# 18.E: Electrochemistry

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### Q18.1

Calculate the oxidation number for nitrogen in the following substances:

1. $$\ce{NH3}$$
2. $$\ce{N2}$$
3. $$\ce{NO2}$$
4. $$\ce{NO3-}$$

### Q18.2

In the oxidation-reduction reaction,

$\ce{Br2(l) + 2 I- (aq) \rightarrow 2 Br- (aq) + I2(s)}$

1. which substance(s) is being reduced?
2. which element(s) is increasing in oxidation number?
3. which element(s) is gaining electrons?
4. which substance(s) is the oxidizing agent?
5. which substance(s) is the reducing agent?

### Q18.3

Classify each of the following substances as an oxidizing agent, reducing agent or both. List the oxidizing agents in order of decreasing strength; list the reducing agents in order of decreasing strength (use SRP Table):

$$\ce{Ni(s)}$$, $$\ce{H+ (aq)}$$, $$\ce{Au(s)}$$, $$\ce{Cl2(g)}$$, $$\ce{Sn^2+ (aq)}$$, $$\ce{Mg(s)}$$, $$\ce{Fe^2+ (aq)}$$

### Q18.4

For each of the following reactions: 1) identify the oxidation and reduction half-equations, and 2) balance the equation (adding $$\ce{H+}$$ and $$\ce{H2O}$$ as needed), 3) find $$\mathrm{E^\circ}$$ (in volts) and 4) determine whether the reaction is spontaneous under standard conditions.

1. $$\ce{Ag(s) + Cu^2+(aq) \rightarrow Ag+(aq) + Cu(s)}$$
2. $$\ce{Ni(s) + MnO_4- (aq) \rightarrow Ni^2+(aq) + Mn^2+(aq)}$$
3. $$\ce{Mn^2+(aq) + NO3- (aq) \rightarrow MnO2(s) + NO(g)}$$

### Q18.5

For each of the following reactions: 1) find $$\mathrm{E^\circ}$$ (in volts) and 2) determine whether the reaction is spontaneous under standard conditions.

1. the reaction between iron and iron(III) ions to give iron(II) ions.
2. the following cell:   $$\mathrm{ I^- \, |\, I_2 \,||\, Zn^{2+} \,|\, Zn}$$

### Q18.6

Which of the following ions will oxidize $$\ce{Br-}$$ ion to $$\ce{Br2}$$?

1. $$\ce{Pb^2+}$$
2. $$\ce{H+}$$
3. $$\ce{Au^3+}$$
4. $$\ce{MnO4-}$$

### Q18.7

A voltaic cell has an aluminum electrode in $$\ce{Al2(SO4)3}$$ solution in one compartment and the other compartment has a lead electrode in $$\ce{PbSO4}$$ solution.

1. Which has a greater tendency to be oxidized, $$\ce{Al}$$ or $$\ce{Pb}$$?  Write a balanced equation for the spontaneous reaction.
2. Draw a diagram of the voltaic cell, including
1. the anode and cathode
2. the direction of flow of electrons, positive ions and negative ions
3. Which electrode will increase in mass?

### Q18.8

The following table contains data obtained by measuring the voltage between two metals in an experiment like the one you did with a lemon:

Voltaic Cell

Anode (-)

Cathode (+)

Cell Voltage (v)

$$\ce{Pb/Ni}$$

$$\ce{Ni}$$

$$\ce{Pb}$$

0.10 volts

$$\ce{Pb/Au}$$

$$\ce{Pb}$$

$$\ce{Au}$$

0.80 volts

$$\ce{Pb/Fe}$$

$$\ce{Fe}$$

$$\ce{Pb}$$

0.25 volts

$$\ce{Ni/Au}$$

$$\ce{Ni}$$

$$\ce{Au}$$

0.90 volts

$$\ce{Ni/Fe}$$

$$\ce{Fe}$$

$$\ce{Ni}$$

0.15 volts

$$\ce{Fe/Au}$$

$$\ce{Fe}$$

$$\ce{Au}$$

1.05 volts

1. From the data in the table above, which metal is the strongest reducing agent?
2. From the data in the table above, which metal is the weakest reducing agent?
3. Using the reduction of lead ($$\ce{Pb}$$) as a reference, construct a half-cell voltage table from the experimental data above.