AP Chemistry : Oxidation-Reduction Reactions

Study concepts, example questions & explanations for AP Chemistry

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Example Questions

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Example Question #1 : Reactions And Equilibrium

What is the balanced equation when heptane is combusted?

Possible Answers:

C3H8 + 5 O2 → 4 H2O + 3 CO2

2 C7H14 + 5 O2 → 3 H2O + 6 CO2

C7H16 + 11 O2 → 8 H2O + 7 CO2

C7H16 + O2 → H2O + CO2

C5H12 + 8 O2 → 6 H2O + 5 CO2

Correct answer:

C7H16 + 11 O2 → 8 H2O + 7 CO2

Explanation:

Heptane: C7H16. Combustion is when a molecule reacts with O2 and the products are CO2 and H2O. Balancing gives 7 CO2, 8 H2O, 1 heptane, and 11 O2

Example Question #1 : Combustion

Which of the following reactions has the most exothermic heat of reaction?

Possible Answers:
The combustion of isobutane
The combustion of propane
The combustion of butane
The combustion of isopropane
The combustion of methane
Correct answer: The combustion of butane
Explanation:

The longer the hydrocarbon chain, the greater the amount of combustion products (CO2 and H20) generated. Branched molecules such as isopropane and isobutane are more difficult to combust than their straight-chain counterparts.

Example Question #3 : Reactions And Equilibrium

Which of the following conditions would describe a combustion reaction?

Possible Answers:

Endergonic with a negative 

Exergonic with a positive 

Endothermic with a positive 

Exothermic with a positive 

Exothermic with a negative 

Correct answer:

Exothermic with a negative 

Explanation:

An exothermic reaction will have a negative  value, indicating that it releases heat. Conversely, an endothermic reaction will have a positive  value, indicating a consumption of heat.

A combustion reaction releases heat; thus it must have a negative  value and be exothermic.

Exergonic reactions have a negative  value, indicating spontaneity, while endergonic reactions are non-spontaneous. While most combustion reactions will be non-spontaneous, it is impossible to draw this conclusion for certain without knowing more about the reaction. The only thing we know for certain is that heat is released, and the reaction is exothermic.

Example Question #1 : Balancing Oxidation Reduction Reactions

The following ReDox reaction takes place in acidic solution:

Fe2+ + Cr2O72– → Fe3+ + Cr3+

What is the sum of coefficients in this redox reaction?

Possible Answers:

36

34

35

33

37

Correct answer:

36

Explanation:

When you balance the redox reaction in acidic conditons, there are 6Fe2+, 1 Cr2O72–, 14 H+, 6 Fe3+, 2 Cr3+, and 7 H2O. Don't forget to add the 1 in front of the Cr2O72–

Example Question #1 : Oxidation Reduction Reactions

For the following unbalanced redox reaction, how many electrons are transferred and which chemical species is being oxidized?

Possible Answers:

One electron is transferred; P is oxidized

Two electrons are transferred; Hg is oxidized

Two electrons are transferred; P is oxidized

One electron is transferred; Hg is oxidized

Correct answer:

Two electrons are transferred; Hg is oxidized

Explanation:

To begin, we will need to separate the given reaction into the two half-reactions by identifying changes in oxidation number. In this case, mercury (Hg) and phosphorus (P) show a change in oxidation number. Mercury begins with an oxidation number of zero, and ends with an oxidation number of . Phosphorus begins with an oxidation number of  and ends with an oxidation number of . Note that the oxidation numbers for fluorine and iodine reamain constant at  for each.

Now we can begin to look at the half-reactions.

Balance the atoms.

Now balance the electrons. We know that each mercury atom loses one electron and each fluorine atom gains one electron.

We can see that two electrons are tranferred. To identify the element being oxidized, we must find the element that is losing electrons. In this case, mercury is being oxidized.

Example Question #1 : Electrochemistry

Consider the following electrolytic cell:

      

What happens at the anode in the electrolytic cell?

Possible Answers:

Nickel is reduced

Iron is oxidized

Nickel is oxidized

Iron is reduced

Correct answer:

Nickel is oxidized

Explanation:

It does not matter if the cell is galvanic or electrolytic; oxidation will always take place at the anode. This means that the nickel loses two electrons and is oxidized at the anode to generate nickel ions.

Nickel ions and iron are products, and are neither oxidized nor reduced during the reaction. Iron ions are reduced at the cathode to generate the iron product.

Example Question #2 : Electrochemistry

Consider the following reaction in a galvanic cell:

 

Which of the following takes place at the anode?

Possible Answers:

No reaction takes place at the anode

Copper is reduced

Zinc is reduced

Copper is oxidized

Zinc is oxidized

Correct answer:

Zinc is oxidized

Explanation:

Oxidation takes place at the anode and reduction takes place at the cathode. You can remember this with the pneumonic "An Ox, Red Cat."

In the equation, zinc loses electrons. It goes from a neutral, elemental charge to a charge of . Since electrons are negative, a loss of electrons will cause an increase in charge. Because zinc loses electrons, it is oxidized. This will take place at the anode.

Example Question #1 : Electrochemistry

Toward which pole do electrons travel in a galvanic cell?

Possible Answers:

Towards the anode

Towards the negative pole

Electrons do not travel; only the protons travel

Towards the cathode

More information is needed

Correct answer:

Towards the cathode

Explanation:

Reduction always occurs at the cathode, and oxidation always occurs at the anode. Since reduction is the addition of electrons, the electrons must flow toward the site of reduction.

In a galvanic cell the positive charge is on the cathode, while the negative charge is on the anode. Since a galvanic cell has a positive potential and is spontaneous, electrons freely flow down their potential gradient. The electrons, which are negatively charged, are traveling towards the cathode, which is positive charged, since opposites attract.

Example Question #1 : Help With Electrolytic Cells

Toward which pole do the electrons travel in an electrolytic cell?

Possible Answers:

Towards the cathode

More information is needed

Towards the positive pole

The electrons do not travel; only the protons travel

Towards the anode

Correct answer:

Towards the cathode

Explanation:

Reduction always occurs at the cathode, and oxidation always occurs at the anode. Since reduction is the addition of electrons, electrons must travel toward the site of reduction.

In an electrolytic cell the negative charge is on the cathode, while the positive charge is on the anode. Since an electrolytic cell requires energy to perpetuate the reaction, we are pushing the electrons against their potential gradient. The electrons, which are negatively charged, are traveling towards the cathode, which is also negatively charged.

Example Question #1 : Electrochemistry

Which of the following differences between galvanic cells and electrolytic cells is false?

Possible Answers:

Electrolytic cells have a positive Gibb's free energy

Electrolytic cells are non-spontaneous

Electrolytic cells have negative voltages

Electrolytic cells have oxidation take place at the cathode

Correct answer:

Electrolytic cells have oxidation take place at the cathode

Explanation:

Electrolytic cells use non-spontaneous reactions that require an external power source in order to proceed. The values between galvanic and electrolytic cells are opposite of one another. Galvanic cells have positive voltage potentials, while electrolytic voltage potentials are negative. Both types of cell, however, have oxidation occur at the cathode and reduction occur at the anode.

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