WarmUp: Oxidation ReviewIn one of Ms. Lyall’s favorite reactions, solid zinc combines with solid sulfur to produce zinc sulfide. Here is the skeleton equation

Zn(s) + S(s) ZnS(s)

a. Label the oxidation number of each species.

b. Label the species that is oxidized and which is is reduced.

c. Label the oxidizing and the reducing agents

d. Determine how many electrons were lost by each atom of the oxidized species. Draw an arrow from the oxidized reactant to the oxidized product and label the arrow with the number of electrons lost each atom. Repeat for the reduced species.

WarmUp: Oxidation ReviewIn one of Ms. Lyall’s favorite reactions, solid zinc combines with solid sulfur to produce zinc sulfide. Here is the skeleton equation

Zn is oxidized

S is reduced

Zn is the reducing agent

S is the oxidizing agent

Zn(s) + S(s) ZnS(s)

a. Label the oxidation number of each species.

0 0 +2 -2

b. Label the species that is oxidized and which is is reduced.

c. Label the oxidizing and the reducing agents

loses 2 e-

gains 2 e-

d. Determine how many electrons were lost by each atom of the oxidized species. Draw an arrow from the oxidized reactant to the oxidized product and label the arrow with the number of electrons lost each atom. Repeat for the reduced species.

e. The number of electrons lost by the reducing agent must equal the number of electrons gained by the oxidizing agent. You can use this ratio to help you balance a redox reaction equation.

Try to balance this skeleton equation:

HNO3(aq) + H3AsO3(aq) --> NO(g) + H3AsO4(aq) + H2O(l)

It can be done.But it’s not easy to do by trial and error.

Balancing Equations Using Oxidation NumbersOxidation numbers can help you!

As is oxidized

N is reduced

HNO3(aq) + H3AsO3(aq) --> NO(g) + H3AsO4(aq) + H2O(l)

1. Label the oxidation number of each species.

+1 -2+5 +1

2. Label which species is oxidized and which is is reduced.3. Determine how many electrons are “lost” by each oxidized atom and how many are “gained” by each reduced atom.

loses 2 e-

gains 3 e-

-2+3 -2+2 +1 -2+5 +1 -2

4. Determine the ratio needed to balance electrons lost with electrons gained.

X 3 = 6 electrons lost

X 2 = 6 electrons gained

Use that ratio to write locked coefficient ratio.

2 3

Balancing Equations Using Oxidation NumbersNow that you have locked the ratio between HNO3 and H3AsO3, the equation is easy to balance.

HNO3(aq) + H3AsO3(aq) --> NO(g) + H3AsO4(aq) + H2O(l)2 3

______ H ____________ N ____________ O ____________ As ______

2

2

2

3

3

3

11 11

15 15

Balancing Equations Using Oxidation NumbersOxidation numbers can help you!

Cu is oxidized

Some N is reduced

Cu(s) + HNO3(aq) --> Cu(NO3)2(aq) + NO(g) + H2O(l)

1. Label the oxidation number of each species.

0 +1

2. Label which species is oxidized and which is is reduced.3. Determine how many electrons are “lost” by each oxidized atom and how many are “gained” by each reduced atom.

loses 2 e-

gains 3 e-

-2+5 -2+5 +2-2 +1 -2

4. Determine the ratio needed to balance electrons lost with electrons gained.

X 3 = 6 electrons lost

X 2 = 6 electrons gained

Use that ratio to write locked coefficient ratio.

23

+2

Balancing Equations Using Oxidation Numbers

Cu(s) + HNO3(aq) --> Cu(NO3)2(aq) + NO(g) + H2O(l)23______ Cu ____________ N ____________ O ____________ H ______

33

38

8

824

4

248 8

Balancing Equations Using Oxidation NumbersOxidation numbers can help you!

Some S is oxidized

Mn is reduced

KMnO4+ H2SO4+ NaHSO3 MnSO4+ K2SO4 + Na2SO4 + H2O

1. Label the oxidation number of each species.

-2

2. Label which species is oxidized and which is is reduced.3. Determine how many electrons are “lost” by each oxidized atom and how many are “gained” by each reduced atom.

loses 2 e-

gains 5 e-

-2+6 -2+4 -2+2 +6-2

4. Determine the ratio needed to balance electrons lost with electrons gained.

X 5 = 10 electrons lost

X 2 = 10 electrons gained

Use that ratio to write locked coefficient ratio.

52

+7+1 +1 +1+1 +6 +1 -2+6+1 -2+1

Balancing Equations Using Oxidation Numbers

KMnO4+ H2SO4+ NaHSO3 MnSO4+ K2SO4 + Na2SO4 + H2O52

______ Mn ____________ K ____________ Na ____________ S ____________ H ____________ O ______

22

22 25

Because of theeven subscriptfor Na in the product youneed to doubleeverything

Balancing Equations Using Oxidation Numbers

KMnO4+ H2SO4+ NaHSO3 MnSO4+ K2SO4 + Na2SO4 + H2O104

______ Mn ____________ K ____________ Na ____________ S ____________ H ____________ O ______

44

44

2

410

Na2SO4

is locked

All the S inproduct is locked.

Because of theeven subscriptfor Na in the product youneed to doubleeverything

5

101313

12

6

1650 50

Balancing Equations Using Oxidation NumbersTry this one on your own

K2Cr2O7 + 6NaI + 7H2SO4

Cr2(SO4)3 + 3I2 + 7H2O + 3Na2SO4 + K2SO4