Chromium Reactions Flashcards by LIVIA BUXTON

What colour is [Cr(H₂O)₆]³⁺/ Cr 3+ ions

Green solution

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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]

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[Cr(H₂O)₆]³⁺ + 3OH- reaction name

Acid base reaction

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What colour is [Cr(H₂O)3(OH)3]

Green precipitate

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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]-

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Why does [Cr(H₂O)2(OH)4]- have a minus 1 charge?

Because 4OH- + Cr3+ = -1 charge

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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

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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)

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Addition of even more concentrated 2 NaOH to [Cr(H₂O)2(OH)4]- forms what?

[Cr(OH)6]3-

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Formation of [Cr(OH)6]3- colour

Green solution

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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

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What colour can [Cr(H₂O)₆]³⁺ be if it’s pure?

Purple

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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-

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[Cr(H₂O)₆]³⁺ + NH3 reaction type?

Acid base reaction

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[Cr(H₂O)₆]³⁺ + 3NH3 product

[Cr(H₂O)3(OH)3] + 3NH4+

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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]

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[Cr(H₂O)3(OH)3] + excess 6NH3 reaction type?

A ligand substitution reaction

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[Cr(H₂O)3(OH)3] + 6NH3 forms?

[Cr(NH₃)₆]³⁺ + 3H2O + 3OH-

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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 reducing 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

What colour is [Cr(NH₃)₆]³

Purple

Oxidation state of Cr in Cr2O7 2-

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+ to make chromate (first step)

2[Cr(OH)6]³⁻ + 3H2O2 ————> 2CrO4²⁻ + 8H2O + 2OH- Produces chromate ions

Change in oxidation state of Cr when alkaline hydrogen peroxide is added to Cr3+ in the form of [Cr(OH)6]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

What colour is Cr2+

Blue solution

How can Cr3+ and Cr2+ be made from Cr2O7²⁻ ?

By reduction using zinc as a reducing agent in ACIDIC conditions

Why can zinc reduce dichromate to Cr3+ then Cr2+?

Strong enough reducing agent due to being easily oxidised (negative electrode potential) So by Ecell being POSITIVE, this reaction is very feasible

Can iron reduce dichromate all the way to Cr2+?

No, stops at Cr3+: not strong enough reducing agent so Ecell isnt positive to go all the way