Constructed Response
Oxidation-Reduction Reactions
Oxidation-Reduction
Reactions
Teacher Guide
Students will need about 20 minutes to complete these constructed response tasks.
Objectives assessed:
• Determine the oxidation number of an element in a compound.
• Apply changes in oxidation number to identify the species oxidized and reduced in a redox
reaction.
• Identify oxidizing and reducing agents in redox reactions.
1) The reaction that occurs when hydrogen sulfide is burned is given in the following reaction:
H2S(g) + 3/2O2(g)  SO2(g) + H2O(g)
• Determine the oxidation numbers of S, O, and H in the reactants.
• Determine the oxidation numbers of S, O, and H in the products.
• Identify the elements that are oxidized and reduced.
• Identify the oxidizing agent.
• Identify the reducing agent.
Correct response(s):
Sample Correct Response: The oxidation numbers are as follows:
Reactants:
H2S (H, +1; S, -2)
O2 (O,0—elemental))
Products:
SO2 (S, +4; O, -2)
H2O (H, +1; O, -2)
Because the oxidation number of S increases from -2 to +4 in the reaction, sulfur is oxidized.
The oxidation number of oxygen decreases from 0 to -2, so oxygen is reduced. The oxidizing
agent is O2 (since oxygen is reduced) and the reducing agent is H2S (since sulfur is oxidized).
Response scoring tool:
Score
Content
5
Includes all of the following:
• correct oxidation numbers for S (-2), O (0), and H (+1) in the
reactants
• correct oxidation numbers for S (+4), O (-2), and H (+1) in the
products
• sulfur is oxidized and oxygen is reduced
• the oxidizing agent is O2
• the reducing agent is H2S
4
Includes four of the points listed above
3
Includes three of the points listed above
2
Includes two of the points listed above
1
Includes one of the points listed above
0
No response, or response not appropriate to the question
Discovery Education Science
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Oxidation-Reduction Reactions
Objectives assessed:
• Determine the oxidation number of an element in a compound.
• Balance redox equations based on the number of electrons transferred.
2) For the following oxidation-reduction reaction:
Mg(s) + AgCl(aq)  MgCl2(aq) + Ag(s)
• Identify the oxidation half-reaction.
• Identify the reduction half-reaction.
• Write the balanced equation for the reaction.
Correct response(s):
Sample Correct Response: The magnesium metal increases in oxidation number from 0 to
+2. The oxidation half-reaction is: Mg(s)  Mg2+(aq) + 2e–. Silver ion, on the other hand, is
reduced in the reaction, with the oxidation number decreasing from +1 to 0. The reduction
reaction is: Ag+(g) +e– Ag(s). Combine both half-reactions to find the equation for overall
reaction, making sure to multiply the proper half-reaction by the correct coefficient to cancel out
the electrons:
Mg(s)  Mg2+(aq) + 2e–
+
2(Ag+(aq) +e– Ag(s))
And, therefore:
Mg(s)  Mg2+(aq) + 2e–
+
2Ag+(aq) +2e– 2Ag(s)
Mg(s) + 2Ag+(aq) 2Ag(s) + Mg2+(aq)
* note that the chloride ions are spectator ions and do not need to be
written in the balanced equation
Response scoring tool:
Score
Content
3
Includes the correct oxidation and reduction half-reactions and the
correct final balanced equation
2
Includes two of the above
1
Includes one of the above
0
No response, or response not appropriate to the question
Discovery Education Science
© Discovery Communications, LLC
Oxidation-Reduction Reactions
Objectives assessed:
Determine the oxidation number of an element in a compound.
Apply changes in oxidation number to identify the species oxidized and reduced in a redox
reaction.
Identify oxidizing and reducing agents in redox reactions.
Compare and contrast elements in an activity series to identify elements most easily oxidized.
3) Consider the following metals: Au(s), Ag(s), Pb(s), and Pt(s)
• Identify the metal(s) that will react in a solution of CuSO4(aq).
• For the metals that react, write the balanced equation that describes the reaction.
• Identify the oxidizing and reducing agents in the above reaction(s).
Correct response(s):
Sample Correct Response: The only metal that is above copper on the reactivity series is
lead. The three other metals lie below copper on the reactivity chart and will not react when
placed in a solution of CuSO4. Therefore, Pb is the metal that will react with CuSO4. The
balanced equation for this reaction can be written:
Pb(s) + CuSO4(aq)  PbSO4(aq) + Cu(s)
In this reaction, lead’s oxidation number increases from 0 in Pb(s) to a +2 in PbSO4(aq). It is
oxidized, so Pb(s) is the reducing agent. Copper’s oxidation number decreased from +2 in
CuSO4 to 0 in Cu(s). It is reduced, so CuSO4 is the oxidizing agent.
Response scoring tool:
Score
Content
3
Correctly includes all of the following:
• identifies Pb as the metal that will react
• includes a balanced equation describing the redox reaction
• identifies the oxidizing and reducing agents as copper sulfate and
lead metal, respectively
2
Correctly includes two of the points above
1
Correctly includes one of the points above
0
No response, or response not appropriate to the question
Discovery Education Science
© Discovery Communications, LLC
Oxidation-Reduction Reactions
Objectives assessed:
Differentiate among acid-base reactions, precipitation reactions, and oxidation-reduction reactions
4. Identify each reaction as an acid-base reaction, an oxidation-reduction reaction, or a
precipitation reaction. Give your reasons.
Correct response(s):
Equation
Type(s) of
reaction
Explain how you know
BaCl2(aq) + Na2SO4(aq) 
Precipitation
Forms insoluble BaSO4
HCl(aq) + NaOH(aq)  H2O + NaCl(aq)
Acid-base
Forms water and a salt
Fe(s) + CuSO4(aq)  FeSO4(aq) + Cu(s)
Oxidationreduction
Oxidation numbers of Fe and
Cu change
H2SO4(aq) + 2KOH(aq)  2H2O + K2SO4(aq)
Acid-base
Forms water and a salt
4Al(s) + 3O2(g)  2Al2O3(s)
Oxidationreduction
Oxidation numbers of Al and O
change
BaSO4(s) + 2NaCl(aq)
Response scoring tool:
Score
Content
5
Correctly identifies type of reaction and explains why for all five
equations.
4
Correctly identifies type of reaction and explains why for four out of five
equations.
3
Correctly identifies type of reaction and explains why for three out of five
equations.
2
Correctly identifies type of reaction and explains why for two out of five
equations.
1
Correctly identifies type of reaction and explains why for one out of five
equations.
0
No response, or response not appropriate to the question
Discovery Education Science
© Discovery Communications, LLC