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    Describe in details the chemical changes (chemical effects) that take place when electric current is passed through a liquid.

    © BrainMass Inc. brainmass.com December 24, 2021, 11:42 pm ad1c9bdddf
    https://brainmass.com/physics/electricity-magnetism/chemical-effects-electric-current-liquid-593885

    SOLUTION This solution is FREE courtesy of BrainMass!

    CHEMICAL EFFECTS OF CURRENT - ELECTROLYSIS

    1. What is meant by chemical effect of current?

    When a current is passed through a solid it produces heating effect, magnetic effects etc. which do not involve any chemical changes. However, when a current is passed through some liquids, under certain conditions, chemical changes take place. These are known as chemical effects of current.

    2. Do all liquids show chemical effects when current is passed through them?

    No, all liquids do not show chemical effects when current is passed through them. Some liquids do not allow the current to pass through at all i.e. these are insulators (examples : distilled water, transformer oil, vegetable oil). Some liquids act as good conductors and allow the current to pass without any chemical effect (example : mercury). However, there are some liquids notably solutions of salts, acids and bases in water or alcohol (called electrolytic solutions) which show chemical reactions when current is passed through them.

    3. What is an electrolyte?

    Many substances even in solid state exist in ionized form. In these substances the +ve and -ve ions hold together due to the electrostatic force of attraction. Such substances are called electrolytes. For example in a molecule of NaCl, each Na atom carries a single +ve charge and Cl atom a single -ve charge. As we know charged atoms are called ions. Thus a molecule of NaCl is made up of one positively charged Na ion (designated as Na+) and one negatively charged Cl ion (designated as Cl-), both held together, in solid state, because of the electrostatic force of attraction. Of course there is the thermal agitation of the molecules which tends to dissociate (break up) the ions even in the solid state. However, the thermal agitation energy is not sufficient to provide the energy required to dissociate them For example : in case of NaCl, the average thermal agitation energy per molecule is 0.03 eV and the energy required to dissociate NaCl ions is 7.9 eV per molecule.

    Acid, bases and salts are the most common examples of electrolytes.

    4. Why do electrolytes dissociate on dissolving in liquids ? What is electrolytic conduction?

    As we had learnt in electrostatics, as per Coulomb's law the force between two charges q1 and q2 at a distance r in a medium with dielectric constant K is given by :

    (see attached file for equations)

    where Є0 is the permittivity of vacuum. Since value of K for vacuum is 1 and for any other medium it is greater than 1, the force of attraction between the ions will reduce when the electrolyte is dissolved in any liquid, extent of reduction depending upon the value of K for the liquid. For example, value of K for water is 81. Thus the force of attraction between Na+ and Cl- ions will be reduced by a factor of 81 when NaCl is dissolved in water. Accordingly the energy required to dissociate the ions also reduces drastically and a large number of ions get dissociated making the solution rich in ions of both the types.

    Now if two metal plates (called electrodes) are dipped partially into the electrolytic solution, and connected to a battery (one plate called anode connected to +ve terminal and the other called cathode to the negative terminal), an electric field is set up inside the solution. Under the influence of this field, positively charged ions start moving towards the negatively charged cathode (these ions are called cations) and negatively charged ions move towards the positively charged anode (called anions). Thus the process of conduction of electric current from the anode to the cathode (since the electrons are moving from cathode to anode) starts inside the solution. This process is called electrolytic conduction.

    5. What is electrolysis?

    The process of dissociating electrolytes and producing continuous chemical activity (such as production of gases or deposition of metals on an electrode) is known as electrolysis. The process of electrolysis is described by Faradays laws of electrolysis as follows :

    First law: The mass of substance produced at a cathode or anode in the process of electrolysis is directly proportional to the amount of electricity (that is total charge) passed through the cell.

    If I is the constant current passing during electrolysis, then total charge passed in time t is given by Ixt and as per Faradays first law we have :

    Total mass of substance deposited or liberated, (see attached file for equations)

    m = zIt = zQ where z is the constant of proportionality and is called the electrochemical equivalent of the substance with units as g/C and Q is the total charge passed. Electrochemical equivalent of a substance is defined as the mass of the substance in grams deposited or liberated when 1 ampere current flows for 1 sec. For example electrochemical equivalent for silver is 0.001118 g/C while that for hydrogen is 0.00001044 g/c.

    Second law: The masses of different substances produced (deposited or liberated) by same quantity if electricity (charge) are proportional to the equivalent masses (chemical equivalents) of the substances.

    Let us consider a substance with atomic mass M and valence p is produced during electrolysis. Let us suppose a mole i.e. M grams of the substance is produced. We know one mole of every substance contains number of atoms equal to Avogadro's number N viz. 6.023 x 1023 . We also know that by definition : Equivalent weight (chemical equivalent) of the substance, E = Atomic mass/Valence. Since M is equal to atomic mass expressed in grams, we can write :

    (see attached file for equations) (1)

    During the process of electrolysis no. of electrons absorbed or released per atom is equal to the valence p of the substance. Thus for production of one mole of the substance no. of electrons required is equal to N.p which is equal to a charge Q = Npe (see attached file for equations) ...(2)
    where e is the electronic charge.

    From first law, for one mole of substance we can write:
    M = z Q = zNpe or z = M/Npe (see attached file for equations) ...(3)

    Substituting E for M/p in (3) [from (1)] we get : z = E/Ne (see attached file for equations) ...(4)
    As N and e are constants we can write (4) as : (see attached file for equations) ...(5)

    Based upon first law and (5) above we can write for two substances with equivalent masses E1 and E2 as follows :

    (see attached file for equations) ...(6) where Q is the charge passed during electrolysis and m1 and m2 are the masses of two substances produced. From above we get :

    (see attached file for equations) ...(7)

    (7) states that the mass of a substance produced during electrolysis is proportional to its equivalent mass which is the statement of the second law.

    6. Describe the process of electrolysis of copper sulphate (CuSO4).

    For electrolysis of CuSO4 the electrodes are made of pure copper and copper sulphate dissolved in water forms the electrolytic solution. This apparatus for electrolysis of CuSO4 is known as copper voltameter.

    (see attached file for equations)

    Between the electrodes are connected a battery, ammeter, rheostat (variable resistor) and a switch. When the process of electrolysis is to be started, the switch is closed and a suitable current is adjusted with the help of the rheostat. Now let us see what happens inside the electrolyte.

    (see attached file for equations)

    When CuSO4 is dissolved in water copper (Cu++ ) and sulphate (SO4--) ions, each with a valence of 2, dissociate and freely move about in the solution. As soon as potential is applied across the plates an electrostatic field is set up in the solution under the influence of which positively charged Cu++ ions start moving towards cathode and negatively charged SO4-- ions move towards the anode. At cathode and Anode following reactions take place :

    At Cathode: Positively charged Cu++ ion receives two electrons from cathode and becomes neutralized and gets deposited on the cathode. This can be written as : Cu++ + 2e -  Cu [this addition of electrons is called reduction reaction][see note below].

    At Anode: i) Simultaneously with transfer of 2 electrons from cathode to Cu++ ion, there is movement of 2 electrons from anode to the positive terminal of the battery which further gets transferred to the negative terminal of the battery, further to the cathode. This transfer of 2 electrons from anode generates a copper ion Cu++ [this giving up of electrons by a copper atom to become a copper ion is known as oxidation reaction][see note below], ii) On the other hand negatively charged SO4-- are moving towards the anode where one SO4-- ion combines with Cu++ ion generated as described at i) above producing CuSO4 molecule. This can be written as: (see attached file for equations)

    Thus a continuous cycle is set up as shown in the diagram.

    (see attached file for equations)

    There is a continuous deposition of copper on the cathode. SO4- ions continuously combine with Cu++ ions being generated at anode (due to transfer of electrons to the cathode via battery) to form CuSO4 and thus maintain the copper sulphate concentration of the solution. Further, due to continuous flow of electrons from anode to cathode through the battery and from cathode to anode inside the solution (carried by SO4-- ions) a continuous flow of current takes place.

    [Note : Oxidation and reduction are two types of extremely important chemical reactions. In an oxidation reaction, electrons are given up, whereas in a reduction reaction, electrons are accepted by the reacting substance. By definition the electrode at which oxidation reaction takes place is called anode and the one at which reduction reaction takes place is called cathode. Thus in the process of electrolysis, anode is the electrode which is connected to the positive terminal of the battery, whereas cathode is the one connected to the negative terminal. It will be seen later in the case of electrochemical cells, the cathode is the positive terminal of the cell and anode the negative terminal.]

    7. Describe the process of electrolysis of water.

    For electrolysis of water The arrangement as shown in the fig is setup.

    (see attached file for equations)

    This is known of a water voltameter. It consists of a container filled with water to which a small quantity of sulphuric acid is added to make it more conducting by due to additional ions available. A tube filled with water is inverted over each electrode to receive the gases liberated at the electrodes. The electrodes are connected to an arrangement of a battery, rheostat and a switch.

    For electrolysis, the switch is closed and current flow is adjusted at a suitable level. Following reactions take place at the two electrodes :

    At Cathode: 4 nos. H2O molecules take 4 electrons from the cathode and a reduction reaction [involving gain of electrons] takes place as follows :

    (see attached file for equations) ...(1)

    Thus hydrogen gas is evolved at cathode and concentration of OH- ions increases around cathode.

    At Anode: 2 nos. H2O molecules release 4 electrons to the anode and oxidation reaction [involving loss of electrons] takes place as follows :

    (see attached file for equations) ...(2)

    Thus oxygen gas is evolved at cathode and concentration of H+ ions increases around anode.

    OH- ions being negatively charged move towards the anode and H+ ions being positively charged move towards the cathode and neutralize. Thus (see attached file for equations) ...(3)

    Thus as can be seen from (1), (2) and (3), for every 6 molecules of water participating in the electrolysis process, 4 molecules come back [equation (3)] while 2 molecules break up into 2 molecules of hydrogen gas and 1 molecule of oxygen gas (i.e. 2 molecules of water have been hydrolyzed). At any given time the volume of hydrogen collected is twice that of oxygen.

    Further, for every two molecules of water hydrolyzed, 4 electrons are taken from cathode [equation (1)] and 4 electrons are given to anode [equation (2)]. Thus a closed cycle of flow of electrons viz. cathode - anode - +ve terminal of battery - negative terminal of battery - cathode and so on is set up constituting flow of current in the opposite direction.

    8. Describe the electrolysis of aqueous solution of silver nitrate (AgNO3).

    Silver voltameter consists of a glass vessel containing solution of silver nitrate in water. Rest of the arrangement is similar to that for electrolysis of copper sulphate described earlier except that the electrodes are made of silver.

    AgNO3 dissociates into Ag+ and NO3- ions.

    At Cathode: When current is passed through the solution Ag+ ions drift towards the cathode where they receive electrons and get neutralized and get deposited on the cathode as pure silver. The reaction at the cathode is : (see attached file for equations)

    At Anode: i) Simultaneously with transfer of 1 electron from cathode to Ag+ ion, there is movement of 1 electron from anode to the positive terminal of the battery which further gets transferred to the negative terminal of the battery, further to the cathode. This transfer of 1 electron from anode generates a silver ion Ag+. ii) On the other hand negatively charged NO3- ions are moving towards the anode where one NO3- ion combines with Ag+ ion generated as described at i) above producing AgNO3 molecule. This can be written as:
    (see attached file for equations)

    Thus a continuous cycle is set up in which silver dissolves from anode and gets deposited in the cathode.

    9. How are Faraday's laws of electrolysis verified experimentally?

    Faraday's laws can be verified as follows:

    First law: In the copper voltameter, to start with the cathode is weighed. Then current is passed for a suitable time after which the cathode is weighed again. This will give the mass of copper deposited. The strength of current is now doubled and passed again for the same time as earlier. Mass of copper deposited second time is determined after weighing the cathode once again. It will be found to be twice that of the first time. This proves that m  I ...(1)

    The experiment is repeated, this time keeping the current constant and increasing the time to twice that of the first time. It will be found that the mass of copper deposited is double of the first time. Thus we can conclude that m  t. ...(2)

    Combining (1) & (2) we get the equation of the Faraday's first law viz. (see attached file for equations)

    Second law: The second law is verified by connecting three voltameters viz. copper voltameter, silver voltameter and water voltameter in series. A constant current is passed for a suitable time after which the masses of copper and silver deposited in copper and silver voltameters are determined through weighing and mass of hydrogen liberated in the water voltameters is determined by measuring the volume of hydrogen and reducing it to NTP and then multiplying the same by the density of hydrogen at NTP. The ratio of the masses of silver, copper and hydrogen will be found to be in the ratio 108 : 31.75 : 1 that is in the ratio of their equivalent masses. This proves the second law.

    10. What are the practical applications of electrolysis?

    Following are some of the applications of electrolysis:

    i) Electroplating

    ii) Extraction of metals from ores

    iii) Purification of metals

    iv) Electrotyping

    v) Production of oxygen and hydrogen

    This content was COPIED from BrainMass.com - View the original, and get the already-completed solution here!

    © BrainMass Inc. brainmass.com December 24, 2021, 11:42 pm ad1c9bdddf>
    https://brainmass.com/physics/electricity-magnetism/chemical-effects-electric-current-liquid-593885

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