We’ve updated our privacy policy so that we are compliant with changing global privacy regulations and to provide you with insight into the limited ways in which we use your data.
You can read the details below. By accepting, you agree to the updated privacy policy.
Thank you!
View updated privacy policy
We've encountered a problem, please try again.
Try this class practical to investigate an equilibrium between chromate(VI), dichromate(VI) and hydrogen ions
In this experiment, students add dilute sulfuric acid to an aqueous solution of potassium chromate(VI). They observe the resulting colour changes, before reversing the reaction using aqueous sodium hydroxide.
The experiment is most appropriate with A-level students, given the potential hazards with solutions containing chromate(VI) and dichromate(VI) ions. Otherwise it could be carried out as a teacher demonstration. The experiment can be carried out individually by students, but the potassium chromate(VI) solution used should be prepared beforehand by the teacher or technician, given the hazards presented by the solid. It should take no more than five minutes.
Equipment
Apparatus
- Eye protection (goggles)
- Test tube
- Test tube holder
- Dropping pipette
Chemicals
- Potassium chromate(VI) solution, 0.2 M (TOXIC, OXIDISING, DANGEROUS FOR THE ENVIRONMENT), about 1 cm3
- Sodium hydroxide solution, 1.0 M (CORROSIVE), about 10 cm3
- Dilute sulfuric acid, 1.0 M (IRRITANT), about 5 cm3
Health, safety and technical notes
- Read our standard health and safety guidance.
- Wear eye protection (goggles) throughout.
- Potassium chromate(VI) solution, K2CrO4(aq) (TOXIC, OXIDISING, DANGEROUS FOR THE ENVIRONMENT) – see CLEAPSS Hazcard HC078a and CLEAPSS Recipe Book RB069.
- Sodium hydroxide solution, NaOH(aq) (CORROSIVE) – see CLEAPSS Hazcard HC091a and CLEAPSS Recipe Book RB085.
- Dilute sulfuric acid, H2SO4(aq), (IRRITANT) – see CLEAPSS Hazcard HC098a and CLEAPSS Recipe Book RB098.
Procedure
- Put 10 drops of potassium chromate(VI) solution in a test tube.
- Add about 5 cm3 of sulfuric acid and shake to mix thoroughly. What do you observe?
- Now add about 10 cm3 of sodium hydroxide solution and shake the mixture. Do you get back to where you started?
- Can you repeat the whole process by adding sulfuric acid and sodium hydroxide alternately all over again?
Teaching notes
The process occurring involves the equilibrium between chromate(VI), dichromate(VI) and hydrogen ions:
2CrO42–(aq) (yellow) + 2H+(aq) ⇌ Cr2O72–(aq) (orange) + H2O(l)
The addition of acid encourages the equilibrium towards the right, producing more orange-coloured dichromate(VI) ions. The addition of hydroxide ions causes the concentration of hydrogen ions to decrease, and this brings the equilibrium back to the left-hand side, regenerating yellow chromate(VI) ions.
It is important that students realize that both ions are always present and exist in equilibrium with one another, but that the yellow chromate(VI) ions predominate under alkaline conditions and the orange dichromate(VI) ions predominate in acidic solutions.
Students may be asked if this is a redox reaction. Their answer is often ‘yes’, but examination of oxidation numbers will show that chromium remains in the +6 oxidation state throughout.
Additional information
This is a resource from the Practical Chemistry project, developed by the Nuffield Foundation and the Royal Society of Chemistry. This collection of over 200 practical activities demonstrates a wide range of chemical concepts and processes. Each activity contains comprehensive information for teachers and technicians, including full technical notes and step-by-step procedures. Practical Chemistry activities accompany Practical Physics and Practical Biology.
© Nuffield Foundation and the Royal Society of Chemistry
Health and safety checked, 2016
Weller, Overton, Rourke & Armstrong: Inorganic Chemistry 6e
Chapter 19: Videos of chemical reactions
Paramagnetism
Diamagnetic zinc chloride is not attracted to the poles of a powerful magnet. Paramagnetic manganese(II) chloride, the manganese(II) ion having five unpaired electrons, is attracted to the magnet.
Click here for Streaming Video
Oxidation states of vanadium
Vanadium shows one of the widest range of commonly-accessible oxidation states of the transition metals. Using zinc as a reducing agent:
Zn (s) ? Zn2+(aq) + 2e-
Pale yellow vanadium(V) (as the dihydrogen vanadate ion) is successively reduced to blue vanadium(IV) (as the vanadyl ion), to green vanadium(III), and finally to violet vanadium(II).
[H2VO4]- (aq) + 4H+(aq) + e-? VO2+(aq) + 3H2O(l)
VO2+ (aq) + 2H+ (aq) + e- ? V3+(aq) + H2O(l)
V3+(aq) + e-? V2+(aq)
Click here for Streaming Video
The chromate-dichromate equilibrium I
The yellow chromate ion and orange dichromate ion are in equilibrium, with the chromate ion predominating in base and the dichromate ion in acid. Addition of hydrogen (hydronium) ion to chromate ion
solution will drive the equilibrium towards dichromate:
2CrO42-(aq) + 2H+(aq) ? Cr2O72-(aq) + H2O(l)
Addition of hydroxide ion to dichromate ion solution will drive the equilibrium towards chromate:
Cr2O72-(aq) + 2OH-(aq) ? 2CrO42- (aq) + H2O(l)
Click here for Streaming Video
The chromate-dichromate equilibrium II
Barium chromate is insoluble while barium dichromate is soluble. Addition of hydroxide ion to dichromate ion solution increases the chromate ion concentration above the solubility product of barium chromate:
Cr2O72-(aq) + 2OH-(aq) ? 2CrO42-(aq) + H2O(l)
Ba2+(aq) + CrO42-(aq) ? BaCrO4(s)
Click here for Streaming Video
Dichromate
ion as an oxidizing agent I
Dichromate ion is a strong oxidizing agent:
Cr2O72-(aq) + 14H+(aq) + 6e-? 2Cr3+(aq) + 7H2O(l)
In this case, dichromate ion oxidizes iodide ion to iodine. The molecular iodine is extracted into dichloromethane to give the characteristic purple color.
2I-(aq) ? I2(aq) + 2e-
Click here for Streaming Video
Dichromate ion as an oxidizing agent II
Yellow dichromate ion is reduced to green chromium(III) ion when ethanol is added.
Cr2O72-(aq) + 14H+(aq) + 6e-? 2Cr3+ (aq) + 7H2O(l)
CH3CH2OH (aq) + H2O(l) ? CH3CO2H (aq) + 4H+ (aq) + 4e-
Click here for Streaming Video
Decomposition
of ammonium dichromate
Orange crystalline ammonium dichromate decomposes on heating to give the ?fluffy? green chromium(III) oxide. The reaction is non-stoichiometric but can be generally represented by the following equation.
(NH4)2Cr2O7(s) ? Cr2O3 (s) + N2(g) + 4H2O(g)
Click here for Streaming Video
A specific test for
chromate and dichromate ion
Cr2O72-(aq) + 3H2O(l) ? 2CrO(O2)2 (ether) + 6H+ (aq) + 8e-
H2O2 (aq) + 2H+ (aq) + 2e-? 2H2O (l)
Click here for Streaming Video
Oxidation states of manganese
Manganese(VII) in the form of the permanganate ion, can be reduced to manganese(VI), manganese(IV), and
manganese(II) by the hydrogen sulfite (acidic solution) and sulfite (basic solution) ions. The product depends upon the pH of the solution.
Under very basic conditions, the green manganate ion is formed:
MnO4-(aq) + e-? MnO42-(aq)
SO32-(aq) + 2OH-(aq) ? SO42-(aq) + H2O(l) + 2e-
Under slightly basic conditions, a deep brown precipitate of manganese(IV) oxide is formed:
MnO4-(aq) + 2H2O(l) + 3e-? MnO2(s) + 4OH-(aq)
SO32-(aq) + 2OH-(aq) ? SO42-(aq) + H2O(l) + 2e-
Under acidic conditions, the colorless manganese(II) ion is formed:
MnO4-(aq) + 8H+(aq) + 5e-? Mn2+(aq) + 4H2O(l)
HSO3-(aq) + H2O (l) ? SO42-(aq) + 3H+ (aq) + 2e-
Click here for Streaming Video
Permanganate ion as an oxidizing agent I
In a kinetically slow reaction, purple permanganate ion oxidizes
oxalate ion to carbon dioxide, itself being reduced to the colorless manganese(II) ion.
MnO4-(aq) + 8H+(aq) + 5e-? Mn2+(aq) + 4H2O(l)
C2O42-(aq) ? 2CO2 (g) + 2e-
Click here for Streaming Video
Permanganate ion as an oxidizing agent II
Purple permanganate ion oxidizes hydrogen peroxide in acidic solution
to oxygen, itself being reduced to the colorless manganese(II) ion.
MnO4-(aq) + 8H+(aq) + 5e-? Mn2+(aq) + 4H2O(l)
H2O2(aq) ? O2(g) + 2H+(aq) + 2e-
Click here for Streaming Video
Permanganate ion as an oxidizing agent III
Purple permanganate ion oxidizes hydrogen peroxide in basic solution (in which it
is present as the HO2- ion) to oxygen, itself being reduced to a precipitate of deep brown manganese(IV) oxide.
MnO4-(aq) + 2H2O(l) + 3e- ? MnO2(s) + 4OH-(aq)
HO2-(aq) + OH-(aq) ? O2(g) + H2O(l) + 2e-
Click here for Streaming Video
Permanganate ion as an oxidizing agent IV
Purple permanganate ion
oxidizes sulfite ion in basic solution to sulfate ion, itself being reduced to the green manganate ion.
MnO4-(aq) + e-? MnO42-(aq)
SO32-(aq) + 2OH-(l) ? SO42-(aq) + H2O(l) + 2e-
Click here for Streaming Video
Reaction of cobalt(II) ion with hydroxide ion
Addition of hydroxide ion to the pink cobalt(II)
chloride solution first results in the formation of a precipitate of blue cobalt(II) hydroxide chloride. warming with excess hydroxide ion gives the pink cobalt(II) hydroxide which upon standing converts to brown-black cobalt(III) hydroxide:
Co2+(aq) + OH-(aq) + Cl-(aq) ? Co(OH)Cl (s)
Co(OH)Cl (s) + OH-(aq) ? Co(OH)2(s) + Cl-(aq)
4Co(OH)2(s) + O2(g) + 2H2O(l) ? 4Co(OH)3(s)
Click here for Streaming Video
Reaction of nickel(II) ion with hydroxide ion
Pale green nickel(II) ion reacts with hydroxide ion to give a gelatinous pale green precipitate of nickel(II) hydroxide.
Ni2+(aq) + 2OH-(aq) ? Ni(OH)2(s)
Click here for Streaming Video
A specific test for nickel(II) ion
Nickel(II) ion gives a characteristic and quantitative red precipitate with the dimethyglyoximate reagent.
Ni2+(aq) + 2C4H6O2N22-(aq) ? [Ni(C4H6O2N2)2] (s)
Click here for Streaming Video
Reaction of copper(II) ion with hydroxide ion
Blue copper(II) ion reacts with hydroxide ion to give a precipitate of gelatinous/flocculent pale blue copper(II) hydroxide. Warming the mixture results in formation of black copper(II) oxide.
Cu2+(aq) + 2OH-(aq) ? Cu(OH)2(s)
Cu(OH)2(s) CuO(s) + H2O(l)
Click here for Streaming Video
Reaction of sulfide ion with copper(II) ion
Blue copper(II) ion reacts with sulfide ion to give a black precipitate of copper(II) sulfide.
Cu2+(aq) + S2-(aq) ? CuS(s)
Click here for Streaming Video
A specific test for cadmium ion
Colorless cadmium ion gives a characteristic yellow to orange (depending upon particle size) precipitate of cadmium sulfide.
Cd2+(aq) + S2-(aq) ? CdS(s)
Click here for Streaming Video