In aqueous solution the chromate(VI) ion and the dichromate(VI) ion exist in equilibrium. The position of the equilibrium depends on how acidic the solution...
Question 1 Report
In aqueous solution the chromate(VI) ion and the dichromate(VI) ion exist in equilibrium. The position of the equilibrium depends on how acidic the solution is.
2CrO42−(aq) + 2H+(aq) ⇌ Cr2O72−(aq) + H2O(l)
(a) Complete the table to give the colour of each solution. [2]
| Solution | colour |
|---|---|
| contains mainly chromate(VI) ions | (i) |
| contains mainly dichromate(VI) ions | (ii) |
(b) A few drops of dilute acid are added to the mixture. Predict the colour change and explain it in terms of the position of equilibrium. [3]
(c) Dilute sodium hydroxide is then added to the acidified solution. Predict the colour change. [1]
(d) Explain, using Le Chatelier's principle, why adding sodium hydroxide has this effect. [2]
(e) State and justify the effect on the position of equilibrium of diluting the mixture with a large volume of water. [2]
Answer Details
This question tests Le Chatelier's principle using the colour of the equilibrium 2CrO42−(aq) + 2H+(aq) ⇌ Cr2O72−(aq) + H2O(l). The chromate(VI) ion is yellow and the dichromate(VI) ion is orange.
(a) The colours are:
| Solution | colour |
|---|---|
| contains mainly chromate(VI) ions | (i) yellow [1] |
| contains mainly dichromate(VI) ions | (ii) orange [1] |
(b) Adding acid raises the concentration of H+, a reactant, so the equilibrium shifts to the right to oppose the increase in H+ [1], forming more orange dichromate(VI) ions [1]; the solution therefore turns from yellow to orange [1].
(c) Adding dilute sodium hydroxide turns the solution back from orange to yellow [1].
(d) The hydroxide ions react with and remove the H+ ions (OH− + H+ → H2O) [1]. Losing H+ makes the equilibrium shift to the left to replace the removed H+, forming more yellow chromate(VI) ions [1].
(e) There are more dissolved ions on the left (2 CrO42− + 2 H+ = 4 particles) than on the right (1 Cr2O72−). Adding a large volume of water dilutes the mixture, so the equilibrium shifts to the side with more dissolved particles, the left [1], forming more yellow chromate(VI) and making the solution more yellow [1].
Exam reminder: for this equilibrium, anything that adds H+ (acid) drives it towards orange, and anything that removes H+ (alkali) drives it towards yellow.
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