MCAT Physical : Electrolytic Cells

Study concepts, example questions & explanations for MCAT Physical

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Example Questions

Example Question #1 : Electrochemistry

In an electrolytic cell, oxidation takes place at the __________.

Possible Answers:

Oxidation does not take place in an electrolytic cell.

salt bridge

cathode

anode

Correct answer:

anode

Explanation:

Electrolytic cells have a negative electromotive force and require an outside energy source to power a non-spontaneous reaction. Galvanic cells, in contrast, have positive potentials and facilitate spontaneous reactions without the need of a power source.

Regardless of cell type, however, oxidation always takes place at the anode and reduction always takes place at the cathode. The flow of electrons is always from the anode to cathode.

Example Question #1 : Electrochemistry

A student conducts an experiment for a chemistry class. The student wishes to explore power generation from different types of voltaic cells. He sets up three different cells, and then compares the amount of energy generated from each one.

One of his cells is shown below as an example. Both remaining cells follow the same layout.

Untitled

 

How would the voltaic cell depicted above differ from an electrolytic cell?

Possible Answers:

An electrolytic cell produces more energy, but only after an input of energy to begin the process

An electrolytic cell requires energy input

An electrolytic cell produces energy based on differences in concentrations between the half cells

An voltaic cell requires energy input to begin the reduction process

An electrolytic cell produces more energy

Correct answer:

An electrolytic cell requires energy input

Explanation:

Electrolytic cells require an input of energy, and are used to plate metals by functionally running a voltaic cell in reverse.

Example Question #2 : Electrochemistry

Which of the following is not true of electrolytic cells?

Possible Answers:

Electricity is conducted by the motion of ions

Oxidation occurs at the anode

The cell potential is negative

Electrons travel toward the cathode

No battery is required for electrolysis to take place

Correct answer:

No battery is required for electrolysis to take place

Explanation:

For electrolytic cells, the cell potential is negative; in contrast, galvanic/voltaic cells have positive cell potentials. Electrolysis reactions can only occur if the total potential is positive. An additional voltage input, such as a battery, is required so that the sum of potentials in the electrolytic cell is greater than zero.

Oxidation always occurs at the anode, regardless of cell type, and electrons always travel toward the site of reduction (the cathode). In a galvanic cell, the cathode is positively charged, naturally drawing the flow of electrons. In an electrolytic cell, the cathode is negatively charged, but still requires the flow of electrons to allow reduction to occur. An induced current from a battery is used to propel these electrons against their natural flow.

Example Question #4 : Electrochemistry

Which of the following is true about electrolysis?

I. It only involves reactions with 

II. It requires a voltage source

III. It is an exothermic process

Possible Answers:

I only

II and III

I and II

II only

Correct answer:

II only

Explanation:

Electrolysis is a specific type of reaction that occurs in an electrolytic cell. An electrochemical cell contains an anode and a cathode that facilitate a redox reaction. In an electrolytic cell (a type of electrochemical cell) the redox reaction that is carried out is a nonspontaneous reaction. Recall that the change in Gibbs free energy for a nonspontaneous, or unfavorable, reaction is always positive; therefore, for electrolysis in an electrolytic cell, the redox reaction has a . Statement I is false.

Nonspontaneous reactions are reactions that are unfavorable. This means that energy is required to carry out the reaction. In an electrolytic cell, energy is provided in the form of voltage input. The voltage provided pushes the reaction in the unfavorable direction; therefore, electrolysis reactions require a voltage source. Statement II is true.

Since it requires energy, an electrolysis reaction is considered to be an endothermic process. Recall that endothermic processes are reactions that take in (or require) energy, whereas exothermic processes are reactions that release energy; therefore, electrolysis is an endothermic process. Statement III is false.

Example Question #3 : Electrochemistry

Consider the following reaction:

How much voltage will you have to apply to carry out this reaction?

Possible Answers:

Less than 

Greater than 

No voltage needs to be applied because this reaction represents a galvanic cell

No voltage needs to be applied because this reaction represents an electrolytic cell

Correct answer:

Greater than 

Explanation:

The question states that the reaction has a negative ; therefore, the reaction is nonspontaneous. Nonspontaneous reactions are carried out in electrolytic cells (as opposed to galvanic cells). A reaction usually proceeds in the spontaneous direction; therefore, to carry out nonspontaneous reactions you must put energy into the system. Without energy, the reaction shown will occur in the reverse direction.

In an electrolytic cell, energy is provided by an external voltage source. Without energy, the electrolytic cell will have a voltage of  and the spontaneous (reverse) reaction will occur. For the nonspontaneous reaction to occur, you must attach a voltage source in such a way that the voltage applied is greater than  and is applied in the opposite direction (nonspontaneous reaction direction). This will force the reaction in the reverse direction and the nonspontaneous reaction will occur; therefore, the external voltage source must provide a voltage greater than .

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