The chemistry of chromium Flashcards
the 3 factors that can affect the colour described of a chromium compound are:
- some compounds have different colours as solids and aqueous solutions
- the colour of a solution depends on concentration
- the presence of dissolved oxygen in an aqueous solution can affect the colour observed
the equation, colour change and type of reaction for the reaction between the hexaaquachromium(III) ion and aqueous sodium hydroxide solution until no further change is seen is:
[Cr(H2O)6]3+ + 3OH- → [Cr(H2O)3(OH)3] + 3H2O
green or violet solution → green precipitate
acid-base reaction
when NaOH is added in excess:
[Cr(H2O)3(OH)3] + OH- → [Cr(H2O)2(OH)4]- + H2O
green precipitate dissolves → green solution
acid-base reaction
if the NaOH is more concentrated, then:
[Cr(H2O)2(OH)4]- + 2OH- → [Cr(OH)6]3- + 2H2O
no colour change
acid-base reaction
these reactions can be reversed by the addition of H+ ions from an acid, showing the amphoteric nature of the neutral complex
the equation, colour change and type of reaction for the reaction between the hexaaquachromium(III) ion and aqueous ammonia until no further change is seen is:
[Cr(H2O)6]3+ + 3NH3 → [Cr(H2O)3(OH)3] + 3NH4+
green or violet solution → green precipitate
acid-base reaction
when NH3 is added in excess:
[Cr(H2O)3(OH)3] + 6NH3 → [Cr(NH3)6]3+ + 3H2O + 3OH-
green precipitate slowly dissolves → violet or purple solution
the equation, colour change and type of reaction for the reaction between the hexahydroxochromium(III) ion in alkaline solution when hydrogen peroxide is added until no further change is seen is:
2[Cr(OH)6]3- + 3H2O2 → 2CrO4 2- + 2OH- + 8H2O
green solution → yellow solution
H2O2 is an oxidising agent which changes the oxidation number of the chromium(III) ion (Cr3+) to the chromate(VI) ion (Cr6+)
CrO4 2- is a complex, but not enclosed in square brackets
chromate(VI) ions are …………. in alkaline solution, but in acidic conditions the dichromate(VI) ion is…………….
(in terms of stability)
chromate(VI) ions are stable in alkaline solution, but in acidic conditions the dichromate(VI) ion is more stable
when an acid is added to an alkaline solution containing chromate(VI) ions, the following reaction and colour change occurs:
2CrO4 2- + 2H+ ⇌ Cr2O7 2- + H2O
yellow solution → orange solution
this reaction is easily reversed by adding an alkali
(also note that square brackets and ligands are left out when they do not undergo redox reactions as this makes it easier to see the redox)
when zinc metal is added to an acid solution containing dichromate(VI) ions, the following reactions and colour changes occur:
Cr2O7 2- + 14H+ + 3Zn → 2Cr3+ + 7H2O + 3Zn2+
first stage of reduction: orange solution → green solution
2Cr3+ + Zn → 2Cr2+ + Zn2+
second stage of reduction: green solution → blue solution
when looking at Eθ values, the best reducing agent relative to a list of species is………………. and the best oxidising agent relative to a list of species is……………….
best reducing agent has the most positive Eθ value, and will be ont he right of the equation
(e.g.: in Zn2+ + 2e- → Zn , Zn is the reducing agent)
best oxidising agent has the most negative value and is on the left
the simplified equation for the oxidation of chromium(III) by hydrogen peroxide in alkaline conditions is:
2Cr(OH)3 + 3H2O2 +4OH- → 2CrO4 2- + 8H2O
the simplified equation below is obtained from which half equations:
2Cr(OH)3 + 3H2O2 +4OH- → 2CrO4 2- + 8H2O
(X) Cr(OH)3 + 5OH- ⇌ CrO4 2- + 4H2O + 3e- (Eθ = -0.13 V)
(Y) H2O2 + 2e- ⇌ 2OH- (Eθ = +1.24 V)
both ionic equations have to be multiplied by 2 and 3 respectively to obtain the full equation
since Eθ value for half equation (X) is more negative than the Eθ value for half equation (Y), Cr(OH)3 is electron releasing with respect to H2O2 and hence the reaction is thermodynamically feasible
this explains the oxidation from +3 + 6
the simplified equation for the reduction of chromium(VI) by zinc in acidic conditions is:
Cr2O7 2- + 14H+ + 3Zn → 2Cr3+ + 7H2O + 3Zn2+
the simplified equation below is obtained from which half equations:
Cr2O7 2- + 14H+ + 3Zn → 2Cr3+ + 7H2O + 3Zn2+
(A) 3Zn ⇌ 3Zn2+ + 6e- (Eθ = -0.76 V)
(B) Cr2O7 2- + 14H+ +6e- ⇌ 2Cr3+ + 7H2O (Eθ = +1.33 V)
the Eθ value for the half equation (A) is more negative than the Eθ value for half equation (B), so Zn is electron releasing with respect to Cr2O7 2- and hence the reaction is thermodynamically feasible
this explains the reduction from +6 + 3
the simplified equation for the reduction of chromium(III) by zinc in acidic conditions is:
2Cr3+ + Zn → 2Cr2+ + Zn2+
the simplified equation below is obtained from which half equations:
2Cr3+ + Zn → 2Cr2+ + Zn2+
(C) Zn ⇌ Zn2+ + 2e- (Eθ = -0.76 V)
(D) 2Cr3+ + 2e-3 ⇌ 2Cr2+ (Eθ = -0.41 V)
the Eθ value for half equation (C) is more negative than the Eθ value for half equation (D), meaning Zn is electron releasing with respect to Cr3+ and hence the reaction is thermodynamically feasible
