Transition Metal Reactions (Chapter 24.4 and 24.5) Flashcards
What is a ligand substitution reaction?
One in which one ligand in a complex ion is replaced by another ligand
What happens when CuSO4 is dissolved in water?
A pale blue solution of the complex ion [Cu(H2O)6] is formed in aqueous solution
What happens when an excess of NH3 is added to [Cu(H2O)6]2+?
- A ligand substitution occurs in which 4 NH3 ligands replace 4 H2O ligands
- [Cu(H2O)6]2+ + 4NH3 => [Cu(NH3)4(H2O)2]2+ + 4H2O
- The solution changes from pale blue to dark blue
What happens when NH3 is added drop-wise to [Cu(H2O)6]2+ (i.e. not in excess)?
1) a pale blue ppt of Cu(OH)2 is formed as it only reacts with the OH- ions: Cu2+ + 2OH- => Cu(OH)2
2) the Cu(OH)2 ppt dissolves in excess NH3 to form the dark blue solution
What happens when an excess of HCl (a source of Cl- ions) is added [Cu(H2O)6]2+?
- A ligand substitution occurs in which 4 Cl- ligands replace 6 H2O ligands
- [Cu(H2O)6] + 4Cl- <=> [CuCl4]2- + 6H2O
- The solution changes from pale blue to yellow
Why is there a change in coordination number and ∴ shape during the ligand substitution of [Cu(H2O)6]2+ with chloride ions?
Bc chloride ligands are larger in size than H2O ligands ∴ fewer chloride ligand fit around the central Cu2+ ion
What happens if HCl is added slowly to [Cu(H2O)6]2+?
There will be an intermediate green solution formed (the blue and yellow mixing)
What happens if water is added to [CuCl4]2-?
A blue solution will form but more dilute and paler than the original blue solution (the substitution has been reversed)
What happens when chromium (III) potassium sulfate (KCr(SO4)2.12H2O) is dissolved in water?
A pale purple (violet) solution of the complex ion [Cr(H2O)6]3+ is formed in aqueous solution
What colour is [Cu(H2O)6]2+?
Pale blue
What colour is [Cr(H2O)6]3+?
Pale purple (violet)
What colour is [Cu(NH3)4(H2O)2]2+?
Dark blue
What colour is Cu(OH)2?
Pale blue ppt
What colour is [CuCl4]2-?
Yellow
What happens when chromium (III) sulfate is dissolved in water?
- A green solution of [Cr(H2O)5SO4]+ is formed (rather than [Cr(H2O)6]3+)
- Cr is still in the +3 oxidation state
What colour is [Cr(H2O)5SO4]+ ?
Green
What happens when excess ammonia is added to [Cr(H2O)6]3+?
- A ligand substitution occurs in which 6 NH3 ligands replace 6 H2O ligands
- [Cr(H2O)6]3+ + 6NH3 => [Cr(NH3)6]3+ + 6H2O
- The solution changes from violet to purple
What happens when ammonia is added drop wise to [Cr(H2O)6]3+ (i.e. not in excess)?
1) a grey-green ppt of Cr(OH)3 is formed
Cr3+ + OH- => Cr(OH)3
2) the Cr(OH)3 ppt dissolves in excess NH3 to form [Cr(H2O)6]3+
What colour is [Cr(NH3)6]3+?
Purple
What colour is Cr(OH)3?
Grey-green ppt
Describe the structure of haemoglobin
- Haemoglobin contains 4 protein chains held together by weak intermolecular forces
- Each protein chain has a haem molecule within its structure
- The central metal ion in a haem group is Fe2+ which can bind to oxygen
What happens to haemoglobin in the body?
1) as blood passes through the lungs, the haemoglobin bonds to oxygen because of the increased oxygen pressure in the capillaries of the lungs
2) oxyhaemoglobin forms, which releases oxygen to body cells
3) haemoglobin in red blood cells can also bond to CO2, which is carried back to the lungs
4) CO2 is then released from RBCs and CO2 is exhaled
Describe what happens with carbon monoxide and Fe2+ in haemoglobin
1) CO can bind to Fe2+ in haemoglobin, forming a complex called carboxyhaemoglobin
2) if CO is breathed in, a ligand substitution reaction takes place where the oxygen in haemoglobin is replaced by CO
3) CO binds to haemoglobin more strongly than oxygen ∴ a small concentration of CO in the lungs can prevent a large proportion of the haemoglobin molecules from carrying oxygen - the process is irreversible bc the bond is so strong
4) if the concentration of carboxyhaemoglobin becomes too high, oxygen transport is prevented, leading to death
What does a precipitation reaction occur?
When two aqueous solutions containing ions react together to form an insoluble ionic solid (ppt)
What reacts with aqueous transition metal ions to form precipitates?
1) aqueous NaOH
2) aqueous NH3
- But some of these ppts will dissolve in excess NaOH or NH3 to form complex ions in solution
Which transition metal ions does NaOH react with to form hydroxide ppts?
Cu2+, Fe2+, Fe3+, Mn2+ and Cr3+
What is special about Cr(OH)3?
It is the ONLY hydroxide ppt which dissolves in excess NaOH
Describe the precipitation reaction of Cu2+ with NaOH
- Cu2+ + 2OH- => Cu(OH)2
- Pale blue solution reacts to form a pale blue ppt
Describe the precipitation reaction of Fe2+ with NaOH
- Fe2+ + 2OH- => Fe(OH)2
- Pale green solution reacts to form a green ppt
What happens to Fe2+ in air?
- Fe(OH)2 => Fe(OH)3
- It turns brown at its surface as Fe (II) is oxidised to Fe (III)
Describe the precipitation reaction of Fe3+ with NaOH
- Fe3+ + 3OH- => Fe(OH)3
- Pale yellow solution reacts to form an orange-brown ppt
Describe the precipitation reaction of Mn2+ with NaOH
- Mn2+ + 2OH- => Mn(OH)2
- Pale pink solution reacts to form a light brown ppt
What happens to Mn(OH)2 when it is standing in air?
It darkens
Describe the precipitation reaction of Cr3+ with NaOH
- Cr3+ + 3OH- => Cr(OH)3
- Violet solution reacts to form a grey-green ppt
What happens to Cr(OH)3 in excess NaOH?
- Cr(OH)3 + 3OH- => [Cr(OH)6]3-
- The grey-green ppt dissolves to form a dark green solution
What happens when Fe2+, Fe3+ and Mn2+ reacts with excess aqueous ammonia?
- They react in the same way as with aqueous NaOH, forming ppts of Fe(OH)2, Fe(OH)3 and Mn(OH)2
- There is no further reaction with aqueous ammonia ∴ these ppts do not dissolve
What is used in the oxidation of Fe2+ to Fe3+?
Manganate (VII) ions - MnO4-
Describe the oxidation of Fe2+ to Fe3+
- A redox reaction occurs between Fe2+ and MnO4- ions in acidic conditions - Fe2+ is oxidised to Fe3+ and MnO4- is reduced to Mn2+
- Fe2+ ions are added to aqueous MnO4-
- The purple solution of MnO4- is decolourised (Mn2+ is colourless)
What is the equation for the oxidation of Fe2+ to Fe3+?
MnO4- + 8H+ + 5Fe2+ => Mn2+ + 5Fe3+ + 4H2O
What is used in the reduction of Fe3+ to Fe2+?
I- ions
Describe the reduction of Fe3+ to Fe2+
- A redox reaction occurs between Fe3+ and I- ions in which Fe3+ is reduced to Fe2+ and I- is oxidised to I2
- When I- ions are added to Fe3+, the orange brown Fe3+ ions change to pale green Fe2+ ions
- However, this colour change is obscured by the formation of iodine, which is brown
What is the equation for the reduction of Fe3+ to Fe2+?
2Fe3+ + 2I- => 2Fe2+ + I2
Why does the oxidation of Fe2+ to Fe3+ take place in terms of standard electrode potentials?
The standard electrode potential for MnO4-/Mn2+ is more positive than for Fe3+/Fe2+ ∴ Fe2+ is oxidised to Fe3+ and MnO4- is reduced to Mn2+
Why does the reduction of Fe3+ to Fe2+ take place in terms of standard electrode potentials?
The standard electrode potential for Fe3+/Fe2+ is more positive than for I2/I- ∴ I- is oxidised to I2 and Fe3+ is reduced to Fe2+
What is used in the reduction of acidified Cr2O72- ions to Cr3+?
Zinc
What is the equation for the reduction of acidified Cr2O72- ions to Cr3+?
Cr2O72- + 14H+ + 3Zn => 2Cr3+ + 7H2O + 3Zn2+
What colour is aqueous Cr3+?
Green
What colour is aqueous Cr2O72-?
Orange
What happens to Cr3+ in an excess of zinc?
- Cr3+ is reduced further to Cr2+
- The solution changes from green to pale blue
What is the equation for the reduction of Cr3+ to Cr2+?
Zn + 2Cr3+ => Zn2+ + 2Cr3+
Why is zinc able to reduce chromium from Cr2O72- to Cr2+?
- The standard electrode potential of Zn2+/Zn is more positive than that of Cr3+/Cr2+ which is more positive than that of Cr2O72-/Cr3+
- ∴ the equilibrium is more likely to gain electrons, shift to the right and undergo reduction
- Zinc is the most powerful reducing agent and is capable of reducing both Cr2O72- to Cr3+ and Cr3+ to Cr2+
What is needed to oxidise Cr3+ to CrO42-?
Hot alkaline hydrogen peroxide
Describe the oxidation of Cr3+ to CrO42-
A redox reaction takes place in which chromium is oxidised from +3 in Cr3+ to +6 in CrO42- and oxygen is reduced from -1 in H2O2 to -2 in CrO42-
What is the equation for the reduction of Cr3+ to CrO42-?
3H2O2 + 2Cr3+ + 10OH- => 2CrO4- + 8H2O
What is needed to reduced Cu2+ to Cu+?
I- ions
Describe the reduction of Cu2+ to Cu+
- A redox reaction occurs in which I- is oxidised to brown iodine and Cu2+ (pale blue) is reduced to Cu+
- Cu+ reacts with I- to form a white ppt copper (I) iodide, which is masked by the brown iodine
What is the equation for the reduction of Cu2+ to Cu+?
2Cu2+ + 4I- => 2CuI + I2
Describe the disproportionation of Cu+ ions
- When solid copper (I) oxide (Cu2O) reacts with hot dilute H2SO4, a brown ppt of Cu and a blue solution of copper (II) sulfate is formed
- In this reaction, Cu+ ions have been simultaneously oxidised and reduced (+1 to 0 and +1 to +2)
What is the equation between [Co(H2O)6]2+ and Cl- ions?
[Co(H2O)6]2+ + 4Cl- => [CoCl4]2- + 6H2O
What is the equation for the disproportionation of Cu+ ions?
Cu2O + H2SO4 => CuSO4 + Cu + H2O
How would you get rid of the brown colour in the mixture of CuI and iodine?
Add thiosulfate: 2S2O32- + I2 => S4O62- + 2I-
What happens to Cu (I) in solution?
It is unstable, so disproportionates to Cu (II) and Cu (0)
Describe the reaction of CuCl2 and Na2SO3 including the equation
- Redox reaction:
Oxidation: H2O + SO32- => SO42- + 2e- +2H+
Reducion: Cu2+ + e- => Cu+ - A white ppt of CuCl is formed
