Chromium Precipitation Reactions Flashcards
What colour is [Cr(H₂O)₆]³⁺
Green solution
What happens when 3OH- is added to [Cr(H₂O)₆]³⁺?
3 water ligands act as base and each donate H+ ion to be accepted by 3OH- (acts as base)
So the complex is left as [Cr(H₂O)3(OH)3]
[Cr(H₂O)₆]³⁺ + 3OH- reaction name
Acid base reaction
What colour is [Cr(H₂O)3(OH)3]
Green precipitate
What happens when 1 more OH- is added to [Cr(H₂O)3(OH)3]?
Further acid base reaction occurs to form
[Cr(H₂O)2(OH)4]-
Why does [Cr(H₂O)2(OH)4]- have a minus 1 charge?
Because 4OH- + Cr3+ = -1 charge
What colour and observation is made when [Cr(H₂O)2(OH)4]- is formed?
From a green precipitate of [Cr(H₂O)3(OH)3] ———> Forms a green solution
Why is [Cr(H₂O)2(OH)4]- soluble? (Formed from addition of more OH- to [Cr(H₂O)3(OH)3])
Because it has a -1 charge so is therefore able to form ion dipole interactions with water which is a polar molecule (H d+ charge on water)
Addition of even more concentrated 2 NaOH to [Cr(H₂O)2(OH)4]- forms what?
[Cr(OH)6]3-
Formation of [Cr(OH)6]3- colour
Green solution
The number of OH- added in each step of Cr3+ solution + OH- acid base reaction
First step adds 3 to form the neutral precipitate (green)
Second step adds 1 to form a solution (green)
Third step adds 2 to form a solution (green) = overall, 6 is added to replace ALL water ligands
What colour can [Cr(H₂O)₆]³⁺ be if it’s pure?
Purple
What type of reaction is [Cr(H₂O)₆]³⁺ + OH-
All occurs is acid base reaction, the ligands are not substituted for another but they donate a H+ (acts as acid) to be accepted by basic OH-
[Cr(H₂O)₆]³⁺ + NH3 reaction type?
Acid base reaction
[Cr(H₂O)₆]³⁺ + 3NH3 product
[Cr(H₂O)3(OH)3] + 3NH4+
What happens with acid base reaction of [Cr(H₂O)₆]³⁺ +NH3
3NH3 acts as weak base and each accepts a H+ from 3 water ligands in the complex (which acts as an acid)
(Acts same as first step of [Cr(H₂O)₆]³⁺ +3OH- )
Forms [Cr(H₂O)3(OH)3]
[Cr(H₂O)3(OH)3] + excess 6NH3 reaction type?
A ligand substitution reaction
[Cr(H₂O)3(OH)3] + 6NH3 forms?
[Cr(NH₃)₆]³⁺ + 3H2O + 3OH-
What happens in ligand substitution reaction of 6NH3 + [Cr(H2O)3(OH)3]
All the water and OH ligands are replaced with NH3 ligands
No coordination number change: Cr still forms 6 dative covalent bonds so remains an octahedral shape
Why is [Cr(NH₃)₆]³⁺ have a +3 charge?
Because the NH3 ligands are neutral and there is no OH- to balance so the Cr3+ stays 3+ charge
Why is [Cr(NH₃)₆]³⁺ soluble to form a (purple) solution?
Because it has a +3 charge which can form ion dipole interactions with O d- on polar water molecules
What colour and observations are made when [Cr(NH₃)₆]³⁺ is formed from adding 6NH3 to [Cr(H2O)3(OH)3]
Green precipitate to form a purple solution
When can we do a ligand substitution reaction of chromium water complex?
Only when [Cr(H2O)3(OH)3] is formed from addition of 3 OH- or 3 NH3 in acid base reaction
Why can Cr2O7²⁻ (Cr 6+)be reduced to Cr³⁺ using oxidising agent Zinc in acidic conditions?
Because the reaction has an overall positive E cell so the reaction is feasible
Why can Cr³⁺ be reduced to Cr2⁺ using zinc and acidic conditions?
Because the reaction has a positive E cell value so is overall positive
Oxidation state of Cr in Cr2O7 2-
+6
How can Cr2O7 2- ion be produced?
In oxidation of Cr3+ in alkaline hydrogen peroxide conditions
Followed by acidification
Why can Cr2O7 2- ion be produced when it is oxidised by hydrogen peroxide?
Because the overall reaction is feasible with a positive E cell value
Overall reaction for alkaline hydrogen peroxide + Cr3+ (first step)
2Cr(OH)3 +4OH- + 3H2O ————> 2CrO4 2- + 8H2O
Produces chromate ions
Change in oxidation state of Cr when alkaline hydrogen peroxide is added to Cr3+ in the form of Cr(OH)3
+3 oxidised to +6 in CrO4 2-
What does hydrogen peroxide act as in production of dichromate?
Oxidising agent of Cr and is reduced itself
How can dichromate ions be produced from chromate 6+ ions after hydrogen peroxide + Cr3+
Acidification
Cr3+ ions in solution written as a complex
[Cr(H2O)6]3+
Cr(OH)3 as a precipitate written as a complex
[Cr(H2O)3(OH)3]
Chromate/dichromate equilibrium
2CrO4²⁻ +2H⁺ <——> Cr2O7²⁻ + H2O
When potassium chromate is dissolved in water
Establishes the dichromate/chromate equilibrium
But the relative concentration of chromate ions is much higher even though some dichromate ions are produced as a result of equilibrium
Predominant colour of a chromate ion solution
Yellow
Relatively much higher concentration of chromate ions relative to dichromate ions
When potassium dichromate is dissolved in water
Establishes the chromate/dichromate equilibrium
Relative concentration of dichromate ion is much higher than concentration of chromate ion (which had been produced as a result of the equilibrium)
Predominant colour of the dichromate solution
Orange because overall there is a much higher concentration of dichromate ions in solution that are orange
Does Cr in chromate and dichromate have the same oxidation state?
Yes of +6
