chemistry

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Standard hydrogen electrode

INTRODUCTION

This is the arrangement by which the electrode at which reaction takes place with respect to standard hydrogen electrode has reduction potential which is given a positive point the electrode at which oxidation takes place with respect to standard hydrogen electrode it expressed as a reduction potential, it will have a negative sign. The various electrodes have thus been arranged in order of their increasing values of standard reduction potentials. This arrangement is called electrochemical series. And it also tells us the reactivity of metals that's why it used also in activity series.

A series in which the metals are listed in the order of their chemical reactivity, the most active at the top and the less reactive or more “noble” metals at the bottom. This series is also known as electrochemical series.

The electrochemical series is shown below and in this lithium is the most reactive element and fluorine is least

ELECTROCHEMICAL SERIES

Half-Reaction

Li+ + e- Li

-3.04

K+ + e- K

-2.92

Ba2+ + 2e- Ba

-2.90

Ca2+ + 2e- Ca

-2.87

Na+ + e- Na

-2.71

Mg2+ + 2e- Mg

-2.37

Al3+ + 3e- Al

-1.66

Mn2+ + 2e- Mn

-1.18

2H2O + 2e- H2 (g) + 2 OH-

-0.83

Zn2+ + 2e- Zn

-0.76

Cr2+ + 2e- Cr

-0.74

Fe2+ + 2e- Fe

-0.44

Cr3+ + 3e- Cr

-0.41

Cd2+ + 2e- Cd

-0.40

Co2+ + 2e- Co

-0.28

Ni2+ + 2e- Ni

-0.25

Sn2+ + 2e- Sn

-0.14

Pb2+ + 2e- Pb

-0.13

Fe3+ + 3e- Fe

-0.04

2H+ + 2e- H2 (g)

0.00

S + 2H+ + 2e- H2S (g)

0.14

Sn4+ + 2e- Sn2+

0.15

Cu2+ + e- Cu+

0.16

SO42+ + 4H+ + 2e- SO2 (g) + 2H2O

0.17

Cu2+ + 2e- Cu

0.34

2H2O + O2 + 4e- 4OH-

0.40

Cu+ + e- Cu

0.52

I2 + 2e- 2I-

0.54

O2 (g) + 2H+ + 2e- H2O2

0.68

Fe3+ + e- Fe2+

0.77

NO3- + 2H+ + e- NO2 (g) + H2O

0.78

Hg2+ + 2e- Hg (l)

0.78

Ag+ + e- Ag

0.80

NO3- + 4H+ +3 e- NO (g) + 2H2O

0.96

Br2 + 2e- 2Br-

1.06

O2 (g) + 4H+ + 4e- 2H2O

1.23

MnO2 + 4H+ + 2e- Mn2+ + 2H2O

1.28

Cr2O72- + 14H+ + 6e- 2Cr3+ + 7H2O

1.33

Cl2 + 2e- 2Cl-

1.36

Au3+ + 3e- Au

1.50

MnO4- + 8H+ + 5e- Mn2+ + 4H2O

1.52

Co3+ + e- Co2+

1.82

F2 + 2e- 2F-

2.87

Characteristics of Electrochemical series

The negative sign of standard reduction potential indicates that an electrode when joined with SHE acts as anode and oxidation occurs on this electrode. For example, standard reduction potential of zinc is -0.76 volt. When zinc electrode is joined with SHE, it acts as anode (-ve electrode) i.e., oxidation occurs on this electrode. Similarly, the +ve sign of standard reduction potential indicates that the electrode when joined with SHE acts as cathode and reduction occurs on this electrode.

The substances which are stronger reducing agents than hydrogen are placed above hydrogen in the series and have negative values of standard reduction potentials. All those substances which have positive values of reduction potentials and placed below hydrogen in the series are weaker reducing agents than hydrogen.

The substances which are stronger oxidizing agents than H+ ion are placed below hydrogen in the series.

The metals on the top (having high negative values of standard reduction potentials) have the tendency to lose electrons readily. These are active metals. The activity of metals decreases from top to bottom. The non-metals on the bottom (having high positive values of standard reduction potentials) have the tendency to accept electrons readily. These are active non-metals. The activity of non-metals increases from top to bottom.

Applications of Electrochemical series

(i)Reactivity of metals:

The activity of the metal depends on its tendency to lose electron or electrons, i.e., tendency to form cation (M"+). This tendency depends on the magnitude of standard reduction potential. The metal which has high negative value (or smaller positive value) of standard reduction potential readily loses the electron or electrons and is converted into cation. Such a metal is said to be chemically active.

The chemical reactivity of metals decreases from top to bottom in the series. The metal higher in the series is more active than the metal lower in the series. For example,

Alkali metals and alkaline earth metals having high negative values of standard reduction potentials are chemically active. These react with cold water and evolve hydrogen. These readily dissolve in acids forming corresponding salts and combine with those substances which accept electrons.

Metals like Fe, Pb, Sn, Ni, Co, etc., which lie a little down in the series do not react with cold water but react with steam to evolve hydrogen.

Metals like Cu, Ag and Au which lie below hydrogen are less reactive and do not evolve hydrogen from water.

(ii)Electropositive character of metals:

The electropositive character also depends on the tendency to lose electron or electrons. Like reactivity, the electropositive character of metals decreases from top to bottom in the electrochemical series. On the basis of standard reduction potential values, metals are divided into three groups:

Strongly electropositive metals: Metals having standard reduction potential near about -2.0 volt or more negative like alkali metals, alkaline earth metals are strongly electropositive in nature.

Moderately electropositive metals: Metals having values of reduction potentials between 0.0 and about -2.0 volt are moderately electropositive. Al, Zn, Fe, Ni, Co, etc., belong to this group.

Weakly electropositive metals: The metals which are below hydrogen and possess positive values of reduction potentials are weakly electropositive metals. Cu, Hg, Ag, etc., belong to this group.

(iii) Displacement reactions:

To predict whether a given metal will displace another, from its salt solution. A metal higher in the series will displace the metal from its solution which is lower in the series, i.e., the metal having low standard reduction poten­tial will displace the metal from its salt's solution which has higher value of standard reduction potential. A metal higher in the series has greater tendency to provide electrons to the cations of the metal to be precipitated.

Displacement of one nonmetal from its salt solution by another nonmetal: A nonmetal higher in the series (towards bottom side), i.e., having high value of reduction potential will displace another nonmetal with lower reduction potential i.e., occupying position above in the series. The nonmetal's which possess high positive reduction potentials have the tendency to accept electrons readily. These electrons are provided by the ions of the nonmetal having low value of reduction potential. Thus, Cl2 can displace bromine and iodine from bromides and iodides.

Cl2 + 2KI --> 2KC1 + I2

21- --> I2 + 2e- (Oxidation)

Cl2 + 2e- --> 2C1- (Reduction)

[The activity or electronegative character or oxidizing nature of the nonmetal increases as the value of reduction potential increases.]

(iv)To calculate the standard EMF of any electrochemical cell:

An electrochemical cell is based on a reaction which can be split into half reactions, viz,

(a) Oxidation half reaction

(b) Reduction half reaction

Standard emf of the cell = [standard oxidation potential of the oxidation half reaction] + [standard reduction potential of the reduction half reaction]

Further, as in the representation of a cell, the electrode on which oxidation takes place (anode) is written on the left hand side and the electrode on which reduction takes place (cathode) is written on the right hand side

E= Eocathode -EOanode

To predict the Spontaneity of a redox reaction:

To check whether a given redox reaction is feasible or not, the emf of the cell based upon the given redox reaction is calculated. For a redox reaction to be spontaneous, the emf of the cell must be positive. If the EMF comes out to be negative. Then it is not spontaneous reaction.

1) Using standard electrode potentials to predict the possibility of reactions

Why can cu2+ oxidize zinc, but zn2+ cannot oxidize cu?

We know that Eo values provide an indication of the relative strengths of oxidizing agents and reducing agents.

The value of Eo for this reaction is:

Cu2+ + 2e- Cu(s) is +0.34 volts

that for the reaction

Zn Zn2+ + 2e- is +0.76 volts

Consequently, the overall potential or standard cell potential for the reaction

Cu2++Zn Cu + Zn2+ is 1.10 volts

The overall positive value for the reaction potential suggests that the process is energetically feasible.

Conversely, the overall potential for the reverse reaction is

Cu + Zn2+ Cu2++Znis -1.10 volts

The negative value indicates that this reaction is unlikely to occur. In general, reactions with an overall positive potential are energetically feasible whereas those with an overall negative value are not so. So from the table any oxidizing agent on the bottom will oxidize any reducing agent top on the table with respect to hydrogen.

To form the battery, the pieces of metal are connected to wires leading to the motor, and then the metals are submersed in cola, or any other fizzy acidic drink. The motor spins, showing that a chemical reaction is occurring between the metals and the acid in the drink. The acid forms an electrolyte - a liquid which takes an active chemical role in the process, and in doing do, allows an electrical circuit to be completed. In schools, the demonstration is usually performed with dilute sulphuric acid. When using cola, it is actually phosphoric acid that plays the role of the electrolyte. The diagram below shows what is happening. Oxidation at the zinc electrode destroys that electrode and liberates electrons in the metal. The zinc ions end up in the electrolyte. Copper and zinc have sufficiently different electrode potentials so as to produce a potential difference of over a volt between the electrodes (copper and zinc have electrode potentials of +0.34V and -0.76V respectively, measured with respect to a standard hydrogen electrode). The electrons liberated in the zinc thus flow to the copper and enter the electrolyte to combine with hydrogen ions from the phosphoric acid. The reduction associated with this reaction creates hydrogen gas.

Simple illustration of a copper and zinc electrode voltaic cell.

The copper electrode should not decay as rapidly as the zinc, although there is a secondary reaction that can go on when the copper oxidizes to produce copper ions, and electrons. This reaction, which is not shown in the diagram, is encouraged by single hydrogen ions H+ combining with water molecules H20 to produce oxonium ions H3O+. The oxonium ions take electrons from the copper, and thus the copper decays also. In addition, this undesirable reaction also serves to reduce the potential produced by the cell, since there is oxidation taking place at both electrodes.

The table shows the electrode potential chart, with the voltage values measured against a standard hydrogen electrode (which is taken to have the arbitrary value of zero). Any two metals sufficiently far apart in the table will form a battery, although some of the more reactive metals oxidize in air too readily and are so not clean enough to work properly when submersed in the cola.

Electrochemical series for some common metals referenced to the standard hydrogen cell.

SUMMARY

The topic is electrochemical series this series helps us to know the reactivity and a series in which the metals are listed in the order of their chemical reactivity, the most active at the top and the less reactive or more “noble” metals at the bottom. This series is also known as electrochemical series. And it has applications and characteristics which is very important to know the basic chemistry. It tells about the electropositive character of metal, displacement reaction and to calculate the standard EMF of any electrochemical cell. And it has many uses one of them is that it predicts the possibility of reaction whether the reaction takes place or not


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