Encuestas aplicadas al personal

7 1 C(s) 8 6 Zn(s) 9

2 MnO4-(aq) 4 Zn2+(aq) 3 K+

(aq) 10 5 NO3-(aq) 11 H+

(aq)

2. Referring to the above cell, find the correct number for the following:

(a) anode ________ (b) cathode ________ (c) anions ________ (d) cations ________

(e) direction of electron flow _________ (f) direction of anion movement ________ (g) direction of cation movement _______

(h) What would we see happening at the cathode ?

(i) What would we see happening at the anode ?

(j) which electrode could be replaced without changing the voltage (k) which solution could be replaced without changing the voltage

45

Label the parts for the following cells: 1. Ag(s) / Ag+(aq) // Cd(s) / Cd2+(aq)

V

2. K2Cr2O7(aq), H+(aq) / C(s) // FeSO4(aq) / Fe(s)

3. HNO3 (aq),H2(g) / Pt(s) // Zn(NO3)2(aq) / Zn(s)

V

4. MnO4-(aq),H+(aq) / C(s) // PbSO4(aq) / Pb(s)

47

Exercise:

1. What is a salt bridge ?

2. Consider the following anode reactions: (a) Cu(s) Æ Cu2+(aq) + 2e- (b) Zn(s) Æ Zn2+(aq) + 2e- (c) H2(g) Æ 2H+(aq) + 2e- (d) Cl-(aq)Æ Cl2 + 2e- (e) Pb(s) Æ Pb2+(aq) + 2e- (f) Ag(s) Æ Ag+(aq) + e-

(i) Which of the above anode reactions involve the "dissolving_{" of a metal ?}

(ii) Anodes often ______________ in size while anodes often _____________ in size 3. Consider the following cathode reactions:

(a) Cu2+(aq) + 2e- Æ Cu(s) (b) Br2(l) + 2e- Æ 2Br-(aq) (c) Al3+(aq) + 3e- Æ Al(s) (d) Ag+(aq) + e- Æ Ag(s)

(e) 2H2O(l) + 2e- Æ H2(g) + 2OH-(aq)

(i) Which of the above reactions involves the production of a solid from a solution ? 4. Complete the prediction section of the following investigation report.

Problem: What is the complete cell description (half-reactions, net reaction, cell potential, electrodes, electrolytes, cathode and anode,

and the electron and ion flow) of the maximum-voltage cell assembled from the materials available ?

Materials: metal strips of iron, copper, chromium, lead and tin.

beakers, 1.0 mol/L solutions of the metal ion nitrates, aqueous potassium nitrate, connecting wires, U-tube salt bridge, porous cup and voltmeter

5. Consider the diagram below and answer the following questions

V

Ag(s) Mg(s)

AgNO3(aq) Mg(NO3)2(aq)

(a) When the cell is operating the anode will be______________

(b) As the cell operates the _______________electrode will gain in mass. (c) The Eo net for this cell is_____________

(d) The anions would be_________________ (e) The cations would be______________

6. If you wish to replate a silver spoon would you make it the anode or the cathode ? ___ 7. A half-cell consisting of a palladium rod dipping into a 1.00 mol/L solution Pd(NO3)2(aq)

solution is connected with a standard hydrogen half-cell. The cell voltage is 0.99 volt and the platinum electrode in the hydrogen half-cell is the anode. Determine the Eo for the reaction: Pd2+

(aq) + 2e- Æ Pd(s)

8. Consider the cell: Ni2+

(aq) / Ni(s) // Cl2(g),Cl-(aq) / C(s) (a) The anode is_____________

(b) The cathode reaction is:__________________________ (c) The net cell voltage is_____________

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Cell Questions:

1. The reduction half reactions and potentials for a silver-cadmium cell are given below. Write down and label the anode and cathode half reactions, the net equation, and determine the Eo for the cell.

2AgO(s) + H2O(l) + 2e- Æ Ag2O(s) + 2OH-(aq) Eo = +0.57 V Cd(OH)2(s) + 2e- Æ Cd(s) + 2OH-(aq) Eo = -0.76 V

2. The reduction half reactions and potentials for a methane-oxygen fuel cell are given below. Write down and label the anode and cathode half reactions, the net equation, and determine the Eo net for the cell.

CO3 2-

(l) + 7H2O(g) + 8e- Æ CH4(g) + 10OH- Eo = +0.17 V

O2(g) + 2H2O(g) + 4e- Æ 4OH-(l) Eo = +0.40 V

3. The equations involved in the operation of a silver-zinc battery are given below along with their oxidation potentials. Write and label the anode and cathode half reactions and write the net ionic equation. Include the Eo values and the Eo net.

2Ag(s) + 2OH-(aq) Æ Ag2O(s) + H2O(l) + 2e- Eo = -0.342 V

4. Scientists isolated a previously undiscovered metal of which they managed to produce a few thin strips. They predicted that the metal would fit between tin and lead on the table of standard electrode potentials. Assume they give you the metal strips and you have access to a lab with various common metals and solutions of their ions. Use M(s) to represent the new metal.

(a) List two steps you could use as an experimental procedure that would test the scientists prediction.

(b) Assume that the scientists prediction was verified and state the observations that you would expect for each step.

(c) Assume that the scientists prediction was verified and write a possible balanced equation for each reaction that occurred involving the unknown metal.

(d) Give a probable Eo value for the reduction half-reaction of the metal.

5. Explain the function of the salt bridge in the voltaic cell.

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Fuel Cells

A fuel cell is a voltaic cell for which the reactants are continuously supplied.

Example:

Methane is usually combined with oxygen to produce carbon dioxide and water vapor and heat energy.

CH4(g) + 2O2(g) Æ CO2(g) + H2O(g) + 803 kJ

Usually the energy from this reaction is used to warm houses or run machines. In a fuel cell designed to use this reaction the energy is used to produce an electric current. The electrons flow from the reducing agent (CH4(g)) to the oxidizing agent (O2(g)).

The U.S. space program uses a fuel cell based on the reaction of hydrogen and oxygen to produce water. A diagram of such a cell is

e- _{wet, acidified plastic membrane}

H2(g) Æ Å O2(g)

H+

Carbon and platinum

Steam Anode reaction: Cathode reaction: Net Reaction: Disadvantages: Advantages: