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REVISION QUESTIONS
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Redox Reactions
1. Define the following terms:
(a) Redox Reactions
(b) Oxidation
(c) Reduction
(d) Oxidant (oxidiser or oxidising agent)
(e) Reductant (reducer or reducing agent)
2. Identify which of the following reactions are redox reactions and for the redox reactions clearly identify the species that is oxidised and its oxidation product; the species that is reduced and its reduction product; the oxidant (oxidiser) and the reductant (reducer). Also show the number of electrons transferred.
- 2 Mg + O 2 ® 2 MgO
- Cl 2 + 2 KI ® 2 KCl + I 2
- CH 4 + O 2 ® 2H 2O + CO 2
- AgNO 3 + NaCl ® AgCl + NaNO 3
- Zn + CuSO 4 ® ZnSO 4 + Cu
- 2Na + 2H 2O ® 2NaOH + H 2
- 2 HCl + CaCO 3 ® CaCl 2 + H 2O + CO 2
- Mg + 4HNO 3 ® Mg(NO 3) 2 + 2 NO 2 + 2 H 2O
- H 2SO 4 + NaOH ® Na 2SO 4 + H 2O
- Mg + 2 HCl ® MgCl 2 + H 2
- MnO 2 + 4HCl ® Cl 2 + 2H 2O + MnCl 2
3. Use the 7 step ion-half-cell method to write oxidation and reduction half-equations and a fully balanced overall equation.
(a). When concentrated nitric acid reacts with silver, the nitrate ion is reduced to brown nitrogen dioxide gas (NO 2), while silver is oxidised to silver ions (Ag+). .
(b). When hot concentrated sulfuric acid reacts with copper, the sulfate ion is reduced to sulfur dioxide gas (SO 2), while the copper is oxidised to copper (II) ions.
(c) When dilute nitric acid reacts with potassium iodide solution, the iodide ion (I-) is oxidised to iodine (I 2), while the nitrate ion is reduced to colourless nitric oxide gas (NO). .
(d). Chlorine gas oxidises potassium bromide solution. Bromide ions (Br- are oxidised to bromine liquid (Br 2), while the chlorine gas is reduced to chloride ions (Cl -).
(e). Acidified potassium permanganate solution oxidises sulfur dioxide gas (SO 2) to the sulfate ion. The solution becomes colourless due to the reduction of the purple permanganate ion (MnO 4 -) to colourless manganese (II) ion (Mn 2+).
(f). Acidified hydrogen peroxide (H 2O 2) oxidises sodium sulfide (Na 2S) solution. The sulfide ion (S 2-) is oxidised to sulfur (S), while the hydrogen peroxide is reduced to water.
(g). Orange acidified potassium dichromate solution oxidises ethanol (alcohol in wine, beer, spirits etc.) with a formula of CH 3CH 2OH to ethanal CH 3CHO. The solution turns green as the orange dichromate ion (Cr 2O 7 2-) is reduced to the green chromium (III) ion (Cr 3+)
4. The oxidation state of an element is its ____________________ number. It is assigned according to a set of rules. Complete the following set of rules by inserting the appropriate oxidation state
RULE: The oxidation number of an atom of a free element is ____________
RULE : The oxidation number of hydrogen in all compounds and ions is _____ unless it is in a metal hydride when its oxidation state is -1.
RULE : The oxidation state of oxygen in all compounds and ions, except peroxides, is __________.
RULE : The oxidation number of a monatomic ion is equal to its _________________.
RULE : The algebraic sum of the oxidation numbers in the formula of a compound is ______.
RULE : The algebraic sum of the oxidation numbers in the formula of a polyatomic ion is equal to the ______________________________________.
5. Find the oxidation numbers of the elements highlighted in the following species.
KClO 3 NaCl HClO KMnO 4 MnO 2 Mn 2+ Mn
H 2S SO 2 SO 3 S 8 H 2SO 4 H 2SO 3 S 2O 7 2- HSO 4 - NH 4NO 3
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