Chemistry Chapter (9) Electrochemistry

Electrochemical Cell: An electrochemical cell is a device, which permits the interconversion of chemical and electrical energy. There are two kinds of electrochemical cell.

i)                  Galvanic Cell: In which chemical energy is converted into electrical energy.

ii)               Electrolytic Cell: In which electrical energy is converted into chemical energy.

·       Spontaneous Reaction and the Galvanic Cell: Consider the following redox reaction.

                Zn_(s) + Cu^2- → Zn²  + Cu_(s)

    The above reaction is a spontaneous reaction. If we place a rod of zinc metal in a solution of cupric sulphate, a dark deposit is formed on the zinc and obscuring its bright surface. This deposit consists of finely divided particles of copper metal and grows to form a thick layer. Gradually, the characteristic blue colour of the CuSO₄ solution becomes paler showing that hydrate cupic ions are used up in the reaction. The reaction is spontaneous; zinc is oxidized and cupric ions are reduced.

For this reaction, ∆G⁰ equal -212 KJ, this large negative value indicating a strong tendency for electrons to be transferred from zinc metal to Cu^2- ions, atleast when reactants and product are in their standard state (pure metal and 1M ionic concentration).

The free energy change for a reaction depends only on the  nature and state of the reactant and products and not on how the    reaction take place. In other words, as we provide the means for    electrons to be transferred from Zn to Cu^2- the transfer will occur.

If we separate the zinc rod from the solution of cupric sulphate i.e if we place zinc rod in a solution of zinc sulphate and a copper rod is immersed in a solution of cupric sulphate and the two rods are electrically connected by a wire outside the apparatus. Athis is one form of a galvanic cell, also called a voltaic cell. The two halves of the cell are called compartments and are separated by a porous partition. The zinc and copper rods are called electrodes and provide surface at which oxidation and reduction half reactions can take place.

Galvanic Cell: two versions.

  a)    Cell with porous partition.

b)   Cell with salt bridge.

If zinc and copper electrodes are connected directly to each other through the external circuit, electrons leave the zinc metal and travel through this circuit to the copper electrode, where they are accepted by Cu^2- ions at its surface. These ions are reduced and the resulting copper atoms deposit on the surface of copper electrode in a process called plating out. In this cell, the zinc electrode is anode i.e. the electrode at which oxidation take place.

                                           Zn(s) →Zn^2- + 2e^- (at anode; oxidation).

The copper electrode is a cathode i.e. the electrode at which reduction take place.

                                Cu^2- + 2e - → Cu(s) (at cathode; reduction).

The porous partition in the cell serves to keep Cu² ion away from zinc anode and thereby prevents electrons from transferring directly from zinc to cupric ions, instead of passing through the internal circuit. As the cell reaction occur, zinc ions migrate away from the zinc anode and toward the copper electrode, as do cupric ions. Positive ions are called cations and negative ions are called anions. If we place a voltmeter in the external circuit, it reads the electrical potential difference or voltage between the two electrodes.

Now if we use an alternate way of constructing a galvanic cell in the porous partition is replace by a salt bridge, a U-tube filled with a solution of potassium chloride. In the salt bridge, Cl^- ions migrate towards the anode and K^- ions towards the cathode, as the cell discharges.

A salt bridge serves three functions:

  1.    It physically separated the electrode compartments.

  2.    It provides electrical continuity (a path for migrating anions and cations) within the cell.

  3.    It reduces the so-called liquid-junction potential, a voltage produced where two dissimilar solutions are in contact. This voltage arises because of unequal rates of anion and cation migration across the contact region or junction. A salt bridge usually contains ions which migrate at almost equal rates, thus minimizing the potential. In both versions of the  galvanic cell, the electrode and overall cell reactions are same.

In both versions of the galvanic cell, the electrode and overall reaction are same.

Cell Notation: Galvanic cells are commonly represented by a shorthand notation called cell notation.

                           Zn(s) | Zn^2- (aq,1M) |  |  Cu^2- (aq, 1M) | Cu(s)

The convention usually followed is to show the anode at left of cell notation. This means that electrons leave the cell from the electrode written at the left. Salt bridge is indicated by a double line.

Voltage and Spontaneity: when a voltmeter is connected to a galvanic cell, it show the voltage produced by the cell and give it a positive algebraic sign. The ‘plus’ sign indicates that the cell reaction proceeds spontaneously but in which direction?

Consider a cell with A – A^- ion electrode and a B – B^- ion electrode connected by a salt bridge. To determine which electrode is the anode, we connect the ‘-’ lead of a voltmeter to the A electrode, the ‘+’ lead to the B electrode and observe the indicated voltage. If the voltmeter reads a ‘positive’ voltage, it means that we have connected the voltmeter correctly and that the A electrode was negatively charged and the B electrode was positively charged, if an electrode appears negatively charged (to the voltmeter), it means that electrons tend to emerge from the cell at that point. So, it is concluded that a electrode in the cell considered is then anode and so the cell diagram is:

                                             A_(s) |A^- | | B^-   |  B_(s)

Cell Reaction are:

                                 An anode       A_(s) → A^- + e^-                oxidation

                                At cathode   B^- + e^-→B_(s)                     reduction

                                                   A_(s)+B^-→A^- +B_(s)

For this cell we associate a positive voltage with the spontaneous reaction.

A negative voltage is associated with a non-spontaneous reaction. For example, the voltage for:

                               Cu(s) + Zn² → Cu² + Zn(s)

Is – 1.10V (1M concentration, 25⁰C). Negative sign in voltage  indicates that copper metal is not oxidized by zinc ions.

Hydrogen Electrode: It is not possible to measure the potential of a single electrode (half cell), however, it is measured with respect to some standard electrode which is usually hydrogen electrode. Hydrogen electrode is half cell developed when H₂(g) is in contact with H^- ions in solution.

Standard Hydrogen Electrode: It is an electrode when H₂ gas at one atmosphere pressure is in contact with H^- ions of 1M concentration at 25⁰C.

“Hydrogen electrode is assigned a potential of 0.000 volts”.

Electrochemical Series (ECS): The electrochemical series is the arrangement of various electrodes in the increasing order of their standard (reduction) electrode potentials.

Characteristics Of Electrochemical Series:

·       In series, if electrode reactions are written as reduction, then electrode potential is called reduction potential and so as for oxidation reaction, it is oxidation potential.

·       Oxidation and reduction potentials are equal in magnitude but are of opposite sign.

·       Substance which are strong reducing agents than H₂ gas are placed above hydrogen gas in the series and their reduction potentials have negative sign.

·       Substance which are weaker reducing agents than hydrogen gas are placed below hydrogen gas and their reduction potentials have positive sign.

·       In this series, electronegative character of non-metals decreases as we move from bottom to the top.

·       Metals which are more electropositive are stronger reducing agent and more reactive.

·       This series helps in predicting the spontaneity or feasibility of a cell reaction. If the emƒ of the cell is positive, the cell reaction is spontaneous, otherwise not.

Electrolytic Cell: In this type cell, electrolysis take place in which the passage of electricity through a solution or fused state of electrolytes provides sufficient energy to cause a non-spontaneous oxidation-reduction reaction to take place.

        An electrolytic cell consists of electrodes that dip into an electrodes and in which a chemical generates electric current.

The movement of anions and cations towards their respective electrodes accompanying all chemical changes in an electrolytic solution under the influence of electric current is known as electrolysis.

         Electrolysis is a redox reaction.

         In the electrolysis of molten sodium chloride, Na^- ions travel to the cathode and sodium metal is liberated.

                                       Na^- + e^- → Na          (reaction)

Chloride ions move to the anode and chlorine is given off.

                                       2Cl – 2e^- → Cl₂           (oxidation)

         i.e reduction always takes place at the negative electrode (cathode) and oxidation always takes place at positive electrode (anode).

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