Chemistry of Chromium

[ Related blog post: http://www.wewwchemistry.com/2014/06/chemistry-of-chromium ]
These slides briefly describe the chemistry of chromium within the scope of the Singapore-Cambridge GCE A Level 9647 H2 Chemistry syllabus. Topics include the unusual electronic configuration of chromium, the various oxidation states of chromium and the reactions of common chromium compounds like chromium(III) chloride, chromium(III) nitrate, chromium(III) hydroxide,potassium chromate(VI) and potassium dichromate(VI).

1. CHEMISTRY OF CHROMIUM

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2. THE ELECTRONIC
CONFIGURATION

>> OF CHROMIUM

3. The ‘Exceptional’ Electronic
Configuration of Chromium

Electronic Configuration

d-Block Element

scandium 3d! 4s?
titanium Ar] 3d? 4s?
vanadium Ar] 3d? 4s?
NOT
_ | [Ar] 3d* 4s?!
manganese Ar] 3d° 45?

Ar] 3d* 45?
3d” 45?

iron

cobolt

Ar] 3d8 45?
3d10 4s!
3d! 45?

nickel

copper

— NOT
[Ar] 3d? 4s?
Another exception!

To be discussed under
Chemistry of Copper

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4. Why the ‘Exception’?

» The 3d and 4s orbitals of chromium are very
close in energies.

» Thus, the occupancy of orbitals in chromium
is determined more by inter-electronic
repulsions rather than the energy differences
between orbitals.

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5. Why the ‘Exception’?

» The configuration [Ar] 3d* 4s? results in
strong repulsion between two 4s electrons
within the same orbital.

» To lower energy, one 4s electron shifts to
occupy a 3d orbital.

» This gives rise to an energetically more stable
configuration of [Ar] 3d5 4s!.

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6. THE OXIDATION STATES

>> OF CHROMIUM

7. Oxidation States of Chromium

4s

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3d

» Since 4s and 3d orbitals

v

are close in energies, it is
possible for a chromium
atom to lose one to six
electrons.

Possible oxidation states
of chromium are 0, +1,
+2, +3, +4, +5 and +6.

> Note:

+ It is also possible for chromium
to adopt a negative oxidation
state, but this is beyond the
scope of H2 Chemistry!

8. Oxidation States of Chromium

» Although chromium exhibits a wide range of
oxidation states, the +2, +3 and +6 are the
most commonly observed states in chromium
compounds.

» The +1, +4 and +5 states are rare.

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9. THE REACTIONS

>> OF CHROMIUM

10. Summary of Reactions
a.

chromium HC{aq) = = 1. excess NaOH(aq)
. = Cr?+(aq) ——— + Cr3*(aq)
meta in absence Blue: in presence green 2. H20,(aQ),
HA of oxygen HA of oxygen e.g. [CrCKH,0),): Warm
in CrCh(aq)
NaOH(aq) NH;(aq)
Na,CO; (ai 1. Filter
BON ar a cr(OH),(s) faa) comas] cro.: (ac)]]
violet 2. To residue, grey-green ppt grey-green ppt yellow
{Cr(H,0),)* in add HNOs(aq)
Cr(NO3)3(aq)
excess excess
NaOH(aq) NaOH(aq) NT CD H*(aq) || OH” (aq)
Cor excess NaOH(aq)
Cr(OH)3(s) —————————» [Cr(OH),]-(aq) [Cr(NH),_]3*(aq) Cr20,? (aq)

grey-green ppt green violet orange

à a a m

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11. The Unstable Chromium ll)

> Cr(ll) ions form blue solutions.
» However Cr(ll) are unstable as they are strong
reducing agents.
» For example,
° they decompose water slowly, producing
hydrogen;
° they are readily oxidised by atmospheric oxygen
to Cr(lll) ions.
» As such, they are rarely encountered in
qualitative analysis.

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12. The Colours of Chromiumtlil)
Solutions

> Cr(lll) ions are stable.
» In aqueous solution, they are either green or
violet.

chromiumiIIl) chloride, \
CrC/, solution

chromium(lll) nitrate,
Cr(NO;); solution

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13. The Colours of Chromium(I!I)
Solutions

» In violet solutions like chromium(lll) nitrate,
the [Cr(H,O),]3+ complex ion is present.

» In green solutions, one or more of the water
molecules around the Cr(lll) ion is replaced by
a negative ion like C/-, OH- or SO,2-.

» E.g. chromiumiIIl) chloride and chromium(III)
sulfate solutions are green.

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14. The Acidity of Chromium(lil)
Solutions

» When Na,CO; (aq) is added to an aqueous
solution containing chromiumiIll) ions,
carbon dioxide is produced.

2Cr3+(aq) + 3CO32-(aq) + 3H20(/) > 2Cr(OH)3(s) + 3CO,(g)

» Why are chromium(lll) solutions acidic?

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15. The Acidity of Chromium(lil)
Solutions

» A Cr3+ ion has a very high charge density.

» It draws electron density from the oxygen
atoms of surrounding water molecules that
are dative covalently bonded to itself as
ligands.

» This polarises the O—H bonds of these water
molecules further, and makes their H atoms
more electrophilic (and more acidic).

» These H atoms become more easily
abstracted as H+ ions by free water molecules
(functioning as Lewis bases) to form H;Ot.

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16. The Acidity of Chromium(lil)
Solutions

3+ 2+
HH, _
Va |
. H .
H208 u, ee OH, + id x H208 m, L “20H, 4 Hot
nore | oto, H noe Se son,
H,08 H,0
[Cr(H20)g]9+* + fo = [Cr(OH)XH20)5]?+* + 307

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17. The Acidity of Chromium(lil)
Solutions

» The H,0* ions produced then react with the
basic carbonate ions to form carbon dioxide.

CO3?-(aq) + H30+(aq) > CO,(g) + H,0()

» Subsequent abstraction of H* ions from
[Cr(OH)(H,O),]?* produces the grey-green
chromium(lll) hydroxide precipitate,
Cr(OH);(H,O);, commonly written as Cr(OHs)3.

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18. The Amphoteric Nature of
Chromium(l!l) Hydroxide

» Chromium(lil) hydroxide, Cr(OH)3(H2O)3, or
simply Cr(OH);, is a grey-green solid which
reacts with both acids and alkalis.

» It is therefore amphoteric.

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19. The Amphoteric Nature of
Chromium(l!l) Hydroxide

5
— o»
H+

Grey-green solid of Suspension containing Grey-green solid of
Cr(OH);(H,0); dissolves grey-green solid of Cr(OH);(H,0); dissolves
in acid to form a violet Cr(OH);(H,0); in excess alkali to form
solution of [Cr(H,0),]3* a dark green solution of

[Cr(OH),]3

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20. The Chromate(VI) — Dichromate(VI)
Equilibrium

2CrO,2- + 2H+ = Cr,0,2- + H,O

» The dichromate(Vl) ion, Cr,0,2-, exists in
equilibrium with the chromate(Vl) ion, CrO,2-.

» An acidic medium favours the right-side of
the above equilibrium, i.e. solution is orange
due to the higher concentration of Cr,0,2-
relative to CrO,2-.

» Conversely, an alkaline medium favours the
CrO,2- ion, resulting in a yellow solution.

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21. The Chromate(VI) — Dichromate(Vl)
Equilibrium

An orange A yellow
solution of solution of
K,Cr,0, K,CrO,
is obtained is obtained
when alkali is when acid is
added toa added toa
solution of solution of
K,CrO, K2Cr207

21

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22. The Dichromate(Vl) as an

Oxidising Agent

» The dichromate(VI) ion, Cr,0,?-, is a good
oxidising agent.

» Its reduction half-equation is as follows:

Cr,0,2-(aq) + 14H* + 6e- — 2Cr3*(aq) + 7H,0

» À colour change is
observed as orange
Cr,0,2- is reduced

After

id ,
to green Cr3*. CH,CO;H

and Cr+
are formed

23. The Dichromate(Vl) as an
Oxidising Agent
» Potassium dichromate(Vl), K¿Cr,O,, is
commonly used as an oxidising agent.
» It is a weaker oxidising agent than potassium
manganate(VIl), KMnO,.
» In organic chemistry, it is often used to
° oxidise primary alcohols to aldehydes (by heating
with distillation) or to carboxylic acids (by heating
under reflux);
° oxidise secondary alcohols to ketones.

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