Determination of EMF of a Cell

Most students are unfamiliar with the idea of electromotive force, or EMF. However, it is inextricably tied to the more well-known idea of voltage. Understanding the differences between the two and what EMF stands for provides us with the skills we need to address a variety of physics and electronics challenges. It will also cover the concept of a battery’s internal resistance. The EMF indicates the voltage of the battery without taking into account the internal resistance. The emf formula will be explained with examples in this article. Let’s learn it together!

What is the electromagnetic field (EMF)?

When there is no current flowing through the battery, the electromotive force is defined as the potential difference across the terminals. Although it may not appear as if it makes a difference, every battery has internal resistance. It’s comparable to typical resistance in that it lowers current in a circuit, but it’s found inside the battery.

When there is no current running through the cell, this internal resistance has no effect because there is no current to slow it down. The EMF can be thought of as the maximum potential difference across the terminals in an idealised environment in this way.

The energy delivered by a battery or a cell per coulomb (Q) of charge travelling through it is known as the electromotive force. When there is no current flowing through the circuit, the amount of emf is equal to V (potential difference) across the cell terminals

What is the distinction between EMF and Potential Difference?

EMF is the amount of energy converted into electrical energy per coulomb of charge. The potential difference, on the other hand, is the amount of electrical energy converted into other kinds of energy per coulomb of charge. Sources of emf include a cell, a solar cell, a battery, a generator, a thermocouple, a dynamo, and so on.

The EMF Formula is a formula that is used to calculate the EMF.

To compute EMF, there are two primary equations. The quantity of joules of energy that each coulomb of charge absorbs as it goes through the cell is the basic definition.

ε=E/Q

emf (electromotive force) is a term used to describe

E is the circuit’s energy.

The circuit’s charge is Q.

We can calculate the resulting energy and the amount of charge flowing through the cell if we know the resultant energy and the amount of charge passing through the cell. It is the most straightforward method of calculating the EMF.

Instead, we can use the definition V = IR, which is similar to Ohm’s law. So here’s the formula:

ε=I(R+r)

I Current

The cell’s electromotive force.

In the circuit, R stands for resistance.

r= A cell’s internal resistance.

V= Voltage

Now, let’s build on this:

ε=IR+Ir

ε=V+Ir

This demonstrates that if we know the voltage across the terminals, the current flowing, and the cell’s internal resistance, we can compute the EMF.

A Cell’s EMF

The electromotive force of the cell, also known as the EMF, is the highest potential difference that exists between the two electrodes of a cell. The net voltage between the oxidation and reduction half-reactions is also known as this.

Electrochemical Cell Types

Galvanic Cell (Galvanic Cell)

A galvanic cell, also known as a voltaic cell, is a device that uses a spontaneous redox reaction to create electricity.

The reaction of zinc metal with aqueous copper sulphate solution is employed in this experiment.

Zn(s)+Cu2+(aq)→Zn2+(aq)+Cu(s)

It is made up of a zinc Electrode and a copper Electrode that have been bathed in zinc sulphate and copper sulphate solutions, respectively. The anode is the zinc electrode, and the cathode is the copper electrode. A zinc metal rod (anode) is immersed in a zinc sulphate solution in the container on the left. The copper rod (cathode) is immersed in a copper sulphate solution in the container on the right. The zinc and copper electrodes are connected by a copper connection. A salt bridge connects the solution in the anode and cathode compartments to the potassium sulphate solution.

In the anode compartment, the oxidation half-reaction occurs.

Zn(s)→Zn2+(aq)+2e−

The cathode reaction takes place as follows:

Cu2+(aq)+2e−→Cu(s)

Electrons are transferred from the zinc electrode to the copper cathode. Zn2+ is formed when zinc dissolves in the anode solution.

Cu2+ ions

In a cathode half cell, ions absorb electrons and transform them to Cu atoms. Simultaneously, SO42

Ions travel from the cathode half cell to the anode half cell through the salt bridge. Zn2+ is a similar case.

From the anode half cell to the cathode half cell, ions travel. Ions are transferred from one half-cell to the other to complete the circuit, ensuring a steady power source. The cell will keep running until either the zinc metal or the copper ions are depleted.

Daniel Cell 

It’s the same thing as a Galvanic Cell. The Copper-Zinc cell is also the same as the Galvanic Cell. The only difference is that Daniel’s cell can only use Zinc and Copper as Electrodes, whereas Galvanic cells can use a variety of metals as Electrodes.

The electrolytes used in the Daniel Cell are copper(II) sulphate and zinc sulphate, whereas the electrolytes used in the Galvanic Cell are the salts of metals of each Electrode.

What is the definition of a salt bridge?

An inverted U-tube filled with a concentrated solution of an inert electrolyte serves as the salt bridge. The inert electrolyte does not react with the fluids in the two half cells and is not involved in any chemical changes. Salts such as KCl, KNO3, and NH4NO3 are commonly employed as electrolytes.

How is a Salt Bridge constructed?

Agar-agar or gelatin is mixed with a heated concentrated electrolyte solution and poured into a U-tube to make a salt bridge. The solution cools to the point where it forms a gel inside the U-tube, preventing the fluids from mixing. To reduce dispersion, the two ends of the U-tube are filled with cotton wool.

Salt Bridge’s Importance

Its major purpose is to prevent potential differences between the two solutions from arising when they come into contact. The liquid junction potential is the difference in potential between two liquids.

By linking the electrolytes in the two half cells, it completes the electrical circuit.

It stops solutions from diffusing from one half cell to the other.

It ensures that the solutions in the two half cells are electrically neutral.

Using a salt bridge, how is the electrical neutrality of the solutions in the two half cells maintained?

When the positive ions that are generated flow into the solution, they accumulate positive charge in the anodic half cell. The salt bridge delivers negative ions to maintain electrical neutrality.

In a Daniell cell, for example, zinc oxidises at the anode and flows into the solution as Zn2+ ions, resulting in a positive charge accumulation in the solution. The salt bridge delivers negative ions to maintain the solution’s electrical neutrality (may be NO3- or Cl-).

Due to the reduction of positive ions, there will be an accumulation of negative ions in the cathodic half cell. The salt bridge delivers positive ions to preserve electrical neutrality.

Cu2+ ions from the CuSO4 solution, for example, are reduced by the electron generated by zinc oxidation and deposited on the copper cathode in a Daniell cell. As a result, the concentration of Cu2+ ions in the solution drops while the concentration of SO42- ions (sulphate ions) rises. As a result, negatively charged sulphate ions will accumulate around the cathode. The salt bridge delivers positive ions (K+ or NH4+) to preserve electrical neutrality.

Potential of an electrode

A potential difference develops at the metal solution interface when a metal electrode is dipped in a solution containing its metal ions. The electrode potential is the difference in potential between two electrodes.

When a zinc rod is dipped in a solution containing Zn2+ ions, it oxidises, and the Zn2+ ions move from the zinc rod to the solution, leaving the zinc rod with an excess of electrons. As a result, the zinc rod becomes negatively charged in relation to the solution, resulting in a potential difference between the zinc rod and the solution. The electrode potential of zinc is the difference in potential. When a copper rod is dipped in a solution containing Cu2+ ions, the Cu2+ ions take electrons from the copper rod, leaving the copper rod with a positive charge. As a result, a potential difference exists between the copper rod and the solution, which is referred to as the copper electrode potential.

The anode has a negative potential, whereas the cathode has a positive potential in an electrochemical cell.

It is impossible to determine the potential of any individual half cell. Only the difference in potential between the two half cells can be measured.

A half cell’s potential can be determined by attaching it to a Standard Hydrogen Electrode (SHE). A SHE’s standard electrode potential is presumed to be zero.

The standard electrode potential is defined as the electrode potential at standard conditions such as 25°C temperature, 1 atm pressure, and 1 M electrolyte concentration. The symbol E0 is used to represent it. Electrochemical series refers to the arrangement of electrodes in increasing order of their standard reduction potential.

Conclusion

An electrochemical cell is a device that uses a chemical reaction to generate electricity. A device that transforms chemical energy into electrical energy is known as a chemical energy converter. An electrochemical cell can only function if there is a chemical reaction that involves the exchange of electrons. Redox reactions are the name for these types of reactions. The voltage of a cell is what distinguishes it. Regardless of cell size, a certain type of cell generates the same voltage. If the cell is operated under ideal conditions, the chemical composition of the cell is determined by the cell voltage