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Oxidation & Reduction
Reactions in which some elements change their oxidation number.
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Oxidation Numbers
• An oxidation number is the charge an atom would have if electrons in its bonds belonged completely to the more electronegative atom.
ClO2
Cl can have several oxidation numbers: -1,+1,+3,+5,+7
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Redox Reaction
• A chemical reaction in species change their oxidation number:
Na + Cl2 => NaCl
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What’s the point ?
• Electrical production (batteries, fuel cells)
REDOX reactions are important in …
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Rules
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Determine the oxidation number of each atom in each of the following formulas:
(a) O2
(b) CH4
(c) NaCl
(d) SO4-2
(e) KMnO4
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Practice Assigning Oxidation Numbers
NO2
N2O5
HClO3
HNO3
Ca(NO3)2
KMnO4
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Practice Assigning Oxidation Numbers
Fe(OH)3
K2Cr2O7
CO32-
CN-
K3Fe(CN)6
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Practice Assigning Oxidation Numbers
NO2N= +4, O = -2
N2O5 N = +5, O = -2
HClO3 H=+1, Cl=+5, O = -2
HNO3 H=+1, N = +5, O = -2
Ca(NO3)2 Ca=+2, N =+5, O= -2
KMnO4 K=+1, Mn=+7, O= -2
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Practice Assigning Oxidation Numbers
Fe(OH)3 Fe =+3, O=-2, H=+1
K2Cr2O7 K=+1, Cr=+6, O=-2
CO32-
C=+4, O =-2
CN- C=+4, N=-5
K3Fe(CN)6 K=+1, Fe=+3, C=+4, N=-5
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Lab: Corrosion of Iron• An important aspect of the use of some metals,
particularly of iron, is the possibility of corrosion.• Rust is apparently a hydrated form of iron(III)oxide
Fe2+(aq) + O2(g) + H2O(l) → Fe2O3•H2O(s)
rust
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Galvanic Corrosion• Galvanic corrosion is an electrochemical
process in which one metal corrodes preferentially to another when both metals are in electrical contact.
Zinc
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What do we do with oxidation numbers?
• The oxidation numbers of atoms in an equation allow us to determine which element is oxidized and which is reduced in a redox reaction.
6Cd + 2 FeBr3 => 3 Cd2Br2 + 2 Fe
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LEO
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LEO says
GER!
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LEO says
Loss of Electrons = Oxidation
Gain of Electrons = Reduction
GER!
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Oxidation and Reduction
• Apply oxidation numbers to all particles and identify the oxidized and the reduced elements.
Mg + HCl => MgCl2 + H2
F2 + AlBr3 => AlF3 + Br2
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• An increase in the oxidation number indicates that an atom has lost electrons and therefore oxidized.
• A decrease in the oxidation number indicates that an atom has gained electrons and therefore reduced
Zn + CuSO4 ZnSO4 + Cu
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Using Oxidation Numbers
• An increase in the oxidation number indicates that an atom has lost electrons and therefore oxidized.
• A decrease in the oxidation number indicates that an atom has gained electrons and therefore reduced
Zn + CuSO4 ZnSO4 + Cu
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Exercise
For each of the following reactions find the element oxidized and the element reduced
Cl2 + KBr KCl + Br2 Cu + HNO3 Cu(NO3)2
+ NO2 + H2O
HNO3 + I2 HIO3 + NO2
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ExerciseFor each of the following reactions find the element oxidized and the element reducedCl2 + KBr KCl + Br2
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Exercise For each of the following reactions find the element oxidized and the element reduced
Cu + HNO3 Cu(NO3)2 + NO2 + H2O
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Oxidation-Reduction Reactions (Redox)
• All oxidation reduction reactions have element reduced and one one element oxidized.
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Why is the Statue of Liberty green?
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Replica – Ellis Island
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• Acid Rain causes the copper to turn into copper oxide, copper sulfate, copper hydroxide or copper chloride because of the oxidation-reduction reaction and this is basically copper salts
or in other words, tarnish.
Things like statues, pennies,
or pipes made of Copper turn green
over a period of time.
This happens due to oxidation.
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Corrosion of steel, whether in atmospheric, soil, water, or chemical exposure, annually costs the economy about 3.1% of the Gross Domestic Product (GDP). Estimates show 25-30% of corrosion could be eliminated if proper corrosion protection methods, such as: galvanizing, painting, stainless or weathering steel were employed.
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Steel
• Steel is an alloy made by combining iron and another element, usually carbon. When carbon is used, its content in the steel is between 0.2% and 2.1% by weight, depending on the grade. Other alloying elements sometimes used are manganese, chromium, vanadium and tungsten.
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Corrosion of Iron• When iron rusts a spontaneous redox
reaction occurs, between the oxygen and iron. If water is added the rusting occurs more rapidly.
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Oxidation :
Fe(s) → Fe2+(aq) + 2e- Equation 1
Reduction : O2(g) + 2H2O(l) + 4e- → 4OH-(aq) Equation 2
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4Fe(OH)2(s) + O2(g) + xH2O(I)→ 2Fe2O3•(x)H2O(s) Equation 5
RUST
3Fe2+(aq) + 2Fe(CN)6 3-(aq) → Fe3[Fe(CN)6]2 (s) Equation 6 Yellow Prussian blue
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Corrosion of Iron
• Corrosion of iron is called rusting. Rust is Fe2O3x.H2O.
Many metals like iron, steel, polymers, ceramic etc are damaged due to some natural process.Corrosion is one of them.
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http://tdwhs.nwasco.k12.or.us/staff/bfroemming/CorrosionIron.html