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Oxidation-Reduction

AKA Redox

OB: Pages 158-172

Aim: How do oxidation numbers tell us what is

happening in a reaction?Objectives1.Students should be able to determine the oxidation states of elements alone or in a compound.

2.Students should be able to identify reactions that are Redox.

3.Students should be able to determine the species being oxidized and the species being reduced in a Redox reaction.

4.Students should be able to identify oxidizing agents and reducing agents in a reaction.

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Oxidation Number Rules• Uncombined elements : Total charge = • Compounds: Total charge = • Polyatomic ions: Total charge =• Elements within compounds and polyatomic ions =

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Oxidation Number Practice• What are the oxidation numbers of each of the

elements in the following substances?

a) Na b) NaCl c) Cl2 d) H2SO4

e) HNO3 f) O2 g) Cr2O7-2 h) PCl5

i) NO2 j) LiH k) H2O l) SCN-

HW: OB Pg. 160-161, Q 1-10; pg. 169 Q 1,9,13-15

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Regents Questions

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Recognizing Redox Reactions

• Not all reactions are Redox!!

• If any element’s oxidation state on the reactant side is different from the oxidation state on the product side, it is a redox rxn.

Are these redox reactions?MnO2 + 4HCl MnCl2 + Cl2 + 2H2O

HCl + KOH H2O + KCl

Zn + 2HCl ZnCl2 + H2

2HCl + FeS FeCl2 + H2SHW: OB Pg. 162, Q 11-14; Pg. 169-170, Q 6,16-17

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During Redox reactions, are electrons lost or gained?

• Loss of e- = Oxidation (LEO) = charge _____• Gain of e- = Reduction (GER) = charge _____

Na + Cl2 NaCl Which one is being oxidized? ______Which one is being reduced? ______Redox = whenever an atom loses an electron, another

atom has to gain it. So oxidation does not happen without reduction.

The total charge of the products = the total charge of the reactants.

LEO says GER!

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Agents• In a reaction the element

that is oxidized is the reducing agent.

oxidized = reducing agent

• The element that is reduced is the oxidizing agent.

reduced = oxidizing agent

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oxidized = reducing agentreduced = oxidizing agent

HW: OB Pg. 162, Q 16-31

Exit• Write a redox reaction that we learned about

in the acid/base unit.• What are the oxidation states of all of the

species in your reaction?• Which species was reduced? Which

species was oxidized?• Which species loses electrons? Which

species gains electrons?• Which species is the reducing agent and

which species is the oxidizing agent?10

Aim: How is Redox demonstrated in a voltaic cell? Objectives:

1.Students should know that there are two types of electrochemical cells and which one is spontaneous and which one requires energy, in which one is chemical energy converted to electrical energy and in which one electrical energy is converted to chemical energy.

2.Students should be able to draw and label all the parts of a voltaic cell set-up.

3.Students should know where oxidation occurs and where reduction occurs in a voltaic cell.

4.Students should be able to determine the direction and location of electron flow in a voltaic cell.

5.Students should know the purpose of the salt bridge.

6.Students should be able to identify which anode gains mass and which one loses mass and why.

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Electrochemical Cells: Application of Redox Reactions

• Electrochemical cells involve a chemical reaction and a flow of electrons. During a redox reaction there is an exchange (flow) of electrons. An electrochemical cell is used to channel and control this flow of electrons for use as electricity.

• There are two types of electrochemical cells: voltaic (galvanic) cells (spontaneous) in which chemical energy is converted to electrical energy and electrolytic cells (require energy) in which electrical energy is converted to chemical energy.

• There are two metal bars or surfaces in an electrochemical cell that can conduct electricity called electrodes.

• Oxidation and reduction occur at the electrodes.

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Components of Voltaic Cells• Electrodes: 2 metals, one in each beaker of

solution containing that metal ion. Ex. Zn metal in a beaker containing Zn2+ ions and Cu metal in a beaker containing Cu2+ ions.

• Wires connecting the two pieces of metal and attached to a voltmeter. Electrons flow through the wire and the voltmeter measures the flow.

• Salt bridge connecting the two beakers containing different types of ions. The purpose of the salt bridge is to provide a path for ions to flow.

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Diagram of a Voltaic (Galvanic) Cell

http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/galvan5.swf

Quick CheckWhat am I?

1.I am like a battery…I convert chemical energy into electrical energy.

2.In a voltaic cell, I am a metal bar where oxidation or reduction takes place.

3.In a voltaic cell, electrons flow through me from one metal bar to the other.

4.Ions flow through me from one beaker to another.

5.I measure the flow of electrons.

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Voltaic Cells

Anode Cathode

Oxidation

AN OX

Reduction

RED CAT

( - ) ( + )

Higher on Table J

Lower on Table J

Electron Flow:

Anode Cathode

• In a voltaic cell, a spontaneous chemical reaction (a reaction that does not require an input of energy) produces a flow of electrons. Chemical energy is converted to electrical energy. A battery is an example.• Voltaic = spontaneous

redox rxn

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Table J• When given two metals, oxidation occurs at the metal that is ___________on Table J.• In a spontaneous redox reaction, metals will replace metal ions below them on Table J.For example: Li + KCl LiCl + K spontaneousK + LiCl no reaction

Are the following reactions spontaneous?Zn + HClCo + FeSO4

Rb + KClCu + PbSO4

Quick Check1. In a voltaic cell, a _______ _______ reaction occurs

where _________energy is converted to _________energy.

2. An example of a voltaic cell is a _______.

3. In a voltaic cell, the electrode where oxidation takes place is called the _________. Reduction takes place at the _________.

4. In a voltaic cell, the charge of the anode is _____. The charge of a cathode is _____.

5. The metal that will be the anode is _______ active than the metal that will be the cathode. That means the anode will appear ______ on Table J.

6. Electrons flow from the ___________ to the ________.

7. The anode will _____ mass and the cathode will _____ mass.

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Voltaic Cell Diagram

HW: pg. 168, Q 37-41, pg. 170-172, Q 24, 26-30

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Answers

Aim: How is redox demonstrated in an electrolytic cell?Objectives:

1.Students should be able to write redox half-reactions when provided with complete reactions.

2.Students should be able to identify parts of an electrolytic cell, including the energy source.

3.Students should know where oxidation occurs and where reduction occurs in an electrolytic cell.

4.Students should be able to determine the direction of electron flow in an electrolytic cell.

5.Students should be able to identify which anode gains mass and which one loses mass and why.

6.Students should understand how electrolytic cells are used for electroplating.

7.Students should be able to distinguish differences between voltaic cells and electrolytic cells.

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Half-Reactions

• Shows either the oxidation part or the reduction part.

Example: Cu + Ag+1 Cu+2 + AgOxidation #s: ___ ___ ___ ___

Oxidation Half:

Reduction Half:

Balance the equation:

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Half-Reactions Practice Ni + Sn+4 Ni+2 + Sn+2

Ox. Half:Red. Half:

Zn + H+ Zn+2 + H2

Ox. Half:Red. Half:

S-2 + O2 S+4 + O-2

Ox. Half:Red. Half:

HW: OB Pg. 164-165 Q 32-36, Pg. 169-170 Q 11,21,22,31,32

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Sketch a galvanic cell based on the half-reactions shown below.Al3+(aq) + 3e- → Al(s)Ni2+(aq) + 2e- → Ni(s)

a. Label the anode and the cathode.b. Indicate where oxidation and reduction are occurring. c. Show the direction of flow of the electrons.d. Show the direction of the movement of the ions.

e. Write a balanced ionic equation for the reaction

Quick Check: Half-Reactions and Voltaic Cells

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Electrolytic Cell• An electrolytic cell requires an electric current to

force a nonspontaneous chemical reaction to occur. • Electrolytic = requires energy• Used for electrolysis: alternator in a car, separate water

and salts into their elements, electroplating

Anode Cathode

Oxidation

AN OX

Reduction

RED CAT

(+) (-)

Electron Flow: Anode Cathode

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Diagram of an Electrolytic Cell

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Electroplating

HW: OB Pg. 137, Q 23,25,40 WB Pg. 9:33, Q 22,23,30,31

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Characteristic Voltaic Electrolytic

Use Redox Rxns?

Charge of the anode

Charge of the cathode

Oxidation occurs at the…

Reduction occurs at the…

Direction of e- flow

Spontaneous?

Has a battery?

HW: Pg. 172, Q 33-47