Transition Metals - Topic 15 Flashcards
Characteristics of transition metals
•complex formation,
•formation of coloured ions,
•variable oxidation state
•catalytic activity.
Why is Zn not a transition metal
Zn can only form a +2 ion. In this ion the Zn2+ has a complete d orbital and so does
not meet the criteria of having an incomplete d orbital in one of its compounds.
Why is Sc not a transition metal?
Sc can only form a +3 ion. In this ion the Sc3+ has an empty d orbital and so
does not meet the criteria of having an incomplete d orbital in one of its ions.
Describe the trend in ionisation energy for elements Sc to Zn
The successive ionization energies for the elements Sc to 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 the inner core electrons
What is coordinate bonding
the shared pair of electrons in the covalent bond
come from only one of the bonding atoms.
What is a complex
a central metal ion surrounded by ligands
What is a ligand
An atom, ion or molecule which can donate a lone electron pair
monodentate
Where ligands can form one coordinate bond per ligand
Bidentate
Ligands can have two atoms with lone pairs
and can form two coordinate bonds per ligand
Multidentate
Ligands can have more than two atoms with lone pairs
and can form more than two coordinate bonds per ligand
Draw EDTA
Page 1 Chemrevise
What cause colour change in transition metals
- oxidation state,
- co-ordination number
- ligand
How does colour arise in transition metals
- Colour arises from electronic transitions from the ground state to excited states: between different d
orbitals - A portion of visible light is absorbed to promote d electrons to higher energy levels. The light that is not
absorbed is transmitted or reflected to give the substance a complementary colour.
How is the colour of a transition metal changed
Changing a ligand or its shape, changing the coordination number or changing the oxidation state of a metal ion will alter the energy split between the d- orbitals, changing delta E and hence change the frequency of light absorbed.
Why does Sc3+ not have colour
As it hasn’t got any d electrons left to move around. So there is not an energy transfer equal to that of visible light
Why doesn’t Zn2+ and Cu+ not have any colours
Zn2+ ions and Cu+ ions the d shell is full
e.g.3d10 so there is no space for electrons to transfer. So there is not an energy transfer equal to that of visible light
What is Spectrophotometry used for in transition metal complexes
It is used to determine the concentration of coloured ions by measuring their absorption of visible light
How does Spectrophotometry work
1) light at increasing frequency is passed through a sample of a coloured complex ion
2) some light is absorbed
3) the amount of light absorbed is proportional to the ion concentration
4) a ligand may be added to intensify colour of complexes with pale colour and that do not absorb light strongly
What steps are involved in using Spectrophotometry to determine ion concentration
1) add appropriate ligand to intensify colour
2) Make up solutions of known concentration
3) Measure absorption or transmission
4) Plot graph of results or calibration curve
5) Measure absorption of unknown and compare
Shapes of complex ions
- octahedral (usually with complexes with small ligands e.g NH3 and H2O)
- tetrahedral (usually with complexes with larger ligands e.g Cl-)
- square planar
- linear (usually just Ag+)
Draw structure of cisplatin
Pg 3 chem revise
What is cisplatin used for
An anticancer drug
How does cisplatin act as an anticancer drug
- the two chloride ions are displaced and the molecule joins on to the DNA of cancer cells by dative and hydrogen bonding. In doing this it stops the replication of cancerous cells.
- It prevents the replication of healthy cells by bonding on to healthy DNA which may lead to unwanted side effects like hair loss
What happens to cisplatin in the body
One chloride ligand is substituted by a water molecule
Pt(NH3 )2Cl2 + H2O -> [Pt(NH3)2Cl(H2O)]+ + Cl–
Why is CO toxic to humans
CO can form a strong coordinate bond with haemoglobin and replaces oxygen in a ligand exchange reaction. This is a stronger bond than that made with oxygen and so it prevents the oxygen attaching to the haemoglobin
Where do transition metals lose their electrons first when they form ions
In the 4s orbital before the 3d orbital
Why can transition metals form various oxidation states
- This is because the ions contain partially filled d sub-shells so d electrons that can easily lose or gain electrons.
- The energy differences between the oxidation states are small.
Trends of oxidation states
•Relative stability of +2 state with respect to +3 state increases across the period
•Compounds with high oxidation states tend to be oxidising agents e.g MnO4
•Compounds with low oxidation states are often reducing agents e.g V2+ & Fe2+
What are vanadium oxidation states and colours
VO2^+ Oxidation state +5 ( a yellow solution)
VO^2+ Oxidation state + 4 (a blue solution)
V^3+ Oxidation state + 3 (a green solution)
V^2+ Oxidation state + 2 (a violet solution)
Exaplain What will happen if you add zinc to vanadium in ACIDIC solution
It reduce the vanadium down through each successive oxidation state, and the colour would successively change from yellow to blue to green to violet, this is because zinc has a more negative electrode potential than all the vanadium half equations (E values)
What will tin metal reduce vanadium to
Tin metal will only reduce the vanadium to the +3 state as its electrode potential is less negative than the electrode potential of vanadium +2 state
What happens when acid is added to solid ammonium vandate (NH4VO3)
It dissolves and forms a yellow VO2+ solution as the vanadium remains in the +5 oxidation state
Oxidation states and colours of chromium
Cr3+ (green) and Cr2+ (blue)
How is Cr3+ (green) formed into Cr2+ (blue)
formed by reduction of Cr2O72- (orange) by the strong reducing agent zinc in (HCl) acid solution
What will Fe2+ reduce dichromate to and why
Fe2+ is a less strong reducing agent so will only reduce the dichromate to Cr3+
Why must zinc/dichromate be kept under a hydrogen atmosphere?
To reduce it to Cr²⁺, as O₂ in air will oxidize Cr²⁺ to Cr³⁺.
How can the Cr²⁺ state be stabilized?
By forming a stable complex ion with a ligand such as the ethanoate ion.
What happens when blue Cr²⁺ ions are bubbled through sodium ethanoate?
A stable red precipitate of chromium(II) ethanoate is formed.
why are transition metals with low oxidation states easier to OXIDISE in ALKALINE conditions rather than ACIDIC conditions
It is easier to remove an electron from a negatively charged ion due to electron electron repulsion
acid base equation for the conversion of chromate CrO4^2- to dichromate Cr2O7^2-
2 CrO42- + 2H+ <—-> Cr2O72- + H2O
Yellow solution —-> orange solution
chromium summary mind map page 6 chemrevise
all colours of chromium ions
- Cr2O7^2- Orange solution
- CrO4^2- Yellow solution
- [Cr(NH3)6]3+ purple solution
- [Cr(H2O)6]3+ green solution
- Cr(H2O)3(OH)3 green ppt
- [Cr(OH)6]3- green solution
- [Cr(H2O)6]2+ blue solution
whats a lewis acid and base
Lewis acid: electron pair acceptor
Lewis base: electron pair donator
Explain what is the lewis base and acid in a complex ion
the ligand is the Lewis base because it is donating a pair of electrons in the dative covalent bond and the metal ion is the Lewis acid.
What is a metal-aqua ion?
A metal ion surrounded by water ligands, e.g., [M(H₂O)₆]²⁺ or [M(H₂O)₆]³⁺.
Why are [M(H₂O)₆]³⁺ ions more acidic than [M(H₂O)₆]²⁺?
because the 3+ metal ions have higher charge density and have greater polarising power. The greater the polarising power, the more strongly it attracts the water molecule. This weakens the O-H bond so it breaks more easily releasing H+ ions.
What colour is [Fe(H₂O)₆]²⁺ in solution? (dissolved in water)
pale Green
What colour is [Co(H₂O)₆]²⁺ in solution? (dissolved in water)
Pink
What colour is [Cu(H₂O)₆]²⁺ in solution? (dissolved in water)
Blue
What colour is [Cr(H₂O)₆]³⁺ in solid hydrated salts?
Ruby
What colour is [Fe(H₂O)₆]³⁺ in solid hydrated salts? (dissolved in water)
Violet
Why do Cr(III) and Fe(III) appear green and yellow/brown in solution?
Due to hydrolysis reactions.
What happens when limited OH⁻ or NH₃ is added to metal-aqua ions?
Hydroxide precipitates ([M(H2O)4(OH)2]) form by deprotonation.
What is the colour of Cu(OH)₂(H₂O)₄ precipitate?
Blue
What is the colour of Co(OH)₂(H₂O)₄ precipitate?
Blue
What is the colour of Fe(OH)₂(H₂O)₄ precipitate?
Green
What is the colour of Cr(OH)₃(H₂O)₃ precipitate?
Green
What is the colour of Fe(OH)₃(H₂O)₃ precipitate?
Brown
Write the hydrolysis reactions of Cr³⁺ with OH⁻.
1) [Cr(H₂O)₆]³⁺ + OH⁻ → [Cr(H₂O)₅(OH)]²⁺ + H₂O
2) [Cr(H₂O)₅(OH)]²⁺ + OH⁻ → [Cr(H₂O)₄(OH)₂]⁺ + H₂O
What happens when excess NaOH is added to Cr(OH)₃(H₂O)₃?
It dissolves, forming [Cr(OH)₆]³⁻ (a green solution).
Cr(H₂O)₃(OH)₃ + 3OH⁻ → [Cr(OH)₆]³⁻ + 3H₂O
Why is Cr(H2O)3(OH)3 amphoteric?
because it can react and dissolve in both acids and bases.
What happens when excess NH₃ is added to Cu, Co and Cr
Cr becomes [Cr(NH3)6]3+ purple solution
Co becomes [Co(NH3)6]2+ pale yellow solution (oxidises to +3 in air).
Cu becomes [Cu(NH3)4(H2O)2]2+ deep blue solution
Give the equation for Cr(OH)₃(H₂O)₃ reacting with excess NH₃.
Cr(OH)₃(H₂O)₃ + 6NH₃ → [Cr(NH₃)₆]³⁺ + 3H₂O + 3OH⁻
Give the equation for Cu(OH)₂(H₂O)₄ reacting with excess NH₃.
Cu(OH)₂(H₂O)₄ + 4NH₃ → [Cu(NH₃)₄(H₂O)₂]²⁺ + 2H₂O + 2OH⁻
Give the equation for Co(OH)2(H2O)4(s) reacting with excess NH₃.
Co(OH)2(H2O)4(s) + 6NH3 (aq) —> [Co(NH3)6]2+(aq) + 4H2O (l) + 2OH- (aq)
What happens when a high concentration of Cl⁻ (from conc HCL or saturated NaCl) is added to an aqueous metal ion?
A ligand substitution reaction occurs, and the coordination number may change due to Cl⁻ being larger than H₂O and NH₃.
What happens when solid copper chloride dissolves in water?
it forms the aqueous [Cu(H2O)6]2+ complex and not the chloride [CuCl4]2- complex
this applies to other metal chlorides aswell
Write the equation for Cu²⁺ reacting with Cl⁻ and their colours
[Cu(H₂O)₆]²⁺ + 4Cl⁻ → [CuCl₄]²⁻ (yellow/green solution) + 6H₂O
Write the equation for Co²⁺ reacting with Cl⁻
[Co(H₂O)₆]²⁺ + 4Cl⁻ → [CoCl₄]²⁻ (blue solution) + 6H₂O
What shape do [CuCl₄]²⁻ and [CoCl₄]²⁻ have?
Tetrahedral
What happens when monodentate ligands are replaced by bidentate or multidentate ligands?
A more stable complex is formed
Why does the chelate effect increase stability?
here is an increase in entropy (ΔS) because more molecules are formed, increasing disorder.
e.g
[Cu(H₂O)₆]²⁺ + EDTA⁴⁻ → [Cu(EDTA)]²⁻ + 6H₂O
The copper complex ion has changed from having monodentate ligands to a multidentate ligand
In this reaction there is an increase in the entropy of the system because there are more moles of products than reactants (from 2 to 7), creating more disorder so there is more stability
Why are EDTA complexes useful?
- remove poisonous heavy metal ions from rivers as the EDTA complexes are not toxic
- Helps lathering in shampoos by removing Ca²⁺ from hard water
Write the equation for Co²⁺ reacting with ethane-1,2-diamine.
[Co(NH₃)₆]²⁺ + 3NH₂CH₂CH₂NH₂ → [Co(NH₂CH₂CH₂NH₂)₃]²⁺ + 6NH₃
Why is the enthalpy change (ΔH) in the reaction between Co²⁺ and ethane-1,2-diamine close to zero?
The number and type of coordinate bonds remain the same.
Why does the reaction between Co²⁺ and ethane-1,2-diamine form a more stable complex ion
this reaction has an increase in entropy because of in the increase in moles from 4 to 7 in the reaction. ΔSsystem is positive. Therefore ΔSTotal will be positive and the complex formed is stable
why can’t NH₂NH₂ act as a bidentate ligand?
Its lone pairs are too close together to fit around the metal ion.
common ligands and their formulas
Ethane-1-2-diamine - NH2CH2CH2NH2
Ethanedioate - C2O42-
when does Partial substitution of ethanedioate ions occur
when a dilute aqueous solution containing ethanedioate ions is added to a solution containing aqueous copper(II) ions
Cu(H2O)62+ + 2C2O42- —>[Cu(C2O4)2(H2O)2]2- + 4H2O
In this reaction four water molecules are replaced and a new complex is formed.
Why is silver nitrate used to test for chloride ions in complex compounds?
It only reacts with free chloride ions outside the complex, forming a white AgCl precipitate
2+ and 3+ ion summary
pg 10 chemrevise
what is a heterogenous and a homogenous catalyst
- A heterogeneous catalyst is in a different phase from the reactants
- A homogeneous catalyst is in the same phase as the reactant
how does the strength of adsorption help determine the catalytic activity
Too strong → Products can’t be released.
Too weak (e.g. Ag) → Reactants don’t adsorb enough.
Ideal strength → Effective catalysis.
what do catalytic convertors do
These remove CO, NOx and unburned hydrocarbons from the exhaust gases, turning them into ‘harmless’ CO2, N2 and H2O.
What metals are used in catalytic converters?
Platinum (Pt), Palladium (Pd), and Rhodium (Rh), coated on a ceramic honeycomb for high surface area.
Steps in Heterogeneous Catalysis
1) CO and NO are adsorbed onto the surface and form bonds with active sites on the catalyst surface.
2) The bonds in the reactants weaken and break.
3) New bonds form between reactants held close together.
4) Desorption of CO₂ and N₂ products from the catalyst surface
How has catalyst development improved over time?
✔ Greater selectivity
✔ Cheaper catalysts
✔ Milder conditions
✔ Improved separation processes
Why can transition metals act as homogeneous catalysts?
- They have variable oxidation states.
- They can donate and accept electrons so are able to oxidize and reduce
- They have partially filled d-orbitals, making electron transfer easier.
How does homogeneous catalysis work?
The catalyst forms an intermediate species with a different oxidation state, which later regenerates the original catalyst.
Why is the uncatalyzed reaction between I⁻ and S₂O₈²⁻ slow?
Both reactants are negatively charged, causing repulsion and requiring a high activation energy.
what is the overall equation for the Reaction between iodide and persulfate ions and the catalyst used
catalyst: Iron (Fe²⁺ / Fe³⁺) ions
overall equation: S₂O₈²⁻ + 2I⁻ → 2SO₄²⁻ + I₂
What are the steps in the catalyzed mechanism for the Reaction between iodide and persulfate ions?
stage 1: S₂O₈²⁻ + 2Fe²⁺ → 2SO₄²⁻ + 2Fe³⁺
stage 2: 2I⁻ + 2Fe³⁺ → 2Fe²⁺ + I₂
Why does Fe³⁺ also act as a catalyst in the reaction between iodide and persulfate ions
The reaction steps can occur in any order, so Fe³⁺ can also donate and accept electrons.
What must be true for a substance to act as a homogeneous catalyst?
Its electrode potential must lie between the two reactants’ electrode potentials so it can first reduce one reactant, then oxidize the other.
Why does using E° values not guarantee a faster reaction?
E° values only show if catalysis is possible, not whether the rate of reaction will increase.
What is autocatalysis?
A reaction in which one of the products acts as a catalyst, speeding up the reaction.
What is the overall reaction in the autocatalysis of ethanedioate and manganate(VII) ions?
2 MnO4- + 5 C2O42- + 16 H+ —> 2Mn2+ + 10 CO2 + 8 H2O
Why does the uncatalyzed reaction between Ethanedioate and Manganate ions slow and have a high activation
The initial uncatalysed reaction is slow because Both MnO₄⁻ and C₂O₄²⁻ are negatively charged so they can’t collide as they repel eachother leading to a high activation energy
steps of the Autocatalytic Reaction between Ethanedioate and Manganate ions
Step: 4Mn^2+ +MnO4- + 8H+ —> 5Mn^3+ + 4H2O
Step2: 2Mn^3+ + C2O42- —> 2Mn^2+ + 2CO2
How does Mn²⁺ act as an autocatalyst in the Reaction between Ethanedioate and Manganate ions
- Mn²⁺ lowers activation energy by allowing a different reaction route.
- This speeds up the reaction as more Mn²⁺ is produced.
- The reaction slows down when MnO₄⁻ runs out.
How can the reaction rate be measured in the Reaction between Ethanedioate and Manganate ions
✔ Titration – Remove samples at set times and titrate to determine MnO₄⁻ concentration.
✔ Spectrophotometry – Measure the intensity of MnO₄⁻’s purple color.
Why is spectrophotometry preferred?
- Non-destructive (doesn’t use up reactants).
- Faster than titration.
write the electron config for chromium and copper
cr = [Ar] 4s¹ 3d⁵
cu = [Ar] 4s¹ 3d¹⁰
Both exceptions arise because half-filled (Cr) and fully filled (Cu) d-orbitals provide extra stability due to reduced electron repulsions
How many coordinate bonds can chlorine form
4
Structure of heamoglobin and cisplatin
heamoglobin - octahedral
cisplatin - square planar (90 degrees bond angle)
Amphoteric
Can act as a base and acid
