Redox reactions: basic concepts.
Hi Steemians!
A subject of chemistry that is difficult for many people to understand is the subject of REDOX reactions. In this post I will try to simplify the understanding of this topic, this article is an elementary introduction. Here are some concepts:
Redox reactions
The word REDOX is an acronym of oxide-reduction, which summarizes this type of reactions: one substance is oxidized and another is reduced. In general, the substance that is oxidized is called the reducing agent (because it causes the reduction of the other substance), while the substance that is reduced is called oxidizing agent (causes the oxidation of the other substance) . But, what does this type of reaction consist of? A REDOX reaction consists of the transfer of electrons from a substance X (reducing agent) to a substance Y (oxidizing agent). An application in everyday life of this type of reactions are the batteries we use daily in various devices, such as alarm clocks, calculators, watches, cell phones, etc.
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It should be noted that in this type of reactions, both oxidation and reduction occur simultaneously (at the same time). In spite of that, the way in which its chemical equation is balanced separates both situations, forming what are called half-reactions (oxidation and reduction, respectively). In the first, all the substances involved in the oxidation are noted, that is, in the part where the reactants should go, the species to be oxidized is noted, while the oxidized species is noted on the side of the products. Analogously, a similar thing is done for the reduction half-reaction (S.R.R).
- Oxidizing and reducing substances
An oxidant is a chemical species (atom, molecule or ion) that produces the oxidation of another, called reductive.
Now, for a substance to oxidize another it has to capture electrons, which come from that other substance, so that its oxidation number will decrease and therefore, it will reduce itself. And conversely, the reducer has to give up electrons and, therefore, oxidizes. Redox reactions occur, then, between oxidation-reduction pairs, called conjugated pairs of oxidation reduction or, simply, redox pairs.
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- Semi-reactions
Something that characterizes Redox reactions is that they are capable of being produced when the reagents are in separate compartments, such as those that occur in a galvanic cell or cell. When a cell is analyzed, the existence of a Redox reaction that can be divided into two half-reactions is evident; that is, to the reactions that occur in the vicinity of the anode and the cathode of a cell, independently.
The concept of half-reaction is very functional, raised in an equation can help balance the equation; they are the framework used to determine the strength of the different agents present in the reaction.
- Hydrogen electrode
We have already said that it is impossible to measure absolute potentials and we can only determine potential differences. For this reason, to assign values to the potentials of the different electrodes, one must be chosen as a reference and assigned an arbitrary value.
By agreement, it has been established that the reference electrode is the standard or normal hydrogen electrode (EEH), to which a potential equal to zero is assigned. It consists of a platinum and hydrogen gas electrode at 1 bar (or 1 atm) of pressure, immersed in a solution containing H + ions in a concentration of 1 M (at 25 ºC).
If we form galvanic batteries combining different electrodes with the standard hydrogen electrode, we can observe that:
• Some electrodes are more prone to oxidation than hydrogen. In these cases the hydrogen is reduced and acts as a cathode and the other electrode acts as an anode.
• Other electrodes are more prone to be reduced than hydrogen. In these cases the hydrogen is oxidized and acts as an anode. The non-hydrogen electrode acts as a cathode.
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Since the reference electrode can act as an anode or cathode, depending on the electrode it faces, the measurement of the potential will provide information on the tendency of said electrode to be reduced or oxidized, respectively.
- Normal potentials
The potential of electrode, or potential reduction of electrode of an element, is represented as Eº, is the difference between the potential that has a cell, formed by an electrode, and an electrode known as hydrogen standard, when the activity carried out by the ions that participate in the process, is 1 mol / L, at a pressure of an atmosphere, and with a temperature of 25ºC (298ºK).
The electrode potential (normal) is usually represented by the letter Eº, measured in volts (V), following the International System of Units.
- Voltaic battery or galvanic battery
Much of the redox reactions (oxidation-reduction reactions) can be performed in a way that generates electricity. In principle it can be carried out if the redox reactions are spontaneous and the oxidizing and reducing agents are not the same. Needed:
- The oxidizing and reducing agents are separated into so-called half-cells, each of which contains a solution and an electrode (metallic conductor).
- The oxidizing agent or reducer of each half cell can be:
a. The electrode
b. A solid substance deposited around the electrode.
c. A gas that bubbles near the electrode.
d. One of the solutes of the dissolution. - The two semicélulas are connected:
a. By means of a device that allows the circulation of the ions between both: porous substance, salt bridge, etc.
b. Through a conductor that joins both electrodes and allows the passage of electric current when establishing a potential difference between both.
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- Electrolysis
Endothermic redox reactions that occur when establishing an electrical potential difference between two electrodes. The electric potential difference is established thanks to an external battery.
The electrode where the reduction occurs (gain of electrons), is called the cathode (where the cations go when the electrode is negative).
The electrode where oxidation occurs (gain of electrons) is called the anode (where the anions go when the electrode is positive).
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- Accumulator
It is a reversible battery. During the discharge it is a voltaic battery that produces electricity. When charged, it acts as an electrolysis cell in which the reverse reaction occurs.
Greetings! Thanks for your attention.
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