PSI AP Chemistry
Name _______________________
Chapter Problems Part E: Oxidation-Reduction Reactions
Classwork:
1. The standard breathalyzer test for BAC (blood alcohol content) involves the
reaction of the dichromate ion with alcohol in the breath.
Cr2O72-(aq) + CH3CH2OH(aq)  CH3COOH + H2O(l) + Cr3+(aq)
Orange
green
a) What is the oxidation state of the following elements as reactants and
products?
i) Cr Reactant
Product
ii)C Reactant
Product
b) Which species got oxidized?
c) Which species was the oxidizing agent?
d) How many electrons were transferred in the reaction?
e) Write the balanced equation for this process in acidic solution:
f) How many grams of alcohol were present in the breathalyzer sample if
3.0 mL of 0.025 M potassium dichromate solution was needed to react
with all of the alcohol?
g) A sample that after reaction possessed an orange likely came from
someone that has been drinking heavily. Do you agree with this
statement? Justify your answer.
2. For each of the following:
a) Identify the elements oxidized and reduced:
b) Identify the oxidizing agent and reducing agent:
c) Balance the reaction in acidic medium
Reaction 1: H3AsO3(aq) + Br2(l)  Br-(aq) + H3AsO4(aq)
Reaction 2: H2C2O4(aq) + MnO4-(aq)  MnO2(s) + CO2(g)
3. For each of the following:
a) Identify the elements oxidized and reduced:
b) Identify the oxidizing agent and reducing agent:
c) Balance the reaction in basic medium
Reaction 1: H2(g) + S(s) + OH-(aq)  S2-(aq) + H2O(l)
Reaction 2: H2O2(aq) + Cl2O7(aq)  O2(g) + ClO2-(aq)
4. Explain why nitric acid (HNO3) is a more effective oxidizing agent than
nitrous acid (HNO2):
5. Explain why K is a more effective reducing agent than Ca.
6. Propose why oxygen carries a negative charge in all compounds except when
it is chemically bonded to fluorine.
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AP Chemistry
Mole Concept, Reactions, Stoichiometry
Answers
1. The standard breathalyzer test for BAC (blood alcohol content) involves the
reaction of the dichromate ion with alcohol in the breath.
Cr2O72-(aq) + CH3CH2OH(aq)  CH3COOH + H2O(l) + Cr3+(aq)
Orange
green
a) What is the oxidation state of the following elements as reactants and
products?
i) Cr Reactant +6
Product +3
ii)C Reactant -2
Product 0
b) Which species got oxidized? CH3CH2OH
c) Which species was the oxidizing agent? Cr2O72d) Write the balanced equation for this process in acidic solution:
2Cr2O72-(aq) + 3CH3CH2OH(aq) + 16H+(aq)  3CH3COOH + 11H2O(l) +
4Cr3+(aq)
e) How many electrons were transferred in the reaction? 12 electrons
f) How many grams of alcohol were present in the breathalyzer sample if
3.0 mL of 0.025 M potassium dichromate solution was needed to react
with all of the alcohol?
0.0052 grams
g) A sample that after reaction possessed an orange likely came from
someone that has been drinking heavily. Do you agree with this
statement? Justify your answer.
Disagree, the orange color demonstrates a high concentration of
dichromate indicating very little of it was reduced by alcohol.
2. For each of the following:
a) Identify the elements oxidized and reduced:
b) Identify the oxidizing agent and reducing agent:
c) Balance the reaction in acidic medium
Reaction 1: H3AsO3(aq) + Br2(l)  Br-(aq) + H3AsO4(aq)
As: Oxidized
Br: Reduced
H3AsO3 = oxidized, reducing agent Br2 = reduced, oxidizing agent
H2O(l) + H3AsO3(aq) + Br2(l)  2Br-(aq) + H3AsO4(aq) + 2H+(aq)
Reaction 2: H2C2O4(aq) + MnO4-(aq)  MnO2(s) + CO2(g)
C: Oxidized
Mn: Reduced
H2C2O4 = oxidized, reducing agent MnO4- = reduced, oxidizing agent
3H2C2O4(aq) + 2MnO4-(aq) + 2H+(aq)  2MnO2(s) + 6CO2(g) + 4H2O(l)
3. For each of the following:
a) Identify the elements oxidized and reduced:
b) Identify the oxidizing agent and reducing agent:
c) Balance the reaction in basic medium
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AP Chemistry
Mole Concept, Reactions, Stoichiometry
Reaction 1: H2(g) + S(s) + OH-(aq)  S2-(aq) + H2O(l)
H2 = oxidized, reducing agent S = reduced, oxidizing agent
H2(g) + S(s) + 2OH-(aq)  S2-(aq) + 2H2O(l)
Reaction 2: H2O2(aq) + Cl2O7(aq)  O2(g) + ClO2-(aq)
O: Oxidized
Cl: Reduced
H2O2 = oxidized, reducing agent
Cl2O7 = reduced, oxidizing agent
4H2O2(aq) + Cl2O7(aq) + 2OH-(aq)  4O2(g) + 2ClO2-(aq) + 5H2O(l)
4. Explain why nitric acid (HNO3) is a more effective oxidizing agent than
nitrous acid (HNO2):
Nitric acid has an N with a +5 oxidation state while nitrous acid carries a
N with a +3 oxidation state which is NOT as effective at attracting
electrons
5. Explain why K is a more effective reducing agent than Ca.
K has a smaller nuclear charge for a given amount of shielding so loses
its electrons easier thereby promoting reduction of other species better.
Additional reasons could include K having a lower first ionization energy
and a half full as opposed to full s orbital.
6. Propose why oxygen carries a negative charge in all compounds except when
it is chemically bonded to fluorine or another oxygen.
Fluorine is more electronegative than oxygen and thus carries the
negative oxidation state leaving oxygen with a positive oxidation state.
The only two elements electronegative enough to remove electrons from
Oxygen are Fluorine and Oxygen.
www.njctl.org
AP Chemistry
Mole Concept, Reactions, Stoichiometry