Given two half equations, we need to be able to write a fully balanced redox equation both under acidic and alkaline conditions, using changes in oxidation state to guide us. This is not straightforward so the methods set out below are worth learning and practising 😬.
Under acidic conditions
During the reaction between sulphur (II) oxide and acidified potassium chromate (VI), the sulphur (II) oxide is oxidised to sulphate (VI) ions and the chromate (VI) ions are reduced to chromium (III) ions. Construct half equations and then a balanced overall equation for the reaction.
Under alkaline conditions
Thiosulphate ions are oxidised to sulphate (VI) ions in alkaline conditions. Write a half equation to show this.
You can now combine this oxidation half equation with any reduction half equation using the first method, from step 3 😊
Practice questions
- Write the oxidation and reduction half equations for each of the following reactions. Identify the oxidising and reducing agent in each case
(a) 2Ag+(aq) + Cu(s) ⇾ Cu2+(aq) + 2Ag(s)
(b) Ni(s) + Sn2+(aq) ⇾ Ni2+(aq) + Sn(s)
(c) Zn(s) + 2Ag+(aq) ⇾ Zn2+(aq) + 2Ag(s)
(d) 2Al(s) + 6H+(aq) ⇾ 2Al3+(aq) + 3H2(g)
(e) Zn(s) + S(s) ⇾ ZnS(s)
2. Chlorine is used as antimicrobial agent in solution
Cl2(aq) + H2O(l) ⇌ HClO(aq) + HCl(aq)
(a) What is the oxidation state of chlorine in HClO and HCl?
(b) Give two half equations to show the oxidation and reduction reactions.
3. (a) What is the equation for the reaction between acidified dichromate (VI) ions, Cr2O72-, and sulphur dioxide in solution? The chief products are chromium (III) ions, Cr3+, and sulphate (VI) ions, SO42-.
(b) In acidic solution, manganate (VII) ions, MnO4–, oxidise sulphate (IV) ions, SO32-, to sulphate (VI). The manganese is reduced to Mn2+ ions. Write a balanced equation.
(c) In acidic solution, dichromate (VI) ions oxidise tin (II) to tin (IV) ions. Chromium (III) ions are also produced. Write a balanced equation.
4. In the reaction between hydrogen iodide, HI, and sulphuric acid, H2SO4, the hydrogen iodide is oxidised to iodine, I2, and the sulphuric acid is reduced to hydrogen sulphide, H2S.
(a) give the oxidation state of iodine in HI and I2
(b) give the oxidation state of sulphur in H2SO4 and H2S
(c) use your answers to write a balanced equation for the reaction between HI and H2SO4
5. Some bacteria can oxidise methane to carbon dioxide in the absence of oxygen
CH4(g) + 2H2O(l) ⇾ CO2(g) + 8H+(aq) + 8e–
The process involves a reaction between methane and nitrite ions, NO2–, in acidic conditions.
(a) Write a half equation for the reduction of NO2– to N2 in acidic conditions.
(b) Balance the overall equation by combing the two half equations
….. CH4 + …..NO2– + ……H+ ⇾ …..CO2 + …..N2 + …..H2O
(c) This reaction is thermodynamically stable but won’t occur under standard laboratory conditions. Suggest the role of the bacteria.
6. Manganate(VII) ions, MnO4–, react with iodide ions under alkaline conditions to form manganese(IV) oxide, MnO2, and iodine. Write a balanced redox equation for the reaction.
Answers
1.
| Oxidation | Reduction | Oxidising agent | Reducing agent | |
| (a) | Cu ⇾ Cu2+ + 2e– | Ag+ + e– ⇾ Ag | Ag+ | Cu |
| (b) | Ni ⇾ Ni2+ + 2e– | Sn2+ + 2e– ⇾ Sn | Sn2+ | Ni |
| (c) | Zn ⇾ Zn2+ + 2e– | Ag+ + e– ⇾ Ag | Ag+ | Zn |
| (d) | Al ⇾ Al3+ + 3e– | 2H+ +2e– ⇾ H2 | H+ | Al |
| (e) | Zn ⇾ Zn2+ + 2e– | S + 2e– ⇾ S2– | S | Zn |
2. (a) chlorine has an oxidation state of +1 in HClO and -1 in HCl
(b) oxidation: Cl2(aq) + H2O(l) ⇾ 2HClO(aq) + 2H+(aq) + 2e–
reduction: Cl2(aq) + 2H+(aq) + 2e– ⇾ 2HCl(aq)
4. (a) iodine changes from -1 to 0
(b) sulphur changes from +6 to -2
(c) 8HI(aq) + H2SO4(aq) ⇾ 4I2(aq) + H2S(g) + 4H2O(l)
5. (a) NO2–(aq) + 3e– + 4H+(aq) ⇾ ½N2(g) + 2H2O(l) (or double up)
(b) 3CH4(g) + 8NO2–(aq) + 8H+(aq) ⇾ 3CO2(g) + 4N2(g) + 10H2O(l)
(c) catalyst
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