Redox Reactions Or How Batteries Work

REDOX Reactions The simultaneous transfer of electrons between chemical species. – Actually 2 different reactions occurring at the same time. Oxidation:a loss of electrons Reduction:a gain of electrons

ReDox Na +Cl  NaCl Na o +Cl o  Na + Cl - Which element was Oxidized? Na Which element was Reduced? Cl

How do we remember ReDox? mnemonics

5 LEO growls GER Losing Electrons Oxidation Gaining Electrons Reduction

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7 OIL RIG Oxidation IsIs Losing Reduction IsIs Gaining

How do we know if there is a RedOx Reaction occurring? We first must find the Oxidation states.

What is the oxidation state of each of the following species? Cl in Cl 2 0 Mg in MgO +2 Na in NaBr +1 F in SrF 2 Sr in SrF 2 +2 O in O 2 0

J Deutsch Regents Question Which particles are gained and lost during a redox reaction? (1)Electrons (2) Protons (3) Neutrons (4) Positrons

J Deutsch 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

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Rules for Determining Oxidation Numbers 1.Free Elements have an oxidation number of zero. Ex: Na, S 8, H 2 2.All Metals in group 1 have a +1 Oxidation # in compounds. Ex: Na, K, Li 3.All Metals in group 2 have a +2 Oxidation # in compounds. Ex: Mg, Ca, Ba 4.Oxygen has a –2 Oxidation Number 1.Unless it is in a Peroxide like H 2 O 2

Continued Rules 5.Hydrogen has a +1 oxidation #. 6.Halogens have a -1 oxidation #. 7.For any neutral compound, the sum of the oxidation numbers of the atoms in the compound must equal zero. For ex: H 2 SO 4 8.For a polyatomic ion, the sum of the oxidation #’s must equal the ionic charge of the ion. Ex: SO 4 2-

CuF 2 PBr 3 HNO 3 C2O4C2O4 SO 4 -2 CO 3 -2 C 12 H 22 O 11 H2OH2O

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Now we will take a deeper look into Oxidation. Can be defined as: – The loss of electrons – The gain of oxygen – The loss of hydrogen

OXIDATION The loss of electrons Na (s)  Na + + e - The sodium ion has been oxidized to the sodium cation. Na + Cl Sodium loses electron to chlorine - ClNa + +

OXIDATION Gain of oxygen – Combustion reactions are classic examples C (s) + O 2(g)  CO 2(g) (burning of coal) 2Fe (s) + 3O 2(g)  2Fe 2 O 3(s) (rusting of iron)

OXIDATION Loss of hydrogen – Oxidation can sometimes be best seen as the loss of hydrogen CH 3 OH (l)  CH 2 O (l) + H 2(g) Methyl alcohol has been oxidized into formaldehyde

Which one of the following elements were oxidized? Cl in Cl 2 0 → NO Mg in MgO +2 → YES Na in NaBr +1 → YES F in SrF 2 -1 → NO Sr in SrF 2 +2 → YES O in O 2 0 → NO

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Now we will take a deeper look into Reduction. 3 Definitions – The gain of electrons – The loss of oxygen – The gain of hydrogen

Gain of electrons – The process of silver electroplating Ag + + e -  Ag Silver cation has gained an electron and has been reduced to silver metal. REDUCTION

Loss of oxygen – Reduction can also be seen as the loss of oxygen in going from reactant to product. Fe 2 O 2 (s) + 3 CO (g)  2 Fe (s) + 3 CO 2(g) Iron ore is reduced to iron metal in a blast furnace with carbon monoxide

REDUCTION Gain of hydrogen A reduction can also be described as the gain of hydrogen atoms going from reactant to product. CO (g) + 2 H 2(g)  CH 3 OH (l) Carbon monoxide has been reduced to methyl alcohol

One’s loss is another’s gain Neither oxidation or reduction can take place without the other. – When those electrons are lost something else has to gain them. When trying to remember which is which think of LEO the lion goes GEROIL RIG Lose Electrons OxidationOxidation is Losing Gain Electrons ReductionReduction is Gaining

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Use a chart to determine Oxidation States. Elements Oxidation State Number of Atoms Total Charge

First we will do K 2 PtCl 6 ElementsKPtClSum of Oxidation States Oxidation State Number of Atoms Total Charge

Now do the following: What is the oxidation state of S in MgSO 4 ? What is the oxidation state of N in NaNO 3 ? What is the oxidation state of Cl in KClO 3 ?

Now do the following: What is the oxidation state of C in CO 3 - What is the oxidation state of Cl in ClO 2 - What is the oxidation state of Cr in CrO 4 2- What is the oxidation state of Cr in CrO 7 2-

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Review of Terms oxidation-reduction (redox) reaction: – involves a transfer of electrons from the reducing agent to the oxidizing agent. oxidation: loss of electrons reduction: gain of electrons Reducing Agent get oxidized Oxidizing agent get reduced

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An oxidation-reduction, (redox), reaction involves the transfer of electrons. The oxidation numbers of the atoms will change…. one goes up (oxidation) and one goes down (reduction) Sodium transfers its electrons to chlorine

Reduction is the gain of electrons. Nonmetals gain electrons to form – ions The oxidation number goes down (reduces)

A half-reaction can be written to represent reduction. Cl 0 + 1e - Cl -1 In reduction half reactions, electrons are written on the left because electrons are gained

Oxidation is the loss of electrons. Metal atoms lose electrons to become + ions The oxidation numbers go up (increases)

A half-reaction can be written to represent oxidation. Na 0 Na e - In oxidation half reactions, electrons are written on the right because electrons are lost

Half-Reactions Show either the oxidation or reduction portion of a redox reaction and the electrons gained or lost.

Steps for Writing Half-Reactions Cu 0 + Ag +1 N +5 O -2 3 Cu +2 (N +5 O -2 3 ) 2 + Ag 0 1.Assign an oxidation number to each element. 2.Write a partial half-reaction to show the change in oxidation state: Oxidation: Cu 0 Cu +2 Reduction: Ag +1 Ag 0

Steps Continued… 3. Show the number of electrons needed to explain how the oxidation number changed. Oxidation: Cu 0 Cu e- Reduction: Ag +1 +1e- Ag 0 4. Last, but not least, achieve conservation of charge: 2Ag +1 +2e- 2Ag 0

Practice Problems 1.Mg + Cl 2 MgCl 2 2.Cu + 2Ag + Cu Ag 3.Al 0 + Cr 3+ Al 3+ + Cr 0

Mg + Cl 2 MgCl 2

Cu + 2Ag + Cu Ag

Al 0 + Cr 3+ Al 3+ + Cr 0

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) (3) Cl – (aq) (4) H 2 (g) LEO growls GER LEO

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 + (3) C 2+ (4) H 2 (g) LEO growls GER GER

Regents Question: 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

Regents Question: Given the equation: 2 Al + 3 Cu 2+ 2 Al Cu The reduction half-reaction is (1) Al Al e – (2) Cu e – Cu (3) Al + 3e – Al 3+ (4) Cu 2+ Cu + 2e –

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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. Any metal above H is more active than H and will react with an acid to produce H 2 (g) – The higher up the table, the more readily the replacement will take place.

Regents Question: 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

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 + ZnSO 4 No Reaction Copper cannot replace zinc Oxidation is on top Reduction is on bottom

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Electrochemical Cells Electrolytic vs voltaic cells.

An electrochemical cell can be either voltaic or electrolytic. In an electrochemical cell, oxidation occurs at the anode and reduction at the cathode. Red Cat – An Ox Reduction at the Cathode Oxidation at the Anode

A voltaic cell spontaneously converts chemical energy to electrical energy. Batteries are voltaic cells

61 An Ox -oxidation takes place…electrons are lost. Red Cat -reduction takes place…electrons are gained. Electrons flow from the anode (- electrode) to the cathode (+ electrode) through the wire in a voltaic cell. Zn Zn e - Cu e - Cu Electrons released here by oxidation Electrons needed here for reduction e-

62 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 Cu(s)

63 The salt bridge completes the circuit allows (-) ions to flow from one ½ cell to the other ½ cell to maintain neutrality. Zn Zn e - Cu e - Cu 0 - +

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.

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An electrolytic cell requires electrical energy to produce chemical change. This process is known as electrolysis.

Regents Question: 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.  Write the half-reaction for the oxidation that occurs.  Balance the equation using the smallest whole- number coefficients. Cr e- Cr 0 Zn 0 Zn e- The number of e-s gained = the number of e-s lost 3Zn + 2Cr 3+ 2Zn Cr

Regents Question: 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?  Which half-reaction occurs at the cathode?  State what happens to the number of protons in a Zn atom when it changes to Zn 2+ as the redox reaction occurs. Zn 0 loses electrons, Cr 3+ gains electrons Hint: Red Cat Cr e- Cr 0 The number of protons remains the same.

70 Regents Question: Indicate with one or more arrows the direction of electron flow through the wire. Write an equation for the half-reaction that occurs at the zinc electrode. Explain the function of the salt bridge. e- Zn Zn e - The salt bridge completes the circuit and allows ions to flow from one ½ cell to the other. This keeps the ½ cells neutral. Hint: Zinc is above copper on table J so Zn is oxidized

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

Regents Question: 06/02 #22 Which process requires an external power source? (1)neutralization (2) synthesis (3) fermentation (4) electrolysis

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Voltaic Vs Electrolytic Cell (Similarities) Both use redox reactions Anode is the site of oxidation & Cathode is the site of reduction (RED CAT & AN OX) Electrons flow through the wire from the anode to the cathode

Differences PROPERTIESVOLTAIC CELLELECTROLYTIC CELL REDOX REACTION SPONTANEOUSNON SPONTANEOUS ANODENEGATIVEPOSITIVE CATHODEPOSITIVENEGATIVE

Anode and Cathode OXIDATION occurs at the ANODE. REDUCTION occurs at the CATHODE.

Which is Which? ELECTROLYTIC CELL VOLTAIC CELL

Electroplating