15.2.3 - The chemistry of Chromium Flashcards
What observation when a solution containing the hexaaquachromium(III) ion reacts with sodium hydroxide?
A green or violet solution forms a green precipitate.
What is the equation for the reaction between the hexaaquachromium(III) ion and ammonia?
(Cr(H2O)6)^3+ + 3NH3 = (Cr(H2O)3(OH)) + 3NH4+
What observations can be made if sodium hydroxide is added in excess to a solution containing the hexaaquachromium(III) ion?
The green precipitate will dissolve to form a green solution.
What happens when aqueous ammonia is added to the green precipitate: (Cr(H2O)3(OH)3)
The green precipitate dissolves to a purple or violet solution.
What colour is the chromate ion and what colour is the dichromate ion?
Chromate ion = Yellow
Dichromate ion = Orange
In acidic and alkaline conditions, which out of the dichromate and chromate ion is more stable?
Alkaline conditions: chromate ions are more stable
Acidic conditions: dichromate ions are more stable
When dichromate (VI) ions are reduced by zinc in acidic conditions Chromium (III) ions form. What colour change can be seen?
orange to green
Chromium (III) ions can be further reduced to Chromium (II) ions by zinc. What colour change can be seen?
Green to blue
What is the equation for the oxidation of chromium (III) by hyrdogen peroxide in alkaline conditions?
Cr(OH)3
And what is the colour change?
2Cr(OH)3 + 3H2O2 + 4OH- = 2CrO4^2- + 8H2O
Green to yellow solution
State the colours of these oxidation states of chromium.
a) 6+ dichromate
b) 6+ chromate
c) 3+
d) 2+
a) orange
b) yellow
c) Green
d) Blue
What conditions and reagents are required to oxidise chromium(III) to chromate(VI) ions?
hydrogen peroxide
alkaline conditions
hot
What conditions and regents are required to oxidise chromate ions to dichromate ions?
Adding H+ ions
Chromate ions are more stable in alkaline conditions
What is the reagent and conditions required to reduce dichromate ions to chromium (III) ions?
Zinc metal
Acid
