Writing half-equations (pH <7) - OCR Chemistry A-level

What is the half-equation when bromide is oxidised to bromine?

• Br₂ + 2e^- -> 2Br^-

• Br₂ -> 2Br^- + 2e^-

• 2Br^- + 2e^- -> Br₂

• 2Br^- -> Br₂ + 2e^-

What is the half-equation when iron (III) is reduced to iron?

• Fe³⁺ + e^- -> Fe²⁺

• Fe³⁺ + 2e^- -> Fe²⁺

• Fe³⁺ + 3e^- -> Fe

• Fe³⁺ -> Fe + 3e^-

What is the half-equation when sulfur dioxide is oxidised to sulfate?

• SO₂ + 2H₂O -> SO₄^2- + 4H⁺ + 2e^-

• SO₂ + 2H₂O + 2e^- -> SO₄^2- + 4H⁺

• SO₂ + 4H⁺ -> SO₄^2- + 2e^- 2H₂O

• SO₂ + H₂O + e^- -> SO₄^2- + 2H⁺

What is the half-equation when H₂S is oxidised to sulfate?

• H₂S + 10H⁺ -> SO₄^2- + 8e^- 4H₂O

• H₂S + 4H₂O -> SO₄^2- + 8e^- + 10H⁺

• H₂S + 2H₂O -> SO₄^2- + 8e^- + 6H⁺

• H₂S + 4H₂O -> SO₄^2- + 8e^- + 8H⁺

What is the half-equation when chromium (III) is oxidised to dichromate (VI)?

• 2Cr³⁺ + 14H₂O -> Cr₂O₇^2- + 28H⁺ + 16e^-

• 2Cr³⁺ + 7H₂O -> Cr₂O₇^2- + 12H⁺ + 4e^-

• Cr₂O₇^2- + 14H⁺ + 6e^- -> 2Cr³⁺ + 7H₂O

• 2Cr³⁺ + 7H₂O -> Cr₂O₇^2- + 14H⁺ + 6e^-

Consider this reaction:

HI is oxidised to I₂ by conc H₂SO₄ - which is reduced to H₂S.

What is the balanced equation?

• H₂SO₄ + 4HI -> 2I₂ + H₂S + 2H₂O

• H₂SO₄ + 8HI -> 4I₂ + H₂S + 4H₂O

• H₂SO₄ + 4HI -> 4I₂ + H₂S + 4H₂O

• H₂SO₄ + 8HI -> 4I₂ + H₂S + 2H₂O

• H₂SO₄ + 8HI -> 2I₂ + 2H₂S + 4H₂O

Answer explanation

Consider this reaction.

What is the balanced equation?

(Hint: work out the two half-equations, then combine them - remembering the electrons need to balance)

• 2HBr + 2H₂SO₄ -> Br₂ + 2SO₂ + 2H₂O

• 4HBr + H₂SO₄ -> 2Br₂ + SO₂ + 2H₂O

• 2HBr + H₂SO₄ -> Br₂ + SO₂ + 2H₂O

• 4HBr + H₂SO₄ -> 2Br₂ + SO₂ + 4H₂O

Consider this reaction.

What is the balanced equation?

(Hint: work out the two half-equations, then combine them - remembering the electrons need to balance)

• 3H₂S + 2HNO₃ -> 3S + 2NO + 4H₂O

• H₂S + 2HNO₃ -> S + 2NO + 2H₂O

• 2H₂S + 2HNO₃ -> 2S + 2NO + 4H₂O

• 3H₂S + 2HNO₃ -> 3S + NO + 4H₂O