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NOTES of CHEMISTRY OF ' S ' BLOCK

Quality Education At Every level

 Class :XII                          Chapter #03 

CHEMISTRY of 'S' BLOCK 

 Prepared by:

Lecturer. Syed Fayyaz Hussain

Email address: syed.fayyaz.hussain396@gmail.com

Manufacturing Of Sodium By Down's Process

Introduction:

                  J.C.Down developed a process of preparing sodium metal by electrolysis of fussed sodium chloride. This process is called "Down's Process". Electrolysis of sodium chloride is carried out in Down's cell.

Raw Materials:

                  Fused sodium chloride is used as electrolyte in this process. Since sodium chloride has melting point 801^oC, therefore some amount of calcium chloride is used to lower the melting point of sodium chloride to about 600^oC.

Construction Of Down's Cell:

                  The Down's cell is made up of steel lined with fire bricks. The cell has central graphite anode & a surrounding iron cathode. The two electrodes are separated by means of iron gauze diaphragm to prevent reaction between chlorine gas & sodium metal.

                                    2Na + Cl₂                                 2NaCl

            Liquid Sodium metal collects in the inverted trough under oil and then tapped off through iron vessel,

Working Of Down's Cell:

                  When electricity is passed through the molten mixture of sodium chloride and calcium chloride, Na⁺ & Cl^- move towards their respective electrodes & form neutral particles.

                        At Cathode      Na⁺ + e^-                        Na

                        At Anode         2Cl^-                  Cl₂ + e^-

            Some calcium ions are also reduced into calcium metal at cathode they can easily be separated by means difference in their densities.

Reactions Of Alkali Metals And Alkaline Earth Metals

            Alkali metals and alkaline earth metals have one and town electrons in their outer most shell respectively therefore they readily loose their electrons to attain the stable configuration of Noble gases and hence very reactive and acts as reducing agent.

Reaction With Hydrogen:

Alkali metals and Alkaline earth metals reacts with hydrogen at 250^oC to form ionic hydrides.

                  M + H₂                               2MH    (M = Alkali metal)

                  M + H₂                               M₂H     (M = Ca, Sr, Ba)

Reactions With Nitrogen:

                  Alkali metals and Alkaline earth metals react with nitrogen to form nitride except Be.

                  6M + N₂                             2M₃N   (M = Alkali metal)

                  3M + N₂                             M₃N₂    (M = Alkaline earth metal except Be)

Reactions With Oxygen:

                  Alkali metals and Alkaline earth metals react with oxygen to form variety of compound like oxide, peroxide and super oxide. The product depends upon the metal and conditions.

                  Li + O₂                               2Li₂O   (Lithium oxide)

                  2Na + O₂                            Na₂O₂  (Sodium peroxide)

                  M + O₂                               MO­₂     (Super oxides M= K, Rb, Cs)

                  2M + O₂                             2MO­    (Oxides M= Be, Mg, Ca)

                  M + O₂                               MO₂     (Peroxides M = Sr, Ba)

Reactions With Halogens:

                  Alkali metals and Alkaline earth metals react with halogens to form halides.

                  2M + X₂                             2MX     (M = Alkali metal)

                  M + X₂                               MX₂     (M = Alkaline earth metals)

Reaction With Water:

                  Alkali metals and Alkaline earth metals react with water violently. They react with water to form hydrogen gas and an alkali. Be and Mg form a protective coating of hydrated metal oxide on their surface and are protected form extensive corrosion.

                  2M + 2H₂O                                    2MOH + H₂      (M = Alkali metals)

                  M + H₂O                            M(OH)₂ + H₂    (M = Ca, Sr, Ba)

                  M + H₂O                            MO + H₂          (M = Be, Mg)

            Magnesium reacts with boiling water to form hydrogen gas.

                  Mg + H₂O                          Mg(OH)₂ + H₂

Industrial Preparation Of Sodium Carbonate Solvay Process

Introduction:

                  Sodium Carbonate is generally called Soda Ash of washing soda and Sodium bicarbonate is also known as Baking soda. Ammonia-Solvay process is used for the preparation of sodium carbonate in industries.

            In 1860s Ernest Solvay developed a process for preparation of Sodium Carbonate. Almost all of the sodium carbonate and sodium bicarbonate produced in the world is manufactured by Solvay Process.

Raw Materials:

                  Raw materials used in this process are as follows.

1.      Sodium chloride (Brine Solution)

2.      Ammonia gas

3.      Lime stone (for producing CO₂)

Ammonia-Solvay Process:

                  This process consists of three steps.

      Step # 01               Ammoniation of Brine

      Step # 02               Carbonation of Ammoniated Brine.

      Step # 03               Conversion to Na₂CO₃.

Step # 01  Ammoniation Of Brine:

                        This step is carried out in ammoniating tower. In this step saturated solution of sodium chloride or Brine is further saturated with ammonia, obtained from the reaction of Ammonium Chloride and Calcium oxide. The brine is allowed to flow down & ammonia passing up the tower. To ensure the proper saturation of brine mushroom-shaped baffles plate is used to control the flow of brine.

Step # 02  Carbonation of Ammoniated Brine:

                        This step is carried out in Carbonating tower also called "Solvay Tower". In this tower ammoniated brine trickle down & carbon dioxide, obtained by thermal decomposition of lime stone, going upward. The baffle plates are used to control the flow of brine & break up carbon dioxide into small bubbles. As a result NH₄⁺ and HCO₃^- are obtained.

           

Following reactions take place in Solvay tower.

                  NH₃ + CO₂ + H₂O                                       (NH₄⁺¹)₂CO₃^-2

                  (NH₄)₂CO₃ + CO₂ + H₂O                             NH₄HCO₃

                  Na⁺Cl^- + (NH₄⁺)HCO₃^-                                 NH₄Cl + NaHCO₃

            The bicarbonate ions form in a solution that has a high concentration of Na⁺ ions. Since sodium bicarbonate is slightly soluble in cold solution. These reaction are exothermic the temperature rises which increases the solubility of NaHCO₃. Therefore lower part of tower is cooled down to increase precipitation of NaHCO₃. NaHCO₃ Filtered off by vacuum filtration.

Step # 03  Conversions to Na₂CO₃:

            In this step thermal decomposition of NaHCO₃ gives anhydrous sodium carbonate.

                        2NaHCO₃                                 Na₂CO₃ + H₂O + CO₂

            Soda ash is re-crystallized and yields deca hydrated sodium carbonate (Na₂CO₃. 10H₂O) called "Washing Soda". CO₂ obtained is recycled to the carbonating tower.

Uses:

            Sodium carbonate has extensive uses.

1.      It is used in manufacturing of glass.

            Na₂CO₃ + SiO₂                   Na₂SiO₃ + CO₂

2.      Washing soda is used as water softener.

            Ca⁺² + Na₂CO₃                   CaCO₃ + 2Na⁺

3.      Sodium carbonate is used in preparations of soap, detergents, paper and other chemicals.

Industrial Preparation Of Sodium Hydroxide

Castner-Kelner Process

Introduction:

                        Sodium hydroxide is very important chemical in industries. It is also called Caustic soda. Sodium hydroxide is prepared by electrolysis of Brine. This process is called Castner-Kelner Process.

Process:

                  Castner-Kelner process consist of two steps.

      Step # 01   Electrolysis of Brine

      Step # 02   Formation of Sodium hydroxide

Step # 01  Electrolysis Of Brine:

                        Electrolysis of Brine is carried out in Castner-kelner cell.

Construction Of The Cell:

                  Castner-Kelner cell is made up of steel tank. In the cell Titanium block are used as anode. Flowing mercury is used as cathode to prevent the reaction between hydroxyl ion and chlorine gas.

                        6OH^- + Cl₂                                ClO₃^- + Cl^- + 3H₂O

            When electricity is passed through castner kelner cell at Titanium anode chloride ions are oxidized to form chlorine gas.

                        2Cl^-                  Cl₂ + 2e^-

            At flowing mercury cathode Na⁺ ions discharged to form sodium metal. This sodium metal dissolved in mercury to form amalgam.

                        Na⁺ + e^-                        Na

                        Na + Hg                       Na/Hg

            H⁺ ions in brine solution also attracted towards mercury but due to high voltage of H⁺ ions on mercury surface, sodium ions are more easily discharged on mercury surface.

Step # 02  Formation Of Sodium Hydroxide:

                        In 2^nd step mercury containing sodium metal is taken to DENUDER. In denuder sodium metal reacts with water to form sodium hydroxide and hydrogen gas.

                  2Na/Hg + 2H₂O                 2NaOH + H₂ + Hg

            In denuder graphite blocks are used to control the flow of flowing mercury.

Disadvantages Of The Process:

1.      This process consumes large quantity of electricity.

2.      Some mercury vapors may escape from the factory and contaminates sea water, resulting in pollution of food chain.

Physical Properties:

                  Physical properties of sodium hydroxide are as follows.

1.      It is a white solid.

2.      It is deliquescent. (absorbs moisture from atmosphere and forms solution)

3.      It melting point is 322^oC with decomposition.

4.      It is highly soluble in water and evolves large quantity of heat when it is dissolved.

Chemical Properties:

                  Chemical properties of sodium hydroxide are as follows.

1.      As An Alkali:

                  Sodium hydroxide is a strong alkali and ionizes fully in its aqueous solutions. It neutralizes acids to form salt and water.

                  Na⁺OH^- + H⁺Cl^-                         Na⁺Cl^- + H₂O

2.      Reaction With Ammonium Salts:

            Sodium hydroxide reacts with ammonium salts on warning and liberates ammonia.

            NaOH + NH₄Cl                         NH₄⁺OH^- + NaCl

3.      Reaction With Metal Ions:

            It reacts with metal ions and forms precipitate of insoluble metal hydroxides.

            Fe⁺³ + 3OH^-                              Fe(OH)₃

      When precipitated hydroxides are amphoteric, then they re-dissolve in excess of sodium hydroxide forming complex.

            Zn⁺² + 2OH^-                              Zn(OH)₂

            Zn(OH)₂ + 2OH^-                       [Zn(OH)₄]^-2

Uses:

            Sodium hydroxide has following uses.

1.      It is used in preparation of phosphine, sodium chlorate(v)

2.      It is used in soap, rayon and paper.

3.      It is used to open drains due to its dissolving and caustic effects.

4.      It is used in textile for bleaching and dyeing processes.

5.      It is used in refining of petroleum.

6.      Its solution gives silky finish to mercerized cotton.

7.      It is used in laboratory as an alkali and for absorbing carbon dioxide.

Magnesium Sulphate

            Magnesium sulphate occurs in nature as Kieserite (MgSO₄. H₂O). The hepta hydrated magnesium sulphate (MgSO₄. 7H­₂O) is called EPSOM.

Preparation:

                  Magnesium sulphate is prepared by the reaction of sulphuric acid on magnesium metal, its oxide, hydrosice or carbonates.

            Mg + H₂SO₄                             MgSO₄ + H₂

            MgO + H₂SO₄                          MgSO₄ + H₂O

            Mg(OH)₂ + H₂SO₄                    MgSO₄ + 2H₂O

            MgCO₃ + H₂SO₄                       MgSO₄ + H₂O + CO₂

Properties:

                  Magnesium sulphate has following properties.

1.      Magnesium sulphate is a white crystalline solid.

2.      It is soluble in water.

3.      It loses its water molecules at 200^oC to form anhydrous magnesium sulphate.

                  MgSO₄. 7H₂O                          MgSO₄ + 7H₂O

Uses:

                  Magnesium sulphate has following uses.

1.      It is used in manufacturing of ceramics, paper, soap and cement.

2.      It is used as purgative in medicines.

Caustic Soda

Calcium Sulphate

            Calcium sulphate occurs in nature as dehydrate (CaSO₄. 2H₂O) called GYPSUM.

Preparation:

                  Gypsum can be prepared by the reaction dilute sulphuric acid with calcium carbonate or chloride.

                  CaCO₃ + H₂SO₄                              CaSO₄ + CO₂ + H₂O

                  CaCl₂ + H₂SO₄                                CaSO₄ + 2HCl

Properties:

                  Calcium sulphate has following properties.

1.      It is sparingly soluble in water, producing permanent hardness in water.

2.      At 100^oC gypsum is converted to hemi hydrated calcium sulphate, which is called Plaster of Paris.

                  CaSO₄. 2H₂O                            (CaSO₄)₂ H₂O + 2H₂O

3.      At 200^oC gypsum forms anhydrous calcium sulphate.

                  CaSO₄. 2H₂O                            CaSO₄ + 2H₂O

Uses:

                  Calcium sulphate has following uses.

1.      It is used in making of black board chalk.

2.      It is used as fertilizer for saline soil.

3.      It is used for filling and glazing of paper.

Bleaching Powder

            Prof. Odling suggested the formula of bleaching powder Ca(OCl)Cl with the help of percentage composition of chlorine.

Preparation:

                  Bleaching powder is prepared by Hasen clever process. In this process chlorine gas reacts with slaked lime to form bleaching powder.

                        Ca(OH)₂ + Cl₂                          CaOCl₂ + H₂O

Properties:

                  Bleaching powder has following properties.

1.      It is a white amorphous solid with smell of chlorine gas.

2.      It dissolves in water and form chlorine gas.

                  CaOCl₂ + H₂O                          Ca(OH)₂ + Cl₂

3.      It reacts with CO₂ in the presence of moisture and form hypochlorous acid which is responsible for bleaching action of Bleaching powder.

                  CaOCl₂ + CO₂ + H₂O                            CaCO₃ + CaCl₂ + 2HOCl

                              HOCl                            HCl + [O]

4.      It reacts with acid and librates chlorine gas.

                  CaOCl₂ + 2HCl                         CaCl₂ + H₂O + Cl₂

Uses:

                  Bleaching powder has following uses.

1.      It is used for bleaching of cotton, linen and paper pulp.

2.      It is used for sterilization of drinking water.

3.      It is used for the preparation of chloroform.

Sodium Chloride

Occurance:

                  Sodium chloride is also known as common salt or table salt. It occurs as Rock salt. Large deposits of rock salt are found in Pakistan at Khewra. It also occurs in sea water to the extent of about 3%. The salt is mined as solid or pumped from under ground deposit as a saturated solution known as Brine.

Purification:

                  In tropical regions, sodium chloride is obtained by the solar evaporation of sea water; the impurities such as calcium and magnesium are removed by treating sea water with sodium carbonate and sodium hydroxide to precipitate these metals.

                  Ca⁺²Cl₂^-_(aq) + Na⁺²CO₃^-2_(aq)                           CaCO₃ + 2Na⁺Cl^-_(aq)

                  Mg⁺²Cl₂^-_(aq) + 2Na⁺OH^-_(aq)                           Mg(OH)₂ + 2Na⁺Cl^-_(aq)

            This purified product is fit for industrial use but requires further purification for its use as table salt.

Uses:

                  Sodium chloride has following uses.

1.      It is an essential part of our daily life.

2.      It is used as food preservative.

3.      In chemical industry it is used in the manufacture of sodium metal, chlorine gas, sodium hydroxide, sodium carbonate, sodium hypochlorite (I), sodium chlorate(V).

4.      It is used in glazing earthen ware.

5.      It is used in regeneration of water softener.

6.      It is used in salting out of soap.

Reasons

1)      Lithium and Beryllium markedly differ from other member of their family.

Reason:

Due to their small atomic size, charge density of Li⁺ ion and Be⁺⁺ ions are higher than their group members. These high charge densities result strong polarizing effect and high heats of hydration.

2)      First ionization enthalpies of Alkali metals and Alkaline earth metals are generally low. However, ionization enthalpies of IIA elements are higher than IA group elements.

Reason:

Alkali metal and Alkaline earth metals have one and two electrons in their valence orbits respectively. They loose their valence electrons to have the stable configuration of preceding noble gas. Hence 1^st ionization enthalpies of IA and IIA group elements are generally low.

Since each Alkaline earth metal has one extra proton than corresponding alkali metal. This higher nuclear charge attracts electrons more strongly; hence increase the ionization enthalpies of IIA group.

3)      Ionization potential decreases Lithium to Cesium.

Reason:

Down the group from Lithium to Cesium, atomic size increases due to increasing number of orbits. Therefore valence electrons, in heavier atoms of IA group, are loosely attracted by nucleus and hence low ionization energy is required to remove valence electrons.

4)      Alkali metals have larges covalent radii.

Reason:

Alkali metals have smallest nuclear charge in their respective periods and force of attraction of nucleus on valence electrons is weakest. Therefore they have largest covalent radii.

5)      Alkali metals and Alkaline earth metals easily form cations.

Reason:

Alkali metals and Alkaline earth metal, having large atomic size and low ionization potential, easily lose their valence electrons to attain the electronic configuration of preceding noble gas and form cations of +1 and +2 charges respectively.

            K                      K⁺ + e^-

      [Ar] 4s¹                  [Ar]

            Ca                    Ca⁺ + e^-

      [Ar] 4s²                  [Ar] 4s¹

            Ca⁺                   Ca⁺⁺ + e^-

      [Ar] 4s¹                  [Ar]

6)      Na⁺ ions are smaller than sodium atom.

Reason:

Sodium like other alkali metals looses its valence electron, forming Na⁺ ion, to attain stable electronic configuration of Neon. After removal of electrons third orbit is empty and in Na⁺ ion number of protons exceeds number of electrons. Therefore strong hold of nucleus over electrons causes decrease in size of sodium ion than sodium atom.

7)      Alkaline earth metal ions are more strongly hydrated than alkali metal ions.

Reason:

Hydration of ions depends upon charge density and ionic radii. Due to small ionic radii and more positive charge on ions of group IIA, strong electric field is produced around these ions. Therefore Alkaline earth metal ions are more strongly hydrated than Alkali metal ions.

8)      Li⁺ ions are more readily hydrated than K⁺ ions.

Reason:

Hydration depends upon charge density of the ion Lithium with small ionic radii, has high chare density as compared to K⁺ ion. Therefore Li⁺ ions are more readily hydrated than potassium ion.

9)      Alkali metals are powerful reducing agent.

Reason:

Substances having tendency to loose electrons are called reducing agent. Alkali metal wit their largest size and low ionization potential values, can easily loose their electrons.

            M                     M⁺ + e^- (M= Li, Na, K, Rb, Cs)

Therefore Alkali metals are powerful reducing agent.

10)  Li⁺/Li couple has exceptionally high negative electrode potential.

Reason:

Substances, having tendency to loose electrons, have highly negative values of standard electrode potential. Li⁺/Li couple has very high negative electrode potential value because high value of heat of hydration for Lithium eases the oxidation of Li and Li⁺.

11)  Alkali metal can not be used in voltaic cells.

Reason:

High negative values of standard electrode potential of Alkali metals indicate ease of oxidation. Since in voltaic cells water is used as solvent and Alkali metals are readily oxidized in water. Therefore they can not be used in voltaic cells.

12)  Alkaline earth metals are harder than Alkali metals.

Reason:

Alkali metals and Alkaline earth metals form positively charged M⁺ and M⁺⁺ ions respectively. Due to the greater charge attraction of metal ions and electron gas of Alkaline earth metal crystal, they are much harder than Alkali metals.

13)  In manufacturing of Sodium, the two electrodes are separated by Iron-gauze diaphragm.

Reason:

In manufacturing of Sodium, Sodium deposits on cathode in molten state and Chlorine gas is obtained on anode. Iron diaphragm is used to separate cathode from anode to prevent reaction between molten sodium and chlorine gas.

            2Na + Cl₂                     2NaCl

14)  Alkali metals are highly reactive.

Reason:

Since all Alkali metals have one electron in their valence shell. They have great tendency to loose their electron to attain electronic configuration of preceding noble gas. Therefore Alkali metals are highly reactive.

15)  Na⁺ ions are discharged at cathode in preference to H⁺ ion in the manufacturing of Sodium hydroxide.

Reason:

In the manufacturing of NaOH, Na⁺ ions are more easily deposited on mercury cathode.

                        Na⁺ + e^-                                    Na

                        Na + Hg                                   Na/Hg

Due to high voltage of H⁺ ion on the surface of mercury, Na⁺ ions are more easily deposited on mercury cathode.

16)  How the given reaction is avoided during preparation of Sodium hydroxide.

                        6OH^- + Cl₂                    ClO₃^- + Cl^- + 3H₂O

Reason:

In the preparation of NaOH, NaOH obtained on cathode and Chlorine gas on anode. To prevent above reaction floating mercury is used as cathode.

17)  Zinc hydroxide is soluble in excess of Sodium hydroxide.

Reason:

If NaOH in excess reacts with insoluble Zinc hydroxide as a result complex compound Tetrahydroxozincate(II) ion is formed which is soluble in water.

                        Zn(OH)₂ + 2OH^-                       [Zn(OH)₄]^-

                                                                        Tetrahydroxozincate(II)

18)  Plaster of Paris is used in making plaster coats and moulds.

Reason:

Plaster of Paris when mixed with water sets with expansion in few minutes. This setting with expansion property helps in use of Plaster of Paris in plaster coats and moulds.

Group Trends

Atomic Radii OR Atomic Size:

      It may be defined as,

            “The distance between the outer electrons and nucleus is called Atomic radii or Atomic size.”

Atomic size is measured by diffraction of x-rays through substance in solid state. It is measured in A^o (Angstrom).

                        1A^o = 10^-10 m = 10^-8 cm

Factors Affecting Atomic Size:

                  There are three factors which may affect the atomic size.

1)      Number of Shell:

Increase in number of shell will increase distance between outer electrons and nucleus (Atomic size). Therefore down in a group atomic size increases due to increase in number of shell in an atom. Greater increase in atomic size occurs due to increase in number of shell.

2)      Nuclear Charge:

Due to increase in nuclear charge the attraction between outer electrons and nucleus is increased, which will cause a decrease in atomic size. Therefore atomic size decreases across in a period with increasing nuclear charge. Small variation in atomic size of elements occurs due to variation in their nuclear charge.

3)      Screening Effect of Shielding Effect:         

Electrons in inner shells will tend to shield electrons in outer most shell from nucleus therefore effective nuclear charge is less than actual nuclear charge; this effect of inner electrons is called Screening or Shielding effect.

Shielding effect of inner electrons will weaken the force of attraction between outer electrons and nucleus therefore increases due to increase in inner electrons therefore atomic size increases down in a group.

Ionization Potential

Definition:

            It may be defined as,

                        "The minimum amount of energy required to remove one mole of electrons from one mole of gaseous atoms of the element to from one mole of gaseous cations is called Ionization Potential."

            Ionization potential is the measure of strength of electrostatic attractions between nucleus and outer electrons. Ionization potential is measured in KJ/mole.

            When first electron is removed from the atom the energy required is called 1^st I.P.

            When electron is removed form singly charged cation then the amount of energy is called 2^nd I.P.

Factors Affecting Ionization Potential:

            There are three factors on which ionization potential of elements.

1)      Atomic Size:

                  Increase in atomic size of elements will cause increase in distance between the nucleus and outer electrons. This wills weak the force of attraction and therefore removal of outer electrons becomes easier and ionization potential decreases. Down in a group ionization potential decreases with increasing atomic size. Hence element at the top of each group has highest value of I.P. and element at the bottom has least value.

2)      Nuclear Charge:

                  Increase in nuclear charge will cause stronger attraction between nucleus and outer electrons therefore ionization potential increases with increasing nuclear charge across in a period. Hence element at the left most in periodic table has least value of I.P. and element at the right most in periodic table has highest value of I.P.

3)      Screening Effect OR Shield Effect:

                  Electrons in inner shells will tend to shield electrons in outer most shell from nucleus therefore effective nuclear charge is less than actual nuclear charge; this effect of inner electrons is called Screening or Shielding Effect.

            Shielding effect of inner electrons will weaken the force of attraction between outer electrons and nucleus therefore increase in shielding effect decreases ionization potential. Down in a group shielding effect increases due to increase in inner electrons therefore ionization potential decreases down in a group.

Electro-Negativity

Definition:

                  "The ability of an atom to attract shared pair of electrons to itself is called Electro-negativity."

            Electro-negativity depends upon the electrostatic attraction between outer electrons and nucleus.

Factors Affecting Electro-Negativity:

            Electro-Negativity depends upon following factors.

1.      Atomic Size:

                  Increase in atomic size of elements will cause increase in distance between the nucleus and outer electrons. This wills weak the force of attraction and as result electro negativity decreases. Down in a group electro negativity decreases with increasing atomic size. Hence element on the top of each group has highest and element at the bottom has lowest electro negativity.

2.      Nuclear Charge:

                  Increase in nuclear charge will cause stronger attraction between nucleus and outer electrons therefore electro negativity increases with increasing nuclear charge across in a period. Hence element at left most of the periodic table has least value of electro negativity in that period and element at the right most has highest value.

Heat Of Hydration

Definition:

      "The process in which water molecules surround other substance is called Hydration."

                        "The amount of heat released during hydration is called Heat of Hydration."

            The heat of hydration depends upon the charge density of the ions present. More the charge density more will be the attraction of ions with water molecules, therefore ions having more charge density has more heat of hydration.

            Charge density of the ion depends upon the size of charge and volume of ions, since alkaline earth metal ions have more charge and less volume than alkali metal ions therefore alkaline earth metal ions more easily hydrated than alkali metal.

Electrode Potential

Definition:

            "The potential difference between metal and its salt solution is called Electrode potential."

            Electrode potential is the measure of the conversion of metal and its ion. Ease of conversion causes high electrode potential.

            Alkali metals and alkaline earth metals are strong reducing agent. The values of electrode potentials are used to predict the ease of conversion of metal into its ion. Electrode potentials of these metals are more negative because these metals undergo oxidation very easily.

            Electrode potential of Lithium couple (Li⁺/Li) has exceptionally high value of electrode potential because Lithium has very value of heat of hydration which promotes the oxidation of Lithium into its ion.

            The electrode potential decreases down the group due to increase in atomic size. The increase in atomic size will decrease ionization potential, due to which metal are easily into their ions.

Density, Melting Point & Boiling Point:

            Density, melting point and boiling point of a substance depends upon inter molecular attractions between their particles. Stronger the attractions more will be the density, melting point and boiling point.

            Density of elements in a group increases down in a group with increasing inter atomic attraction between their atoms in the crystal. Therefore element at the top of each group has least density and element at the bottom has highest density.

            Alkaline earth metals are denser than alkali metals due to the presence of M⁺² ions in their structure which causes greater inter atomic attraction.

            Melting and boiling point also depends upon inter atomic attractions. Down in a group melting and boiling point decreases due to decrease in inter atomic attraction.

            Melting and boiling point of alkaline earth metals are more than alkali metals because alkaline earth metals have more inter atomic attractions than alkali metals.

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