Oxidation ReductionOxidation Reduction

Oxidation Reduction

DefinitionsDefinitions► Oxidation- an element has lost electrons to another Oxidation- an element has lost electrons to another

element.element.► Reduction- an element gains electrons from another Reduction- an element gains electrons from another

element.element.► Oxidizing agent- a substance which causes another Oxidizing agent- a substance which causes another

substance to be oxidized. itself is reduced.substance to be oxidized. itself is reduced.► Oxidizer- same as oxidizing agent.Oxidizer- same as oxidizing agent.► Reducing Agent- a substance which causes another Reducing Agent- a substance which causes another

substance to be reduced. Itself becomes oxidized.substance to be reduced. Itself becomes oxidized.► Reducer or reductant- same as above.Reducer or reductant- same as above.► Anion- (-) ion.Anion- (-) ion.► Cation- (+) ionCation- (+) ion

► ..

►Electrolyte- The salt solution in a Electrolyte- The salt solution in a battery or electrochemical cell.battery or electrochemical cell.

►Electrodes- are in batteries or an Electrodes- are in batteries or an electrochemical cell. Location for electrochemical cell. Location for either reduction or oxidation.either reduction or oxidation. Anode: charged electrode where oxidation Anode: charged electrode where oxidation

occurs.occurs. Cathode: charged electrode where Cathode: charged electrode where

reduction occurs.reduction occurs.

Rules for determining the Rules for determining the oxidation number(state) of a oxidation number(state) of a

substance.substance.►Page 159 in ZumdahlPage 159 in Zumdahl►1. The oxidation state of an atom in its 1. The oxidation state of an atom in its

elemental state is 0. For ex:, the elemental state is 0. For ex:, the oxidation state of each atom in the oxidation state of each atom in the substances Na(s), Osubstances Na(s), O22(g), O(g), O33(g), and Hg(l) (g), and Hg(l) is 0.is 0.

►2. The oxidation state of a monatomic ion 2. The oxidation state of a monatomic ion is the same as its charge. For example, is the same as its charge. For example, the oxidation state of the Na ion is +1 the oxidation state of the Na ion is +1 and of the Cl ion is -1and of the Cl ion is -1

Rules continued…Rules continued…►3. Oxygen is assigned an oxidation 3. Oxygen is assigned an oxidation

state of -2 in its covalent compounds state of -2 in its covalent compounds such as CO, COsuch as CO, CO22, SO, SO22, and SO, and SO33. An . An exception to this rule occurs in exception to this rule occurs in peroxides( compounds containing the Operoxides( compounds containing the O22 group), where each oxygen is assigned group), where each oxygen is assigned an oxidation state of -1. The best known an oxidation state of -1. The best known example of a peroxide is hydrogen example of a peroxide is hydrogen peroxide ( Hperoxide ( H22OO22) )

Rules continued…Rules continued…►4. In its covalent compounds with 4. In its covalent compounds with

nonmetals, hydrogen is assigned an nonmetals, hydrogen is assigned an oxidation state of +1. For ex: in the oxidation state of +1. For ex: in the compounds HCl, NHcompounds HCl, NH33, H, H22O, and CHO, and CH44, , hydrogen is assigned an oxidation hydrogen is assigned an oxidation state of +1.state of +1.

►5. In its compounds, fluorine is always 5. In its compounds, fluorine is always assigned an oxidation state of -1.assigned an oxidation state of -1.

Rules continued….Rules continued….► 6. The sum of the oxidation states must be zero 6. The sum of the oxidation states must be zero

for an electrically neutral compound. For an ion, for an electrically neutral compound. For an ion, the sum of the oxidation states must equal the the sum of the oxidation states must equal the charge of the ion. For ex: the sum of the charge of the ion. For ex: the sum of the oxidation states for the hydrogen and oxygen oxidation states for the hydrogen and oxygen atoms in water is 0; the sum of the oxidation atoms in water is 0; the sum of the oxidation states for the carbon and oxygen atoms in COstates for the carbon and oxygen atoms in CO33 is is -2; and the sum of oxidation states for the -2; and the sum of oxidation states for the nitrogen and hydrogen atoms in NHnitrogen and hydrogen atoms in NH44

++ is +1 is +1 ► 7. Group 1 and 2 metals: oxidation state = ionic 7. Group 1 and 2 metals: oxidation state = ionic

charge.charge.

Determining an oxidation Determining an oxidation numbernumber

Examples:Examples:KK22CrCr22OO77

CHCH44

HH22

Br Br ––

FeOFeOFeFe22OO33

CC22HH55OHOHCuClCuCl22

Determining what was oxidized Determining what was oxidized and what was reducedand what was reduced

►Examples:Examples:►Mg + 2HCl Mg + 2HCl MgCl MgCl22 + H + H22

►NN2 2 + 3H + 3H22 2NH 2NH33

►CHCH44 + 2O + 2O22 CO CO22 + 2H + 2H22OO►2Na + 2H2Na + 2H22O O NaOH + H NaOH + H22

►FeFe22OO3 3 + 3CO + 3CO 2Fe + 3CO 2Fe + 3CO22

►NaNa22SOSO4 4 + BaCl + BaCl22 BaSO BaSO44 + 2NaCl + 2NaCl

Relative reactivitiesRelative reactivities►Ultimate goal:Ultimate goal:

To be able to predict whether a reaction To be able to predict whether a reaction will occur or not (be spontaneous or not)will occur or not (be spontaneous or not)

To predict the products produced in a To predict the products produced in a spontaneous reaction spontaneous reaction

To be able to determine relative To be able to determine relative reactivities for a series of elements.reactivities for a series of elements.

Determining if a reaction is Determining if a reaction is spontaneous and the products spontaneous and the products A reactivity chart is usedA reactivity chart is used The more easily reduced substance must The more easily reduced substance must

end up in its reduced state, the other end up in its reduced state, the other substance must end up in its oxidized substance must end up in its oxidized state.state.

If the substances are already in their most If the substances are already in their most reduced or oxidized state than no reaction reduced or oxidized state than no reaction occurs.occurs.

continuedcontinued► Oxidized state:Oxidized state:► For metals:For metals:

Cation state: ex. MgCation state: ex. Mg+2+2, , NaNa++,,

transition metals transition metals must have their must have their highest oxidation highest oxidation number. Ex Fenumber. Ex Fe+3+3, not , not FeFe+2+2

► For nonmetals: For nonmetals: Diatomic molecules:ex Diatomic molecules:ex

FF22, O, O2, 2, ClCl2,2,, etc., etc.

► Reduced state:Reduced state:► For metals:For metals:

Solid state: ex. Mg, Solid state: ex. Mg, Na, Fe etc.Na, Fe etc.

► For nonmetals:For nonmetals: Anion state: ex. F-, Anion state: ex. F-,

Cl-, OCl-, O-2-2, N, N-3-3 etc. etc.

continuedcontinued►Hydrogen, even though it is a Hydrogen, even though it is a

nonmetal, has these states:nonmetal, has these states: Oxidized state: HOxidized state: H++

Reduced state: HReduced state: H22

Determining whether a reaction Determining whether a reaction is spontaneousis spontaneous

►Examples:Examples: Cu(s) + ZnCu(s) + Zn+2 +2 CuCu+2+2 + Zn(s) + Zn(s)

II22 + 2Cl + 2Cl-- 2I2I- - + Cl + Cl22

Zn + HZn + H+ + Cu + HCu + H+ +

Determining relative Determining relative reactivitiesreactivities

►ExampleExample Rank the following elements from Rank the following elements from

strongest reducing agent to weakest strongest reducing agent to weakest reducing agent.reducing agent.

A + BA + B+2+2 no reaction no reaction A + CA + C+2+2 A A+2+2 + C + C C + DC + D+2 +2 no reaction no reaction A + DA + D+2+2 A A+2+2 + D + D

►Example:Example:► rank the elements from strongest rank the elements from strongest

oxidizer to weakest oxidizer.oxidizer to weakest oxidizer. ZnZn+2+2 + Cu + Cu no reaction no reaction ZnZn+2 +2 + Mg + Mg Zn + Mg Zn + Mg+2+2

ZnZn+2+2 + Pb + Pb no reaction no reaction Pb + CuPb + Cu+2 +2 Pb Pb +2+2 + Cu + Cu

The two electrochemical cellsThe two electrochemical cells► Battery or voltaic Battery or voltaic

cellcell Uses a spontaneous Uses a spontaneous

redox reactionredox reaction Produces electricityProduces electricity

► Electrolytic cellElectrolytic cell Called electrolysisCalled electrolysis USES A USES A

NONSPONTANEOUS NONSPONTANEOUS REDOX reactionREDOX reaction

Uses electricityUses electricity Practical use is to obtain Practical use is to obtain

elements in a pure elements in a pure reduced form that reduced form that cannot be obtained in cannot be obtained in another way.another way.

Battery: Voltaic cell or galvanic Battery: Voltaic cell or galvanic cellcell

►Produces electricityProduces electricity►Must use a spontaneous redox Must use a spontaneous redox

reactionreaction►Oxidation occurs at the anode (-)Oxidation occurs at the anode (-)►Reduction occurs at the cathode (+)Reduction occurs at the cathode (+)

Voltaic cellVoltaic cell► AnodeAnode

Is negatively chargedIs negatively charged Oxidation occursOxidation occurs The more easily The more easily

oxidized element is oxidized element is located here.located here.

e- are removed from e- are removed from the neutral metal.the neutral metal.

Ex. ZnEx. Zn(s)(s) Zn Zn+2+2 + + 2e-2e-

► CathodeCathode Is positively chargedIs positively charged Reduction occursReduction occurs The more easily The more easily

reduced metal is reduced metal is located here.located here.

e- are added to the e- are added to the cation of the metal to cation of the metal to produce a pure produce a pure elemental substance.elemental substance.

2e- + Cu2e- + Cu+2+2 Cu Cu(s)(s)

A battery with zinc and A battery with zinc and coppercopper

►Zinc will spontaneously lose e- to the Zinc will spontaneously lose e- to the copper.copper.

►Anode: Anode: Solid piece of Zn metalSolid piece of Zn metal Zn has e- removed from it.Zn has e- removed from it. The e- move through the wire to the The e- move through the wire to the

cathode.cathode. Zinc ions are formed and are in the Zinc ions are formed and are in the

solution.solution.

►Cathode:Cathode: Copper ions are gaining e- and being reduced.Copper ions are gaining e- and being reduced. Solid Cu forms around the outside of the Cu Solid Cu forms around the outside of the Cu

cathode.cathode. Other info: the electrolyte is needed to Other info: the electrolyte is needed to

complete the circuit, and must be free moving.complete the circuit, and must be free moving. There must be a barrier between the electrodes There must be a barrier between the electrodes

or the copper will get e- directly from the zinc or the copper will get e- directly from the zinc and the e- will not move through the wires. and the e- will not move through the wires.

How a voltaic cell worksHow a voltaic cell works

Predicting the battery Predicting the battery arrangement between lead and arrangement between lead and

silversilver►For a reaction between lead and silver:For a reaction between lead and silver:

Determine what will be oxidized and what Determine what will be oxidized and what will be reducedwill be reduced

Draw a diagram of a simple battery. Label Draw a diagram of a simple battery. Label the anode and the cathode including its the anode and the cathode including its corresponding charge.corresponding charge.

Write out the half reaction that occurs at Write out the half reaction that occurs at each electrode.each electrode.

Write the over all redox reaction that occurs.Write the over all redox reaction that occurs.

Diagram of a batteryDiagram of a battery

Electrolytic cellElectrolytic cell► CathodeCathode► Negative chargeNegative charge► Attracts the cationAttracts the cation► Electrons are given Electrons are given

to the cationto the cation► Reduction occursReduction occurs► Ex: NaEx: Na++ + 1e + 1e-- Na Na(s)(s)► A build up of a fuzzy A build up of a fuzzy

metal, or a gas metal, or a gas appears appears

► AnodeAnode► Positive chargePositive charge► Attracts anionsAttracts anions► Electrons are Electrons are

removed from the removed from the anions. Neutral anions. Neutral elements are formedelements are formed

► Oxidation occursOxidation occurs► 2Cl- 2Cl- Cl Cl22 + 2e- + 2e-► A gas appears A gas appears

Electrolytic cell reactionsElectrolytic cell reactions►When NaCl is used:When NaCl is used:

2(Na2(Na++ + 1e + 1e-- Na) reduction Na) reduction 2Cl2Cl-- Cl Cl22 + 2e + 2e-- oxidation oxidation note: the electrons must cancel out. note: the electrons must cancel out.

There must be the same # of eThere must be the same # of e-- in in reduction as there is an oxidation. reduction as there is an oxidation.

continuedcontinued

►When water undergoes electrolysisWhen water undergoes electrolysis 2H2H22O O 2H 2H22 + O + O22

2(2H2(2H++ + 2e + 2e-- H H22) reduction) reduction 2O2O-2-2 O O22 + 4e + 4e- - oxidationoxidation

Electolytic cell: electrolyisis of Electolytic cell: electrolyisis of NaClNaCl