Galvanic Cells. SCH4U7 Ms. Lorenowicz. Tuesday, December 6, 2011

Transcription

1 Galvanic Cells SCH4U7 Ms. Lorenowicz 1

2 Electrochemistry Concepts 1.Redox reactions involve the transfer of electrons from one reactant to another 2.Electric current is a flow of electrons in a circuit Electrochemistry - study of processes involved in converting chemical energy into electrical energy 2

3 Electrochemical Cells Redox reactions take place in electrochemical cells. Many reduction-oxidation reactions occur spontaneously, giving off energy. Other reduction-oxidation reactions are nonspontaneous, requiring energy. Galvanic/Voltaic Cells Spontaneous reactions occur Electrolytic Cells Non-spontaneous reactions occur 3

4 Galvanic Cell (Voltaic Cell) converts chemical energy into electrical energy want electrons to do work prevent direct contact of reactants of redox reactions electrons flow through an external circuit 4

5 spontaneously, zinc is displaced by copper, releasing energy this energy can be made to do useful work by an galvanic cell Galvanic Cells Two compartments or half-cells, each composed of an electrode dipped in a solution of electrolyte to keep the reactions separate 5

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7 Oxidation Half-Cell : ANODE Zn (s) Zn 2+ (aq) + 2 e - Reduction Half-Cell : CATHODE Cu 2+ (aq) + 2 e - Cu (s) Nothing will happen unless the two half-cells are connected Connect metal electrodes with a conducting wire to allow electrons to flow When they are connected, an imbalance of electrical charge will be produced. The anode will become more positive (Zn 2+ (aq) produced) The cathode will become more negative (Cu 2+ (aq) removed) Solve imbalance of electrical charge by using a salt bridge connecting the two cells 7

8 Salt Bridge porous barrier that allows the migration of ions in both directions to maintain electrical neutrality Electrodes solid metals electrical conductors carry electrons in and out of the cell electrical conductors e.g. Zn, Cu Electrolytes substances that conduct electricity when dissolved in water due to ion movements and loss/gain of electrons with electrodes e.g. Cu 2+, Zn 2+, SO 4 2-, salts in bridge, ie. Cl -, NH 4 + 8

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10 Daniell Cell Type of Galvanic (Voltaic Cell) Anode (Zn) releases electrons anode is the negative electrode releasing cations to solution over time, anode loses mass Cathode (Cu) accepting electrons cathode is the positive electrode cations removed from solution over time, cathode gains mass External circuit forces electrons to move from anode to cathode (between 2 electrodes) and powers any electrical device Porous barrier or Salt Bridge prevents contact of Cu 2+ & Zn allows the movement of ions to maintain neutrality of the solution e.g. NH 4 Cl, Na 2 SO 4, KNO 3 10

11 Cell Notation allows for a short-hand way to represent a galvanic cell phase boundary (solid aqueous) porous barrier or salt bridge, comma between ions if no phase boundary between two ions occurs in the following order anode cation of anode cation of cathode cathode reducing agent oxidizing agent Spectator ions are usually omitted Zn Zn 2+ Cu 2+ Cu Standard Cell Both electrolyte solution concentrations are 1 mol/l Temperature is 25 C 11

12 Inert Electrodes electrode made from a material that is neither a reactant nor a product of cell redox reaction allows for redox reaction of gasses or dissolved electrolytes oxidation: Pb Pb e - reduction: Fe 3+ + e - Fe 2+ e.g. Pb Pb 2+ Fe 3+, Fe 2+ Pt inert electrode 12

13 Disposable Batteries Dry Cell Battery Zn Zn 2+ Mn 4+, Mn 3+ C - paste with electrolytes - portable, cheap (A, AA, C, D) (a) Disposable Battery (Primary) - stop producing electricity when reactants are used up (b) Rechargeable Battery (Secondary) - regenerate reactants Anode: Zn container OX: Zn Zn e - Paste: MnO 2, ZnCl 2, NH 4 Cl, C Cathode: C graphite (inert cathode) RED: MnO 2 + H 2 O + 2e - Mn 2 O 3 + 2OH - 13

14 Summary of Galvanic Cell e- e- e- e - Anode e - Reducing Agent Cathode e - Oxidizing Agent 14

15 Summary of Galvanic Cells The ANODE... The CATHODE... supplies electrons to the external circuit (wire) accepts electrons from the external circuit (wire) is the negative pole of the battery is the positive pole of the battery is the site of OXIDATION is the site of REDUCTION is written on the left hand side if the convention is followed is the half-cell with the lowest electrode potential is written on the right hand side if the convention is followed is the half-cell with the highest electrode potential 15