[Chemistry Class Notes] on Cathode and Anode Pdf for Exam

Before we learn about the terms cathode and anode, it is important to understand what an electrode is. According to the general definition, an electrode is a substance that helps in electricity conduction wherein the electric current either leaves or enters the non-metallic medium such as an electrolytic cell. In other words, an electrode can be defined as a conductor that helps to establish electrical contact with a non-metallic part of the circuit. Electrodes consist of two major points called cathode and anode, which describe the direction of the flow of current.

Anode and Cathode Explained

Let us discuss what cathode and anode exactly mean. Both these terms can be defined by the flow of current. A cathode is an electrode from which the current exits a polarized electrical device. In the same way, an anode can be described as an electrode from which the current enters into the polarized electrical device.

 

The terms Cathode and Anode were finalized in 1834 by William Whewell. He adapted the words from the Greek word (named, kathodos), ‘way down’ or ‘descent.’ William had consulted with Michael Faraday in coining the terms.

 

The pictorial representation of both cathode and anode is given below.

 

What is Cathode?

When we speak about the cathode in chemistry, it is said to be the electrode where the reduction takes place. In an electrochemical cell, this is common. Here, the cathode is negative due to the electrical energy that is supplied to the cell, which results in the decomposition of chemical compounds. However, it can also be positive similar to the case of a galvanic cell where the chemical reaction tends to the generation of electrical energy. The flow of current in the interface of the cathode is actually the flow of electrons from the surface of the cathode to the solution. 

 

Also, a cathode is said to be either a cold cathode or a hot cathode. A cathode that is heated in the presence of a filament to emit electrons using thermionic emission is called a hot cathode, whereas the cold cathodes are not heated with any filament. In general, a cathode is flagged as “cold” if it emits more electrons than the ones generated by thermionic emission alone.

 

Example of cathode

• Electrolytic cell: In an electrolytic cell the negative polarity appears at the cathode. The pure metal or hydrogen gas from the metal ions are the result of the reduction at the cathode. While considering the relative reducing power of the two redox reactions, the one that makes the most reducing species is more cathodic as compared to the other that itself get reduced easily.

• Galvanic cell: Cathode acts as a positive electrode in the galvanic cell. It is completely connected to allow the circuit to be completed. The anode of the galvanic cell releases electrons that reach the cell through the circuit wire cathode.

• Electroplating Metal Cathode (Electrolysis): Once the metal ions are reduced, they form the pure metal surface on the cathode. The material that is needed to be plated with pure metal is attached to the cathode and becomes part of the cathode in the solution.

 

What is Anode?

An anode in electrochemistry is defined as the point where oxidation reaction takes place. In general, at an anode, anions, or negative ions, due to their electrical potential, tend to react and give off the electrons. Then, these electrons move up and into the driving circuit.

 

If we consider a galvanic cell, the anode exhibits a negative nature, and mostly the electrons move towards the external part of the circuit. Whereas, in an electrolytic cell, it is given as positive. In addition, an anode can be a wire or a plate having an excess positive charge.

 

Example of anode

1. The anode acts as a negatively charged terminal in the galvanic cell or in the discharge battery as the current flows into the cell through the cathode. The current that is flowing inward is carried by the electrons that are flowing outwards. The negative charge that is flowing in one direction is equivalent to the positive charge flowing in the opposite direction. 

2. In a recharging battery, the anode acts as a positive terminal that receives the current from the external generator. The flow of current within the battery is in the opposite direction to the direction of current during discharge in the recharging battery. Thus, the cathode during the time of the discharge of the battery becomes the anode while the battery is charged by the external generator. 

3. The anode also acts as a positive terminal where the current flows into the device in a diode. On the basis of the flow of current in the forward direction, the naming of the electrodes of the diode is determined. 

4. The anode is the terminal while the current enters the tube in vacuum tubes or gas-filled tubes.  

 

Let us discuss some key differences between anode and cathode:

 

Difference Between Anode And Cathode

 

Charge on Anode and Cathode

At the anode, there is an oxidation response. The oxidized species would lose the electrons, leaving this electrode with an electron accumulation. Thus, the anode is negatively charged. But, in contrast to the cathode, there is a reduction response where the species of decreased ones would obtain electrons. Hence, the electrode, which means, the cathode, lacks electrons and is thus charged positively.

 

Reason Behind the Cathode as Negative and Anode as Positive

It is because of the way these were defined originally, which dates to a time before the electron discovery. The cathode was derived from the Greek word, “kathodos,” means “way down,” as the terminal from which the conventional current (“flows” from the positive to a negative electrode, opp
osite the electron travel direction) leaves a device (with the anode being the terminal, where the conventional current enters).

 

The cathode is not always positive or negative. But, in the discharging battery, the cathode acts as a positive terminal, while in a vacuum tube or diode, the cathode acts as a negative terminal, since the conventional current enters these via anode or positive terminal.

 

Charges of a Cathode and Anode of a Voltaic Cell

The redox reactions in both half-cells in a voltaic cell are spontaneous. At the cathode, a reduction takes place, creating a positive charge, and using up the electrons. Whereas, at the anode, oxidation reaction takes place and excess electrons create a negative charge.