Under acidic conditions the bromate ion (BrO3–) is reduced to the bromide ion. The balanced half-equation for this process is:

Question

BlackTom AI · Accepted Answer

Reduction changes the oxidation state: bromine in BrO3– is +5 (O is −2, overall −1 → Br = +5) and in Br− it is −1, so each bromine atom gains 6 electrons. This key oxidation-number change determines the number of electrons in the balanced half-equation.

1) Relevant concepts
- Oxidation states to determine electrons transferred.
- Balancing atoms (Br, O, H) and charge in acidic solution: add H2O to balance O, H+ to balance H, and e− to balance charge.

2) Step-by-step balancing
- Write the unbalanced reduction: BrO3– → Br−.
- Balance Br: 1 Br on each side — OK.
- Balance O by adding H2O: BrO3– → Br− + 3 H2O (adds 3 O atoms on right).
- Balance H by adding H+ (acidic medium): there are 3 H2O → 6 H atoms, so add 6 H+ on the left: BrO3– + 6 H+ → Br− + 3 H2O.
- Balance charge by adding electrons: left charge = (−1 from BrO3–) + (+6 from 6H+) = +5. Right charge = −1 (Br−). To reduce left net (+5) to right net (−1) you must add 6 e− to the left (each e− is −1): +5 + (−6) = −1.

3) Intermediate calculations
- Oxidation change: +5 → −1 = gain of 6 e−.
- Charge check: left: −1 + 6(+1) + 6(−1) = −1; right: −1 + 3×0 = −1. Atom check: Br 1 = 1; O 3 = 3; H 6 = 6.

4) Why this answer is correct
The half-equation BrO3– + 6 H+ + 6 e− → Br− + 3 H2O satisfies conservation of mass (Br, O, H) and charge, and correctly reflects the six-electron reduction implied by the oxidation-number change.

5) Why other options are incorrect
- Any option with a different number of electrons (e.g., 3 e−, 8 e−, 10 e−) fails the oxidation-number and charge balance.
- Options that produce Br2 or O2 instead of Br− are describing different redox processes (disproportionation or oxidation) and do not represent reduction to bromide.
- Incorrect stoichiometries of H2O and H+ mean oxygen or hydrogen atoms are not conserved.

6) Conclusion
The balanced acidic half-equation is BrO3– + 6 H+ + 6 e− → Br− + 3 H2O, which matches the provided correct answer (a).

Under acidic conditions the bromate ion (BrO3–) is reduced to the bromide ion. The balanced half-equation for this process is:单项选择题

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