What is an Electrochemical Cell?
An electrochemical cell is used to generate electricity from a chemical reaction. It can be defined as a device that converts chemical energy into electrical energy. A chemical reaction that involves the exchange of electrons is required for an electrochemical cell to operate. These kinds of reactions are called redox reactions.
A cell is characterized by its voltage. A particular type of cell generates the same voltage irrespective of the size of the cell. The chemical composition of the cell depends on the cell voltage, given the cell is operated at ideal conditions. The cell voltage may differ due to several factors such as temperature difference, change in concentration, etc.
The Nernst equation by Walther Nernst can be used to calculate the EMF value of a given cell, providing the standard cell potential of the cell.
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Michael Faraday proved that chemical reactions at each of two electrodes–electrolyte interfaces provide the "seat of emf" for the voltaic cell, around 1830. These reactions that drive the current in the circuit which is an open case, charge separation continues until the electrical field from the separated charges is sufficient enough to arrest the reactions.
Electromotive force or EMF of a cell is the maximum potential difference between two electrodes of a cell. The electromotive force of a cell can also be defined as the net voltage between the oxidation and reduction half-reactions. The electromotive force of a cell is mainly used to determine whether an electrochemical cell is galvanic or not.
Types of Electrochemical Cell
The Daniell cell is composed of copper electrodes immersed in copper sulfate and zinc electrodes immersed in zinc sulfate solutions. Two half cells are connected together using a salt bridge. The zinc electrode acts as an anode and the copper electrode acts as a cathode. The Daniell cell is an adaptation of the galvanic cell.
On comparing copper and zinc, zinc is at the top position in the electrochemical series, owing to its higher value of oxidation potential. Hence, zinc undergoes oxidation, consequently, the solution of its own ions, two electrons and a zinc ion are generated. This electrode acquires a negative potential due to the release of electrons when compared to the other electrode. We call it an anode.
However, copper undergoes reduction, owing to its higher reduction potential capacity. The copper ion in the solution of the copper half cell accepts two electrons from the electrode and becomes copper metal and gets deposited at the electrode. As this electrode uses up electrons, we consider this electrode as a positive electrode, and we call it cathode.
Anode reaction is represented as follows:
Zn(s) → Zn2+ (aq) + 2e–
The cathode reaction is represented as follows:
Cu2+ (aq) +2e– → Cu(s)
The combined cell reaction or as we can say the overall cell reaction is as follows.
Zn(s) + Cu2+(aq) → Zn2+ (aq) + Cu(s)
Galvanic Cell is named after Luigi Galvani, an Italian scientist. It is an important electrochemical cell that forms the base of many other electrochemical cells like the Daniell cell. It is made up of two different metallic conductors called electrodes immersed in their own ionic solutions. Each of these arrangements is a half cell. A half cell is not able to generate a potential difference alone. So when combined, they generate a potential difference. To combine the two cells chemically a salt bridge is used. It donates the required amount of electrons to the electron-deficient half cell and accepts electrons from the electron-rich half cell.
When an electrode is immersed in the solution of its own ions, a potential difference is set up across the interface. This potential difference is called the electrode potential.
The presence of electrons in the electrode and ions in the solution creates a potential difference. Consider a case where the zinc electrode is immersed in a zinc sulfate solution. The zinc metal gets oxidized by releasing two electrons and becomes dissipated in the solution. In the same way, copper develops a positive potential. The combination of these two cells owes for the cell potential.
In reality, we are not able to determine the potential of a single half cell. To determine the potential, we always need a standard half cell whose potential value is already known. A standard half cell is then connected with the other unknown half cell to determine the overall potential.
This overall potential is the difference between the potentials of the two half cells. The standard hydrogen electrode is an example of such a standard half cell. The potential value of a standard hydrogen electrode is inherently set to zero volts. The standard hydrogen electrode is connected with an unknown half cell and the potential difference is measured. As the standard hydrogen electrode has zero volts, the measured value will be the potential difference of the unknown half cell.
Q1. Explain the EMF of a Cell.
Ans: EMF or electromotive force is equal to the potential difference across the terminals of the cell when no current is present. EMF is the energy provided by a cell or battery per coulomb of charge passing through it and it is measured in volts(V).
Q2. Write the Factors on Which the EMF of a Cell Depends.
Ans: The EMF of a cell is independent of the shape of electrodes, the distance between electrodes, and the amount of electrolyte. It depends on the material of electrodes and the electrolyte used in the cell.
Q3. Write the Units of EMF.
Ans: EMF is measured in Volts. It is equivalent to a joule per coulomb.
Q4. Give the Formula for EMF.
Ans: Electromotive force is equal to work done on the charge per unit(∈=dWdq) if there is no current flowing. As the unit for work is joule and the unit for charge is Coulomb, so the unit for emf is the volt (1V=1J/C).
Q5. What is the EMF of a Dry Cell?
Ans: A zinc-carbon cell which is dry, has an emf of 1.54 Volts and it is produced as a single cell or in various combinations to form voltage of the cell.