Packet 9: Oxidation-Reduction Reactions Many reactions are oxidation-reduction reactions A.k.a redox Reaction where one atom loses electrons and another atom gains electrons Atoms that lose electrons are Ex: An atom losing an electron can be written like this: Atoms that gain electrons are Ex: An atom losing an electron can be written like this: Oxidation means Reduction means Remember: OIL RIG Determining oxidation and reduction: Mg + Br 2 MgBr 2 Oxidation numbers are used to identify the path of electrons in redox reactions. Each element in the compound must be assigned an oxidation number. The following rules MUST be memorized: 1. The sum of all oxidation numbers in the compound must equal ZERO o Ex: o 2. The sum of the oxidation numbers in a polyatomic ion must equal the CHARGE o Ex: 1
Assign oxidation numbers to the following substances: Mg Br 2 MgBr 2 Na NaCl In some compounds you must assign oxidation numbers according to the rules and then SOLVE for elements that are unknown. Ex: KClO 3 K Cl O Determine the oxidation number of each element in the following compounds: NF 3 N F K 2 CO 3 K C O NO 3 - N O Purple Book p. 91 Jan 2007-45 What is the oxidation number of chromium in the chromate ion, CrO4 2? (1) +6 (2) +3 (3) +2 (4) +8 Jan 2006-21 What is the oxidation number assigned to manganese in KMnO4? (1) +7 (2) +3 (3) +2 (4) +4 Jan 2003-4 In which compound does chlorine have the highest oxidation number? (1) NaClO (2) NaClO 3 (3) NaClO 2 (4) NaClO 4 Jan 2009-44 What is the oxidation number of sulfur in Na 2 S 2 O 3? (1) -1 (2) +6 (3) +2 (4) +4 June 2009-44 Given the balanced equation representing a reaction: Fe 2 O 3 + 2Al--> Al 2 O 3 + 2Fe During this reaction, the oxidation number of Fe changes from (1) +2 to 0 as electrons are transferred (2) +2 to 0 as protons are transferred (3) +3 to 0 as electrons are transferred (4) +3 to 0 as protons are transferred Agents of The substance that is oxidized is a agent. (Think of it as the substance that is oxidized is the agent of reduction.) The substance the is reduced is the agent. (Think of it as the substance that is reduced is the agent of oxidation) Agents will always be reactants Remember: REDUCED = OXIDIZING AGENT OXIDIZED = REDUCING AGENT Recognizing Redox Equations: If oxidation numbers change as you go from left to right in a chemical equation, then a redox reaction has occurred. 2
Oxidation occurs when there is an in oxidation number (ex. From - 1 to +2, from -3 to -1). An element becomes more positive by losing electrons. Reduction occurs when there is a in oxidation number (ex. From +2 to 0, from +7 to +2). An element becomes more negative by gaining electrons. The number REDUCES. Picture a number line: NOTE: The balancing coefficients in the equation DO NOT affect the value of the oxidation numbers. Ex: Determine the oxidation numbers of the elements on both sides of the equation. Then determine the substance oxidized, the substance reduced, the oxidizing agent and the reducing agent. To Solve: NH 3 + O 2 NO + H 2 O 1) Assign an oxidation number to each element: -3 +1 0 +2-2 +1-2 NH 3 + O 2 NO + H 2 O 2) Substance oxidized Substance reduced 3) Oxidizing agent Reducing agent You can easily recognize a redox redaction when there is a free element in the equation Past Regents Redox Reactions Questions Aug 2008-42 Which equation represents an oxidation reduction reaction? (1) CH 4 + 2O 2 ==> CO 2 + 2H 2 O (2) H 2 SO 4 + Ca(OH) 2 ==> CaSO 4 + 2H 2 O (3) MgCrO 4 + BaCl 2 ==> MgCl 2 + BaCrO 4 (4) Zn(NO 3 ) 2 + Na 2 CO 3 ==> 2NaNO 3 + ZnCO 3 3
June 2006-23 Which balanced equation represents a redox reaction? (1) AgNO 3 + NaCl ==>AgCl + NaNO 3 (2) BaCl 2 + K 2 CO 3 ==> BaCO 3 + 2KCl (3) CuO + CO==> Cu + CO 2 (4) HCl + KOH ==>KCl + H 2 O Aug 2004-26 Which reaction is an example of an oxidation reduction reaction? (1) AgNO 3 + KI >AgI + KNO 3 (2) Cu + 2 AgNO 3 >Cu(NO 3 ) 2 + 2 Ag (3) 2 KOH + H 2 SO 4 >K 2 SO4 + 2 H 2 O (4) Ba(OH) 2 + 2 HCl >BaCl 2 + 2 H 2 O Practice: http://evanschemistrycorner.com/ws/electrochemistry/ws10-1-2_analyzing_oxidation- Reduction_Reactions.pdf HALF- REACTIONS Each redox reaction consists of two parts: the oxidation reaction and the reduction reaction. Each part is called a HALF-REACTION and separate equations can be written for each. In each half-reaction, both mass and charge must be balanced. In the two half-reactions, the number of electrons lost must equal the number of electrons gained. Any coefficients in the original equation must also be used when writing half-reactions. Ex: Mg + Cl 2 MgCl 2 1) Assign oxidation numbers to each element and determine which is the oxidized substance and which is the reduced substance. 0 0 +2-1 Mg + Cl 2 MgCl 2 Substance oxidized Substance reduced 2) Write a separate equation for oxidation and reduction, balancing both mass and charge. 4
Oxidation: Mg 0 Mg +2 + 2e- (Think: Mg break down into ) Reduction: Cl 0 2 + 2e- 2Cl - (Think:Cl 2 gains to become) Writing Half-Reactions Summary: Oxidation Half-Reactions: in oxidation number Electrons on the side Reduction Half-Reactions: in oxidation number Electrons on the side Practice: Note: Ions that are neither oxidized nor reduced are not included in the half-reactions. These are called spectator ions and have no effect on oxidation or reduction Substance oxidized Substance reduced Cu + AgNO 3 Cu(NO 3 ) 2 + Ag Oxidation half-reaction: Reduction half-reaction: Write a the half-reactions for: Mg + O 2 MgO Oxidation half-reaction Reduction half-reaction Zn(s) + Cu 2+ (aq) Cu(s) + Zn 2+ (aq) Oxidation half-reaction Reduction half-reaction Purple Book p. 92 (ignore coefficients) Electrochemical Cells Practical use of redox reactions Involves a chemical reaction and a flow of Have two surfaces where reactions occur called There are two types of electrodes o : where oxidation occurs o : where reduction occurs o Remember: 5
Two common types of electrochemical cells: Voltaic Cell Electrolytic Cell Voltaic Cell: Spontaneous Reactions Spontaneous reaction produces a flow of electrons Ex: a battery Energy is converted from to Anode is the positive electrode, cathode is negative strip in a solution of. Zn is and Cu +2 is reduced. connects the two beakers. The salt bridge. Makes a complete circuit Electrons are lost during at the They travel through the to the cathode. At this surface, the material that is reduced its electrons. The copper ions are reduced (not metal) or Cu +2 NOT Cu. Watch :http://www.kentchemistry.com/links/redoxlinks.htm 6
Metals that can be used in a voltaic cell: The Table J Activity Series The metal that is on the table will be oxidized therefore it will be the. The lower metal will be the, therefore it will be reduced. Table J can be used to predict if a redox reaction is spontaneous. In order for a redox reaction to be spontaneous, the metal (free element) must be on Table J than the ion or metal in the compound. The most reactive metals lose electrons most easily and therefore are most easily oxidized. The most reactive nonmetals gain electrons most easily and therefore are most easily reduced Part of Table J Activity Series Metals Metals Nonmetals Nonmetals Activity Most easily Most easily reduced Activity decreases as oxidized decreases as you go down Table J. Most Li F2 you go down Most Table J. Metals on top are most Rb Cl2 Nonmetals on top are most easily easily oxidized. K Br2 reduced. Ca I2 As you go down, less Na As you go down, less easily oxidized Na Mg easily reduced ess Fe ess Bottom is least easily eeeeeaeasily H2 Cu Ag Bottom is least easily oxidized Least Au Least reduced Least easily Least easily oxidized reduced (Look at Table J, above.) 1. Which reaction will take place spontaneously? (1) Ni +2 + Pb Ni + Pb +2 (2) Au +3 + Al Au + Al +3 (3) Sr +2 + Sn Sr + Sn +2 (4) Fe +2 + Cu Fe + Cu +2 2. Tell whether the reaction will occur spontaneously or not: a. K + LiClà b. Ca+ MgCO 3 à 3. Which metal is most easily oxidized? 1) Li 2) K 3) Ca 4) Na 4. Which metal is least easily oxidized? 1) Ca 2) Ni 3) Cu 4) Au Click here: http://evanschemistrycorner.com/ws/electrochemistry/ws10-2- 1_Applying_the_Activity_Series.pdf 7
Electrolytic Cells: NON-spontaneous reaction Electrolytic cells are used to force a They do not produce electricity they USE electricity to force a chemical reaction to occur. Electrical energy is converted into chemical energy. The following table compares electrochemical and electrolytic cells: In an electrolytic cell, the polarities of the electrodes are reversed: the ANODE is and the CATHODE is. Two types of electrolytic cells are 1. 2. 1. Electrolysis Electrolysis means to break apart using electricity. Oxidation occurs at the anode (+ electrode) Reduction occurs at the cathode(- electrode). There is no salt bridge. A source of electricity is required to force a chemical reaction to occur. Negative ions are attracted to the anode where they are oxidized. Positive ions are attracted to the cathode where they are reduced. 2. Electroplating is a technique in which a thin layer of a desired metal is used to coat or plate another object. 3. The object to be plated is the. Reduction of ions occurs here. 4. The anode is a bar of the desired metal. Oxidation of the metal atoms occurs here. 5. The solution used is a salt containing ions of the desired metal. 6. Electrons will flow from to. 8
Past Regents Questions Jan 2009-23 Which statement describes electrolysis? (1) Chemical energy is used to produce an electrical change. (2) Chemical energy is used to produce a thermal change. (3) Electrical energy is used to produce a chemical change. (4) Thermal energy is used to produce a chemical change Aug 2008-45 Given the balanced equation representing a reaction occurring in an electrolytic cell: 2NaCl (l) ==> 2Na (l) + Cl 2(g) Where is Na(l) produced in the cell? (1) at the anode, where oxidation occurs (2) at the anode, where reduction occurs (3) at the cathode, where oxidation occurs (4) at the cathode, where reduction occurs Energy conversion Spontaneous or not Charge of anode Charge of cathode Salt bridge or not Oxidation at anode Reduction at cathode Electricity Balancing Redox Reactions VOLTAIC CELL ELECTROLYTIC If a redox reaction contains no ions, the equation can be balanced by simply adding coefficients to balance out the number of atoms (conservation of mass) (See Packet 5a!) Recall: Mg + Cl 2 à MgCl 2 9
If a redox reaction does contain ions, BOTH mass AND charge need to be balanced! 1. Balance the charge (electrons). The # of electrons lost must equal the # of electrons gained!! 2. Balance the mass (# of atoms: a ) Ex: Mg + Cr 3+ à Mg 2+ + Cr Practice Problems 1. Cr + Pb 2+ à Cr 3+ + Pb 2. Sn + Ag + à Sn 2+ + Ag 3. Al + Cu 2+ à Cu + Al 3+ Find a comprehensive review/practice redox Regents questions (and answers) at: http://www.mrbeauchamp.com/redoxregentsreview.pdf 10