MR. SURRETTE
VAN NUYS HIGH SCHOOL
CHAPTER 15: OXIDATION AND REDUCTION
WORKSHEET SOLUTIONS
1. __________ reactions always involve the transfer of electrons.
1A. Oxidation reduction
2. An atom that loses electrons is:
2A. oxidized
3. An atom that gains electrons is:
3A. reduced
4. __________ help determine which element is oxidized and which is reduced.
4A. Oxidation numbers
5. If the oxidation number increases, then the element is:
5A. oxidized
6. If the oxidation number decreases, then the element is:
6A. reduced
7. __________ is an atom’s ability to attract electrons.
7A. Electronegativity
8. Halogens are assigned a _____ oxidation number.
8A. – 1
Questions 9 - 11. Draw a Lewis Dot Structure (LDS) diagram for the following:
9. Hydrogen
9A.
10. Nitrogen
10A.
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CHEMISTRY
MR. SURRETTE
VAN NUYS HIGH SCHOOL
11. Ammonia (NH3)
11A.
12. Assign oxidation numbers to hydrogen and nitrogen based on the LDS diagram for ammonia.
12A.
The hydrogen atoms are each assigned a + 1 oxidation number. Since there are three hydrogen atoms,
they have a combined + 3 oxidation number. Ammonia has a net 0 charge. Therefore, the nitrogen
atom must have a - 3 oxidation number. Double check charges: + 3 - 3 = 0
Questions 13 - 15. Assign oxidation numbers to the following common polyatomic ions:
13. Carbonate (CO3-2)
13A.
Oxygen is assigned a - 2 oxidation number. Since there are three oxygen atoms, they have a combined
oxidation number of – 6. The net charge on the carbonate ion is – 2, so the carbon must have a + 4
oxidation number. Double check charges: - 6 + 4 = - 2 (CO3-2).
14. Phosphate (PO4-3)
14A.
The four oxygen atoms in PO4-3 each have a – 2 oxidation number, for a combined – 8 oxidation
number. Since the net charge on the phosphate ion is – 3, phosphorus must have a + 5 oxidation
number. Double check charges: - 8 + 5 = - 3 (PO4-3).
15. Ammonium (NH4+)
15A.
The four hydrogen atoms in NH4+ are each assigned a + 1 oxidation number, for a total of + 4.
Since the net charge on the ammonium ion is + 1, the nitrogen atom must have a – 3 oxidation
number. Double check charges: + 4 – 3 = + 1 (NH4+).
16. What is the oxidation number of nitrogen in magnesium nitride (Mg3N2)?
16A.
Magnesium is an alkaline metal, so it is assigned a + 2 oxidation number. Since there are three
magnesium atoms, they have a combined + 6 oxidation number. In order to balance out this + 6
oxidation number, each of the two nitrogen atoms must have a – 3 oxidation number.
Double check charges: + 6 + 2(- 3) = 0.
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CHEMISTRY
MR. SURRETTE
VAN NUYS HIGH SCHOOL
17. Find the oxidation numbers for iodine, oxygen, and carbon in the following redox reaction:
I2O5 + 5 CO  I2 + 5 CO2
17A.
Reactants:
For I2O5: Oxygen is assigned a – 2 oxidation number. Since there are five oxygen atoms, they have a
combined – 10 oxidation number. In order for the two iodine atoms to balance out the five oxygen
atoms, each iodine atom must have a + 5 oxidation number.
Reactants:
For CO: Oxygen is assigned a – 2 oxidation number. Therefore, the carbon must have a
+ 2 oxidation number.
Products:
For I2: I2 is a pure element, so each iodine atom has a + 0 oxidation number.
Products:
For CO2: Oxygen is assigned a – 2 oxidation number. Since there are two oxygen atoms in CO2, they
combine to a – 4 oxidation number. The carbon atom has a + 4 oxidation number.
I2(+5)O5(-2)
Final oxidation numbers
+ 5 C(+2)O(-2)  I2(0) + 5 C(+4)O2(-2)
18. For the above reaction, identify the element that is oxidized and the element that is reduced.
18A.
Since carbon loses electrons, it is oxidized. Since iodine gains electrons, it is reduced.
19. Zinc metal and chlorine gas react to form zinc chloride: Zn(s) + Cl2(g)  ZnCl2(s). Provide the
before and after oxidation numbers for zinc and chlorine.
19A.
Reactants:
For Zn: Pure zinc metal has a + 0 oxidation number.
For Cl2: Pure chlorine gas has a + 0 oxidation number.
Product:
For ZnCl2: Chlorine is a halogen, so it is assigned a – 1 oxidation number. Since there are two chlorine
atoms, they have a combined – 2 oxidation number.
19A. (continued…)
Product:
The zinc atom must have a + 2 charge to balance out the chlorine atoms.
Double check charges: - 2 + 2 = 0.
Final oxidation numbers
Zn(0) + Cl2(0)  Zn(+2)Cl2(-1)
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CHEMISTRY
MR. SURRETTE
VAN NUYS HIGH SCHOOL
CHAPTER 15: OXIDATION AND REDUCTION
QUIZ SOLUTIONS
1. An atom that loses electrons is:
1A. oxidized
2. An atom that gains electrons is:
2A. reduced
3. __________ help determine which element is oxidized and which is reduced.
3A. Oxidation numbers
4. If the oxidation number increases, then the element is:
4A. oxidized
5. If the oxidation number decreases, then the element is:
5A. reduced
6. __________ is an atom’s ability to attract electrons.
6A. Electronegativity
7. Hydrogen is assigned a _____ oxidation number.
7A. + 1
8. Alkaline metals are assigned a _____ oxidation number.
8A. + 2
9. Draw a Lewis Dot Structure (LDS) diagram for the following:
9a. Carbon
9b. Fluorine
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CHEMISTRY
MR. SURRETTE
VAN NUYS HIGH SCHOOL
9c. Carbon tetraflouride (CF4)
10. Assign oxidation numbers to carbon and fluorine based on the LDS diagram for carbon
tetrafluoride.
10A.
Fluorine is a halogen, so it is assigned a -1 oxidation number. Since there are four fluorine atoms in
CF4, they combine to form a – 4 oxidation number. The carbon atom must have a + 4 oxidation
number to balance out the fluorine oxidation numbers. Double check charges: - 4 + 4 = 0
(CF4 is a neutral molecule).
Questions 11 - 13. Assign oxidation numbers to the following polyatomic ions.
11. Sulfite (SO3-2)
11A.
Oxygen is assigned a – 2 oxidation number. Since there are three oxygen atoms in SO3-2, they have a
combined – 6 oxidation number. The net charge on the sulfite ion is – 2, so the sulfur must have a + 4
oxidation number. Double check charges: - 6 + 4 = - 2 .
12. Permanganate (MnO4-)
12A.
Oxygen is assigned a -2 oxidation number. Since there are four oxygen atoms in MnO4-, they have a
combined – 8 oxidation number. Since the net charge on the permanganate ion is – 1, manganese
must have a + 7 oxidation number. Double check charges: - 8 + 7 = - 1 (MnO4-).
13. Nitrite (NO2-)
13A.
Oxygen is assigned a - 2 oxidation number. Since there are two oxygen atoms, they have a combined –
4 oxidation number. Since the net charge on nitrite is - 1, the nitrogen atom must have a + 3 oxidation
number. Double check charges: - 4 + 3 = - 1 (NO2-)
14. What is the oxidation number of phosphorus in phosphoric acid (H3PO4)?
14A.
Hydrogen is assigned a + 1 oxidation number. Since there are three hydrogen atoms, they have a
combined oxidation number of + 3. Oxygen is assigned a – 2 oxidation number. Since there are four
oxygen atoms in phosphoric acid, they have a combined oxidation number of – 8. Phosphoric acid has
a net charge of 0. Therefore, the phosphorus atom must have a + 5 oxidation number.
Double check charges: + 3 – 8 + 5 = 0.
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CHEMISTRY
MR. SURRETTE
VAN NUYS HIGH SCHOOL
15. Find the oxidation numbers for sulfur, oxygen, and carbon in the following redox reaction:
CO2(g) + SO2(g)  CO(g) + SO3(g)
15A.
Reactants:
For CO2: Oxygen is assigned a – 2 oxidation number. Since there are two oxygen atoms in CO2, they
combine to a – 4 oxidation number.
Reactants:
Since CO2 has a 0 net charge, the carbon atom in CO2 have a + 4 oxidation number.
Reactants:
For SO2: Oxygen is assigned a – 2 oxidation number. Since there are two oxygen atoms in SO2, they
combine to a – 4 oxidation number.
Reactants:
Since SO2 has a 0 net charge, the sulfur atom must have a + 4 oxidation number.
Products:
For CO: Oxygen is assigned a – 2 oxidation number. Since CO has a 0 net charge, the carbon atom
must have a + 2 oxidation number.
Products:
For SO3: Oxygen is assigned a – 2 oxidation number. The three oxygen atoms have a combined – 6
oxidation number.
Products:
Since SO3 is a neutral molecule, sulfur must have a + 6 oxidation number.
C(+4)O2(-2)
Final oxidation numbers
+ S(+4)O2(-2)  C(+2)O(-2) + S(+6)O3(-2)
16. For the above reaction, identify the element that is oxidized and the element that is reduced.
16A.
Since sulfur loses electrons, it is oxidized. Since carbon gains electrons, it is reduced.
17. Predict the oxidation numbers for the chromate ion, CrO4-2.
17A.
Oxygen is assigned a – 2 oxidation number. Since there are four oxygen atoms in CrO4-2, they must
combine to a - 8 oxidation number. It might be tempting to assign the chromium atom a + 8 oxidation
number, but the chromate ion is not neutral. It has a -2 net charge. Therefore, the chromium must
have a + 6 oxidation number. Double check charges: - 8 + 6 = - 2.
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CHEMISTRY
MR. SURRETTE
VAN NUYS HIGH SCHOOL
18. Sulfur dioxide and oxygen react to form sulfur trioxide:
2 SO2(g) + O2(g)  2 SO3(g)
18a. Provide the before and after oxidation numbers for sulfur and oxygen.
Reactants:
For SO2: Each oxygen atom is assigned a – 2 oxidation number. Since there are two oxygen atoms in
SO2, they have a combined – 4 oxidation number.
Reactants:
Since SO2 is a neutral molecule, sulfur must have a + 4 oxidation number.
Reactants:
For O2: Pure oxygen has a 0 oxidation number.
Product:
For SO3: Non-elemental oxygen is assigned a – 2 oxidation number. Since there are three oxygen
atoms in SO3, they have a combined – 6 oxidation number.
Product:
Since SO3 is a neutral molecule, sulfur must have a + 6 oxidation number.
18a. (continued…)
Final oxidation numbers
2 S(+4)O2(-2)(g) + O2(+0)(g)  2 S(+6)O3(-2)(g)
18b. Identify the element that is reduced.
A. Since oxygen gains electrons, it is reduced.
18c. Identify the element that is oxidized.
A. Since sulfur loses electrons, it is oxidized.
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CHEMISTRY