Standard hydrogen electrode

Introduction

This is the agreement by which the electrode at which reaction takes topographic point with regard to standard H electrode has decrease potency which is given a positive point the electrode at which oxidization takes topographic point with regard to standard H electrode it expressed as a decrease potency, it will hold a negative mark. The assorted electrodes have therefore been arranged in order of their increasing values of standard decrease potencies. This agreement is called electrochemical series. And it besides tells us the responsiveness of metals that ‘s why it used besides in activity series.

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

The electrochemical series is shown below and in this Li is the most reactive component and F 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- Cadmium

-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 ( cubic decimeter )

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

Features of Electrochemical series

The negative mark of standard decrease potency indicates that an electrode when joined with SHE acts as anode and oxidization occurs on this electrode. For illustration, standard decrease potency of Zn is -0.76 V. When Zn electrode is joined with SHE, it acts as anode ( -ve electrode ) i.e. , oxidization occurs on this electrode. Similarly, the +ve mark of standard decrease potency indicates that the electrode when joined with SHE acts as cathode and decrease occurs on this electrode.

The substances which are stronger cut downing agents than H are placed above H in the series and have negative values of standard decrease potencies. All those substances which have positive values of decrease potencies and placed below H in the series are weaker cut downing agents than H.

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

The metals on the top ( holding high negative values of standard decrease potencies ) have the inclination to lose negatrons readily. These are active metals. The activity of metals lessenings from top to bottom. The non-metals on the underside ( holding high positive values of standard decrease potencies ) have the inclination to accept negatrons readily. These are active non-metals. The activity of non-metals additions from top to bottom.

Applications of Electrochemical series

( I ) Responsiveness of metals:

The activity of the metal depends on its inclination to lose negatron or negatrons, i.e. , inclination to organize cation ( M ” + ) . This inclination depends on the magnitude of standard decrease potency. The metal which has high negative value ( or smaller positive value ) of standard decrease possible readily loses the negatron or negatrons and is converted into cation. Such a metal is said to be chemically active.

The chemical responsiveness of metals lessenings from top to bottom in the series. The metal higher in the series is more active than the metal lower in the series. For illustration,

Alkali metals and alkalic Earth metals holding high negative values of standard decrease potencies are chemically active. These react with cold H2O and germinate H. These readily dissolve in acids organizing matching salts and combine with those substances which accept negatrons.

Metallic elements like Fe, Pb, Sn, Ni, Co, etc. , which lie a small down in the series do non respond with cold H2O but react with steam to germinate H.

Metallic elements like Cu, Ag and Au which lie below H are less reactive and do non germinate H from H2O.

( two ) Electropositive character of metals:

The positively charged character besides depends on the inclination to lose negatron or negatrons. Like responsiveness, the positively charged character of metals lessenings from top to bottom in the electrochemical series. On the footing of standard decrease potency values, metals are divided into three groups:

Strongly positively charged metals: Metallic elements holding standard decrease potency near about -2.0 V or more negative like alkali metals, alkalic Earth metals are strongly positively charged in nature.

Reasonably positively charged metals: Metallic elements holding values of decrease potencies between 0.0 and about -2.0 V are reasonably positively charged. Al, Zn, Fe, Ni, Co, etc. , belong to this group.

Weakly positively charged metals: The metals which are below H and possess positive values of decrease potencies are decrepit positively charged metals. Cu, Hg, Ag, etc. , belong to this group.

( three ) Supplanting reactions:

To foretell 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 holding low standard decrease poten­tial will displace the metal from its salt ‘s solution which has higher value of standard decrease potency. A metal higher in the series has greater inclination to supply negatrons to the cations of the metal to be precipitated.

Supplanting of one nonmetal from its salt solution by another nonmetal: A nonmetal higher in the series ( towards bottom side ) , i.e. , holding high value of decrease potency will displace another nonmetal with lower decrease possible i.e. , busying place above in the series. The nonmetal ‘s which possess high positive decrease potencies have the inclination to accept negatrons readily. These negatrons are provided by the ions of the nonmetal holding low value of decrease potency. Thus, Cl2 can displace Br and I from bromides and iodides.

Cl2 + 2KI — & gt ; 2KC1 + I2

21- — & gt ; I2 + 2e- ( Oxidation )

Cl2 + 2e- — & gt ; 2C1- ( Reduction )

[ The activity or negatively charged character or oxidizing nature of the nonmetal additions as the value of decrease potency additions. ]

( four ) To cipher 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 voltage of the cell = [ standard oxidization potency of the oxidization half reaction ] + [ standard decrease potency of the decrease half reaction ]

Further, as in the representation of a cell, the electrode on which oxidization takes topographic point ( anode ) is written on the left manus side and the electrode on which decrease takes topographic point ( cathode ) is written on the right manus side

E= Eocathode -EOanode

To foretell the Spontaneity of a redox reaction:

To look into whether a given oxidation-reduction reaction is executable or non, the voltage of the cell based upon the given oxidation-reduction reaction is calculated. For a redox reaction to be self-generated, the voltage of the cell must be positive. If the EMF comes out to be negative. Then it is non self-generated reaction.

1 ) Using standard electrode potencies to foretell the possibility of reactions

Why can cu2+ oxidise Zn, but zn2+ can non oxidise copper?

We know that Eo values provide an indicant of the comparative strengths of oxidising agents and cut downing agents.

The value of Eo for this reaction is:

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

that for the reaction

Zn Zn2+ + 2e- is +0.76 Vs

Consequently, the overall potency or criterion cell potency for the reaction

Cu2++Zn Cu + Zn2+ is 1.10 Vs

The overall positive value for the reaction possible suggests that the procedure is energetically executable.

Conversely, the overall potency for the contrary reaction is

Cu + Zn2+ Cu2++Znis -1.10 Vs

The negative value indicates that this reaction is improbable to happen. In general, reactions with an overall positive potency are energetically executable whereas those with an overall negative value are non so. So from the tabular array any oxidising agent on the underside will oxidise any cut downing agent top on the tabular array with regard to H.

To organize the battery, the pieces of metal are connected to wires taking to the motor, and so the metals are submersed in Cola, or any other fizzy acidic drink. The motor spins, demoing that a chemical reaction is happening between the metals and the acid in the drink. The acid forms an electrolyte – a liquid which takes an active chemical function in the procedure, and in making do, allows an electrical circuit to be completed. In schools, the presentation is normally performed with dilute sulfuric acid. When utilizing Cola, it is really phosphorous acid that plays the function of the electrolyte. The diagram below shows what is go oning. Oxidation at the Zn electrode destroys that electrode and liberates negatrons in the metal. The Zn ions end up in the electrolyte. Copper and Zn have sufficiently different electrode potencies so as to bring forth a possible difference of over a V between the electrodes ( Cu and Zn have electrode potencies of +0.34V and -0.76V severally, measured with regard to a standard H electrode ) . The negatrons liberated in the Zn therefore flow to the Cu and enter the electrolyte to unite with H ions from the phosphorous acid. The decrease associated with this reaction creates H gas.

Simple illustration of a Cu and Zn electrode Gur cell.

The Cu electrode should non disintegrate every bit quickly as the Zn, although there is a secondary reaction that can travel on when the Cu oxidizes to bring forth Cu ions, and negatrons. This reaction, which is non shown in the diagram, is encouraged by individual H ions H+ uniting with H2O molecules H20 to bring forth oxonium ions H3O+ . The oxonium ions take negatrons from the Cu, and therefore the Cu decays besides. In add-on, this unwanted reaction besides serves to cut down the potency produced by the cell, since there is oxidization taking topographic point at both electrodes.

The tabular array shows the electrode possible chart, with the electromotive force values measured against a standard H electrode ( which is taken to hold the arbitrary value of nothing ) . Any two metals sufficiently far apart in the tabular array will organize a battery, although some of the more reactive metals oxidize in air excessively readily and are so non clean plenty to work decently when submersed in the Cola.

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

Summary

The subject is electrochemical series this series helps us to cognize the responsiveness and a series in which the metals are listed in the order of their chemical responsiveness, the most active at the top and the less reactive or more “noble” metals at the underside. This series is besides known as electrochemical series. And it has applications and features which is really of import to cognize the basic chemical science. It tells about the positively charged character of metal, displacement reaction and to cipher the standard EMF of any electrochemical cell. And it has many utilizations one of them is that it predicts the possibility of reaction whether the reaction takes topographic point or non

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