The Foundations of Chemistry

Chromium(VI) species are oxidizing agents. Basic solutions containing chromate ions,

CrO 42 , are weakly oxidizing. Acidification produces the dichromate ion, Cr 2 O 72 , and

chromium(VI) oxide, both powerful oxidizing agents.

Cr 2 O 72 
14H

 6 e88n2Cr^3 

7H 2 O E^0 
1.33 V

Chromate–Dichromate Equilibrium

Red chromium(VI) oxide, CrO 3 , is the acid anhydride of two acids: chromic acid, H 2 CrO 4 ,

and dichromic acid, H 2 Cr 2 O 7. Neither acid has been isolated in pure form, although chro-

mate and dichromate salts are common. CrO 3 reacts with H 2 O to produce strongly acidic

solutions containing hydrogen ions and (predominantly) dichromate ions.

2CrO 3 
H 2 O88n[2H

Cr 2 O 72 ] dichromic acid (red-orange)

From such solutions orange dichromate salts can be crystallized after adding a stoichio-

metric amount of base. Addition of excess base produces yellow solutions from which

only yellow chromate salts can be obtained. The two anions exist in solution in a pH-

dependent equilibrium.


 6 
 6

2CrO 42 
2H
 34 Cr 2 O 72 
H 2 O Kc4.2 1014

yellow orange

[Cr 2 O 72 ]



[CrO 42 ]^2 [H
]^2

Adding a strong acid to a solution that contains CrO 42 /Cr 2 O 72 ions favors the reac-

tion to the right and increases [Cr 2 O 72 ]. Adding a base favors the reaction to the left

and increases [CrO 42 ].

Cr

O

O

O

O

Cr

O

O

O

O Cr

O

O

O

dichromate ion, Cr 2 O 72 

chromate ion, CrO 42 

2 

2 

TABLE 23-8 Some Compounds of Chromium

Ox. State Oxide Hydroxide Name Acidic/Basic Related Salt Name

 2 CrO Cr(OH) 2 chromium(II) basic CrCl 2 chromium(II)

black hydroxide anhydr. colorless chloride

aq. lt. blue

3Cr 2 O 3 Cr(OH) 3 chromium(III) amphoteric CrCl 3 chromium(II)

green hydroxide anhydr. violet chloride

aq. green

KCrO 2 potassium

green chromite

 6 CrO 3 H 2 CrO 4 or chromic acid weakly acidic K 2 CrO 4 potassium

dk. red [CrO 2 (OH) 2 ] yellow chromate

H 2 Cr 2 O 7 or dichromic acid acidic K 2 Cr 2 O 7 potassium

[Cr 2 O 5 (OH) 2 ] orange dichromate

Chromate ion, CrO 4 2–

2–

Dichromate ion, Cr 2 O 7 2–

2–