Transition metals Flashcards
What are the rules for writing electronic configurations?
- Unless asked to, you don’t need to give the full configuration; start with [Ar]
- 4s subshell comes before 3d
- Copper and chromium both only have 1 electron in their 4s subshells
- When forming ions, remove the 4s electrons first
*Why does 4s come before 3d?
The outer edges of the d subshell are further from the nucleus than the s subshell and therefore there is less attraction to the nucleus
*Why do copper and chromium only have 1 electron in their 4s subshells?
A full subshell or half filled subshell is particularly stable
What is the definition of a transition metal?
Transition metals are d-block elements that form one or more stable ions with incompletely filled d-orbitals
Why is zinc not classified as a transition metal?
It can only form a 2+ ion, which has a complete d subshell and so does not meet the criteria of having an incomplete d orbital in its ion
Why is scandium not classified as a transition metal?
It can only form a 3+ ion, which has an empty d orbital and so does not meet the criteria of having an incomplete d orbital in one of its ions
Why do many transition metals have variable stable oxidation states?
- They are able to donate and receive electrons and are able to oxidise and reduce because of the partially filled subshells of d electrons that can easily use and gain electrons
- The 4s orbital and 3d orbitals have very similar energies
- There isn’t a huge jump in energy needed to remove the 3rd electron compared with the first and second
What is the pattern for successive ionisation energies for elements Sc-Zn?
Show a gradual increase in ionisation energy until all the 4s and 3d electrons have been removed. There is then a big jump as electrons start to be removed from inner core electrons
What is a complex ion?
A central metal ion surrounded by ligands
What is a ligand?
An atom or molecule that can donate a lone electron pair
What is coordinate bonding?
It is involved in complex formation and is when the shared pair of electrons in the covalent bond come from only one of the bonding atoms
What is coordination number?
The number of coordinate bonds formed to a central metal ion
What are the 3 types of ligands?
Monodentate, bidentate and multidentate
What is a monodentate ligand and what are some examples?
One that can form one coordinate bond per ligand
e.g. H2O, NH3, Cl^-
What is a bidentate ligand and what are some examples?
One that forms 2 coordinate bonds per ligand as it has 2 atoms with lone pairs
e.g. NH2CH2CH2NH2, C2O4^2-
What is a multidentate ligand and what are some examples?
One that forms 3 or more coordinate bonds per ligand
e.g. edta which forms 6 coordinate bonds per ligand
What changes can give rise to coloured transition metal ions?
- Oxidation state
- Coordination number
- Ligand
Why do transition metal ions give rise to coloured compounds?
Complex ions cause light to be absorbed because ligands split their d orbitals into 2 different energy levels
An electron can then be promoted from the lower energy level by absorbing a particular wavelength of light. The remaining wavelengths are then transmitted and that results in the colour of the complex. The wavelength absorbed depends on the size of the energy gap.
Complex ions can also have different shaps (e.g. octahedral and tetrahedral) and these split the d orbital differently, which can lead to different colours being seen
Why do differently-shaped complex ions affect the colour of a transition metal compound?
The ligands repel different d-orbitals which means there is a different energy requirement from the different amounts of repulsion, meaning different wavelengths are absorbed and therefore different colours are seen
How can the colour be changed?
Changing a ligand or the coordination number will alter the energy split between the d orbitals, changing ΔE, hence changing the frequency of light absorbed
Why are scandium ions uncoloured?
The Sc^3+ ion has no d electrons left to move around, so there cannot be an energy transfer equal to that of visible light as no energy will be absorbed to promote electrons
Why are Zn^2+ and Cu^+ ions uncoloured?
Both have 3d10, so there is no space for electrons to transfer, so no energy transfer equal to that of visible light is possible
What feature allows a substance to be a ligand?
A lone pair of electrons
Why do chloride ions form tetrahedral complexes when water and ammonia form octahedral complexes?
Chloride ions are bigger and negatively charged, so not as many can fit around the central ion and they repel each other to a greater degree
Why do most metal ions not form square planar complexes?
It is usually a less favourable shape for complexes as repulsion is not minimised
Why is cis-platin supplied as a single isomer for cancer treatment?
It works as 2 chloride ions are displaced, as it undergoes ligand exchange, and the molecule joins on to the same strand of DNA, stopping the replication of cancer cells. Both chlorides have to be on the same side for this to happen, so cis-platin is the only isomer for which this happens, and trans-platin is harmful
Why do chloride ions only work as monodentate ligands, despite having 4 lone pairs?
Lone pairs have to be far enough apart for dative covalent bonds to form from the metal ion
Why is CO toxic to humans?
Oxygen binds to the haem which is an iron (II) complex, but CO can replace oxygen as a ligand, and this coordinate bond is stronger so it binds irreversibly and prevents the oxygen bonding to it, reducing the blood’s capacity to carry oxygen
Zn will reduce V from 5^+ to 2^+ in several stages. What are the colour changes and the standard electrode potentials?
VO2+ (+5) -> VO^2+ (+4) Yellow -> blue (+1.76V) VO^2+ (+4) -> V^3+ Blue -> green (+1.1V) V^3+ -> V^2+ Green to lilac (+0.50V) V^2+ -> V Electrode potential is -0.37V
What colour is vanadium when its oxidation number is +5?
A yellow solution
What colour is vanadium when its oxidation number is +4?
A blue solution
What colour is vanadium when its oxidation number is +3?
A green solution
What colour is vanadium when its oxidation number is +2?
A violet solution
What colour is chromium in acidic, aqueous solution when its oxidation number is +2?
Blue
What colour is chromium in acidic, aqueous solution when its oxidation number is +3?
Green
What colour is chromium in acidic, aqueous solution when its oxidation number is +6?
Orange
What colour is chromium in alkali solution when its oxidation number is +3?
Green
What colour is chromium in alkali solution when its oxidation number is +6?
Yellow
How can yellow chromate (VI) ions be converted to orange dichromate (VI) ions?
Adding acid
2CrO4^2- + 2H^+ -> Cr2O7^2- +H2O
What is the reaction of [Fe(H2O)6]^3+ and NaOH?
[Fe(H2O)6]^3+ + 3OH^- -> Fe(OH)3(H2O)3 + 3H2O
What is the reaction of [Fe(H2O)6]^3+ and ammonia?
[Fe(H2O)6]^3+ + 3NH3 -> Fe(OH)3(H2O)3 + 3NH4^+
What colour is Zn^2+ in aqueous solution?
Colourless
What colour is Cr^3+ in aqueous solution?
Green
What colour is Co^2+ in aqueous solution?
Pink
What colour is Fe^2+ in aqueous solution?
Pale blue-green
What colour is Fe^3+ in aqueous solution?
Yellow
What colour is Cu^2+ in aqueous solution?
Blue
What colour is the precipitate formed when Zn^2+ reacts with a small amount of NaOH?
White precipitate
What colour is the precipitate formed when Cr^3+ reacts with a small amount of NaOH?
Green precipitate
What colour is the precipitate formed when Co^2+ reacts with a small amount of NaOH?
Blue precipitate that turns pink in air
What colour is the precipitate formed when Fe^2+ reacts with a small amount of NaOH?
Pale green precipitate that turns brown in air
What colour is the precipitate formed when Fe^3+ reacts with a small amount of NaOH?
Red-brown precipitate
What colour is the precipitate formed when Cu^2+ reacts with a small amount of NaOH?
Blue precipitate
What happens when excess NaOH is added to the precipitate formed when a small amount of NaOH was added to Zn^2+?
Precipitate dissolves to form a colourless solution
What happens when excess NaOH is added to the precipitate formed when a small amount of NaOH was added to Cr^3+?
Precipitate dissolves to form a green solution
What happens when excess NaOH is added to the precipitate formed when a small amount of NaOH was added to Co^2+?
Precipitate is insoluble, so the blue precipitate that turns pink in air is unchanged
What happens when excess NaOH is added to the precipitate formed when a small amount of NaOH was added to Fe^2+?
Precipitate is insoluble, so the pale green precipitate that turns brown in air is unchanged
What happens when excess NaOH is added to the precipitate formed when a small amount of NaOH was added to Fe^3+?
Precipitate is insoluble, so the red-brown precipitate is unchanged
What happens when excess NaOH is added to the precipitate formed when a small amount of NaOH was added to Cu^2+?
The precipitate is insoluble, so the blue precipitate remains unchanged
What colour is the precipitate formed when Zn^2+ reacts with a small amount of NH3?
White precipitate
What colour is the precipitate formed when Cr^3+ reacts with a small amount of NH3?
Green precipitate
What colour is the precipitate formed when Co^2+ reacts with a small amount of NH3?
Blue precipitate that turns pink in air
What colour is the precipitate formed when Fe^2+ reacts with a small amount of NH3?
Pale green precipitate that turns brown in air
What colour is the precipitate formed when Fe^3+ reacts with a small amount of NH3?
Reddish-brown precipitate
What colour is the precipitate formed when Cu^2+ reacts with a small amount of NH3?
Blue precipitate
What happens when excess NH3 is added to the precipitate formed when a small amount of NH3 was added to Zn^2+?
Precipitate dissolves to form a colourless solution
What happens when excess NH3 is added to the precipitate formed when a small amount of NH3 was added to Cr^3+?
Precipitate dissolves to form a green solution
What happens when excess NH3 is added to the precipitate formed when a small amount of NH3 was added to Co^2+?
Precipitate dissolves to form a brown solution that turns yellow
What happens when excess NH3 is added to the precipitate formed when a small amount of NH3 was added to Fe^2+?
The precipitate is insoluble, so the pale green precipitate that turns brown in air remains unchanged
What happens when excess NH3 is added to the precipitate formed when a small amount of NH3 was added to Fe^3+?
The precipitate is insoluble, so the reddish-brown precipitate remains unchanged
What happens when excess NH3 is added to the precipitate formed when a small amount of NH3 was added to Cu^2+?
Precipitate dissolves to form a deep blue solution
What is the general formula for the acid-base reaction (amphoteric behaviour) with sodium hydroxide?
[M(H2O)6]^n+ (aq) + nOH^- (aq) ⇌ M(H2O)6-n(OH) (s) + nH2O (l)
What is the general formula for the acid-base reaction (amphoteric behaviour) with ammonia?
[M(H2O)6]^n+ (aq) + nNH3 (aq) ⇌ M(H2O)6-n(OH) (s) + nNH4^+ (l)
How can the acid-base reactions with ammonia or sodium hydroxide be reversed?
By adding acid (H^+ ions)
Why is [Fe(H2O)6]^3+ more acidic than [Fe(H2O)6]^2+?
The lone pair on the oxygen in H2O ligands is pulled more strongly to the more highly charged ion, meaning the O-H bond is weaker and easier to remove. This means H^+ ions are lost more easily, so it’s more acidic as there are more H^+ ions in solution
What is the reaction of chromium with hydroxide ions, and what type of reaction is it?
Cr(OH)3(H2O)3 (s) + 3OH^- (aq) -> [Cr(OH)6]^3- (aq) + 3H2O (l)
Amphoteric behaviour
What is the reaction of chromium with ammonia and what type of reaction is it?
Cr(OH)3(H2O)3 (s) + 6NH3 (aq) -> [Cr(NH3)6]^3+ + 3H2O (l) + 3OH^- (aq)
Ligand exchange
What is the reaction of cobalt with ammonia and what type of reaction is it?
Co(OH)2(H2O)4 (s) + 6NH3 (aq) -> [Co(NH3)6]^2+ (aq) + 4H2O (l) + 2OH^- (aq)
Ligand exchange
What is the reaction of copper with ammonia and what type of reaction is it?
Cu(OH)2(H2O)4 (s) + 4NH3 (aq) -> [Cu(NH3)4(H2O)2]^2+ (aq) + 2H2O (l) + 2OH^- (aq)
Why does the coordination number of the central ion sometimes change in ligand exchange?
When the ligand changes, it can change to larger or more highly charged ligands, such as when H2O ligands are exchanged with Cl^- ligands, as the chloride ligands are bigger so less fit around the central ion
How many dative bonds does EDTA form, and where are they?
- They form on the 2 nitrogen atoms, and the lone pair on the oxygen in the -OH
Why is edta very good at replacing other ligands?
It can form 6 dative bond from 1 individual molecule, so there are always more moles on the RHS of the equation, so entropy of the system increases
What is the process of a heterogeneous catalyst speeding up rate of reaction?
- Adsorption: the reactant molecules stick to the surface of the catalyst
- Bond weakening - The bonds in the reactants are weakened by the catalyst so the reaction can happen
- Desorption - The product molecules leave the surface of the catalyst, making way for more reactants
What are the key examples of where a heterogeneous catalyst is used?
- The contact process to manufacture sulfuric acid (uses a vanadium oxide catalyst)
V2O5 + SO2 -> V2O4 + SO3 (regenerated by adding 1/2 O2 to V2O4) - Catalytic converters. They contain a platinum, rhodium or palladium surface for the absorption of gases, so 2CO + 2NO -> 2CO2 + N2 can take place
What are the key examples of where a homogeneous catalyst is used?
- Fe^2+ to catalyse the reaction between I^- and S2O8^2-
S2O8^2- + 2I^- -> 2S2O4^2- + I2
(Iron catalyses in a 2 step process: S2O8^2- + 2Fe^2+ -> 2SO4^2- + 2Fe^3+ and 2Fe^3+ + 2O^- -> 2Fe^2+ + I2) - Mn^2+ to catalyse the reaction between MnO4^- and C2O4^2-
2MnO4^- + 5C2O4^2- + 16H^+ -> 2Mn^2+ + 10CO2 + 8H2O
What is autocatalysis?
When a reaction is catalysed by products of the reaction
What is the d-block?
The middle of the periodic table, includes elements from columns 3 to 12
What is oxidation number?
The hypothetical number that indicates how ‘oxidised’ a given atom is - how many electrons it has lost
What is a ligand?
Ion or molecule attached to a metal atom by coordinate bonding
What is coordinate bonding?
A covalent bond in which both shared electrons come from the same atom - also known as a dative bond
What is a complex ion?
A central metal ion surrounded by ligands
What is a shell?
The principle levels in electron configuration
What is a sub-shell?
What shells are divided into
What is an orbital?
What the sub-shell is divided into
What is coordination number?
The number of atoms or ions immediately surrounding a central atom in a complex - determined by counting the other atoms to which it is bonded
What is ligand exchange?
A ligand exchange is a type of reaction in which a ligand in a compound is replaced by another
What is haemoglobin?
The iron-containing oxygen-transport protein in the red blood cells of all vertebrates. Oxygen forms a dative bond to an Fe^2+ ion in the centre of the molecule
What is amphoteric behaviour?
An amphoteric compound is a molecule or ion that can react as an acid as well as a base. Many metals form amphoteric oxides or hydroxides
What is a heterogeneous catalyst?
The form of catalysis where the phase of the catalyst differs from that of the reactants
What is a homogeneous catalyst?
Catalytic reactions where the catalyst is in the same phase as the reactants - usually catalysis in solution by a soluble catalyst
What is a catalytic converter?
Something that converts toxic gases and pollutants in exhaust gases to less harmful products by catalysing a redox reaction
