11

Redox Redox ReactionsReactions

::

HALF HALF REACTIONSREACTIONS

2

REDOX REACTIONS

REDOX REACTIONS

Day 1: UsingDay 1: UsingOxidation numbers

Electrochemical ReactionsElectrochemical Reactions

REACTIONSREACTIONS

33Terminology; Redox Reactions

Terminology; Redox Reactions

• OXIDATION—loss of electron(s)

increase in oxidation number.

Or called REDUCING AGENT.

• REDUCTION—gain of electron(s);

decrease in oxidation number.

Or called OXIDIZING AGENT

• OXIDATION—loss of electron(s)

increase in oxidation number.

Or called REDUCING AGENT.

• REDUCTION—gain of electron(s);

decrease in oxidation number.

Or called OXIDIZING AGENT

44

LEO says GER

Lose electrons: OXIDATION

Gain electrons: REDUCTION

5

Redox reactions are characterized byRedox reactions are characterized by ELECTRON TRANSFERELECTRON TRANSFER between an between an electron donor and electron acceptor.electron donor and electron acceptor.

Transfer leads to— Transfer leads to—

1. 1. increase in oxidation numberincrease in oxidation number of some element = of some element = OXIDATIONOXIDATION

2.2. decrease in oxidation numberdecrease in oxidation number of some element = of some element = REDUCTIONREDUCTION

REDOX REACTIONSREDOX REACTIONSREDOX REACTIONSREDOX REACTIONS

66

Assigning oxidation numbers

88

Copper + Silver IonCopper + Silver Ion

99

HALF- REACTIONSHALF- REACTIONS

Cu(s) + 2 Ag+(aq) ---> Cu2+(aq) + 2 Ag(s) Oxidized?

Cu(s) ---> Cu2+(aq)

Reduced?

2 Ag+(aq) ---> 2 Ag(s)

1010Balancing Balancing EquationsEquations

Balance BOTH mass and CHARGE

Cu + Ag+ ----> Cu2+ + Ag

Step 1:: Divide into half-reactions:

oxidation and reduction.

Ox Cu ---> Cu2+

Red Ag+ ---> Ag

1111Balancing Balancing EquationsEquations

1. Balance each for mass (coefficients).

Already done in this case.

2. Balance each half-reaction for charge

Ox Cu ---> Cu2+ + 2e-(Oxidation electrons OUT: Product)

Red Ag+ + e- ---> Ag(Reduction electrons IN: Reactant)

1212Balancing Balancing EquationsEquations

3 Multiply each half-reaction so the electrons lost equal the number gained

Cu ---> Cu2+ + 2e- 2 Ag+ + 2 e- ---> 2 Ag

Add the two half reactions to the overall equation.

Cu + 2 Ag+ ---> Cu2+ + 2Ag

balanced for BOTH charge and mass.

1313

Space Fight• Identify two half reactions

• ID which element reduced and which is oxidized

• Why are liquid fuel preferred over gaseous?

• Why use hydrogen and oxygen as fuels if solid

fuels provide better propulsion? D8 C10

1414

2 Al(s) + 3 Cu2+(aq) --> 2 Al3+(aq) + 3 Cu(s)

Redox Redox EquationsEquations

1515

Recognizing a Redox Recognizing a Redox ReactionReaction

Recognizing a Redox Recognizing a Redox ReactionReaction

2 Al(s) + 3 Cu2+(aq) --> 2 Al3+(aq) + 3 Cu(s)

Al is OXIDIZED Al is OXIDIZED

Al(s) --> Al3+(aq) + 3 e-

Al Ox. # increases (Al Ox. # increases (more positive):):

loses e- : Oxidation

1616Recognizing a Redox Recognizing a Redox ReactionReaction

Recognizing a Redox Recognizing a Redox ReactionReaction

2Al(s) + 3Cu2+(aq) ’ 2Al3+(aq) + 3Cu(s)

Cu is REDUCED Cu is REDUCED

Cu2+(aq) + 2 e- ’ Cu(s)

•Ox. # of Cu decreases (more neg.)

• gaining electrons Reduction

1717Recognizing a Redox Recognizing a Redox ReactionReaction

Recognizing a Redox Recognizing a Redox ReactionReaction

2 half-reactions add up to give the overall reaction

2 Al(s) --> 2 Al3+(aq) + 6 e-

3 Cu2+(aq) + 6 e- --> 3 Cu(s)

--------------------------------------------------

2 Al(s) + 3 Cu2+(aq) ---> 2 Al3+(aq) + 3 Cu(s)

balanced for mass and charge.

1818

1919

“REDOX on Wheels”: Automobiles

D8 C13

2121Ready for more complex reactions??

No!No!

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