Electrochemistry is part of the chemistry that deals with the relationship between electrical currents and chemical reactions, and the conversion of chemical energy into electrical energy and vice versa. In a broader sense, electrochemistry is the study of chemical reactions that produce electrical effects and of chemical phenomena caused by the action of currents or voltages.
That is why the field of electrochemistry has been divided into two large sections. The first one is Electrolysis, which refers to the chemical reactions that are produced by the action of an electric current. The other section refers to those chemical reactions that generate an electric current, this process is carried out in a galvanic cell or battery.
If a chemical reaction is driven by an externally applied potential difference, it is referred to as electrolysis. On the other hand, if the drop in electrical potential is created as a consequence of the chemical reaction, it is known as an “electrical energy accumulator”, also called a battery or galvanic cell.
The chemical reactions where a transfer of electrons between molecules occurs are known as redox reactions, and their importance in electrochemistry is vital, since through this type of reactions the processes that generate electricity are carried out or otherwise, they are produced as consequence of it.
In general, electrochemistry is responsible for studying situations where oxidation and reduction reactions occur, being separated, physically or temporarily, they are in an environment connected to an electrical circuit. The latter is the subject of study in analytical chemistry, in a subdiscipline known as potentiometric analysis.
In such reactions, the energy released from a spontaneous reaction is converted into electricity or can be used to induce a non-spontaneous chemical reaction.
Adjustment of Redox equations
Electrochemical reactions can be adjusted by the ion-electron method where the overall reaction is divided into two half-reactions (one oxidation and the other reduction), charge and element adjustment is made, adding H+, OH−, H2O and/or electrons to compensate for oxidation changes. Before starting to balance, it must be determined in which medium the reaction occurs, because it proceeds in a particular way for each medium.
It is the device used for the decomposition by electric current of ionized substances called electrolytes. It is also known as a galvanic or voltaic cell, in honor of the scientists Luigi Galvani and Alessandro Volta, who manufactured the first of this type at the end of the 18th century.
Schematic of the Daniell Pile. The salt bridge (represented by the inverted U-shaped tube) contains a KCl solution allowing electrical interaction between the anode and cathode. The tips of this should be covered with pieces of cotton to prevent the KCl solution from contaminating the other containers.
Electrochemical cells have two electrodes: The Anode and the Cathode. The anode is defined as the electrode where oxidation takes place and the cathode where reduction takes place. The electrodes can be of any material that is an electrical conductor, such as metals, semiconductors. Graphite is also widely used due to its conductivity and low cost. To complete the electrical circuit, the solutions are connected by a conductor through which the cations and anions pass, known as a salt bridge (or as a salt bridge).
Dissolved cations move towards the cathode and anions towards the anode. Electric current flows from the anode to the cathode because there is a difference in electric potential between the two electrolytes. That difference is measured with the help of a voltmeter and is known as the cell voltage.
It is also called electromotive force (emf) or as cell potential. In a galvanic cell where the anode is a Zinc bar and the cathode is a Copper bar, both submerged in solutions of their respective sulfates, and joined by a salt bridge is known as Daniell’s Pile. Their half-reactions are these:
Anodic reaction Zn(s)= Zn2+ (ac) + 2 e–
Cathodic Reaction Cu2+ (aq) + 2e– = Cu(s)
Total reaction Zn (s) + Cu2+ (aq) = Zn2+ (aq)+ Cu (s)
The conventional notation for representing electrochemical cells is a cell diagram. Under normal conditions, for the Daniell stack the diagram would be:
This diagram is defined by: anode –> cathode Negative electrode/electrolyte // Electrolyte/positive electrode (the / indicates electron flow and the // means salt bridge)
The vertical line represents the boundary between two phases. The double vertical line represents the salt bridge. By convention, the anode is written first on the left, and the other components appear in the same order they are found when moving from anode to cathode.
Electrochemical corrosion is a spontaneous process that always denotes the existence of an anodic zone (the one that undergoes corrosion), a cathodic zone and an electrolyte, and the existence of these three elements is essential, in addition to a good electrical connection between anodes and cathodes, so that this type of corrosion can take place.
The most frequent corrosion is always of an electrochemical nature and results from the formation on the metal surface of a multitude of anodic and cathodic zones; If the metal is not submerged or buried, the electrolyte is condensed water from the atmosphere, for which the relative humidity must be 70%.
The process of dissolving a metal in an acid is also an electrochemical process. The infinity of bubbles that appear on the metallic surface reveals the existence of infinite cathodes, while the metal dissolves in the anodes.
With the naked eye it is impossible to distinguish between an anodic and a cathodic zone, given their microscopic nature (galvanic micropiles). As the anodic and cathodic zones continually change position, there comes a time when the metal is completely dissolved.