
What shape are ions with a co-ordination number of six normally
Octahedral or square planar
What shape are ions with a co-ordination number of four normally
Tetrahedral
What are aqua ions
- The species that forms when the salt of a transition metal is dissolved in water.
- The positively charged metal ion becomes surrounded by water molecules acting as ligands.
- Normally there are six water molecules in an octahedral arrangement
What are multidentate ligands
Molecules or ions which have more than one atom with a lone pair of electrons which can bond to a transition metal ion
List the three bidentate ligands you need to know
1) Ethane-1,2-diamine
2) Ethanedioate (oxalate) ion
3) Benzene-1,2-diol
How does a ethane-1,2-diamine ligand bind to a transition metal
- Each nitrogen atom has a lone pair which can form a coordinate bond to the metal ion.
- The name of this ligand is often abbreviated to en
- It is a neutral ligand
What is the formula of the ethanedioate (oxalate) ion
C2O4 2-
What is the multidentate ligand you may be asked about in the exam
EDTA 4-
How does EDTA 4- bind to transition metals
It acts as a hexadentate ligand using lone pairs on four oxygen and both nitrogen atoms
What are chelates
Complex ions with polydentate ligands
What can chelates be used for
To remove d-block metal ions from solution
What is the chelate effect and explain it
- Chelate effect= chelate complexes with multidentate ligands are favoured over monodentate ligands.
- This is because when chelates replace water/other monodentate ligands, the number of particles on the product side of the equation increases.
- This means that there is an increase in entropy which drives the reaction to the right.
- Eg. If EDTA 4- replaces six water ligands around a transition metal, there are 6 product molecules as opposed to two reactant molecules.
What type of isomers do transition metal complexes form
Geometrical isomers (E/Z isomers) and optical isomers (non-superimposable mirror images of each other).
In what shape of transition metal complexes does geometrical isomerism occur
In octahedral and square planar complexes
When do transition metal complexes form optical isomers
When there are two or more bidentate ligands in a complex
Why are transition metal complexes coloured
- Transition metal compounds are coloured because they have part filled d-orbitals
- It is therefore 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 electrons move from one d-orbital to another of a higher energy level (called an excited state), they often absorb energy in the visible region of the spectrum equal to the difference in energy between the levels.
- This colour is therefore missing from the spectrum and you see the combination of colours that are not absorbed.
What is the equation which links the frequency of light absorbed by the transition metal to the d-orbital energy difference
- /\E = hV where:
- E is the energy
- V is the frequency
- h is Plancks constant
What colour are Fe 2+ complexes
Green
What colour are Fe 3+ complexes
Pale brown
What colour are Cr 2+ complexes
Blue
What colour are Cr 3+ complexes
Red-violet
What colour are Co 2+ complexes
Brown
What colour are Co 3+ complexes
Yellow
How do you measure the concentration of solutions of coloured transition metal compounds
- A colorimeter is used to detect the amount of light that passes through the solution.
- The more concentrated a solution, the less light passes through the solution.
What is the equation of a managante ion
MnO4-
When does potassium manganate (VII) act as an oxidising agent
In acidic solution
Describe what happens when titrating potassium manganate and a compound containing Fe 2+ ions
- Using a burette, gradually add potassium manganate (VII) solution to a solution containing Fe 2+ ions, acidified with dilute sulfuric acid.
- The purple colour disappears as the MnO 4- ions are converted to pale pink Mn 2+ ions to leave a virtually colourless solution.
- Once just enough MnO 4- ions have been added to react with all the Fe 2+ ions, one more drop of MnO 4- ions will turn the solution purple.
- This is the end point of the titration.
In what ratio do Fe 2+ ions react with manganate ions in a redox titration
5:1
What colour is the Fe 2+ ion in solution
Pale green
What colour are MnO 4- ions in solution
Intense purple
What colour are Fe 3+ ions in solution
Pale violet
What colour are Mn 2+ ions in solution
Pale pink
Why can’t you use hydrochloric acid (as an alternative to sulfuric acid) to supply the H+ ions in the reaction between potassium manganate and Fe 2+ (aq)
- Using electrode potential and EMF values, you can see that the reaction where MnO 4- ions oxidise the Cl- ions in hydrochloric acid is feasible and so occurs.
- This would affect the titration because the manganate ions must only be oxidising the Fe 2+ ions.
- This is not an issue with sulfuric acid because the manganate ions do not oxidise sulfate ions.
What is the typical species which represents how a transition metal exists in an acidic solution
M(H2O)6 2+
What is the typical species which represents how a transition metal exists in a neutral solution
M(H2O)4(OH)2
What is the typical species which represents how a transition metal exists in an alkaline solution
M(H2O)2(OH)4 2-
What are the two types of catalyst
Homogeneous catalyst and heterogeneous catalyst
Describe what a heterogeneous catalyst is and how it works
- A heterogeneous catalyst is a catalyst which is in a different phase (solid/liquid/gas) than the reactants.
- They are usually present as solids, whilst the reactants may be gases or liquids.
- Their catalytic action occurs on the solid surface.
- The reactants pass over the catalyst surface, which remains in place so that the catalyst is not lost and does not need to be separated from the products.
What are two ways in which you can make heterogeneous catalysts more effecient
- Increase their surface area
- Spread the catalyst onto sun inert support medium , or even impreganate it into one.
- This increases the surface-to-mass ratio so that a little goes a long way; the more expensive catalysts are often used in this way.
What is catalytic poisoning (heterogeneous catalysts) and what is another reason why a hetergeneous catalyst may not last forever
- When the surface of the catalyst becomes covers with unwanted impurities that block its action.
- Reason 2: the finely divided catalyst may gradually be lost from the support medium.
Give the equation for the Haber process
N2 (g) + 3H2 (g) <=> 2NH3 (g)
Describe the catalyst used in the Haber process
- The catalyst used is iron which is present as pea-sized lumps to increase its surface area.
- The iron catalyst lasts about five years before it becomes poisoned by impurities in the gas stream such as sulfur compounds and has to be replaced.
What is the equation for the contact process
2SO2 + O2 <=> 2SO3
Describe the catalyst used in the contact process
- The contact process in catalysed by vanadium (V) oxide V2O5 which catalyses in in two steps:
- First the vanadium (V) oxide oxidises sulphur dioxide to sulfur trioxide and is itself reduced to vanadium (IV) oxide V2O4.
- The vanadium (IV) oxide is then oxidised back to vanadium (V) oxide by oxygen.
- The Vanadium (V) oxide is regenerated unchanged.
- Each of the two steps has a lower activation energy than the uncatalysed single step and therefore the reaction goes faster.
Give the two equations that show how vanadium (V) oxide V2O5 catalyses the contact process
SO2 + V2O5 —> SO3 +V2O4
2V2O4 + O2 —> 2V2O5
What is a homogeneous catalyst
A catalyst that is in the same phase as the reactant
What is an example of a homogeneous transition metal catalyst
Peroxodisulfate ions oxidise iodide ions to iodine and this reaction is catalysed by Fe 2+ ions
Describe how Fe 2+ ions ctalayse the oxidation of iodide ions to iodine by peroxodisulfate ions and why it is useful
- The catalysed reaction takes place in two steps
- First the peroxodisulfate ions oxidise iron (II) to iron (III).
- The Fe 3+ then oxidises the I- to I2, regenerating the Fe 2+ ions so that none are used up in the reaction.
- So iron first gives an electron to the peroxodisulfate and later takes on back from the iodide ions.
- This is useful because the uncatalysed reaction takes place between two ions of the same charge which repel, therefore giving as high activation energy.
- Both steps of the catalysed reaction involve reactions between oppositely charged ions which explains the increase in rate of reaction.
Give the two equations which show how Fe 2+ ions catalyse the oxidation of iodide ions to iodine by peroxodisulfate ions
- S2O8 2- (aq) + 2 Fe 2+ (aq) —> 2SO4 2- (aq) + 2Fe 3+ (aq)
- 2Fe 3+ (aq) + 2I- (aq) —> 2Fe 2+ (aq)+ I2 (aq)
What is autocatalysis
- Autocatalysis is when one of the products of the reaction is a catalyst for the reaction.
- These reaction start slowly at an uncatalysed rate- as the concentration of the product that is also the catalyst builds up, the reaction speeds up to the catalysed rate.
- From then on it behaves like a normal reaction.
What is an example of an auto catalysed reaction
The reaction between potassium manganate and ethanedioic acid
What happens when the salt of a transition metal is dissolved in water
- The water molecules cluster around the metal ions so it exists as a complex ion.
- Six water molecules act as ligands bonding to the metal ion in an octahedral arrangement.
- These ions are called aqua ions.
Why are solutions of Fe 2+ not noticeably acidic whereas a solution of Fe 3+ is a stronger acid than ethanoic acid
- The Fe 3+ ion is both smaller and more highly charged than Fe 2+ (it has a higher charge density) making it more strongly polarising.
- So in the [Fe(H2O)6]3+ (aq) ion, the iron strongly attracts electrons from the oxygen atoms of the water ligands, so weakening the O—H bonds in the water molecules.
- This complex ion will then readily release an H+ ion making the solution acidic.
- Fe 2+ is less polarising so fewer O—H bonds break in solution.
What is the general rule relating to the acidity of the aqua ions of transition metals
The aqua ions of M3+ are significantly more acidic than those of M2+
What was Lavosiers (1777) theory of acidity
He proposed that all acids contain oxygen
What was Davy’s theory of acidity (1816)
All acids contain hydrogen
What was Liebig’s theory of acidity
Leibig (1838) defined acids as substances containing hydrogen which could be replaced by a metal.
Why was Liebigs theory an improvement on Davy’s
- It was an improvement on Davy’s theory as it explains why not all hydrogen-containing compounds are acidic eg.ammonia.
- There must be something special about that hydrogen that makes it replaceable by a metal.
What was Arrhenius’ (1887) theory of acidity
Arrhenius thought of acids as producing hydrogen ions, H+
What is the theory of acidity we use today
Brønsted-Lowry
What is the Lewis theory of acidity that is used today (as well as the Brønsted-Lowry)
Lewis theory regards acids as electron pair acceptors and bases as electron pair donors in the formation of coordinate covalent bonds.
What is the general rule for the carbonates of transition metal ions
In general, carbonates of transition metal ions in oxidation state +2 exist, whilst those of ions in the 3+ state do not.
What colour is the solution containing [Fe(H2O)6]3+ (aq)
Pale brown
What colour is the solution containing [Fe(H2O)6]2+
Pale green
What test allows you do distinguish between dilute solutions containing [Fe(H2O)6]3+ and [Fe(H2O)6]2+ where it is hard to tell the difference
- Add dilute alkali, which precipitates the hydroxides whose colours are obviously different:
- Iron II hydroxide is green
- Iron III hydroxide is brown
What does it mean if something is amphoteric
It shows both acidic and basic properties
What is an example of an amphoteric compound
Ammonium hydroxide
What are the possibilities when replacing water as a ligand in ligand substitution reactions
- The water molecules may be replaced by neutral ligands such as ammonia.
- The water molecules may be replaced by negatively charged ligands, such as chloride ions.
- The water molecules may be replaced by bi- or multidentate ligands- this is called chelation.
- Replacement of water ligands may be complete or partial.
Why is there no change in co-ordination number or charge when water ligands are replaced by ammonia ligands
Both ligands are uncharged and of similiar size
What is the complication that’s occurs when M2+ aqua ions undergo ligand substitution reactions with ammonia
Because ammonia is a base as well as a ligand, and therefore contains OH- ions, a precipitate may form and redissolve.
Describe what happens when an aqua ion of cobalt 2+ undergoes ligand substitution with ammonia
- The first step is the formation of a blue hydrated cobalt (II) hydroxide when ammonia is added.
- This is produced by the loss of a proton from each of two of the six water molecules co-ordinated to the Co 2+ ion.
- Here ammonia is acting as a base.
- If you add more of the concentrated ammonia, then both OH- ions and all the four water ligands are replaced by ammonia.
- This causes the blue precipitate to dissolve to form a pale yellow solution.
Why are all four ammonia ligands and both hydroxide ligands replaced by ammonia when more concentrated ammonia is added to Co 2+ aqua ion after two molecules of ammonia have already been added
- Ammonia is a better ligand than water
- The high concentration of ammonia displaces equilibria to the right.
What is the overall equation for the replacement of a cobalt 2+ aqua ion with ammonia
Co(H2O)4(OH)2 + 6NH3 (aq) <=> [Co(NH3)6]2+ (aq) + 4H2O (l) + 2OH- (aq)
Describe what happens when a copper 2+ aqua ion undergoes ligand substitution with ammonia
- The ligand replacement is partial- only four of the water ligands are replaced.
- The ammonia first acts as a base, removing protons from two of the water molecules in [Cu(H2O)6]2+ to form [Cu(OH)2(H2O)4] (s).
- The first thing seen is the pale blue precipitate of copper hydroxide.
- When more of the concentrated ammonia is added, the precipitate dissolves to form a deep blue solution containing [Cu(NH3)4(H2O)2]2+
- The ammonia has replaces both of the OH- ligands and two of the H2O ligands.
What is the overall reaction that occurs when a copper 2+ aqua ion undergoes ligand substitution with ammonia
[Cu(H2O)6]2+ + 4NH3 <=> [Cu(NH3)4(H2O)2]2+ + 4H2O
Why is the shape of the octahedron of the [Cu(NH3)4(H2O)2]2+ ion slightly distorted
Because the Cu—O bonds are longer and therefore weaker than the Cu—N bonds.
Describe what happens when cooper 2+ ions undergo ligand substitution with chloride ions
- When aqueous copper ions react with concentrated hydrochloric acid there is a change in both charge and co-ordination number.
- Concentrated HCl provides a high concentration of Cl- ligands
- The pale blue colour of the [Cu(H2O)6]2+ ion is replaced by the yellow [CuCl4]2- ion. (Although the solution may look green as some [Cu(H2O)6]2+ will remain).
- This replacement takes place in steps.
Why is [Cu(H2O)6]2+ six co-ordinate but [CuCl4-]2- four co-ordinate
Cl- is larger than H2O and fewer ligands can physically fit around the central copper ion.
What forms when carbonate ions are added to M2+ ions
Precipitates of the metal carbonates
What forms when carbonate ions are added to M3+ ions and why
Bubbles of carbon dioxide- this is a reflection of the greater acidity of [M(H2O)6]3+ compared with [M(H2O)6]2+
What transition metal complexes have a linear shape
Ag+ complexes
What shape are all platinum complexes
Square planar
What is the bond angle in a square planar molecule
90 degrees
What is the bond angle in an octahedral molecule
90 degrees
What are the factors that affect the colour of a transition metal complex
1) Identity of metal
2) Oxidation state of metal
3) Identity of ligands
4) Co-ordination number
Why does a change in one of the factors that affects the colour of a transition metal complex cause said change
If any of the factors are changed, then the size of the energy gap between the higher and lower d orbitals changes, and so the frequency of light absorbed changes and so the colour seen changes.
What is the relationship between pH and redox reactions of transition metals
In general, it is easier to:
- Oxidise a transition metal in alkaline conditions
- Reduce a transition metal in acidic conditions.
What type of catalyst is Vanadium V oxide that catalyses the contact process
Heterogeneous
Explain why dilute sulfuric acid is a suitable acid for acidifying manganate ions for a REDOX titration
- Hydrochloric acid cannot be used as the MnO4- would also oxidise Cl- to Cl2 so affect the volume of KMnO4 required in the titration.
- Concentrated sulfuric acid or concentrated nitric acid cannot be used as they are oxidising agents themselves so affect the volume of KMnO4 required in the titration.
- Ethanoic acid cannot be used as it is a weak acid so would not provide enough H+ ions.
Titrations containing manganate ions are autocatalysed- what species acts as the catalyst
Mn 2+
In a redox titration with potassium manganate, what is the setup of the apparatus
- The purple potassium manganate is in the burette.
- The sample being analysed is in a flask with dilute sulfuric acid.
If the iron is not in the +2 oxidation state before a redox titration with manganate, what is done
- if it is the element Fe (0) then it is reacted with sulfuric acid to oxidised it to Fe 2+ for analysis.
- If it is Fe 3+, then it is reacted with zinc to reduce it to Fe 2+ ready for analysis (the remaining Zn must be removed first to stop it reducing Fe 3+ formed in the titration back to Fe 2+ ).
What is the reacting ratio between ethandioate ions and manganate ions in a redox titration
2.5 : 1
What is the bond angle for tetrahedral complexes
109.5
Describe what happens, in terms of transition metals, when Tollens reagent is used to test for an aldehyde
- Tollens reagent contains [Ag(NH3)2]+
- A silver mirror is formed in the presence of aldehydes
- Ag + in [Ag(NH3)2]+ is reduced to Ag (0) in silver mirror.
- The aldehyde is oxidised to a carboxylic acid.
Describe what happens, in terms of transition metals, when Fehlings reagent is used to test for an aldehyde
- Fehlings solution contains Cu 2+
- A brick-red precipitate of (Cu2O) is formed in the presence of Aldehydes.
- Cu 2+ is reduced to Cu + in Cu2O
- Aldehyde is oxidised to a carboxylic acid.
Describe what happens, in terms of transition metals, when acidified potassium dichromate is used to test for primary and secondary alcohols and aldehydes
- The formula for acidified potassium dichromate is H+/K2Cr2O7
- Colour change form orange (Cr2O7)2- to green Cr3+
- Cr 6+ in (Cr2O7)2- is reduced to Cr 3+
- The alcohol or aldehyde is oxidised
What is the colour change in a redox titration using potassium manganate in the burette
Colourless to purple
