How to prepare for JEE mains for Electrochemical Series ?
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The Electrochemical Series
Many of the chemical reactions that occur in biological processes, in the processes of industrial production and in the chemical processes of the geosphere, the atmosphere and the hydrosphere are reactions in which the participating substances undergo oxidation and reduction processes (redox). Since redox reactions are electron exchange processes, these chemical processes can be investigated by measuring electrical variables.
Most elements have been ordered with the electrochemical series according to the possibility that they have to yield electrons to produce electronic displacement. The ability to release electrons is called electrochemical tension. The zero value corresponds to the electrochemical voltage of the hydrogen. There are elements that have electrochemical stresses greater than zero and others, less than zero. This arrangement allows knowing when an electron displacement reaction will occur. Each element is capable of transferring electrons to the ions of any material that is found “below” the electrochemistry series taking into account that the greater the difference of electrochemical stress is. The series of potentials or electrochemistry (tensions) is very useful, since it allows to determine in what sense that there is an oxidation-reduction reaction.
The definition of E0 for each of the metals and phases makes it possible to establish a first approximation of the tendency to become an anode, corrosion, or cathode, reduction, of the two metals that are constituting a micro or macro-corrosion. With the normal hydrogen electrode as origin E0 = 0. With this, it can be predicted in a first approximation which of the two constituent metals of the micro or macropole will be the anode, it will corrode. This function will correspond to the one with the highest normal oxidation potential E0.
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Sometimes the electrochemical series refers to the normal reduction potential, equal but opposite to those of oxidation. In this case, the anode function will be the one with the lowest reduction potential E0.
We will talk about the potential difference that is established between the two electrodes of a galvanic or voltaic battery or between the two semibatteries. This potential difference between the electrodes is called the electromotive force of the battery, although it is also often abbreviated as EMF. Another name that can be given to the electromotive force is the standard potential of the battery, referring to the designation of standard that the solutions have a concentration 1M and the battery is at a temperature of 25ºC. The standard potential of the stack is represented by an E0.
When the table of normal electrode reduction potentials and, therefore, the standard reduction potential of the two electrodes forming the cell is available, the electromotive force of the battery or standard potential is calculated as:
E (battery) = E (cathode) – E (anode)
It will act as cathode (reduction) that electrode whose standard potential is greater and as anode (oxidation) that electrode whose standard potential is lower. That is to say:
-The greater the normal potential of an electrode, the greater its tendency to reduce and, therefore, the greater its oxidizing power.
-The smaller the value of the normal potential of an electrode, the greater its tendency to oxidize and, therefore, the greater its reducing power.
It should also be added that the electromotive force of a battery is always positive.
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Types of cells
- Electrolytic chamber, showing the electrodes and the power supply that generates the electric current.
- The voltaic cell transforms a spontaneous chemical reaction into an electric current, such as batteries. They also receive the names of galvanic cell, galvanic battery or voltaic battery. They are widely used so most of the examples and images are related to them.
- The electrolytic cell transforms an electric current into a chemical oxidation-reduction reaction that does not occur spontaneously. In many of these reactions a chemical substance is decomposed so that this process is called electrolysis. They also receive the names of electrolytic cell or electrolytic cell. Unlike the voltaic cell, in the electrolytic cell, the two electrodes do not need to be separated, so there is only one vessel in which the two half-reactions take place.
Each half cell has a characteristic voltage called half-cell potential or reduction potential. The different substances that can be chosen for each half cell give rise to different potential differences of the whole cell, which is the parameter that can be measured. You can not measure the potential of each half cell, but the difference between the potentials of both. Each reaction is undergoing a balance reaction between the different oxidation states of the ions; When equilibrium is reached, the cell can not provide more tension. In the half-cell that is undergoing oxidation, the closer the equilibrium is to the ion / atom with the more positive oxidation state, the more potential this reaction will give. Similarly, in the reduction reaction, the further away from the equilibrium is the ion / atom with the more negative oxidation state, the higher the potential.
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Potentials of electrode and electromotive force of a battery
The potential or electromotive force of a cell can be predicted through the use of the electrode potentials, the tensions of each half cell. The voltage difference between the reduction potentials of each electrode gives a prediction for the measured potential of the cell.
E(baterry) = E(cathode) + E(anode)
The battery potentials have a possible range from 0 to 6 volts. Batteries that use electrolytes dissolved in water generally have cell potentials less than 2.5 volts, since the very powerful oxidants and reducers, which would be required to produce greater potential, tend to react with water.
It is worth mentioning that the tables presented are reduction tables, so when taking the value of the potential the sign should be changed when it corresponds to the oxidation electrode, to avoid the change of sign the formula to use is the following:
E(baterry) = E(cathode) + E(anode)
A concentration cell, is a galvanic cell in which the two half cells are formed by the same metal in the same solution. The electric current is generated because the concentrations of the solution in the half cells are different, in one the solution will be more concentrated than in the other.
Then, the electrons will tend to flow from the half cell where the solution is more diluted to the more concentrated solution, in order to reduce the dissolved ions, and the concentration of the concentrated solution decreases, at the same time as the concentration of ions in the diluted solution will increase (because it is oxidizing, it is giving up electrons).
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When the concentrations in the solutions are equalized, a potential difference is no longer generated, there is no electron transport, then the battery has run out.
EMF (Electromotor Force)
A few topics above we mentioned the potential of a battery and how it is calculated, assuming allusion to the electromotive force, but what is the electromotive force?
Electromotive force (EMF) is the energy coming from any source, medium or device that supplies electric current. This requires the existence of a potential difference between two points or poles (one negative and the other positive) of said source, which is capable of pumping or driving electrical charges through a closed circuit.
A. Open electric circuit (without load or resistance). Therefore, the circulation of the electric current from the FEM source (the battery in this case) is not established. B. Closed electric circuit, with a load or resistance coupled, through which the circulation of an electric current flow from the negative pole to the positive pole of the FEM source or battery is established.
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