What is a? s a.k.a. Electrochemical cells Aim: To analyze the process of a spontaneous chemical reaction that produces electricity. Voltaic cell: an electrochemical cell where chemical energy is spontaneously converted to electrical energy. The voltage produced by an electrochemical cell is measured in volts. This is called the cell potential. Apr 25 5:02 PM Use Table J to determine which metal is the site of oxidation. Analyzing a The MORE active metal oxidizes and will be the anode, the site of oxidation. Parts of a Two half cells with salt solutions Wire and voltmeter Anode and cathode electrodes Salt bridge The anode must be the negative electrode and the cathode must be the positive electrode. The cathode must attract e lost at the anode and get them to move through the wire so reduction can occur. 1
oxidation half reduction half oxidation half Zn > Zn +2 + 2e reduction half Cu +2 + 2e > Cu As Zn (s) metal oxidizes to form Zn +2 (aq) ions, the Zn +2 (aq) ions dissolve into solution. excess positive charge builds up as Zn +2 ions increase Cu +2 + Zn > Cu + Zn +2 excess negative charge builds up as Cu +2 ions decrease Salt Bridge Purpose: the salt bridge permits the flow of ions between the half cells to reduce the build up of excess charge. As (aq) metal cations leave the solution they reduce into Cu (s) and stick onto the Cu cathode. Zinc ( ) Copper ( ) Zinc ( ) Copper ( ) Cu 2+ Zn0 Zn 2+ Zn2+ Zn0 Cu 0 1) Using Table J, determine which electrode is the anode and which is the cathode. Indicate the direction the electrons flow (current). a. Anode: b. Cathode: c. Direction of electron flow: 2) What occurs at the anode? 3) What happens to the electrons lost by the zinc electrode during oxidation? 4) Write the half reaction that occurs at the anode. 5) What happens to the mass of the anode? (hint: "OIL RIG") ZnCl 2(aq) 6) What occurs at the cathode? 7) What is reduced at the cathode? 8) Write the half reaction that occurs at the cathode. 9) What happens to the mass of the cathode? CuCl 2(aq) 10) How does the concentration of the Cu +2 ions change? How does the concentration of the Zn +2 ions change? Sep 2 12:51 PM Sep 2 12:51 PM 2
How does a work? VIDEOS https://ny.pbslearningmedia.org/resource/rr11.chem.per.atomod.youstart/youstart at the anode/#.wwvy2zeqwdu https://www.youtube.com/watch?v=c320kqgxolm#action=share The wire connects the electrodes and the salt bridge connects the ionic solutions to complete the circuit. The salt bridge promotes the flow of anions to prevent the buildup of charge in either half cell. Oxidation occurs at the ANODE and as electrons are lost, metal cations form and dissolve into the solution, decreasing the anodes mass through corrosion. http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/galvan5.swf Breaking Bad Galvanic Cell The lost electrons from the metal of the anode attract to the positively charged cathode and flow through the wire. The electrons entering the metal cathode allow reduction at the CATHODE. Cations in the solution of the cathode half cell are reduced and they plate onto the cathode electrode, increasing the cathode s mass. The flow of electrons and ions through the completed circuit generates electricity from the chemical reaction. VOLTAIC CELLS ARE SPONTANEOUS. Apr 11 5:02 PM Animations http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/galvan5.swf Apr 25 5:34 PM 3
Example 1: Mg Na Example 2: Fe K Write the oxidation and reduction half reactions for the cell. Write the oxidation and reduction half reactions for the cell. ox 1/2: ox 1/2: red 1/2: MgCl 2(aq) 2) What happens to the mass of both electrodes? Mg(s) mass Na(s) mass NaCl(aq) red 1/2: 2) What happens to the mass of both electrodes? Fe(s) mass K (s) mass FeCl 3 (aq) KCl (aq) 3) What happens to the concentration of each of the following ions? Mg 2+ concentration Na 1+ concentration 4) Which way do the electrons flow? Apr 26 5:27 PM 3) What happens to the concentration of each of the following ions? Fe +3 concentration K +1 concentration 4) Which way do the electrons flow? Apr 26 5:27 PM Cell #1 Cell #2 Cell #3 Cell #1 May 23 9:11 AM May 22 9:50 AM 4
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