Topic 15 Transition Metals Flashcards
What is the definition of transition metals?
d-block elements that form one or more stable ions with incompletely filled d-orbitals
Why aren’t Scandium and Zinc non transition metals?
Zn can only have 2+ charge (lost from 4s orbital, no e-s can be lost from d block, most stable full)
Sc only does 3+ charge (can only form an empty d-block, symmetrical again)
When are d block elements most stable?
Half full (5 electrons)
Or full (10 electrons)
Why can transition metals change oxidation numbers so much?
3d energy level is very close to 4s, transition metals usually lose 4s and capable of losing any unpair 3d e-s as well
What are properties of transition metals?
- form complex ions
- colourful compounds
- catalytic properties (both ions and compounds)
- variable oxidation states
- hard / dense / high mp bp
What does hydrated mean for ions?
Ions surrounded by H2O molecules
How do transition metals interact with water?
covalent bonds
(dative sometimes)
What are properties of transition metal complex ions?
- small
- highly charged = high charge density
What is a ligand?
Ion or molecules w lone pair
forms dative covalent bond with a (transitional) metal ion (except aluminium)
Forming complex ions
How to write a complex ion?
[M(H2O)6] ^3+ (aq)
What do molecules with lone pairs have to be?
nucleophile
What is a monodentate ligand?
Has 1 lone pair
What is a bidentate ligand?
Has 2 lone pairs that are the same distance from each other so they can form 2 co-ordinate bonds
eg H2N-CH2CH2-NH2
What is a coordination number?
number of dative covalent bonds
(NOT number of ligands)
Chromium has oxidation numbers +2, +3, +6.
Why can chromium show variable oxidation numbers? (1 mark)
- 3d and 4s orbitals have similar energy levels to remove an e-
- ionisation energies are similar in value
How to work out transition metal compounds that are unlikely to exist?
Eg electronic configuration 3d5 4s1
There are only 6 electrons to lose
So ox num 7+ and above are unlikely to exist
Are electrons from transition metals lost from 3d or 4s orbital first?
4s ALWAYS
Explain why brass is malleable but sodium chloride is not. (3 marks)
- brass is alloyed with similar sized atoms
- so layers can slide
In NaCl - any shift in ionic lattice will repel
- won’t occur in brass as delocalised e-s prevents repulsion
Why is vanadium classified as a transition metal?
It forms stable ions with incompletely filled d orbitals
Why does Cu have extra high 2nd ionisation energy / big jump from 1st?
2nd e- is from 3d not 4s
What complexes form square planar complex ions?
Pt 2+ and Ni 2+
What angle is octahedral complexes?
90 degrees
Why is Cl- as ligands, forming only 4 single co-ordinate bond with Cu 2+ not 6?
What shape does it make?
Cl- ions are large
So can only fit 4, not 6
Makes tetrahedral, 109.5
What is the most common bidentate ligand and give molecular formula.
1,2-diaminoethane
NH2CH2CH2NH2
What is the most common multidentate ligand we need to know?
EDTA 4-
In Pt(NH3)2Cl2 - it is square planar.
When is it a cis and trans isomer?
cis when Cl are next to each other, NH3 next to each other.
trans when opposite each other
With reference to electrons, explain why aqueous copper(II) ions are blue. (3 marks)
- e-s in 3d shell split into 2 energy levels
- e-s from lower energy level absorbs energy (red) , can be promoted to higher energy level
- remaining light (blue) is reflected/transmitted
State the origin colour of transition-metal complexes. (2 marks)
- e-s absorb energy from visible spectrum in 3d shell
- wavelengths not absorbed produce the colour (transmitted)
Give 3 changes to a transition-metal complex which result in a change of colour. (3 marks)
- oxidation state of metal
- charge density / type of metal
- charge density / type of ligands
How to use colorimeter to determine the conc of iron(III) ions in a solution of unknown conc?
- calibration curve
- use 1 moldm-3 solution of Fe3+, measure, dilute and repeat (known concs)
- measure absorbance of sample after reference cuvette
- measure absorbance of unknown conc and see which matches on graph
- can add ligand to intensify colour
In terms of electrons, explain why an aqueous solution of cobalt(II) sulfate has a red colour. (3 marks)
- Co2+ has 2 energy levels in d orbital
- energy difference corresponds to energy of blue photons - electron promoted to higher orbital when absorbed
- red light reflected
Why is colorimetry preferred over titration?
get results quicker
Explain why the aqueous solution of complex ion has an acidic pH by considering the interaction between metal ions and ligands. / why is the higher the charge in metal aqua ion, the more acidic the solution?
- to lose H+, OH bond must break
- will break when e-s in bond pulled towards O
- higher charge = stronger pull = release H+ more readily = more acidic
Which metal aqua ions + ligand combo would result in redissolving?
If TM begins with ‘C’ (like chromium, copper, cobalt)
Will re dissolve in excess
How many ammonia swap in for metal aqua ions for which metals?
Chromium & cobalt - (NH3)6 swaps in
Copper - only (NH3)4, makes [Cu(NH3)4(H2O)2]2+
Why do sometimes predictions with oxidation/reducing agents don’t work?
Reactions are too slow
(As activation energy is too high)
What are the colour changes from V5+ to V4+ to V3+ to V2+ ?
Yellow
Blue (green first, as it’s a mixture of yellow and blue)
Green
Violet
How to work out which reducing/oxidising agents to use for transition metals?
Use E cell values
For reducing agents, use E cell values higher up (more negative) than itself
Vice versa
How to reduce Cr6+ to Cr2+?
Zinc or H+
What colours are Cr6+, Cr3+ and Cr2+ ?
Orange
Green
Blue
How to convert Cr^3+ to CrO4^2-?
+ H2O2 or OH-
How to convert Cr^2+ to Cr^3+ ?
Oxygen / air
Why is Cr2O7 bad?
Very carcinogenic
How to convert Cr2O7^2- -> CrO4^2- ?
And
How to convert CrO4^2- ->Cr2O7^2- ?
Cr2O7^2- + NaOH → CrO4^2-
CrO4^2- + H2SO4 → Cr2O7^2-
(Think 2 to 1 Chromates, NaOH all singular 1 ; 1 to 2, has H2SO4
What 4 factors affect the transition metal’s colours?
Type of metal
Oxidation state of metal
Type of ligands
Coordination number
Why do changes in factors eg types of metals and ligands affect the colour of the Complex ion?
Size of energy gap between d orbitals would be changed
So frequency of light absorbed would be changed
Hence colour reflected changes
What is a ligand substitution?
Reaction when a ligand replaces another ligand
(No change in ox num at all)
What colours are [Cu(H2O)6]2+ (aq) and [CuCl4]2- (aq) ?
Blue
And
Yellow
What colours are [Co(H2O)6]2+ (aq) and [CoCl4]2- (aq) ?
Pale pink
To
Blue
How do catalysts work?
- reactants adsorb to active site of catalyst
- bonds weaken
- reaction takes place
- desorption
- by providing alternative route with a lower activation energy
What is a heterogeneous catalyst? (1 mark)
catalyst that is in a different phase to reactants.
Why must catalysts not be too strong or too weak?
too strong: risk of poisoning as may bind irreversibly
too weak: not many molecules adsorbed to catalyst, little effect to reaction AND won’t weaken bonds enough
What is the equation and catalyst for the Haber process?
3 H2 + N2 ⇌ 2NH3
Fe catalyst
What are the 2 steps of the contact process and what catalyst is used?
+ give half equation
2 SO2 + O2 ⇌ 2 SO3
V2O5 catalyst
half eq
SO2 + V2O5 → SO3 + V2O4(intermediate)
V2O4 + 1/2 O2 → V2O5
How to maximise efficiency of a catalyst?
Increase the surface area for adsorption
by using a mesh or honeycomb structure
What is catalyst poisoning and how does it happen?
Adsorption is too strong, binding is irreversible
active site is blocked
- catalyst is ruined, v costly
What is a homogeneous catalyst?
catalyst in the same phase as reactants
Why are transition metals great catalysts?
- have variable oxidation states
- are position ions so will bind with anions and have low activation energy
Can catalysts alter the value of equilibrium constant?
NO
What reactions use heterogeneous catalysts?
Haber
Contact
Car exhaust
What is the reaction of the car exhaust system and what catalyst is used?
CO + NO → CO2 + 1/2 N2
Pt catalyst
What is the reducing agent in
CO + NO → CO2 + 1/2 N2 ?
CO
State the general property of transition metals that allows the vanadium in vanadium(V) oxide to act as a catalyst in the Contact process.
variable oxidation states
Suggest why the vanadium(V) oxide is used in small pellets form rather than as large lumps. (1 mark)
increases surface area
so more efficient
State why the reactant should be purified before they some into contact with V2O5 in the contact process. (1 mark)
To remove potential poisons
What is autocatalysis?
When one of the products in a reaction acts as a catalyst.
What is the reaction between iodide ions and peroxodisulfate ions? What catalyst is used?
give half eq
2I- + S2O8 2- → I2 + 2 SO4 2-
Fe2+/Fe3+ as catalyst
2 Fe^2+ + S2O8^2- → 2 Fe^3+ + 2 SO4^2-
2 Fe^3+ + 2I- → 2 Fe^2+ + I2
What is the equation of reaction between manganate(VII) ions and ethanedioate ions? What catalyst?
2 MnO4^- + 14H+ + 5C2O4^2- → 2 Mn^2+ + 8 H2O + 10 CO2
Mn2+ the product
How to prove Mn2+ is the catalyst in the manganate(VII) ions and ethanedioate ions reaction?
Add Mn2+ from the start
Suggest why ions from s block elements do not usually act as catalysts. (1 mark)
No variable oxidation states
Give one reason why, in the absence of a catalyst, the activation energy for the reaction between S2O8^2- ions and I- ions is high. (1 mark)
they are both negative ions
so will repel
How do catalysts catalyse reactions between two negative ions?
Transition metal catalysts are positively charged - so oppositely charged to anions
lowers activation energy, so less energy needed to overcome
[Co(H2O)6]^2+ + ethane-1,2-diamine →
[Co(NH2CH2CH2NH2)3]2+ + 6H2O
(as it is bidentate!!)
Name the ligands
H2O
NH3
Cl-
OH-
CN-
aqua
ammine
chloro
hydroxo
cyano
How to name transition metals in complex ions?
if complex is +ve charged - regular name
if -ve charged - latin name + ‘ate’
What is the formula of the manganese species formed from MnO4- and excess dilute sulfuric acid
From purple to colourless
Mn2+ !
Suggest why zinc does not produce hydrogen with nitric acid.
Using E cell values
It is not feasible (and expand using data equations)
