Mcat Electrochemistry

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MCAT Physical : Electrochemistry

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

Oxidation does not take place in an electrolytic cell.

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 #2 : 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.

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

An voltaic cell requires energy input to begin the reduction process

An electrolytic cell requires energy input

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

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

An electrolytic cell produces more energy

An electrolytic cell requires energy input

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

Example Question #3 : Electrochemistry

Which of the following is not true of electrolytic cells?

No battery is required for electrolysis to take place

Oxidation occurs at the anode

The cell potential is negative

Electricity is conducted by the motion of ions

Electrons travel toward the cathode

No battery is required for electrolysis to take place

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

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 #5 : Electrochemistry

Consider the following reaction:

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

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

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 .

Example Question #1 : Half Reactions And Reduction Potential

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.

Instead of silver, a scientist uses a strip of zinc in the opposite half cell from copper. Which of the following is true when comparing this new cell to the cell in the diagram?

The reduction potential of is 0.34 volts. The reduction potential for is -0.76 volts.

Energy can be produced in both cells

The direction of ion migration in the salt bridge is the same in both cells

Ions are no longer generated in the new cell

The direction of electron flow is the same in both cells

The cathode is the same in both cells

Energy can be produced in both cells

In the new cell, energy can still be produced, but because zinc ions have a lower reduction potential than copper ions, the copper will be reduced and the direction of electron flow will be reversed, as compared to the cell with silver in which copper was oxidized.

Example Question #2 : Half Reactions And Reduction Potential

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.

If there is a net production of copper ions in the half cell on the left as the reaction proceeds, which of the following must be true?

Copper is reduced in the reaction

There is no net electron flow

Copper has a more positive reduction potential than silver

Silver has a more positive reduction potential than copper

Silver is oxidized in the reaction

Silver has a more positive reduction potential than copper

If copper ions are generated as the voltaic cell functions, then the copper is being oxidized, and the silver must be reduced. Reduction and oxidation always occur together in a coupled reaction. This must also mean that the reduction potential for Ag is higher than the reduction potential for Cu.

Example Question #3 : Half Reactions And Reduction Potential

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.

As the difference in the reduction potentials between two half cells increases, what happens to the Gibbs free energy of the reaction?

It only changes with changes in temperature or pressure, and is independent of the chemical species involved

It will increase or decrease, depending on the species involved

It becomes more positive

It becomes more negative

It does not change, as thermodynamics is independent of reduction potential

It becomes more negative

The reduction potential of a cell is directly related to the Gibbs free energy by the equation below.

As the reduction potential of a cell becomes more and more positive, the Gibbs free energy value becomes more and more negative.

Example Question #4 : Half Reactions And Reduction Potential

Imagine a galvanic cell which uses solid zinc and aqueous iron ions to produce a voltage.

What is the standard state cell potential for this reaction?

Keep in mind that a galvanic cell will always have a positive voltage, so you can disregard the negative options. The half reactions show the voltage that will result if the element in question is reduced; however, an oxidation-reduction reaction will always have one element oxidized and another element reduced. In the equation shown, solid zinc (Zn) is oxidized, so the voltage of its half reaction is inverted to +0.76V. Next, you add the voltage of iron’s reduction, resulting in the overall voltage of the galvanic cell.

Example Question #1 : Electrochemistry

Imagine a galvanic cell which uses solid zinc and aqueous iron ions to produce a voltage.

Suppose that this galvanic cell was converted into an electrolytic cell. Which of the following statements would be true?

The cell potential would be negative

The reaction is spontaneous

No electrons would flow from anode to cathode

Oxidation would take place at the cathode

The cell potential would be negative

An electrolytic cell is best thought of as a cell that requires an external power source in order to work. The reaction will go in the opposite direction of a galvanic cell, meaning that the cell potential will also be inversed and the reaction will be non-spontaneous. As a result, cell potential would be negative in an electrolytic cell.

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Electrochemistry for the MCAT: Everything You Need to Know

Learn key MCAT concepts about electrochemistry, plus practice questions and answers

(Note: This guide is part of our MCAT General Chemistry series.)

Table of Contents

Part 1: Introduction to electrochemistry

Part 2: Cell potentials

a) Oxidation and reduction potentials

b) Electromotive force

Part 3: Concentration cells

a) Calculation of electric potential

b) Nernst equation

Part 4: Electrochemical cells

a) Voltaic cells

b) Electrolytic cells

c) Batteries

Part 5: High-yield terms

Part 6: Passage-based questions and answers

Part 7: Standalone questions and answers

Part 1: Introduction to electrochemistry

Electrochemistry drives the batteries in your car, runs your cell phone, and even powers cells within your body! Thus, electrochemistry has high biological and medical importance.

In this article, we’ll go over everything you need to know about electrochemistry for the MCAT. We’ll discuss how these concepts are applied using different electrochemical cells as well as their application to human biology.

Throughout this guide, several important terms are highlighted in bold. At the end of this guide, there are also several AAMC-style practice questions for you to test your knowledge with.

Let’s get started!

Part 2: Cell potentials

a) Oxidation and reduction potentials

Recall that reduction and oxidation reactions are types of reactions in which atoms exchange electrons. Oxidation refers to the loss of electrons from an atom, whereas reduction refers to the gain of electrons by an atom. The mnemonic “OIL RIG,” Oxidation Is Loss and Reduction Is Gain, may be helpful in remembering this distinction. (For more information on these topics, be sure to refer to our guide on oxidation and reduction reactions.)

The standard oxidation potential tells us how likely a species is to be oxidized, or lose electrons, under standard conditions (1M concentrations, 1 atm pressure, 298K). Similarly, the standard reduction potential tells us how likely it is for a species to be reduced, or gain electrons, under standard conditions. Both of these are measured in volts (V).

Reduction and oxidation potentials are measured relative to a reference electrode, such as the standard hydrogen electrode (SHE) or the saturated calomel electrode (SCE). These reference electrodes allow us to calculate the emf of half-reactions in relation to a benchmark value. Thus, a reference electrode is assumed to have zero potential (0 V) in relation to itself.

Oxidation and reduction potentials are critical considerations in analyzing electrochemical cells. Electrochemical cells contain two sets of electrodes: an anode and a cathode. Electrons flow from the anode and toward the cathode. Thus, oxidation occurs at the anode and reduction occurs at the cathode. (Another useful mnemonic is “AN OX, RED CAT”: the first few syllables of the words anode—oxidation, and reduction—cathode.)

b) Electromotive force

The standard electromotive force (E° _cell), or emf, is the difference between the reduction potentials of the cathode and anode of an oxidation-reduction reaction. A spontaneous reaction will have a positive E° _cell, while a nonspontaneous reaction will have a negative E° _cell. Therefore, the free energy change (ΔG) of a reaction and the standard electromotive force of a reaction always has opposite signs.

E° _cell = E° _{reduction of cathode} – E° _{reduction of anode}

A half-reaction shows the chemical oxidation or reduction that takes place at a single electrode. Thus, the net overall equation for an electrochemical cell requires two half-reactions.

When presented with a problem where you have to calculate the standard electromotive force, you will most likely be given a set of half-reactions and their standard reduction potentials. For example, you might see something like this:

A + + e – →A (E°_reduction = 1.2 V)
B + + e – →B (E°_reduction = 0.6 V)

How can these half-reactions be put together to represent the overall electrochemical cell?

These are both reduction reactions. However, when two half-reactions are found in a single electrochemical cell, the one with a higher E°_reduction is the one that can be spontaneously reduced in the overall reaction. The one with a lower E°_reduction is the one that can be spontaneously oxidized in the overall reaction. In our example reactions above, A will be spontaneously reduced by B, and B will be spontaneously oxidized by A.

Thus, the half-reaction involving B would actually occur in the reverse direction. As a result, a better representation of the oxidation of species B is as follows:

B →B + + e – (E°_oxidation = -0.6 V)
Note that the E°_oxidation of a half-reaction is equal to -1 x E°_reduction, and vice-versa.
Thus, the net reaction for our example electrochemical cell is as follows:

The table below provides examples of reduction potentials for several different half-reactions. You do not need to memorize these for test day, as any required values will be provided when needed.

Reaction	E° (V)
Al 3+ + 3e – →Al	-1.66
Zn 2+ + 2e – →Zn	-0.76
Cu 2+ + 2e – →Cu	0.34
Fe 3+ + e – →Fe 2+	0.77
Ag+ + e – →Ag	0.80