How to Write Cell Notation and Calculate EMF Easily
What is EMF?
What is meant by EMF? EMF is the short form of Electromotive force, and it simply refers to the electrical activity produced by a non-electrical source. It is seen that devices provide emf by converting a form of energy into chemical energy. In chemistry, EMF means, the maximum potential difference between the two electrodes of a voltaic or galvanic cell. The devices that can produce an emf are electrochemical cells, thermoelectrical devices, solar cells, electric generators, and transformers.
What is EMF Definition?
EMF stands for Electromotive Force. It is defined differently in Physics and Chemistry. We will now study both the definitions and meaning in detail.
What is the Meaning of EMF in Physics?
Now, what is EMF in Physics? EMF, in Physics, is defined as the energy per unit of an electric charge that is generated by an electric source, such as an electric generator or a battery. We can say that electromotive force or EMF is generated by any device that converts any other form of energy to electrical energy.
What is the Meaning of EMF in Chemistry?
In chemistry, it simply means the maximum potential difference between the electrodes of a voltaic or galvanic cell.
What is an Electrochemical Cell?
In a simple way, if we try to understand an electrochemical cell, we can say that an electrochemical cell is a device that generates electricity from a chemical reaction. An electrochemical cell transforms chemical energy into electrical energy. In order to operate an electrochemical cell, a chemical reaction involving the exchange of electrons is required, such a reaction is known as a redox reaction. Generally, an electrochemical cell can be categorized into two types, and they are; Galvanic Cell and Daniell Cell.
What is a Galvanic Cell?
Now, what is the galvanic cell? A galvanic cell is a device that was developed by an Italian scientist named, Luigi Galvani. It is an important electrochemical cell, and it forms the base of many other electrochemical cells. A galvanic cell is made up of two different kinds of electrodes which are immersed in their ionic solutions. Each of the two electrodes is called a half-cell, and a half-cell is not capable of producing the potential difference. Still, when both the electrodes or half-cells are combined, they can produce the required potential difference. The half-cells are connected using a salt bridge, this bridge provides the required amount of electrons to the electron deficit half-cell, and it also accepts the extra electrons from the electron-rich half-cell.
What is a Daniell Cell?
In simple words, a Daniell cell is a type of galvanic cell that is made using zinc and a copper electrode. Both the electrodes are immersed in their respective ionic solutions, i.e., for the zinc electrode, it is zinc sulfate, and for the copper electrode, it is copper sulfate. The zinc electrode is the anode, and the copper electrode is the cathode, both these half-cells are connected using a salt bridge to produce the maximum potential difference.
What is the EMF of a Cell?
EMF, which is also known as the Electromotive force of a cell, is defined as the maximum potential difference between the electrodes of a cell. EMF of a cell or EMF of a galvanic cell can be calculated by taking the values of electrode potentials of both anode and cathode.
There are usually three ways of calculating the potential difference of a galvanic cell.
First, by observing the oxidation potential at anode and reduction potential cathode.
Second, by taking into account the reduction potential of both the electrodes.
Third, by taking into account the reduction potential of both the electrodes.
Cell Notation
Cell notation or cell line notation is defined as the short-hand expression of any reaction of an electrochemical cell. In this type of expression, the anode and cathode of the cell are separated by using two bars or slashes that represents a salt bridge that connects the two electrodes. The individual solids, liquids, or aqueous solutions are separated using single bars.
Here is an example of cell notation.
Zn | Zn²⁺ || Cl¯ | AgCl | Agᐤ
Cell Notation to Equation
The cell notation of an electrochemical reaction can also be expressed in the form of a chemical reaction by using the following steps.
For this purpose let us take the example of a cell notation.
Ag | Ag⁺ || H⁺ | H₂ | Pt
For the above cell notation, we first take the half-cell reaction of the anode:
Ag → Ag⁺ + e¯
Next, we take the reaction at the cathode:
2H⁺ + 2e¯ → H₂
We multiply both the reactions, and we get the final equation.
2Ag + 2H⁺ → 2Ag⁺ + H₂(g)
FAQs on EMF and Cell Notation: Key Concepts for Chemistry Students
1. What is the Electromotive Force (EMF) of a cell in chemistry?
The Electromotive Force (EMF) of a cell is the maximum potential difference between its two electrodes when no current is being drawn from the cell (i.e., in an open circuit). It represents the driving force for the electron flow in an electrochemical cell and is measured in volts (V). The EMF arises from the difference in the individual potentials of the anode and cathode.
2. How is the standard EMF of a cell (E°cell) calculated using standard electrode potentials?
The standard EMF of a cell is calculated by taking the difference between the standard reduction potential of the cathode and the standard reduction potential of the anode. The formula is:
E°cell = E°cathode - E°anode
It is crucial to use the standard reduction potentials for both half-cells in this calculation, as per IUPAC convention.
3. What is cell notation and what are its key conventions?
Cell notation is a shorthand representation of an electrochemical cell's components and reactions. The main conventions are:
- The anode (oxidation half-cell) is written on the left.
- The cathode (reduction half-cell) is written on the right.
- A single vertical line ( | ) represents a phase boundary (e.g., between a solid electrode and an aqueous solution).
- A double vertical line ( || ) represents the salt bridge, which separates the two half-cells.
- The concentration of aqueous solutions is written in parentheses next to the ion's formula.
4. What is the fundamental difference between EMF and potential difference?
The key difference lies in the circuit condition:
- EMF (Electromotive Force) is the potential difference when the cell is in an open circuit, meaning no current is flowing. It is the maximum possible voltage the cell can provide.
- Potential Difference (Voltage) is the voltage measured when the cell is in a closed circuit and current is flowing. It is always less than the EMF due to the internal resistance of the cell.
5. How would you write the cell notation for a standard Daniell cell involving zinc and copper?
In a Daniell cell, zinc acts as the anode (it gets oxidised) and copper acts as the cathode (it gets reduced). The correct cell notation is:
Zn(s) | Zn2+(aq, 1M) || Cu2+(aq, 1M) | Cu(s)
This shows the zinc electrode in a zinc sulfate solution separated by a salt bridge from a copper electrode in a copper sulfate solution.
6. Why is a salt bridge represented by a double vertical line (||) in cell notation?
The double vertical line (||) is a specific symbol for the salt bridge because it highlights its crucial and distinct function in the cell. It signifies that the two half-cells are physically separated but electrically connected. This connection allows for the migration of ions to maintain charge neutrality in each half-cell, completing the electrical circuit without allowing the bulk solutions to mix.
7. Can the EMF of a cell be negative, and what does a negative value signify?
Yes, the calculated EMF of a cell can be negative. A negative E°cell value signifies that the redox reaction, as written, is non-spontaneous under standard conditions. This means the forward reaction will not occur on its own. Instead, the reverse reaction is spontaneous. A cell with a negative EMF will function as an electrolytic cell, requiring an external energy source to drive the non-spontaneous reaction.
8. How does cell notation help in determining the cathode and anode?
Cell notation provides a clear and standardised way to identify the anode and cathode based on their position. By convention:
- The half-cell written on the left of the salt bridge (||) is always the anode, where oxidation occurs.
- The half-cell written on the right of the salt bridge (||) is always the cathode, where reduction occurs.
