9th Class Chemistry Chapter No. 7 Electrochemistry Notes
Chemistry 9th class chapte 7 i.e. Electrochemistry is a branch of chemistry that explores the relationship between electricity and chemical reactions. It involves reactions known as redox reactions, which are oxidation and reduction reactions. In redox reactions, either electricity is produced spontaneously, or electricity is used to drive non-spontaneous reactions. Spontaneous reactions occur without any external agent, while non-spontaneous reactions occur in the presence of an external agent. These reactions take place in galvanic cells (spontaneous) or electrolytic cells (non-spontaneous).
Oxidation is defined as the addition of oxygen or removal of hydrogen in a chemical reaction, while reduction is the addition of hydrogen or removal of oxygen. Both processes occur simultaneously in redox reactions. Alternatively, oxidation and reduction can be understood in terms of the loss or gain of electrons. Oxidation involves the loss of electrons by an atom or ion, while reduction involves the gain of electrons by an atom or ion.
Long Answer Questions
Oxidation states or oxidation numbers are used to indicate the apparent charge assigned to an atom of an element in a molecule or ion. Rules are followed to assign oxidation numbers to different elements in a compound.
In summary, electrochemistry deals with redox reactions, which involve the transfer of electrons between species. Oxidation is the loss of electrons, while reduction is the gain of electrons. These processes can be described in terms of the addition or removal of oxygen and hydrogen. Oxidation states are used to represent the apparent charge of atoms in a compound, and specific rules guide the assignment of these numbers.
Short Answer Questions
An oxidizing agent is a species that facilitates oxidation in a chemical reaction by accepting electrons from another species. In simpler terms, it is a substance that causes another substance to lose electrons, leading to an increase in the oxidation state of the latter. The oxidizing agent itself gets reduced during this process as it gains electrons.
On the other hand, a reducing agent is a species that promotes reduction in a chemical reaction by donating electrons to another species. In this case, the reducing agent causes another substance to gain electrons, resulting in a decrease in its oxidation state. The reducing agent itself gets oxidized during this process as it loses electrons.
MCQ’s Notes
In redox reactions, both oxidation and reduction processes occur simultaneously. The species that undergoes oxidation is called the reducing agent because it facilitates the reduction of another species. Similarly, the species that undergoes reduction is called the oxidizing agent because it causes the oxidation of another species.
Non-metals tend to be good oxidizing agents because they have a high electronegativity, making them more likely to accept electrons. Conversely, metals are usually good reducing agents because they have a tendency to lose electrons due to their low ionization energy.
Overall, redox reactions play a fundamental role in various chemical processes and are essential in the fields of electrochemistry, corrosion prevention, industrial processes, and many other applications. Understanding the roles of oxidizing and reducing agents is crucial for comprehending these chemical transformations and their significance in practical scenarios.
Electrochemical cells are systems that involve the relationship between electricity and chemical reactions. They can be used as energy storage devices or to drive chemical reactions using electric current (electrolysis) or to generate electricity through spontaneous chemical reactions (galvanic cells). There are two main types of electrochemical cells:
Electrolytic Cells: In an electrolytic cell, a non-spontaneous chemical reaction occurs when electric current is passed through the solution. This process is called electrolysis. The cell consists of two electrodes (anode and cathode) dipped in an electrolyte solution and connected to a battery. The anode is the electrode connected to the positive terminal of the battery, and the cathode is the electrode connected to the negative terminal. Anions move towards the anode and discharge there by losing electrons (oxidation), while cations move towards the cathode and gain electrons (reduction). Electrolysis of water is an example of an electrolytic cell, where water is split into oxygen gas and hydrogen gas.
Galvanic Cells: In a galvanic cell, a spontaneous chemical reaction occurs, and it generates electric current. The cell consists of two half-cells, each containing an electrode dipped in a solution of its own salt, connected by a salt bridge. Anode and cathode reactions take place in each half-cell. In the anode reaction, oxidation occurs, while in the cathode reaction, reduction takes place. Electrons flow from the anode to the cathode through an external circuit, generating electric current. Galvanic cells are used in batteries for various applications.
Electrolytes are substances that can conduct electricity in their aqueous solutions or molten states. They are classified into strong electrolytes and weak electrolytes based on their extent of ionization. Strong electrolytes ionize almost completely and produce more ions, while weak electrolytes ionize to a small extent and produce fewer ions. Non-electrolytes do not ionize in their aqueous solutions and do not conduct electricity.
Electrochemical industries use these cells in various processes. For example, molten sodium metal is obtained by the electrolysis of fused NaCl in the Downs cell. Sodium hydroxide (NaOH) is produced in Nelson’s cell by the electrolysis of aqueous solution of NaCl (brine). These industrial processes are crucial for the production of important chemicals and metals. Overall, electrochemical cells play a significant role in energy storage, chemical synthesis, and various industrial applications.
Nelson’s cell is used for the production of sodium hydroxide (NaOH) from brine, which is an aqueous solution of sodium chloride (NaCl). The cell consists of a steel tank with a graphite anode suspended in the center and a U-shaped perforated iron cathode internally lined with an asbestos diaphragm. Brine is present inside the iron cathode.
The working of the Nelson’s cell involves the process of electrolysis, where an electric current is passed through the cell, causing the migration of ions towards their respective electrodes and redox reactions to take place.
Anode reaction (oxidation)
At the anode, chloride ions (Cl-) are discharged and oxidized to form chlorine gas (Cl2).
Cathode reaction (reduction)
At the cathode, hydrogen ions (H+) are discharged and reduced to form hydrogen gas (H2).
As a result of this redox process, sodium hydroxide (NaOH) is produced, and chlorine (Cl2) and hydrogen (H2) gases are evolved. The NaOH solution slowly percolates into a catch basin.
The Nelson’s cell is an important industrial process for the production of sodium hydroxide, which is used in various applications such as the manufacturing of soaps, detergents, paper, and textiles, as well as in various chemical processes. The cell provides a continuous supply of sodium hydroxide with the electrolysis of brine.