Electrochemistry And Arrhenius Theory Of Ionization

Introduction of electrochemistry

Electrochemistry, electro + chemistry, is a branch of physical chemistry that is related to the interaction between electricity and chemical reaction. Electricity is generated when electrons or ions move, depending upon its nature of conduction whether it is a metal or an electrolyte. During the conduction of electricity through metallic conduction, both chemical reaction and transportation of matters doesn’t happen i.e. only the migration of electrons take place. While conduction of electricity through electrodes undergoes chemical changes and the transfer of ion takes place along with its matter.

While,

electrochemistry is related with those chemical reactions that: -

1. Either involves in those chemical reactions that don't normally occur in the absence of electricity.
2. Or those type of chemical reaction that produces electricity when the chemical reactions occur. Example: - In a battery, the chemical reaction that happens within the cell generate electricity.

Electrolytic conduction

Electrolytic conduction is the ability of an electrolytic solution to let the electric current flow through it when the current is applied to the electrodes.

Electrolyte

The electrolyte is an aqueous solution that conducts electricity or it can also be said as the substance that conducts electricity when dissolved in water (solvent). Only those solutions can be taken as electrolytes that can conduct electricity. Eg: - Salt-water is electrolyte as it conducts electricity while sugar + water is not an electrolyte as it cannot conduct electricity.

Arrhenius Theory of Ionization

1. When an electrolyte is dissolved in water, it gets separated into electrically positive and negative charged ions as cations and anions respectively which is known as ionization or electrolytic dissociation.
2. The sum of positively charged cations and negatively charged anions are equal i.e. the solution formed by the mixture of electrolyte and water is electrically neutral.
3. The conduction of electricity through electrolyte is possible as ions move towards oppositely charged electrodes.
4. There is a state of dynamic equilibrium between ions and non-ionized molecules as the ions are constantly reuniting and the molecules are dissociating. Hence, the process of ionization is reversible. Example: - $$NH_4 OH \rightleftharpoons NH_4^+ +OH^-$$
5. The process of ionization is incomplete and the extent of ionization is expressed in the terms of degree of ionization (α) defined as: - $$\alpha= \frac {number\;of\;moles\;ionized} {total\;number\;of\;moles\;dissolved}$$
6. Strong electrolytes get nearly-ionized hence they have a degree of ionization nearly equal to 1 while weak electrolytes are partially ionized and degree of ionization is considerably less than 1.

Factors that affect a degree of ionization

a. Nature of electrolyte

The degree of ionization differs with accordance to the nature of electrolyte. Ionic compounds that dissolve in water have a high degree of ionization as it ionizes almost completely while weak electrolytes ionize partially and have a very low degree of ionization.

b. Nature of solvent

The degree of ionization increases if the dielectric constant of the solvent is greater and decreases if the dielectric constant of solvent is less.

c. Dilution

If the dilution of electrolyte in the solution is increased electrolytes get ionized more completely than when it is concentrated as the equilibrium between a undissociated molecule and ions shift to the direction of ion formation. Hence, a degree of ionization increases with increase in dilution of an electrolyte.

d. Temperature

The degree of ionization increases with the increase in temperature and decreases with a decrease in temperature. Inorganic salts like NaCl, BaCl, etc. dissolve in hot water easily than in cold water.

Faraday's law of electrolysis

Faraday studied about the quantitative relationship between electricity and chemical change and proposed a phenomena of electrolysis which is today known as Faraday's law of electrolysis. Faraday put forward two laws of electrolysis as: -

Faraday's first law

$$\mathcal Faraday's\;first\;law\;states\;that\;the\;mass\;of\;chemical\;substance\;discharged\;or\;deposited\\at\;the\;electrode\;is\;directly\;proportional\;to\;the\;amount\;of\;charge\;passed\;through\;the\;"electrolyte".$$

Mathematically, $$W ∝ Q$$ $$W∝I t [\because Q=I t]$$ $$W=zIt$$

Where,

z=proportionality constant known as electrochemical equivalent

I = amount of charge/current in ampere

W=mass of chemical substance

t = time in second

Faraday's second law

$$\mathcal Faraday's\;second\;law\;of\;electrolysis\;states\;that\;the\;weight\;of\;a\;chemical\;substance\\discharged\;at\;electrodes\;by\;the\;same\;quantity\;of\;electrical\;energy\;are\;directly\;proportional\;to\;their\;respective\\electrochemical\;equivalent.$$

Mathematically, $$W∝E \qquad \qquad \qquad \qquad$$ $$\qquad W=kE \qquad where,k\;is\;a\;constant$$ $$k= \frac{W}{e} \qquad \qquad \qquad \qquad$$

W= weight of a substance

E=electrochemical equivalent

References: -

(Sthapit and Pradhananga)

Sthapit, Moti Kaji, and Dr.Raja Ram Pradhananga. Foundations Of Chemistry. 5th. Vol. 1. Kathmandu: Supravaha Press, 2010. 3 vols.

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