What causes the colour change from CrO42- (yellow) to Cr2O72- (orange)?

Question

BlackTom AI · Accepted Answer

Chromate–dichromate equilibrium is pH-dependent: 2 CrO4^2- + 2 H+ ⇌ Cr2O7^2- + H2O. This acid–base equilibrium shifts toward dichromate as the solution becomes more acidic, producing the observed yellow → orange colour change.

1. Relevant knowledge points and concepts
- Chromate (CrO4^2-) and dichromate (Cr2O7^2-) form a common acid–base equilibrium in aqueous solution; addition of H+ shifts the equilibrium toward Cr2O7^2-.
- Oxidation state of chromium in both species is +6; therefore no redox (oxidation or reduction) occurs in the interconversion.
- The different colours arise from different electronic structures and ligand environments, giving different light absorption (chromate: yellow; dichromate: orange).

2. Step-by-step reasoning
- Write the equilibrium: 2 CrO4^2- + 2 H+ ⇌ Cr2O7^2- + H2O.
- Adding acid (increasing [H+]) drives the reaction to the right by Le Chatelier’s principle, increasing dichromate concentration and changing solution colour to orange.
- Removing H+ (making solution basic) shifts equilibrium left and restores yellow chromate.

3. Oxidation-state check (intermediate steps)
- For CrO4^2-: x + 4(−2) = −2 → x − 8 = −2 → x = +6.
- For Cr2O7^2-: 2x + 7(−2) = −2 → 2x − 14 = −2 → 2x = 12 → x = +6.
- Since Cr stays at +6, no oxidation or reduction occurs.

4. Why the correct answer is correct
- The colour change is triggered by protonation/deprotonation equilibrium (acid–base chemistry) shifting the relative amounts of chromate and dichromate, not by electron transfer.

5. Why other options are incorrect
- a. Oxidation: would require chromium to lose electrons (change in oxidation state) — not observed.
- b. Reduction: would require chromium to gain electrons (change in oxidation state) — not observed.
- c. Both acid–base and reduction: reduction is not occurring, so this is incorrect.

6. Conclusion
- The correct answer is d. Acid-base reaction, which matches the provided answer.

What causes the colour change from CrO42- (yellow) to Cr2O72- (orange)?单项选择题

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