Osukuuni Practice Questions chem 1151 Oxidation-Reduction (Redox) Reactions




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Osukuuni Practice Questions CHEM 1151 Oxidation-Reduction (Redox) Reactions
Reactions in which electrons are transferred from one substance to another substance.
Oxidation: Loss of electrons (increase in oxidation number).

Reduction: Gain of electrons (decrease in oxidation number).
Rules For Assigning Oxidation Numbers

1. Oxidation number of uncombined elements = 0 {e.g. Na(s), O2(g), H2(g), Hg(l)}

2. Oxidation number of a monatomic ion = charge {e.g. Na+ = +1, Cl- = -1, Ca2+ = +2, Al3+ = +3}

3. Oxygen is usually assigned -2 {e.g. H2O, CO2, SO2, SO3}

4. Hydrogen is usually assigned +1 (-1 when bonded to metals) {e.g. HCl, NH3, H2O, CaH2, NaH}

5. Halogens are usually assigned -1 (F, Cl, Br, I)

6. The sum of oxidation numbers for neutral compound is 0 and for polyatomic ion the charge

{e.g. H2O = 0, CO32- = -2, NH4+ = +1}



Practice Questions

Assign oxidation numbers to all species in the redoc reactions below. Identify the oxidizing and reducing agents.

1. Fe(OH)2 (s) + CrO42- (aq) → Fe2O3(s) + Cr(OH)4-(aq) (basic)
Reactant side; Fe: +2, O: -2, H: +1, O: -2, Cr: +6 Product side; Fe: +3, O: -2, O: -2, H: +1, Cr: +3

Oxidizing agent: CrO42- (aq) Reducing agent: Fe(OH)2 (s)


2. H2O2(aq) + MnO4- (aq) → Mn2+(aq) + O2(g) (acidic)
Reactant side; H: +1, O: -1, Mn: +7, O: -2 Product side; Mn: +2, O: 0

Oxidizing agent: MnO4- (aq) Reducing agent: H2O2(aq)


3. S2O32-(aq) + I2(aq) → S4O62-(aq) + I-(aq) (acidic)
Reactant side; S: +2, O: -2, I: 0 Product side; S: +2.5, O: -2, I: -1

Oxidizing agent: I2(aq) Reducing agent: S2O32-(aq)


4. Al(s) + OH-(aq) → Al(OH)4-(aq) + H2(g) (basic)
Reactant side; Al: 0, O: -2, H: +1 Product side; Al: +3, O: -2, H: +1, H: 0

Oxidizing agent: OH-(aq) Reducing agent: Al(s)


5. PbO2(s) + Cl-(aq) → ClO-(aq) + Pb(OH)3-(aq) (basic)
Reactant side; Pb: +4, O: -2, Cl: -1 Product side; Cl: +1, O: -2, Pb: +2, O: -2, H: +1

Oxidizing agent: PbO2(s) Reducing agent: Cl-(aq)


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