COMMON MISCONCEPTIONS
The following are some of the misconceptions students have about electrochemistry. By doing research for this website, I have come to believe that each one of these misconceptions can be addressed experimentally. As such, I am providing links to some experiments that illustrate the truth behind the misconceptions. The teacher should make sure to address them as the experiment is being performed.
MISCONCEPTION # 1 (Addressed in lessons: Galvanic Cells, Cells and Batteries, and Electrolysis and Electrolytic Cells)
Electrons can flow through aqueous solutions without ions.
Truth: Electrons need ions (ionic attraction) to be able to flow in aqueous solution.
MISCONCEPTION # 2 (Addressed in lessons: Galvanic Cells, Cells and Batteries, and Electrolysis and Electrolytic Cells)
Electrons enter the electrolyte at the cathode, move through the electrolyte and are released at the anode to complete the circuit.
Truth: The electrons move from the minus side to the plus side. The way this happens is that inside the battery itself, a chemical reaction produces the electrons. The speed of electron production by this chemical reaction controls how many electrons can flow between the terminals.
In practice when you connect to a battery, electrons flow from the battery into a wire, and must travel from the negative to the positive terminal for the chemical reaction to take place. That is why a battery can sit on a shelf and still have power. Unless electrons are flowing from the negative to the positive terminal, the chemical reaction does not take place. Once you connect a wire, the reaction starts.
MISCONCEPTION # 3 (Addressed in lessons: Galvanic Cells, Cells and Batteries, and Electrolysis and Electrolytic Cells)
Only negatively charged ions constitute a flow of current in the electrolyte and the salt bridge. That is, current is believed to always involve the movement of electrons.
Truth: Ionic compounds, which make the best electrolytes, dissolve in water, the positive and negative ions originally present in the crystal lattice persist in solution. Their ability to move nearly independently through the solution permits them to carry positive or negative electrical charges from one place to another. Hence the solution conducts an electrical current.
MISCONCEPTION # 4 (Addressed in lessons: Galvanic Cells, Cells and Batteries, and Electrolysis and Electrolytic Cells)
The charge of the cathode and the anode is determined by the physical placement of the half-cells, regardless of the type of electrolyte they are in.
Truth: The anode of an electrolytic cell is positive (cathode is negative), since the anode attracts anions from the solution. However, the anode of a galvanic cell is negatively charged, since the spontaneous oxidation at the anode is the source of the cell's electrons or negative charge. The cathode of a galvanic cell is its positive terminal. In both galvanic and electrolytic cells, oxidation takes place at the anode and electrons flow from the anode to the cathode.
MISCONCEPTION # 5 (Addressed in lessons: Galvanic Cells, Cells and Batteries, and Electrolysis and Electrolytic Cells)
The function of the salt bridge is to supply electrons to complete the circuit.
Truth: The salt bridge serves two related functions. The primary function is to complete the circuit so that charge can flow from one half-cell to the other. The second is to balance the mass by allowing the anion to move to the half-cell where additional cations are being produced.
Ideally, but not necessarily, the anion in the salt bridge would be the same as the anion in the half-cells. For instance, if the solutions surroundings the Zn and Cu electrodes were zinc sulfate and copper (II) sulfate, then the salt bridge would be filled with potassium sulfate. But just about any soluble salt will work in a salt bridge, as long as there are mobile ions.
MISCONCEPTION # 6 (Addressed in lessons: Galvanic Cells, Cells and Batteries, and Electrolysis and Electrolytic Cells)
Ions are attracted to the oppositely charged electrode.
Truth: This is true only in the very thin inter-facial region near the electrode surface. Ionic motion throughout the bulk of the solution occurs mostly by diffusion, which is the transport of molecules in response to a concentration gradient. Migration— the motion of a charged particle due to an applied electric field, is only a minor player, producing only about one non-random jump out of around 100,000 random ones for a 1 Volt cm–1 electric field. Only those ions that are near the interfacial region are likely to undergo migration.
MISCONCEPTION # 7 (Addressed in lessons: Galvanic Cells, Cells and Batteries, and Electrolysis and Electrolytic Cells)
The strongest electron attractor always gets any available electrons from the weakest electron attractor.
Truth: Many chemists refuse to accept the concept of an oxidation potential, rather than a reduction potential. It is not correct to use the phrase "tendency to lose electrons." The reason for this can be explained using the following analogy: when someone is robbed, one does not think that such person has a tendency to lose his or her valuables. Instead, his or her valuables are taken, without their conscious approval.
MISCONCEPTION # 1 (Addressed in lessons: Galvanic Cells, Cells and Batteries, and Electrolysis and Electrolytic Cells)
Electrons can flow through aqueous solutions without ions.
Truth: Electrons need ions (ionic attraction) to be able to flow in aqueous solution.
MISCONCEPTION # 2 (Addressed in lessons: Galvanic Cells, Cells and Batteries, and Electrolysis and Electrolytic Cells)
Electrons enter the electrolyte at the cathode, move through the electrolyte and are released at the anode to complete the circuit.
Truth: The electrons move from the minus side to the plus side. The way this happens is that inside the battery itself, a chemical reaction produces the electrons. The speed of electron production by this chemical reaction controls how many electrons can flow between the terminals.
In practice when you connect to a battery, electrons flow from the battery into a wire, and must travel from the negative to the positive terminal for the chemical reaction to take place. That is why a battery can sit on a shelf and still have power. Unless electrons are flowing from the negative to the positive terminal, the chemical reaction does not take place. Once you connect a wire, the reaction starts.
MISCONCEPTION # 3 (Addressed in lessons: Galvanic Cells, Cells and Batteries, and Electrolysis and Electrolytic Cells)
Only negatively charged ions constitute a flow of current in the electrolyte and the salt bridge. That is, current is believed to always involve the movement of electrons.
Truth: Ionic compounds, which make the best electrolytes, dissolve in water, the positive and negative ions originally present in the crystal lattice persist in solution. Their ability to move nearly independently through the solution permits them to carry positive or negative electrical charges from one place to another. Hence the solution conducts an electrical current.
MISCONCEPTION # 4 (Addressed in lessons: Galvanic Cells, Cells and Batteries, and Electrolysis and Electrolytic Cells)
The charge of the cathode and the anode is determined by the physical placement of the half-cells, regardless of the type of electrolyte they are in.
Truth: The anode of an electrolytic cell is positive (cathode is negative), since the anode attracts anions from the solution. However, the anode of a galvanic cell is negatively charged, since the spontaneous oxidation at the anode is the source of the cell's electrons or negative charge. The cathode of a galvanic cell is its positive terminal. In both galvanic and electrolytic cells, oxidation takes place at the anode and electrons flow from the anode to the cathode.
MISCONCEPTION # 5 (Addressed in lessons: Galvanic Cells, Cells and Batteries, and Electrolysis and Electrolytic Cells)
The function of the salt bridge is to supply electrons to complete the circuit.
Truth: The salt bridge serves two related functions. The primary function is to complete the circuit so that charge can flow from one half-cell to the other. The second is to balance the mass by allowing the anion to move to the half-cell where additional cations are being produced.
Ideally, but not necessarily, the anion in the salt bridge would be the same as the anion in the half-cells. For instance, if the solutions surroundings the Zn and Cu electrodes were zinc sulfate and copper (II) sulfate, then the salt bridge would be filled with potassium sulfate. But just about any soluble salt will work in a salt bridge, as long as there are mobile ions.
MISCONCEPTION # 6 (Addressed in lessons: Galvanic Cells, Cells and Batteries, and Electrolysis and Electrolytic Cells)
Ions are attracted to the oppositely charged electrode.
Truth: This is true only in the very thin inter-facial region near the electrode surface. Ionic motion throughout the bulk of the solution occurs mostly by diffusion, which is the transport of molecules in response to a concentration gradient. Migration— the motion of a charged particle due to an applied electric field, is only a minor player, producing only about one non-random jump out of around 100,000 random ones for a 1 Volt cm–1 electric field. Only those ions that are near the interfacial region are likely to undergo migration.
MISCONCEPTION # 7 (Addressed in lessons: Galvanic Cells, Cells and Batteries, and Electrolysis and Electrolytic Cells)
The strongest electron attractor always gets any available electrons from the weakest electron attractor.
Truth: Many chemists refuse to accept the concept of an oxidation potential, rather than a reduction potential. It is not correct to use the phrase "tendency to lose electrons." The reason for this can be explained using the following analogy: when someone is robbed, one does not think that such person has a tendency to lose his or her valuables. Instead, his or her valuables are taken, without their conscious approval.
How to Address These Misconceptions
As stated above, the misconceptions here presented are ones that can address more effectively through experimentation. It is the teacher's responsibility to talk about them, and address them while the experiments are being performed. Here are some experiments that can be effectively used for this purpose:
The following websites are excellent to reference when addressing the students' misconceptions:
The following videos can also be used to debunk the misconceptions:
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References
- Miller, George. (2013) Addressing Student's Difficulties and Misconceptions About Electrochemistry. AP Central. Retrieved March 21, 2013. From http://apcentral.collegeboard.com/apc/members/courses/teachers_corner/155230.html
- Barke, H.D., Hazar, A., & Yitbarek, S. (2009) Misconceptions in Chemistry: Addressing Perceptions in Chemical Education. New York, NY: Springer.
- Di Guiseppe, M., Haberer, S., Salciccioli, K., Sanader, M, Vavitsas, A. (2012) Chemistry 12. Toronto: Nelson Education Ltd.
- van Kessel, H., Jenkins, F., Davies, L., Plumb, D., Di Guiseppe, M., Lantz, O., Tompkins, D. (2003). Chemistry 12. Toronto: Thomson Canada Limited