Topic 23

1. Topic 23

2. Table of Contents Topic 23 Topic 23: Electrochemistry Basic Concepts Additional Concepts

3. Electrochemistry: Basic Concepts Topic 23 Reviewing Redox • Suppose you could separate the oxidation and reduction parts of a redox reaction and cause the electrons to flow through a wire. • The flow of electrons in a particular direction is called an electrical current.

4. Electrochemistry: Basic Concepts Topic 23 Reviewing Redox • In other words, you are using a redox reaction to produce an electrical current. • This is what occurs in a battery—one form of an electrochemical cell in which chemical energy is converted to electrical energy.

5. Electrochemistry: Basic Concepts • You can reverse the process and use a current to cause a redox reaction to occur. Topic 23 Reviewing Redox

6. Electrochemistry: Basic Concepts Topic 23 Electrolysis • An electrochemical cell consists of two electrodes and a liquid electrolyte. • One electrode, the cathode, brings electrons to the chemically reacting ions or atoms in the liquid; the other electrode, the anode, takes electrons away. • The electrons act as chemical reagents at the electrode surface. • The liquid electrolyte acts as the chemical reaction medium.

7. Electrochemistry: Basic Concepts Topic 23 Electrolysis • You can remember that reduction always occurs at the cathode and oxidation always occurs at the anode by studying this diagram.

8. Electrochemistry: Basic Concepts Topic 23 The Electrolysis Process • Electrolysis takes place in a type of electrochemical cell called an electrolyticcell, in which a source of electricity, such as a battery, is added to an external circuit connecting the electrodes. • The electrolysis process occurs when the electrons are transferred between the electronic conductors—the metal electrodes—and the ions or atoms at the electrode surfaces.

9. Electrochemistry: Basic Concepts Topic 23 The Electrolysis Process Click box to view movie clip.

10. Electrochemistry: Basic Concepts Topic 23 Electrolytic Cell • Electrolysis, the splitting of compounds by electricity, occurs when two electrodes, an anode and a cathode, are inserted into a liquid electrolyte such as molten sodium chloride and connected to a source of electrical energy such as a battery.

11. Electrochemistry: Basic Concepts Topic 23 Electrolytic Cell

12. Electrochemistry: Basic Concepts Topic 23 Electroplating • Reduction of silver ions onto cheaper metals forms silverplate. Click box to view movie clip.

13. Electrochemistry: Basic Concepts Topic 23 Electroplating • The object to be plated is made the cathode. • At the pure silver anode, oxidation of silver metal to silver ions replaces the silver ions removed from the solution by plating at the cathode.

14. Electrochemistry: Basic Concepts Topic 23 Electrolytic Cleaning • Electrolysis can be used to clean objects by pulling ionic dirt away from them. • The electrolysis cell for this cleaning process includes a cathode that is the object itself, a stainless steel anode, and an alkaline electrolyte. • When an electric current is run through the cell, the chloride ions are drawn out. • Hydrogen gas forms and bubbles out, helping to loosen corrosion products.

15. Electrochemistry: Basic Concepts Topic 23 Electrophoresis • Electrophoresis is another electrochemical process that was used to restore some of the ceramic and organic artifacts from the Titanic. • Electrophoresis involves placing an artifact in an electrolyte solution between positive and negative electrodes and applying a current.

16. Electrochemistry: Basic Concepts Topic 23 Electrophoresis • The current breaks up salts, dirt, and other particles as their charged components migrate to the electrodes. • Electrophoresis is also used in laboratories to separate and identify large molecules.

17. Electrochemistry: Basic Concepts Topic 23 Potential Difference • Why do the electrons travel in one direction and not in the reverse? • The electron pressure at the cathode is kept low by the reduction reaction, and the electrons flow from a region of high pressure (negative potential at the anode) to a region of low pressure (positive potential at the cathode). This potential difference between the electrodes causes an electrical current to flow.

18. Electrochemistry: Basic Concepts • In this model of a lemon battery, the level of the electron sea is raised or lowered by the chemical reactions at the electrode surfaces, creating a potential difference across the battery. Topic 23 Potential Difference

19. Electrochemistry: Basic Concepts • A spontaneous oxidation reaction raises the electron pressure (potential) at the anode, and a spontaneous reduction reaction reduces the pressure at the cathode. Topic 23 Potential Difference

20. Electrochemistry: Basic Concepts Topic 23 Potential Difference

21. Electrochemistry: Basic Concepts Topic 23 Potential Difference • Because the redox reactions that take place during electrolysis are not spontaneous, a battery is needed to pump electrons from an area of low potential to one of high potential. Click box to view movie clip.

22. Electrochemistry: Basic Concepts Topic 23 Potential Difference

23. Electrochemistry: Basic Concepts Topic 23 Galvanic Cells • An electrochemical cell in which an oxidation-reduction reaction occurs spontaneously to produce a potential difference is called a galvanic cell. • In a Galvanic cell, chemical energy is converted into electrical energy. • Galvanic cells are sometimes called voltaic cells; both terms refer to the same device. • A galvanic cell that has been packaged as a portable power source is often called a battery.

24. Electrochemistry: Basic Concepts Topic 23 Batteries Perform Work • When a simple galvanic cell does useful work, it is called a battery. Click box to view movie clip.

25. Electrochemistry: Basic Concepts • If the external circuit is connected with a wire, electrons flow from the site of oxidation at the magnesium strip and through the LED to the surface of the copper strip, where reduction of Cu2+ ions takes place. Topic 23 Batteries Perform Work

26. Electrochemistry: Basic Concepts Topic 23 Batteries Perform Work • The voltage pushes electrons through the LED, causing it to light up.

27. Electrochemistry: Basic Concepts Topic 23 Batteries Perform Work

28. Electrochemistry: Basic Concepts Topic 23 Modern Batteries • Modern batteries come in a wide variety of sizes, shapes, and strengths. • Each type of battery serves a different purpose.

29. Electrochemistry: Basic Concepts Topic 23 Modern Batteries

30. Electrochemistry: Basic Concepts Topic 23 Modern Batteries • Although the term battery usually refers to a series of galvanic cells connected together, some batteries have only one such cell. • Other batteries may have a dozen or more cells.

31. Electrochemistry: Basic Concepts Topic 23 Modern Batteries • When you put a battery into a flashlight, radio, or CD player, you complete the electrical circuit of a galvanic cell(s), providing a path for the electrons to flow through as they move from the reducing agent (the site of oxidation) to the oxidizing agent the site of (the reduction).

32. Electrochemistry: Basic Concepts Topic 23 Carbon-Zinc Dry Cell • Whenever you put two or more common D batteries into a flashlight, you are connecting them in series. • They have to be placed in the correct order so that electrons flow through both cells.

33. Electrochemistry: Basic Concepts Topic 23 Carbon-Zinc Dry Cell • These relatively inexpensive batteries are carbon-zinc galvanic cells, and they come in several types, including standard, heavy-duty, and alkaline. • This type of battery is often called a dry cell because there is no aqueous electrolyte solution; a semisolid paste serves that role.

34. Electrochemistry: Basic Concepts Topic 23 Carbon-Zinc Dry Cell • A standard D battery is shown both whole and cut in half to reveal the structure of the carbon-zinc dry cell. • Beneath the outside paper cover of the battery is a cylinder casing made of zinc. • The zinc serves as the anode and will be oxidized in the redox reaction. 

35. Electrochemistry: Basic Concepts • The carbon rod in the center of the cylinder— surrounded by a moist, black paste of manganese (IV) oxide (MnO2) and carbon black—acts as a cathode. Topic 23 Carbon-Zinc Dry Cell • Ammonium chloride (NH4Cl) and zinc chloride (ZnCl2) serve as electrolytes.

36. Electrochemistry: Basic Concepts • Alkaline batteries contain potassium hydroxide (KOH) in place of the ammonium chloride electrolyte, and they maintain a high voltage for a longer period of time. Topic 23 Carbon-Zinc Dry Cell

37. Electrochemistry: Basic Concepts Topic 23 Carbon-Zinc Dry Cell • The flow of electrons from the zinc cylinder through the electrical circuits of an appliance and back into the battery provides the electricity needed to power a flashlight, radio, CD player, toy, clock, or other item. • When electrons leave the casing, zinc metal is oxidized.

38. Electrochemistry: Basic Concepts Topic 23 Carbon-Zinc Dry Cell • The reactions in the carbon rod and the paste are much more complex, but one major reduction that takes place is that of manganese in manganese (IV) oxide.

39. Electrochemistry: Basic Concepts • In this reaction, the oxidation number of manganese is reduced from . Topic 23 Carbon-Zinc Dry Cell • Adding the two half-reactions together gives the major redox reaction taking place in a carbon-zinc dry cell.

40. Electrochemistry: Basic Concepts Topic 23 Carbon-Zinc Dry Cell • Each galvanic cell in a lead-acid battery has two electrodes—one made of a lead (IV) oxide (PbO2) plate and the other of spongy lead metal. • In each cell, lead metal is oxidized as lead (IV) oxide is reduced. • The lead metal is oxidized to Pb2+ ions as it releases two electrons at the anode. • The Pb4+ ions in lead oxide gain two electrons, forming Pb2+ ions at the cathode.

41. Electrochemistry: Basic Concepts Topic 23 Carbon-Zinc Dry Cell • The Pb2+ ions combine with SO42– ions from the dissociated sulfuric acid in the electrolyte solution to form lead (II) sulfate at each electrode. • Thus, the net reaction that takes place when a lead-acid battery is discharged results in the formation of lead sulfate at both of the electrodes.

42. Electrochemistry: Basic Concepts Topic 23 Lead Storage Batteries

43. Electrochemistry: Basic Concepts Topic 23 Lead Storage Batteries • The reaction that occurs during discharge of a lead-acid battery is spontaneous and requires no energy input. • The reverse reaction, which recharges the battery, is not spontaneous and requires an input of electricity from the car’s alternator.

44. Electrochemistry: Basic Concepts Topic 23 Lead Storage Batteries • Current enters the battery and provides energy for the reaction in which lead sulfate and water are converted into lead (IV) oxide, lead metal, and sulfuric acid.

45. Electrochemistry: Basic Concepts Topic 23 Experimental Batteries • Two new experimental types of batteries for use in electric cars show early promise as candidates. • One is a rechargeable, nickel-metal hydride or NiMH battery. • This type of battery is less toxic and has a higher storage capacity than the batteries now used in electric cars.

46. Electrochemistry: Basic Concepts Topic 23 Experimental Batteries • Another experimental battery is a lithium battery with a water-based electrolyte. • Lithium is more easily oxidized than any other metal but has a drawback that has limited its use in batteries: it explodes violently when it comes into contact with water. • Lithium is used in some batteries to power camcorders, but they require an expensive, nonaqueous electrolyte.

47. Electrochemistry: Basic Concepts Topic 23 Aqueous Lithium Battery • How can a lithium battery have an aqueous electrolyte? Two facets of the construction of this new battery keep the lithium metal from reacting with water. • First, the lithium is in the form of individual atoms embedded in a material such as manganese (IV) oxide, rather than as a solid metal.

48. Electrochemistry: Basic Concepts Topic 23 Aqueous Lithium Battery • Second, the electrolyte is full of dissolved lithium salts, so the lithium ions that are produced travel to the site of reduction without reacting with water.

49. Electrochemistry: Basic Concepts Topic 23 Aqueous Lithium Battery

50. Basic Assessment Questions Topic 23 Question 1 What term describes a battery that is not rechargeable?