Before we are going to learn about the terms cathode and anode, first, we are required to understand what an electrode is. According to the general definition, an electrode is a substance that helps in the electricity conduction wherein the electric current either leaves or enters the non-metallic medium such as an electrolytic cell.
In pure terms, 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 basically describe the direction of the flow of current.
Let us discuss what cathode and anode exactly mean. Both these terms can be defined by the flow of current. Thus, a cathode can be considered as 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 has 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.
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When we speak about the cathode in chemistry, it can be said to be the electrode where the reduction takes place. In an electrochemical cell, this is common. Here, the cathode results negative due to the electrical energy that is supplied to the cell results in the decomposition of chemical compounds. However, it can also be said to be positive similar to the case of a galvanic cell where the chemical reaction tends to the generation of electrical energy.
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.
In the most basic form, an anode in electrochemistry is defined as the point where an oxidation reaction takes place. In general, at an anode, anions, or negative ions, due to its 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 in nature, and mostly the electrons move towards the external part of the circuit. Whereas, in an electrolytic cell, it is again given as positive. In addition, an anode can be a wire or a plate having an excess positive charge.
Let us discuss some key differences between 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.
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, opposite 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.
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, an oxidation reaction takes place and excess electrons create a negative charge.
1. How to Identify Cathode and Anode in a Given Circuit?
Ans: In a battery (simply called a galvanic cell), the anode is the electrode from which the electrons leave and go into the external circuit. Of course, the electrons leave from the negative terminal. Therefore, the answer is that the negative (-) electrode can be defined as the anode, whereas the positive (+) electrode can be given as cathode.
If there are any arrows given in the diagram, those represent the direction of electron flow. "Conventional current" however, flows in a different direction. The anode exists where the conventional current flows into the battery.
2. How are the Common Anode and Common Cathode Different?
Ans: A Cathode is a negative electrode, whereas the anode is a positive electrode. They are so-called because the cations, which are positively charged, migrate to the negative cathode. Hence, known as a cathode while the anions migrate to a positively charged anode, and so-known as the anode.
These are used in the electroplating process where the metal ions being positively charged migrate and get deposited at the cathode. Thus, objects placed at the cathode get electroplated with the metal that is present in the electrolyte.
Cathode and anode can also be used in the electrolysis to separate compounds into constituent ions or molecules, as in Kolbe's electrolysis.