Electrochemical cells. Section 21.1

Transcription

1 Electrochemical cells Section 21.1

2 Electrochemical processes Chemical process either release energy or absorb energy This does not have to be solely heat or light - sometimes it can be in the form of electricity Electrochemical processes involve the conversion between chemical and electrical energy

3 Redox reactions If I put a strip of zinc in copper sulfate solution, what will happen?

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5 What is an electrochemical cell? When electrons flow an electric current is generated When two half reactions are separated the redox reactions between zinc atoms and copper ions can be used to produce electrical energy The electrons released by the zinc must flow through a circuit to reach the copper ions - electrical energy is produced This is the basis for an electrochemical cell

6 Voltaic cells A voltaic cell is an electrochemical cell used to convert chemical energy into electrical energy Electrical energy is produced in a voltaic cell by a spontaneous redox reaction within the cell A voltaic cell contains two half cells A half cell is one part of a voltaic cell in which either oxidation or reduction occur This is often made from a piece of metal in a solution with its ions

7 Voltaic cells Salt bridge - A tube containing a strong electrolyte that connects the two half cells of an electrochemical cell K 2 SO 4 is a common electrolyte used The purpose is to allow ions to move without mixing occurring The zinc and copper strips are electrodes Oxidation occurs at negative electrode (anode) - electrons produced Reduction occurs at positive electrode (cathode) - electrons consumed

8 How the Voltaic cell works The two half-reactions can be summed to show the overall reaction. Note that the electrons must cancel. Zn(s) Zn 2+ (aq) + 2e Cu 2+ (aq) + 2e Cu(s) Zn(s) + Cu 2+ (aq) Zn 2+ (aq) + Cu(s)

9 Shorthand notation Single vertical lines are used to represent boundaries between phases that are in contact You can represent the zinc-copper voltaic cell by using the following shorthand form. Zn(s) ZnSO 4 (aq) CuSO 4 (aq) Cu(s)

10 Why are electrochemical cells useful Electrochemical cells form the basis of batteries The zinc-copper voltaic cell is not a common source of electrical energy, instead the following are used: Dry Cells Lead storage batteries Fuel cells

11 Dry Cells In one type of dry cell, a zinc container is filled with a thick, moist electrolyte paste of manganese(iv) oxide (MnO2), zinc chloride (ZnCl2), ammonium chloride (NH4Cl), and water (H2O). A graphite rod is embedded in the paste. The zinc container is the anode, and the graphite rod is the cathode. The thick paste and its surrounding paper liner prevent the contents of the cell from freely mixing, so a salt bridge is not needed. Positive button (+) Graphite rod (cathode) Moist paste of MnO 2, ZnCl 2, NH 4 Cl 2, H 2 O, and graphite powder Zinc (anode) Negative end cap ( )

12 What happens in a dry cell? The graphite rod is the cathode, but is not part of the reaction This reaction is not reversible (the Mn reaction at the cathode) Oxidation: Zn(s) Zn 2+ (aq) + 2e (at anode) Reduction: 2MnO 2 (s) + 2NH 4 + (aq) + 2e Mn 2 O 3 (s) + 2NH 3 (aq) + H 2 O(l) (at cathode)

13 How can this be improved? By changing the electrolyte basis - the basis for an alkaline battery KOH paste is used This change eliminates the buildup of ammonia gas and maintains the zinc electrode, which corrodes more slowly under basic, or alkaline, conditions. Positive button (+) Steel case MnO 2 in KOH paste Graphite rod (cathode) Absorbent separator Zinc (anode) Negative end cap ( )

14 Lead storage batteries A battery is a group of voltaic storage cells connected together Commonly example - car batteries How does a car battery work? A 12 volt battery consists of six voltaic cells connected together Each cell is made of lead grids The cathode is filled with PbO2 The anode is filled with spongey Lead The electrolyte for both is Sulfuric acid As this is the same, no salt bridge is required

15 What is going on inside a car battery? Oxidation: Pb(s) + SO 4 2 (aq) PbSO 4 (s) + 2e Reduction: PbO 2 (s) + 4H + (aq) + SO 4 2 (aq) + 2e PbSO 4 (s) + 2H 2 O(l)

16 How can the battery be recharged The overall spontaneous redox reaction used to start the car is the formation of PbSO 4 - electricity is discharged to start the car Pb(s) + PbO 2 (s) + 2H 2 SO 4 (aq) 2PbSO 4 (s) + 2H 2 O(l) When a car s generator is used to recharge the battery, the reverse reaction occurs 2PbSO 4 (s) + 2H 2 O(l) Pb(s) + PbO 2 (s) + 2H 2 SO 4 (aq)\ Theoretically this process can continue to occur indefinitely, but in practice PbSO 4 falls from the electrode and accumulates at the bottom of the cell

17 Fuel cells A fuel cell is a voltaic cell in which a fuel is oxidized to produce a continuous supply of electrical energy They do no need to be recharged They can be engineered so that they do not give off any pollutants One simple fuel cell uses the reaction of hydrogen gas and oxygen gas to produce water In this cell there are 3 components separated by two electrodes (carbon)

18 Fuel cells Oxygen flows into the cathode Hydrogen flows into the anode compartment The thin membrane allows allows hydrogen ions but not electrons to pass through This membrane acts as a salt bridge Electrons from the oxidation half reaction pass through an external circuit to enter the reduction half reaction

19 The half-reactions are as follows: Oxidation: 2H 2 (g) 4H + (aq) + 4e (at anode) Reduction: O 2 (g) + 4H + (aq) + 4e 2H 2 O(g) (at cathode) The overall reaction is the oxidation of hydrogen to form water. 2H 2 (g) + O 2 (g) 2H 2 O(g)

20 What are fuel cells used for? Fuel cells have been sued since the 1960s as an energy source aboard spacecraft Nowadays they can be used in cars Vehicles run on electric motors powered by fuel cells When the book was published, fuel cell cars ran on hydrogen gas, and were difficult/expensive to make Is this technology outdated? Are there downsides to hydrogen based technology?

21 Electrolysis Section 21.3 (slightly)

22 Electrolysis The process in which electrical energy is used to bring about a chemical change is called electrolysis. You are already familiar with some results of electrolysis, such as gold-plated jewelry, chrome-plated automobile parts, and silver-plated dishes.

23 Electrolytic cells The apparatus in which electrolysis is carried out is an electrolytic cell. An electrolytic cell is an electrochemical cell used to cause a chemical change through the application of electrical energy.

24 Voltaic Cell Anode (oxidation) e e Energy Cathode (reduction) Electrolytic Cell Anode (oxidation) e Battery Energy e Cathode (reduction) In both voltaic and electrolytic cells, electrons flow from the anode to the cathode in the external circuit.

25 Differences between electrolytic and voltaic cells The key difference between voltaic and electrolytic cells is that in a voltaic cell, the flow of electrons is the result of a spontaneous redox reaction, whereas in an electrolytic cell, electrons are caused to flow by an outside power source, such as a battery.

26 Quick lab We are going to set up an electrolytic cell to electroplate copper onto another metal object Copper Sulphate will be our electrolyte solution A copper strip will be attached to the positive terminal A paper clip will be attached to the negative terminal In your lab books I would a like a description of the experimental set up, a labeled diagram of the electrolytic cell, and an overview of the the half equations that occur at each electrode.