Transition Metal Equations

Question 1

Dichromate ions can be reduced using zinc and dilute acid

Answer 1

Cr2O7 2- + 14H+ + 3Zn > 3Zn2+ + 2Cr3+ + 7H2O

Question 2

Zinc will reduce Cr3+ to Cr2+

Answer 2

Need to use inert atmosphere because Cr2+ is unstable so can oxidise back

2Cr3+ + Zn + e- > Zn2+ + 2Cr3+

Question 3

Oxidise Cr3+ to chromate (vi) ions (CrO4 2-) with hydrogen peroxide in alkaline solution

Answer 3

2Cr3+ + 10OH- + 3H2O > 2CrO4 2- + 8H2O

Question 4

If you add some acid to the yellow solution, you form an orange solution

Answer 4

2CrO4 2- (yellow) + 2H+ = Cr2O7 2- (orange) + H2O

Question 5

Mixing an aqueous solution of chromium (iii) ions with aqueous sodium hydroxide

Answer 5

[Cr(H2O)6]3+ + 3OH- > [Cr(OH)3(H2O)3] + 3H2O

GREEN > GREEN GREY

Question 6

Mixing an aqueous solution of chromium (iii) ions with aqueous ammonia

Answer 6

[Cr(H2O)6]3+ + 3NH3 > [Cr(OH)3(H2O)3] + 3NH4+

GREEN > GREEN GREY

Question 7

Chromium hydroxide is

Answer 7

Amphoteric - can react with both acids and bases

Question 8

If you add excess sodium hydroxide to a chromium hydroxide precipitate…

Answer 8

The H2O ligand protonate, and a solution containing [Cr(OH)6]3- forms

[Cr(OH)3(H2O)3] + 3OH- > [Cr(OH)6]3- + 3H2O
GREEN GREY > DARK GREEN

Question 9

If you add acid to a chromium hydroxide precipitate…

Answer 9

The OH- ligand protonate, and a solution containing [Cr(H2O)6]3- forms

[Cr(OH)3(H2O)3] + 3H+ > [Cr(H2O)6]3+ + 3H2O
GREEN GREY > GREEN

Question 10

If you add excess ammonia to the chromium hydroxide, a ligand exchange reaction occurs

Answer 10

[Cr(OH)3(H2O)3] + 6NH3 > [Cr(NH3)6]3+ + 3OH- + 3H2O

GREEN GREY > PURPLE