1. REDOX REACTIONS
REACTIONS
Day 1 Review
Batteries

2. What Are Oxidation and Reduction?
What happens to a substance that undergoes oxidation?
What happens to a substance that undergoes reduction?
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3. Electron Transfer Reactions
oxidation-reduction or redox reactions are Electron transfer reactions.
Redox reactions can result in the generation of an electric current.
Therefore, this field of chemistry is often called ELECTROCHEMISTRY.

4. oxidation-reduction reactions.
A substance that undergoes oxidation gains oxygen. A substance that undergoes reduction loses oxygen.
Original definition
Reactions that involve the processes of oxidation and reduction are called
oxidation-reduction reactions.
Oxidation-reduction reactions are also known as
redox reactions.
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5. What Are Oxidation and Reduction?
MODERN REDOX DEFINITION
Oxidation
Loss of electrons
Reduction
Gain of electrons
Oxidation is now defined to mean complete or partial loss of electrons
Reduction is now defined to mean complete or partial gain of electrons
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6. What Are Oxidation and Reduction?
Redox Reactions
What Are Oxidation and Reduction?
During a reaction between a metal and a nonmetal, electrons are transferred from atoms of the metal to atoms of the nonmetal.
Mg(s) S(s) MgS(s)
heat
metal
nonmetal
IONIC
lose
gain
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7. REDOX Reactions: Metals Lose electrons
Metals (Cu) are “loser”:
oxidized
Metals (K) are oxidized
HNO3 is the reduced
hydrogen is reduced
Cu + HNO3 --> Cu NO2
2 K + 2 H2O --> 2 KOH + H2

8. Assigning Oxidation Numbers
How do we determine which element has gained and which has lost electrons?
oxidation numbers
An oxidation number is a positive or negative number assigned to an atom to indicate its degree of oxidation or reduction.
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9. Assigning Oxidation Numbers
What is the general rule for assigning oxidation numbers?
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10. OXIDATION NUMBERS The electric charge an element
APPEARS to have when electrons
are counted by some arbitrary rules:
1. An element has ox. no. = 0.
Zn O I S8
2. In simple ions, ox. no. = charge on ion.
-1 for Cl for Mg2+

11. OXIDATION NUMBERS 3. In a compound: O has ox. no. = -2
(except in peroxides: in H2O2, O = -1)
4. In a Molecule: Ox. no. of H = +1
(except when H is associated with a metal as in NaH where it is -1)
5. Algebraic sum of oxidation numbers
= 0 for a compound
= overall charge for an ion

12. What is the oxidation number of chromium in each compound?
chromium(III) potassium sulfate (CrK(SO4)2) is purple Cr
CrK(SO4)2 –2
potassium dichromate (K2Cr2O7) is Orange
K2Cr2O7 –2
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13. Oxidation number of F in HF?
OXIDATION NUMBERS
NH N =
ClO Cl =
H3PO P =
MnO Mn =
Cr2O Cr =
C3H C =
Oxidation number of F in HF?

14. Identifying Redox Reactions
What are the two classes of chemical reactions?
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15. How can you tell if this is a redox reaction?
N2(g) + O2(g) → 2NO(g)
During an electrical storm, oxygen molecules and nitrogen molecules in air react to form nitrogen monoxide.
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16. increase in oxidation number of same element = OXIDATION
REDOX REACTIONS
increase in oxidation number of same element = OXIDATION
2. decrease in oxidation number of same element = REDUCTION

17. The iron becomes coated with metallic copper.
This figure illustrates a redox reaction that shows what occurs when a shiny iron nail is dipped into a solution of copper(II) sulfate.
Cu2+SO4 + Fe → Fe2+SO4 + Cu
The iron reduces Cu2+ ions in solution and is simultaneously oxidized to Fe2+.
The iron becomes coated with metallic copper.
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18. Oxidation-Number Changes in Chemical Reactions
What happens when copper wire is placed in a solution of silver nitrate?
Oxidation-Number Changes in Chemical Reactions
2AgNO3(aq) + Cu(s) → Cu(NO3)2(aq) + 2Ag(s)
– –
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19. Use changes in oxidation number to identify which atoms are oxidized and which are reduced in the following reaction.
2HNO3(aq) + 3H2S(g) → 2NO(g) + 4H2O(l) + 3S(s)
+1 +5 – – – –
2HNO3(aq) + 3H2S(g) → 2NO(g) + 4H2O(l) + 3S(s)
Sulfur is oxidized because its oxidation number increases (–2 to 0).
Nitrogen is reduced because its oxidation number decreases (+5 to +2).
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20. Cl2(g) + 2HBr(aq) → 2HCl(aq) + Br2(l)
Use changes in oxidation number to identify which atoms are oxidized and which are reduced in the following reactions.
Cl2(g) + 2HBr(aq) → 2HCl(aq) + Br2(l)
b. C(s) + O2(g) → CO2(g)
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21. Identifying Oxidized and Reduced Atoms
Use changes in oxidation number to identify which atoms are oxidized and which are reduced in the following reaction.
Zn(s) + 2MnO2(s) + 2NH4Cl(aq) →
ZnCl2(aq) + Mn2O3(s) + 2NH3(g) + H2O(l)
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22. Which of the following are redox reactions?
A. NH3 + HCl → NH4Cl
B. SO3 + H2O → H2SO4
C. NaOH + HCl → NaCl + H2O
D. H2S + NHO3 → H2SO4 + NO2 + H2O
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23. Which of the following are redox reactions?
A. NH3 + HCl → NH4Cl
B. N2 + 3H2 → 2NH3
C. NaOH + HCl → NaCl + H2O
D. H2S + NHO3 → H2SO4 + NO2 + H2O
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24. The element whose oxidation number increases is oxidized
If changes in oxidation number occur, the reaction is a redox reaction.
The element whose oxidation number increases is oxidized
The element whose oxidation number decreases is reduced
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25. All chemical reactions can be assigned to one of two classes.
One class of chemical reactions is oxidation-reduction (redox) reactions, in which electrons are transferred from one reacting species to another.
The other class includes all other reactions, in which no electron transfer occurs.
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26. single-replacement, synthesis, decomposition, and combustion reactions are redox reactions.
Potassium metal reacts violently with water to produce hydrogen gas (which ignites) and potassium hydroxide.
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27. Identifying Redox Reactions
Examples of reactions that are not redox reactions include
double-replacement reactions
and acid-base reactions.
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28. Redox: Electron transfer
TRANSFER REACTIONS
Atom/Group transfer (not)
HCl + H2O ---> Cl H3O+
Redox: Electron transfer
Cu(s) Ag+(aq) ---> Cu2+(aq) Ag(s)

29. Balancing Redox Reactions Chapter 20:
To play the movies and simulations included, view the presentation in Slide Show Mode.

30. Cu + Ag+ --give--> Cu2+ + Ag Need to Balance BOTH mass and CHARGE
Balancing Equations
Cu + Ag+ --give--> Cu Ag
Need to Balance BOTH mass and CHARGE
Step 1: Divide into half-reactions:
one for oxidation and the other for reduction.
Ox Cu ---> Cu2+
Red Ag+ ---> Ag

31. Already done in this case. charge by adding electrons.
Balancing Equations
Step 2: Balance each for mass.
Already done in this case.
Step 3: Balance each half-reaction for
charge by adding electrons.
Ox Cu ---> Cu e-
Red Ag+ + e- ---> Ag

32. Balancing Equations
Step 4: Multiply each half-reaction by a factor to have the electrons lost equal to number gained
Cu ---> Cu e-
2 Ag e- ---> 2 Ag
Step 5: Add to give the overall equation.
Cu Ag > Cu Ag
The equation is now balanced for BOTH charge and mass.

33. Use the half-reaction method to balance the following redox equation.
FeCl3 + H2S → FeCl2 + HCl + S
Oxidation: H2S → 2H+ + S + 2e–
Reduction: 2Fe3+ + 2e– → 2Fe2+
2FeCl3 + H2S → 2FeCl2 + 2HCl + S
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34. The equation must be balanced for BOTH charge and mass
BIG IDEA
Reactions
Redox equations can be balanced by balancing the oxidation and reduction half-reactions.
The equation must be balanced for BOTH charge and mass
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