Physical Chemistry : Physical Chemistry

Study concepts, example questions & explanations for Physical Chemistry

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Example Questions

Example Question #1 : Galvanic And Electrolytic Cells

The Gibbs free energy of a redox reaction is __________ for a galvanic cell and is __________ for an electrolytic cell.

Possible Answers:

positive . . . positive

negative . . . positive

positive . . . negative

negative . . . negative

Correct answer:

negative . . . positive

Explanation:

Gibbs free energy is the “free” energy available that can be used to perform useful work. A negative Gibbs free energy corresponds to a spontaneous reaction, whereas a positive Gibbs free energy corresponds to a nonspontaneous reaction. Recall that a galvanic cell involves a spontaneous reaction, whereas an electrolytic cell involves a nonspontaneous reaction; therefore, the Gibbs free energy of the reaction is negative for a galvanic cell and positive for an electrolytic cell. 

Example Question #2 : Galvanic And Electrolytic Cells

What type of energy input is required for a galvanic cell? 

Possible Answers:

Both electrical and chemical energy

Electrical energy

Galvanic cells do not require energy

Chemical energy

Correct answer:

Galvanic cells do not require energy

Explanation:

Recall that galvanic cells carry out spontaneous reactions; therefore, they do not require energy. They release free energy that can be used to do work such as powering an electrical device (like cell phones).

Electrolytic cells, on the other hand, require energy because they carry out nonspontaneous reactions. They require the input of electrical energy. 

Example Question #171 : Physical Chemistry

Listed below are standard reduction potentials for a few elements.

Lithium: 

Iron (with  oxidation state): 

Aluminum: 

Given this information, which of the following redox reactions can be found in a galvanic cell? 

I. 

II. 

III. 

Possible Answers:

I and II

I and III

III only

I only 

Correct answer:

I and II

Explanation:

Galvanic cells are electrochemical cells that are characterized by a spontaneous redox reaction. To solve this question, we need to find the standard potential for each of the given reactions. Note that each reaction is a redox reaction; therefore, there is an oxidation half-reaction and a reduction half-reaction for each one.

The first reaction has a ferrous ion being reduced (gaining electrons) and lithium being oxidized (losing electrons). From the given information, we can deduce the standard potential for each half-reaction. The standard potential for reduction of iron is . Since the given standard potentials are for "reduction," the oxidation of iron is the opposite of the given value: . The total standard potential for this reaction is the sum of the standard potential for the half-reaction; therefore, the standard reaction potential for the first reaction is:

Since its standard reaction potential is positive, this reaction is spontaneous and occurs in a galvanic cell.

The second reaction has aluminum being oxidized () and a ferrous ion being reduced (). The standard reaction potential is ; therefore, this reaction can also occur in a galvanic cell.

The third reaction has a lithium ion being reduced () and an aluminum being oxidized (). The standard reduction potential is ; therefore, this reaction is nonspontaneous and cannot occur in a galvanic cell.

Example Question #5 : Galvanic And Electrolytic Cells

What will happen when the two given half-cells are connected by a salt bridge to form a galvanic cell? 

 

 

Possible Answers:

No reaction will take place. 

Lead metal will be oxidized at the anode; copper ions will be reduced at the cathode.

Copper metal will be oxidized at the cathode; lead ions will be reduced at the anode.

Copper metal will be oxidized at the anode; lead ions will be reduced at the cathode.

Lead metal will be oxidized at the cathode; copper ions will be reduced at the anode.

Correct answer:

Lead metal will be oxidized at the anode; copper ions will be reduced at the cathode.

Explanation:

 has a higher standard reduction potential, and will therefore be reduced, meaning that lead will be oxidized. The anode is where oxidation takes place, and the cathode is where reduction occurs. Therefore, we assign the cathode to copper and the anode to lead.

Example Question #6 : Galvanic And Electrolytic Cells

             

      

 

For an electrolytic cell containing the two half-cells shown, the __________ reaction will take place and the  will be __________.

Possible Answers:

nonspontaneous . . . 

nonspontaneous . . . 

nonspontaneous . . . 

spontaneous . . . 

spontaneous . . . 

Correct answer:

nonspontaneous . . . 

Explanation:

In an electrolytic cell, the non-spontaneous reaction occurs. This means that Na is reduced (making it the cathode) and Mn is oxidized (making it the anode). The standard cell potential can then be calculated as follows:

Example Question #1 : Batteries

A battery converts __________ energy to __________ energy.

Possible Answers:

chemical . . . electrical

electrical . . . chemical

chemical . . . kinetic

electrical . . . kinetic

Correct answer:

chemical . . . electrical

Explanation:

Electronic devices such as cell phones utilize batteries for power. A discharging battery converts the chemical energy associated with redox reactions to electrical energy that can be utilized to power the device. Kinetic energy is the energy associated with motion and is irrelevant to this question.

Example Question #1 : Batteries

Which of the following statements about a charging battery is true? 

Possible Answers:

Oxidation occurs at the cathode and reduction occurs at anode

The battery behaves as a galvanic cell

The reaction occurring is exothermic

The Gibbs free energy of the reaction in the battery is positive

Correct answer:

The Gibbs free energy of the reaction in the battery is positive

Explanation:

A charging battery behaves as an electrolytic cell. The process involves a nonspontaneous reaction; therefore, energy input is needed for charging a battery. This energy is provided by the electrical outlets that we use to charge our electronic devices, such as cell phones. Since it is nonspontaneous, the reaction occurring during charging has a positive Gibbs free energy.

Oxidation always occurs at the anode and reduction always occurs at the cathode, regardless of the type of cell. As mentioned previously, the charging battery requires energy input and behaves as an electrolytic cell. Exothermic reactions are characterized by a negative change in enthalpy. A reaction happening in an electrolytic cell is nonspontaneous. This means that the Gibbs free energy is always negative; however, other thermodynamic variables such as enthalpy and entropy can be either positive or negative. 

Example Question #11 : Electrochemistry

A charging battery is acting as a(n) __________ cell and a discharging battery is acting as a(n) __________ cell. 

Possible Answers:

electrolytic . . . electrolytic

electrolytic . . . galvanic

galvanic . . . galvanic

galvanic . . . electrolytic

Correct answer:

electrolytic . . . galvanic

Explanation:

A charging battery consumes energy from a power source like an electrical outlet, whereas a discharging battery releases energy and powers a device; therefore, a charging battery behaves as an electrolytic cell whereas a discharging battery behaves as a galvanic cell. 

Example Question #12 : Electrochemistry

Lithium batteries consist of lithium compounds and manganese dioxide. The reduction potential for lithium is , whereas the reduction potential for manganese dioxide is . Which of the following reaction(s) occur(s) when a lithium battery is discharging?

I. Oxidation of manganese

II. Reduction of lithium

III. Oxidation of lithium

Possible Answers:

II and III

I and II

III only

I only

Correct answer:

III only

Explanation:

A discharging battery behaves like a galvanic cell; therefore, it is characterized by a spontaneous reaction. This means that the reaction happening in a discharging battery has a positive standard reaction potential. Given the information in the question, we will get a positive standard reaction potential if the lithium atom is oxidized (half reaction potential = ) and the manganese ion is reduced (half reaction potential = ). The total standard reaction potential is the sum of the half reaction potentials, . The oxidation of manganese and the reduction of lithium will yield a negative potential, not characteristic of a discharging battery. Note that one reaction has to be oxidation and one reaction has to be reduction.

Example Question #1 : Other Electrochemistry Concepts

         

A  current is passed through a silver ion solution. How much silver is deposited after 10 minutes? 

Possible Answers:

Correct answer:

Explanation:

First we need to convert minutes to seconds (10min=600s), and knowing that , we can calculate the amount of silver deposited as follows:

If we follow the units carefully, we see that all of the units cancel out to give us a value in grams of silver, which is what we are looking for. 

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