Find the oxidation numbers of each element in a reaction and see which ones have changed. Rules for oxidation numbers: An element that is not in a compound has an oxidation number of zero (0) Group 1 Metals are always 1+ Group 2 Metals are always 2+ Fluorine is always 1- Oxygen is always 2- except when combined with F (OF 2 ) or the peroxide ion (O 2 2- ) J Deutsch 2003 2
An oxidation-reduction (redox) reaction involves the Sodium transfers its electrons to + The oxidation numbers of the atoms will change. one goes up (oxidation) and one goes down (reduction) J Deutsch 2003 3
Reduction is the gain of electrons. Nonmetals The oxidation number goes (reduces) The element that is reduced is called the. J Deutsch 2003 4
A can be written to represent reduction. Cu+ 2 + 2e- Cu 0 In reduction half reactions, electrons are written on the J Deutsch 2003 5
Oxidation is the Metal atoms The oxidation numbers go (increases) The element that is oxidized is called the J Deutsch 2003 6
A can be written to represent oxidation. Zn 0 Zn 2+ + 2e - In oxidation half reactions, electrons are written on J Deutsch 2003 7
LEO growls GER L G E E O R J Deutsch 2003 8
Table J tells us if a redox reaction can occur between an atom and an ion. A more active metal will replace an ion on Table J. A more active nonmetal will replace an ion on Table J. The, the more readily the replacement will take place. J Deutsch 2003 9
A metal will replace a metal from its compound. Zn + CuSO 4 Cu + ZnSO 4 Zinc replaces copper because zinc is more active than copper. Cu + ZnSO4 No Reaction Copper cannot replace zinc J Deutsch 2003 10
Regents Question: 01/03 #46 According to Reference Table J, which of these metals will react most readily with 1.0 M HCl to produce H 2 (g)? (1) Ca (2) K (3) Mg (4) Zn J Deutsch 2003 11
During redox, one atom is while another atom is. Reduction and oxidation happen together. The oxidation numbers of the atoms will change: one goes up (0 to +1) and one goes down (0 to -2) 0 0 2H 2 + O 2 Oxidation + 2H 2 O Reduction 2- J Deutsch 2003 12
In a redox reaction the The number of electrons must be Al + O 2 Al 2 O 3 Al 0 Al 3+ + 3e- oxidation - 3 electrons lost O 0 2 + 4e- 2O 2- reduction - 4 electrons gained Balance the electrons 0 0 3+ 2-4(Al 0 Al 3+ + 3e-) multiply by 4 3(O 2 0 + 4e- 2O 2-( multiply by 3 12 electrons are transferred in this reaction J Deutsch 2003 13
Regents Question: Given the reaction: Mg(s) + 2H + (aq) + 2Cl (aq) Mg 2+ (aq) + 2Cl (aq) + H 2 (g) Which species undergoes oxidation? (1) Mg(s) (2) H + (aq) LEO growls GER LEO (3) Cl (aq) (4) H 2 (g) J Deutsch 2003 14
Regents Question: 06/03 #28 Given the equation: 2 Al + 3 Cu 2+ 2 Al 3+ + 3Cu The reduction half-reaction is (1) Al Al 3+ + 3e (2) Cu 2+ + 2e Cu (3) Al + 3e Al 3+ (4) Cu 2+ Cu + 2e J Deutsch 2003 15
Regents Question: Which particles are gained and lost during a redox reaction? (1) electrons (2) Protons (3) Neutrons (4) positrons J Deutsch 2003 16
Regents Question: As a Ca atom undergoes oxidation to Ca 2+, the number of neutrons in its nucleus (1) decreases (2) increases (3) remains the same J Deutsch 2003 17
Regents Question: 08/02 #22 In any redox reaction, the substance that undergoes reduction will (1) lose electrons and have a decrease in oxidation number (2) lose electrons and have an increase in oxidation number (3) gain electrons and have a decrease in oxidation number (4) gain electrons and have an increase in oxidation number J Deutsch 2003 18
Regents Question: Which particles are gained and lost during a redox reaction? (1) electrons (2) Protons (3) Neutrons (4) positrons J Deutsch 2003 19
Regents Question: As a Ca atom undergoes oxidation to Ca 2+, the number of neutrons in its nucleus (1) decreases (2) increases (3) remains the same J Deutsch 2003 20
An oxidation-reduction (redox) reaction involves the transfer of electrons (e - ). Sodium transfers its electrons to chlorine + The oxidation numbers of the atoms will change. one goes up (oxidation) and one goes down (reduction) J Deutsch 2003 21
Reduction is the gain of electrons. Nonmetals gain electrons to form ions The oxidation number goes down (reduces) The element that is reduced is called the oxidizing agent. J Deutsch 2003 22
A half-reaction can be written to represent reduction. Cu+ 2 + 2e- Cu 0 In reduction half reactions, electrons are written on the left because electrons are gained J Deutsch 2003 23
Oxidation is the loss of electrons. Metal atoms lose electrons to become + ions The oxidation numbers go up (increases) The element that is oxidized is called the reducing agent... J Deutsch 2003 24
A half-reaction can be written to represent oxidation. Zn 0 Zn 2+ + 2e - In oxidation half reactions, electrons are written on the right because electrons are lost J Deutsch 2003 25
LEO growls GER Losing Electrons Gaining Electrons Oxidation Reduction J Deutsch 2003 26
Regents Question: Given the reaction: Mg(s) + 2H + (aq) + 2Cl (aq) Mg 2+ (aq) + 2Cl (aq) + H 2 (g) Which species undergoes oxidation? (1) Mg(s) (2) H + (aq) LEO growls GER LEO (3) Cl (aq) (4) H 2 (g) J Deutsch 2003 27
Regents Question: Given the equation: C(s) + H 2 O(g) CO(g) + H 2 (g) Which species undergoes reduction? (1) C(s) (2) H + LEO growls GER (3) C 2+ (4) H 2 (g) J Deutsch 2003 28
Regents Question: 08/02 #22 In any redox reaction, the substance that undergoes reduction will (1) lose electrons and have a decrease in oxidation number (2) lose electrons and have an increase in oxidation number (3) gain electrons and have a decrease in oxidation number (4) gain electrons and have an increase in oxidation number J Deutsch 2003 29
Regents Question: 06/03 #28 Given the equation: 2 Al + 3 Cu 2+ 2 Al 3+ + 3Cu The reduction half-reaction is (1) Al Al 3+ + 3e (2) Cu 2+ + 2e Cu (3) Al + 3e Al 3+ (4) Cu 2+ Cu + 2e J Deutsch 2003 30
Oxidation numbers can be assigned to atoms and ions. Changes in oxidation numbers indicate that oxidation and reduction have occurred. 0 2+ 5+ 2-2+ 5+ 2-0 Zn + Cu(NO 3 ) 2 Zn 0 Zn 2+ + 2e - Oxidation Cu 2+ + 2e - Cu 0 Reduction Zn(NO 3 ) 2 + Cu Find the oxidation numbers and see which ones change. J Deutsch 2003 31
Regents Question: What is the oxidation number of chromium in K 2 Cr 2 O 7? (1) +12 (2) +2 (3) +3 (4) +6 J Deutsch 2003 32
Regents Question: In which substance does chlorine have an oxidation number of +1? (1) Cl 2 (2) HCl (3) HClO (4) HClO 2 J Deutsch 2003 33
During redox, one atom is oxidized while another atom is reduced. Reduction and oxidation happen together. The oxidation numbers of the atoms will change: one goes up (0 to +1) and one goes down (0 to -2) 0 0 2H 2 + O 2 Oxidation + 2H 2 O Reduction 2- J Deutsch 2003 34
In a redox reaction the number of electrons lost is equal to the number of electrons gained. The number of electrons must be balanced! Al + O 2 Al 2 O 3 Al 0 Al 3+ + 3e- oxidation - 3 electrons lost O 0 2 + 4e- 2O 2- reduction - 4 electrons gained Balance the electrons 0 0 3+ 2-4(Al 0 Al 3+ + 3e-) multiply by 4 3(O 2 0 + 4e- 2O 2-( multiply by 3 12 electrons are transferred in this reaction J Deutsch 2003 35
Table J tells us if a redox reaction can occur between an atom and an ion. A more active metal will replace an ion below it on Table J. A more active nonmetal will replace an ion below it on Table J. The higher up the table, the more readily the replacement will take place. J Deutsch 2003 36
A more active metal will replace a less active metal from its compound. Zn + CuSO 4 Cu + ZnSO 4 Zinc replaces copper because zinc is more active than copper. Cu + ZnSO4 No Reaction Copper cannot replace zinc Oxidation is on top Reduction is on bottom J Deutsch 2003 37
Regents Question: 01/03 #46 According to Reference Table J, which of these metals will react most readily with 1.0 M HCl to produce H 2 (g)? (1) Ca (2) K (3) Mg (4) Zn J Deutsch 2003 38
An electrochemical cell can be either voltaic or electrolytic. J Deutsch 2003
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The Electrochemical Cell. In an electrochemical cell, oxidation occurs at the anode and reduction at the cathode. (3.2j) Red Cat An Ox Reduction at the Cathode Oxidation at the Anode J Deutsch 2003 41
A voltaic cell spontaneously converts chemical energy to electrical energy. (3.2k) Batteries are voltaic cells J Deutsch 2003 42
Electrons flow from the anode (- electrode) to the cathode (+ electrode) through the wire in a voltaic cell. An Ox -oxidation takes place electrons are lost. Red Cat -reduction takes place electrons are gained. Zn Zn 2+ + 2e - e- e- e- e- Cu 2+ + 2e - Cu 0 - + e- e- e- e- Electrons released here by oxidation e- e- e- Electrons needed here for reduction J Deutsch 2003 43
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http://group.chem.iastate.edu/greenbowe/sectio ns/projectfolder/flashfiles/electrochem/voltaic CellEMF.swf http://www.kentchemistry.com/moviesfiles/units/redo x/voltaiccelll20.htm http://www.mhhe.com/physsci/chemistry/ess entialchemistry/flash/galvan5.swf http://wn.com/galvanic_voltaic_cell_using_cop per_and_zinc_electrodes J Deutsch 2003 45
Regents Question: 06/03 #46 A diagram of a chemical cell and an equation are shown below. When the switch is closed, electrons will flow from (1) the Pb(s) to the Cu(s) (3) the Pb 2+ (aq) to the Pb(s) (2) the Cu(s) to the Pb(s) (4) the Cu 2+ (aq) to the J Deutsch 2003 46
The salt bridge completes the circuit allows ions to flow from one ½ cell to the other ½ cell to maintain neutrality. Zn Zn 2+ + 2e - Cu 2+ + 2e - Cu 0 - + J Deutsch 2003 47
Regents Question: 06/03 #27 Which statement is true for any electrochemical cell? (1) Oxidation occurs at the anode, only. (2) Reduction occurs at the anode, only. (3) Oxidation occurs at both the anode and the cathode. (4) Reduction occurs at both the anode and the cathode. J Deutsch 2003 48
An electrolytic cell requires electrical energy to produce chemical change. This process is known as electrolysis. (3.2l) J Deutsch 2003 49
Regents Question: 06/02 #70-75 Base your answers to the next six questions on the following redox reaction, which occurs spontaneously in an electrochemical cell. Zn + Cr 3+ Zn 2+ + Cr Write the half-reaction for the reduction that occurs. Cr 3+ + 3e- Cr 0 Write the half-reaction for the oxidation that occurs. Zn 0 Zn 2+ + 2e- Balance the equation using the smallest wholenumber coefficients. The number of e-s gained = the number of e-s lost 3Zn + 2Cr 3+ 2Zn 2 + + 3Cr J Deutsch 2003 50
Regents Question: 06/02 #70-75 Base your answers to the next five questions on the following redox reaction, which occurs spontaneously in an electrochemical cell. Zn + Cr 3+ Zn 2+ + Cr Which species loses electrons and which species gains electrons? Zn 0 loses electrons, Cr 3+ gains electrons Which half-reaction occurs at the cathode? Cr 3+ + 3e- Cr 0 Hint: Red Cat State what happens to the number of protons in a Zn atom when it changes to Zn 2+ as the redox reaction occurs. The number of protons remains the same. J Deutsch 2003 51
Regents Question: 01/03 #53-55 Indicate with one or more arrows the direction of electron flow through the Hint: wire. Zinc is above copper on table J Write an equation for the half-reaction that occurs at the zinc electrode. Zn Zn 2+ + 2e - so Zn is oxidized Explain the function of the salt bridge. The salt bridge completes the circuit and allows ions to flow from one ½ cell to the other. This keeps the ½ cells neutral. e- J Deutsch 2003 52
Uses of Electrolytic cells Recharging a battery Electroplating During copper plating, Cu 2+ ions are reduced to Cu 0 metal at the cathode (Red Cat) which is the negative electrode Electrolysis The Hoffman apparatus uses electricity to break water apart into hydrogen + oxygen J Deutsch 2003 53
Regents Question: 06/02 #22 Which process requires an external power source? (1) neutralization (2) synthesis (3) fermentation (4) electrolysis J Deutsch 2003 54
The cathode is negative and is the site of reduction in an electrolytic cell. Cu 2+ ions are reduced to Cu 0 metal at the negative electrode. J Deutsch 2003 55