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PRE-AP CHEMISTRY CHAPTER 23 ELECTROCHEMISTRY

The interconversions of chemical and electrical energy are called electrochemical processes, all of which involve the transfer of electrons. This process occurs in a device called an electrochemical cell. . Remember the activity series of metals in Chapter 8? We used it to determine reactivity in single replacement reactions. The most active metals are the group I

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PRE-AP CHEMISTRY CHAPTER 23 ELECTROCHEMISTRY

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    1. PRE-AP CHEMISTRY CHAPTER 23 ELECTROCHEMISTRY

    2. The interconversions of chemical and electrical energy are called electrochemical processes, all of which involve the transfer of electrons. This process occurs in a device called an electrochemical cell.

    3. Remember the activity series of metals in Chapter 8? We used it to determine reactivity in single replacement reactions. The most active metals are the group I&II metals + Al, then the transition metals, H is by itself, and the least active are the "jewelry metals (Cu, Hg, Ag, Au). The more active metals (higher on the activity series) can replace the less active metals (lower on the activity series) in a chemical reaction.

    5. These single replacement reactions are one type of redox reaction. Electrons are lost by the more active metals and gained by the less active metals. The higher a metal is on the activity series, the easier it is oxidized. Ag will not react with NaCl because Ag can not spontaneously transfer electrons to Na. Ag is less active than Na.

    6. Electrochemical cells that convert chemical energy into electrical energy are called voltaic or galvanic cells. The energy is produced by spontaneous redox reactions.

    7. Voltaic cells can be separated into two half-cells, one in which oxidation occurs and one in which reduction occurs. A half cell consists of a metal rod or strip immersed in a solution of its ions.

    8. The two half-cells are connected by a porous partition or a salt bridge. A salt bridge is a tube containing a conducting solution. Ions pass through the salt bridge to keep the charges balanced. Electrons pass through an external wire.

    9. The metal rods in voltaic cells are called electrodes. Oxidation occurs at the anode and reduction occurs at the cathode. (An Ox and a Red Cat)

    11. Species undergoing reduction receive electrons from the cathode. Species undergoing oxidation donate electrons to the anode. The direction of electron flow is therefore from the anode to the cathode.(FAT CAT)

    13. In a voltaic cell, the anode is assigned a negative charge and the cathode is assigned a positive charge. Line notation is a shorthand method for representing electrochemical cells. The general form is: anode|anode ion || cathode ion | cathode The double vertical line represents the salt bridge.

    14. For a voltaic cell involving Cu and Zn as shown on the top of pg 680, the line notation would be represented as: Zn(s) | Zn2+(aq) || Cu2+(aq) | Cu(s)

    16. When only ions (rather than solids) are involved in the redox process, "inert" electrodes such as graphite or platinum are used. Ag+ + Fe2+ ? Fe3+ + Ag(s)

    18. Galvanic cell: Ag+ + Fe2+ ? Fe3+ + Ag(s) Line notation:Pt | Fe2+,Fe3+ || Ag+ | Ag Fe2+ is oxidized, but we can’t make a solid piece of Fe2+. Ag+ is reduced. Oxidation half-reaction: Fe2+ ? Fe3+ + e- Reduction half-reaction: Ag+ + e- ? Ag

    19. A dry cell is a voltaic cell in which the electrolyte is a paste. The flashlight battery is a common example. The inexpensive dry cell batteries consist of a zinc container which acts as the anode, a graphite rod which acts as the cathode and an electrolyte paste of MnO2, ZnCl2, NH4Cl and H2O. The pH of this solution is acidic and tends to corrode the zinc case. Zinc is oxidized and manganese is reduced.

    21. An alkaline battery has the NH4Cl replaced with KOH or NaOH and is basic instead of acidic. This makes the battery last longer.

    22. A group of cells connected together make up a battery. A lead storage battery is commonly used in an automobile. In this type of battery the anode is lead. The cathode is lead dioxide. The electrolyte (battery acid) is sulfuric acid.

    26. A fuel cell is an electrochemical device that, in most cases, combines hydrogen and oxygen to produce electricity, with water and heat as its by-product.  As long as fuel is supplied, the fuel cell will continue to generate power. 

    28. What are some uses for fuel cells? H2 powered cars spacecraft military submarines

    29. The electromotive force (emf) or cell potential (Eo) of a galvanic cell is a combination of the potentials of the two half-reactions. The standard reference that all half-cells are measured against is the standard hydrogen electrode (SHE), 2H+ + 2e- ? H2 Eo = 0.00 Volts

    31. Table 23-2 in your book has a list of the reduction potentials of many reduction reactions. To change a reaction from reduction to oxidation, simply reverse the reaction and change the sign of the reduction potential to make it an oxidation potential. Reduction Potential Table

    32. Adding the potentials together for the oxidation and the reduction half-reactions will give you the overall cell potential.

    33. For example, using the Zn and Cu cell we used earlier, we can calculate the cell potential. Zn ? Zn2+ + 2e- Eo = + 0.76 Cu2+ + 2e- ? Cu Eo = + 0.34 ----------------- Eocell = 1.10 V

    34. Galvanic cells require Eocell > 0 V

    35. The more positive a reduction potential is, the more easily that substance can be reduced. The more negative a reduction potential is, the more easily that substance can be oxidized.

    36. One of the reduction potentials will have to be reversed (to form an oxidation half-reaction in every Eo calculation. To determine which reaction is to be reversed, the sum of the oxidation and reduction half-reactions must be > 0 V in a galvanic cell. When you reverse a reaction, Eo gets the opposite sign. When you multiply a reaction by a coefficient (for purposes of balancing), the Eo is NOT changed. If the Eocell is negative, the reaction is not spontaneous.

    37. Write the half-reactions and calculate the cell potential for the following reaction: Co2+ + Fe ? Fe2+ + Co oxidation: Fe ? Fe2+ + 2e- +0.44V reduction: Co2+ + 2e- ? Co -0.28V 0.16V

    38. Given the following half-cells, decide which is the anode and the cathode and calculate Eocell. Ni2+ + 2e- ? Ni Eo = -0.23 V O2 + 4H+ + 4e- ? 2H2O Eo = +1.23 V We must reverse the equation with the less positive potential. anode Ni ? Ni2+ + 2e- Eo = + 0.23V cathode O2 + 4H+ + 4e- ? 2H2O Eo = +1.23 V Eocell = +1.46V

    39. B. Ce4+ + e- ? Ce3+ Eo = +1.70 V Sn2+ + 2e- ? Sn Eo = -0.14 V anode Sn ? Sn2+ + 2e- Eo = +0.14 V cathode Ce4+ + e- ? Ce3+ Eo = +1.70 V Eocell = +1.84V

    40. The process of forcing a current through a cell to produce a chemical change is electrolysis. In order for electrolysis to occur, you must apply an external voltage that is greater than the potential of the galvanic cell if you want to force the reaction in the opposite (electrolytic) direction. The direction of electron flow is always from anode to cathode. "FAT CAT"

    42. An electrolytic cell is used to change electrical energy into chemical energy. In an electrolytic cell, the anode is assigned a positive charge and the cathode is assigned a negative charge.

    43. When water is electrolyzed, an electrolyte such as sulfuric acid or sodium sulfate must be added to make the water conduct electricity. The two half reactions that occur in the electrolysis of water are: anode: 2H2O ? O2 + 4H+ + 4e- cathode: 2H2O + 2e- ? H2 + 2OH- The overall reaction is: 2H2O ? 2H2 + O2

    44. When brine (salt water) is electrolyzed, chlorine gas, hydrogen gas, and sodium hydroxide are produced. Solid sodium metal is not produced because water is more easily reduced than is the sodium ion. The two half-reactions that occur in the electrolysis of brine are: anode: 2Cl- ? Cl2 + 2e- cathode: 2H2O + 2e- ? H2 + 2OH- Overall reaction: 2Cl- + 2H2O ? Cl2 + H2 + 2OH-

    45. When molten sodium chloride is electrolyzed, chlorine gas and sodium metal are produced. The two half-reactions that occur in the electrolysis of molten sodium chloride are: anode: 2Cl- ? Cl2 + 2e- cathode: Na+ + e- ? Na Overall reaction: 2Na+ + 2Cl- ? 2Na +Cl2

    46. What is electroplating? Electroplating is the deposition of a thin layer of a metal on an object in an electrolytic cell. The cathode is the object to be plated and the anode is usually a block of the metal that will be plated onto the object. The electrodes are immersed in a solution of the metallic salt.

    48. True or False 1. The half-cell that gains electrons contains the substance that is oxidized. 2. The electrode at which oxidation occurs is the anode. 3. In a galvanic cell, electrons flow from the half-cell where reduction occurs to the half-cell where oxidation occurs.

    49. 4. A substance with a high positive reduction potential is a good oxidizing agent. 5. In electrolysis, oxidation always occurs at the anode.

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