1. Indicate whether the following balanced equations involve oxidation-reduction

Question

1. Indicate whether the following balanced equations involve oxidation-reduction. If they do, identify the elements that undergo changed in oxidation number:

2. A voltaic cell is constructed that is based on the following reaction:

a) if the concentration of Sn2+ in the cathode half-cell is 1.00 M and the cell generates an emf of +0.22 V, wht is the concentration of Pb2+ in the anode half-cell? b) If the anode half-cell contains [SO42-] = 1.00 M in equilibrium with PbSO4(s), what is the Ksp of PbSO4?

3. A volatic cell is constructed with two silver-silver chloride electrodes, each of which is based on the following half reaction:

AgCl(s) - e- ---> Ag(s) + Cl-(aq)

The two half cells have [Cl-] = 0.0150 M and [Cl-] = 2.55 M respectively. a) Which electrode is the cathode of the cell? b) What is the standard emf of the cell? c) What is the cell emf for the concentration given? d) For each electrode. predict whether [Cl-] will increase, decrease, or stay the same as the cell operates.

4. An iron object is plated with a coating of cobalt to protect against corrosion. Does the cobalt protect iron by cathodic protection. Explain?

5. Metallic magnesium can be made by the electrolysis of molten MgCl2. a) What mass of Mg is formed by passing a current of 4.55 A through molden MgCl2 for 4.50 days? b) How many minutes are needed to plate out 25.00 Mg from molten MgCl2 using 3.50 A of current?

Explanation / Answer

1.)

(a)PBr3(l) + 3H2O(l) -----> H3PO3(aq) + 3HBr(aq)

Here, phosphorous is in the +3 state both as a product and as a reactant; neither H nor O nor Br change oxidation states, so this reaction does not involve oxidation-reduction.

(b) NaI(aq) + 3HOCl(aq) ----> NaIO3 + 3HCl(aq)

We calculate the oxidation numbers of Na, I, O and Cl for the products and the reactants:

On the reactant side, we have: Na +1, I -1, H +1, O -2, Cl +1

On the product side, we have: Na +1, I +5, O -2, H +1, Cl-1

I is oxidized from -1 to +5, and Cl is reduced from +1 to -1.

(c) 3SO2(g) + 2HNO3(aq) + 2H2O(l) ----> 3H2SO4(aq) + NO(g)

Here, the S in SO2 is in the +4 state; the nitrogen atom in HNO3 is in the +5 state. The nitrogen in NO is in the +2 state, and the sulfur in sulfuric acid is in the +6 state, so sulfur is oxidized and nitrogen reduced.

(d)2H2SO4(aq) + 2NaBr(s) -----> Br2(l) + SO2(g) + Na2SO4 + 2H2O(l)

Here, S is reduced from +6 (in H2SO4) to +4 (in SO2); Br is oxidized from -1 (in NaBr) to 0 (in Br2).

2.)

Red: Sn2+ + 2 e

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