1 2 Oxidation Number 3 The oxidation number (oxidation state) of an atom represents the number of electrons lost, gained, or unequally shared by an.

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Oxidation NumberOxidation Number

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The oxidation number (oxidation state) of an atom represents the number of electrons lost,

gained, or unequally shared by an atom.

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Oxidation numbers can be zero, positive, negative or fractional.

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An oxidation number of zero means the atom has the same number of electrons assigned to it as there are in the free neutral atom.

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A positive oxidation number means the atom has fewer electrons assigned to it than in the neutral atom.

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A negative oxidation number means the atom has more electrons assigned to it than in the neutral atom.

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The oxidation number of an atom that has gained or lost electrons to form an ion is the same as the positive or negative charge of the ion.

NaCl

Sodium has lost an electron.

Chlorine has gained an electron.

The charge on sodium is +1.

The oxidation number of

sodium is +1.

The charge on chlorine is –1.

The oxidation number of

chlorine is -1.

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In covalently bonded substances, oxidation numbers are assigned by an arbitrary system based on relative electronegativities.

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For symmetrical covalent molecules each atom is assigned an oxidation number of 0 because the bonding pair of electrons is shared equally between two like atoms of equal electronegativity.

Electronegativity 2.1


Oxidation Number 0

Oxidation Number0

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When the covalent bond is between two unlike atoms, the bonding electrons are shared unequally because the more electronegative element has a greater attraction for them.



shared pair of electrons

unequal electron sharing

both shared electrons are assigned to chlorine

there is a partial transfer of an

electron to chlorine

after assignment chlorine has one more electron than neutral chlorine

Oxidation Number -1

after assignment hydrogen has one less electron than neutral chlorine

Oxidation Number+1

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N2 N2O NO N2O3 NO2 N2O5

N oxidation number

0 +1 +2 +3 +4 +5 +5

Many elements have multiple oxidation numbers

-3NO

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Step 1 Write the oxidation number of each known atom below the atom in the formula.

Step 2 Multiply each oxidation number by the number of atoms of that element in the compound.

Step 3 Write an expression indicating the sum of all the oxidation numbers in the compound. Remember: The sum of the oxidation numbers in a compound must equal zero.

Rules for Determining the Oxidation Number of an Element Within a Compound

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Determine the oxidation number for sulfur in sulfuric acid.

H2SO4Step 1 -2+1

Write the oxidation number of each known atom below the atom in the formula.

Multiply each oxidation number by the number of atoms of that element in the compound.

Step 2 4(-2) = -82(+1) = +2

Step 3 +2 + S + (-8) = 0

Write an expression indicating the sum of all the oxidation numbers in the compound.

Step 4 S = +6 (oxidation number for sulfur)

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Determine the oxidation number for manganese in potassium permanganate.

KMnO4Step 1 -2+1



Step 2 4(-2) = -81(+1) = +2

Step 3 +1 + Mn + (-8) = 0


Step 4 Mn = +7 (oxidation number for Mn)

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Determine the oxidation number for nitrogen in the nitrate ion.

Step 1 -2



Step 2 3(-2) = -6

Step 3 1N + (-6) = -1 (the charge on the ion)


Step 4 1N = +5 N = +5 (oxidation number for nitrogen)

NO3-

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Determine the oxidation number for carbon in the oxalate ion.

Step 1 -2



Step 2 4(-2) = -8

Step 3 2C + (-8) = -2 (the charge on the ion)


2-2 4C O

Step 4 2C = +6 C = +3 (oxidation number for carbon)

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Determine the oxidation number for carbon in ethane.

C2H6Step 1 +1



Step 2 6(+1) = +6

Step 3 2C + (+6) = 0


Step 4 C = - 3 (oxidation number for carbon)

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Determine the oxidation number for carbon in propyne.

C3H4Step 1 +1



Step 2 4(+1) = +4

Step 3 3C + (+4) = 0


Step 4 3C = - 4 or C = - 4/3

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Oxidation-ReductionOxidation-Reduction

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Oxidation-reduction (redox) is a chemical process in which the oxidation number of an

element is changed.

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Redox may involve the complete transfer of electrons to form ionic bonds or a partial

transfer of electrons to form covalent bonds.

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• Oxidation occurs when the oxidation number of an element increases as a result of losing electrons. OIL

• Reduction occurs when the oxidation number of an element decreases as a result of gaining electrons. RIG

• In a redox reaction oxidation and reduction occur simultaneously.

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Leo the Lion says

GER

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Balancing Oxidation-Balancing Oxidation-Reduction EquationsReduction Equations

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The Loop MethodThe Loop Method

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Balance the equation Sn + HNO3 → SnO2+ NO2+H2O

Step 1 Assign oxidation numbers to each element to identify the elements being oxidized and those being reduced. Write the oxidation numbers above or below each element..

Sn + HNO3 → SnO2 + NO2 + H2O0 +1 +5 -2 +4 -2 +4 -2 +1 -2

oxidation number of tin increases

oxidation number of nitrogen decreases

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Balance the equation

Sn + HNO3 → SnO2+ NO2+H2O

Sn + HNO3 → SnO2 + NO2 + H2O+ 4+ 50 + 4

gain 1e-

lost 4 e-

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Step 2 Multiply the two equations by the smallest whole numbers that will make the electrons lost by oxidation equal to the number of electrons gained by reduction.

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Sn + HNO3 → SnO2+ NO2+H2O

Sn + HNO3 → SnO2 + NO2 + H2O+ 4+ 50 + 4

gain 1e-

lost 4 e-

41 4 2

Step 3 Balance the remaining elements that are not oxidized or reduced to give the final balanced equation.

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Balance the equation C3H4 + O2 → CO2 + H2O

C3H4 + O2 → CO2 + H2O- 4/3 + 40 - 2

gain 2e-2 4e-

lost 16/3 e- 3 16e- 4 4

4 1

41 3 2

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MnO2 + HCl → MnCl2 + Cl2 + H2O+ 2-1+4 0

lost 1e-

gain 2 e-

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2 2 2 2MnO HCl MnCl Cl H O

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2+2

4 2Cl-1

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Balancing Practice

2 2Ca HCl CaCl H

2 2ZnS O SO ZnO

3 2 4 4 2Cu HNO H SO CuSO H O NO

2 2 2 2MnO HCl MnCl Cl H O

3 3 2 2Fe HNO Fe(NO ) NO H O

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Multiple Change Balancing

3 2 2 4 4 2CrBr NaOH Cl Na CrO NaBrO NaCl H O

This one’s hard!

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Answer to Multiple Change

3 2

2 4 4 2

2 CrBr 64 NaOH 27 Cl

2 Na CrO 6 NaBrO 54 NaCl 32 H O

+3 -1 0

+6 +7 -1

Oxidation numbers above the three elements that change oxidation numbers in RED

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Activity SeriesActivity Seriesof Metalsof Metals

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activity series: A listing of metallic elements in descending order of reactivity.

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Sodium (Na) will displace any element below it from one of its compounds.

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ng

acti

vity

Mg(s) + PbS(aq) MgS(aq) + Pb(s)

PbH2

KBaCaNaMgAlZnCrFeNiSn

Cu

Magnesium is above lead in the activity series.

Magnesium will displace lead from one of its compounds.

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acti

vity

Silver is below copper in the activity series.

Silver will not displace copper from one of its compounds.

Ag(s) + CuCl2(aq) no reactionBa

PbH2

NaMgAlZnCrFeNiSn

CuAgHg

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Electrolytic andElectrolytic andVoltaic CellsVoltaic Cells

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electrolysis The process whereby electrical energy is used to bring about a chemical change.

electrolytic cell: An electrolysis apparatus in which electrical energy from an outside source is used to produce a chemical change.

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cathode The negative electrode.

anode The positive electrode.

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Electrolysis ofHydrochloric Acid

Electrolysis ofHydrochloric Acid

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In an electrolytic cell electrical energy from the voltage source is used to bring about nonspontaneous redox reactions.

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H3O+ + 1e- → Ho + H2O

Ho + Ho → H2

Cathode Reaction

Hydronium ions migrate to the cathode and are reduced.

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17.3

Cl-→ Clo + e-

Clo + Clo→ Cl2

Anode Reaction

Chloride ions migrate to the anode and are oxidized.

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The hydrogen and chlorine produced when HCl is electrolyzed have more potential energy than was present in the hydrochloric acid before electrolysis.

electrolysis2HCl(aq) H2(g) + Cl2(g) electrolysis

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The Zinc-Copper Voltaic Cell

The Zinc-Copper Voltaic Cell

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voltaic cell: A cell that produces electrical energy from a spontaneous chemical reaction. (Also known as a galvanic cell).

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When a piece of zinc is put in a copper(II) sulfate solution, the zinc quickly becomes coated with metallic copper. This occurs because zinc is above copper in the activity series.

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Zn(s) + CuSO4(aq) ZnSO4(aq) + Cu(s)

incr

easi

ng

acti

vity

Zinc is above copper in the activity series.

Zinc will displace copper from one of its compounds.

PbH2

KBaCaNaMgAlZnCrFeNiSn

Cu

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If this reaction is carried out in a voltaic cell, an electric current is produced.

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Zno(s) → Zn2+

(aq) + 2e-anode oxidation

Cu2+(aq) + 2e- → Cuo

(s) reductioncathode

loss of electrons

gain of electrons

Zno(s) + Cu2+

(aq) → Zn2+(aq) + Cuo

(s)

Net ionic reaction

Zno(s) + CuSO4(aq) → ZnSO4(aq) + Cuo

(s)

Overall equation

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LeClanche Cell• The LeClanche Cell was

described by Georges LeClanche (1839-1882) in 1867. The two electrodes are carbon and zinc, with a sal ammoniac electrolyte. The carbon electrode is mixed with manganese peroxide. This battery was used mainly for intermittent service, such as ringing electric bells.

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Dry Cell batteryAnode: Zn Zn+2 + 2 e–

Zn+2 + 2 NH3 Zn(NH3)2+2

Removal of NH3

Cathode: 2 NH4

+ + 2 e– 2 NH3 + H2

H2 + MnO2 MnO + H2O

Removal of H2

Uses: Portable radios, toys, flashlights.

Advantages: Inexpensive, safe, many sizes.

Disadvantages: High current drain, NH3 builds up causing voltage drop, short shelf life.

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Alkaline Battery

Uses: Same as dry cell.

Advantages: No voltage drop, longer shelf life.

Disadvantage: Expensive

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Super-iron batteryNew type of Alkaline Battery

Uses: Same as dry cell.

Advantages: works well in high-drain-rate electronics.

Disadvantages: Expensive.

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Lead Storage Battery

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Mercury Battery

Used in calculators, watches, hearing aids, cameras, and devices where small size isneeded.

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Lithium Battery

Because it is light in weight, and has a large voltage (3.4 Vper cell) these are usedin pacemakers, cell phones,laptops, camcorders.

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Hydrogen Fuel Cell

Honda FCX Clarity Fuel Cell Vehicle

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Downs cell for sodium production

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Copper Electrolysis

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Electrorefining of copper metal

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Zinc strips help protect the iron hull of an oil tanker from oxidization. This strip is attached to the hull’s interior surface.

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Tarnish on silverware is a coating of silver sulfide (Ag2S). Tarnish begins when silver atoms come into contact with hydrogen sulfide (H2S) in the air. The silver ions and sulfide ions combine to form blackish silver sulfide. Aluminum atoms can help restore the silver to its shiny self.

Directions at:http://faculty.chemeketa.edu/lemme/CH%20122/handouts/Removing Silver Tarnish.pdf

http://newterra.chemeketa.edu/faculty/lemme/CH%20122/handouts/Removing%20Silver%20Tarnish.doc

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Corrosion

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Corrosion Prevention

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Dental Voltaic Cell

Al → Al+3 + 3e– O2 + 4 H+ + 4e– → 2 H2O

The short circuit between the Al foil and the filling produces a current that is sensed by the nerve of the tooth.

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• In 1936, while excavating ruins of a 2000-year-old village near Baghdad, workers discovered mysterious small vase. A 6-inch-high pot of bright yellow clay dating back two millennia contained a cylinder of sheet-copper 5 inches by 1.5 inches. The edge of the copper cylinder was soldered with a 60-40 lead-tin alloy comparable to today's solder. The bottom of the cylinder was capped with a crimped-in copper disk and sealed with bitumen or asphalt. Another insulating layer of asphalt sealed the top and also held in place an iron rod suspended into the center of the copper cylinder. The rod showed evidence of having been corroded with an acidic agent.

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The ancient battery in the Baghdad Museum

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The jar was found in Khujut Rabu just outside Baghdad and is composed of a clay jar with a stopper made of asphalt. Sticking through the asphalt is an iron rod surrounded by a copper cylinder. When filled with vinegar – or any other electrolytic solution - the jar produces about 1.1 volts.

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Atlantis Light-Bulb? From Egypt Hieroglyphics

Light BlubsFilament

Cord

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http://www.world-mysteries.com/sar_lights_fd1.htm

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