Chapter 18 - Electrochemistry

18.1 - Redox Reactions

The discipline of chemistry that deals with the interconversion of electrical and chemical energy are known as electrochemistry.
During oxidation, the electron loss through an element is marked by an increased oxidation number of the element.
The oxidation number is reduced as a result of the electrode gain by an element.
The Mg metal was oxidized and H+ ions were reduced in the preceding reaction; the Cl− ions were spectator ions.

The experimental method for generating electricity through a spontaneous reaction is known as a galvanic cell or voltaic cell.
- After the Italian scientists Luigi Galvani and Alessandro Volta who built early versions of the technology.
The Daniell cell is a specific configuration of electrodes (Zn and Cu) and solutions (ZnSO4 and CuSO4).
The anode of a galvanic cell is the electrode where oxidation takes place, whereas the cathode is the electrode where reduction takes place.
- The cell voltage, or cell potential, is the voltage across the electrodes of a galvanic cell.
- The electromotive force, or emf (E), is another term for cell voltage that, despite its name, is a measure of voltage rather than force.

E° is the standard reduction potential, or the voltage associated with a reduction reaction at an electrode when all solutes are 1 M and all gases are at 1 atm, and the superscript “°” denotes standard-state conditions.
The surface of the reduction is provided by the Pt electrode.
When all reactants (H2 in 1 atm, H+ and Zn2+ in 1 M) in their standard states, the cell emf is 0.76 V in 25°C.
A cell with a copper electrode and a SHE [Figure 18.4(b)] can be used to achieve the standard electrode potential for copper similarly.
- Here, the copper electrode is the cathode, since its mass increases while the cell operates, as the decrease reaction is consistent: Cu2+(aq) + 2e− ⟶ Cu(s)

The Faraday constant (F) is the charge of one mole of electrons, named after the English chemist and scientist Michael Faraday.
The total charge can now be expressed as nF, in the total redox process equation n is the number of moles of electrons exchanged between the oxidizing agent and the reduction agent
The emf (Ecell) measured is the cell's maximum voltage.
The electrical labor performed, wele, the maximum work possible (wmax), is therefore given by the overall charge product and cell emf: wmax = wele = −nFEcell

Under nonstandard-state conditions, there is a mathematical relationship between the emf of a galvanic cell and the concentration of reactants and products in a redox reaction.
The electrons flow to the cathode during the operation of a galvanic cells, which leads to the formation of products and a decline in the amount of reactants.
- Therefore, Q is increasing, meaning E is decreasing.
- The cell eventually strikes a balance.
At balance, there is no net electron transfer, so E = 0 and Q = K where K is a constant balance.
A dilute hydrochloric acid solution contains a silver wire covered with silver chloride.
The possibilities between both ends of the membrane can be monitored by means of the reference electrode when the electrode is placed in a solution with a pH different from that of the inner solution.
A voltmetre, measured in pH units, measures the emff of the cell made up of the glass electrode and the reference electrode.

A battery is a galvanic cell or a group of galvanic cells, that can be utilized as a constant-voltage source of direct electric current.
A fuel cell is a galvanic cell that requires a constant supply of reactants to function.
The battery of mercury is widely used in medicine and in electronics and is costlier than the common dry cell.
The Mercury battery is made up of a zinc anode (mercury amalgamated) that comes with a strongly alkaline electrolyte containing zinc oxide and mercury(II) oxide.

Corrosion is the term used to describe the electrochemical process that causes metals to corrode.
Cathodic protection is a corrosion-prevention technique in which the metal to be protected becomes the cathode in a galvanic cell.
The iron is not limited to metallic corrosion. Take aluminum, a metal for many helpful items, including aircraft and drink cans.
- Aluminum has a much greater tendency to oxidize than iron does.
A number of methods for protecting metals from corrosion have been developed.
The aim of most of these methods is to prevent the formation of rust.
- The clearest approach is to paint the metal surface.
But if the color is scraped, punched or dentated, even the smallest part of bare metal is exposed, rust forms under the layer of paint.
A process called passivation can make the surface of iron metal inactive.
- When a strong oxidizing agent such as concentrated nitric acid treats the metal, a thin oxide layer forms.
- In the refrigeration systems and radiators a sodium chromate solution is often added to prevent the formation of rust.
A layer of other metal like tin or zinc can cover the iron container.
The application of a thin layer of tin over iron can produce a "tin."
The formation of rust is avoided while the tin layer remains unbroken.
- But once the surface is scratched, rusting happens quickly

Electrolysis is a process in which electrical energy is utilized to cause a non-spontaneous chemical reaction to occur, as opposed to spontaneous redox reactions
- which result in the transfer of chemical energy into electrical energy.
- An electrolytic cell is a device that performs electrolysis.
The discrepancy between the electrode potential and the actual voltage necessary to initiate electrolysis is known as minimumovervoltage.
The battery is used as a "electron pump" which leads to the cathode where there is a reduction and removes electrons from the anode where oxidation takes place.

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