7. Notes of Solution and Electrolytes

Quality Education At Every level

 Chapter #07        SOLUTION AND ELECTROLYTES

Prepared by:

Lecturer  S.Fayyaz Hussain

A homogenous or single phase mixture of the molecules, atoms or ions of two or more components is called solution .The component that predominates in the solution is referred as solvent.

Solution may be classified into           

1) Electrolytic solution

2) Non-electrolytic solution        

CONCENTRATION UNITS OF SOLUTION:

1.      Molar Concentration ( Molarity)

The number of moles of solute dissolved per Litre/dm³ of solution is called molarity. It is denoted by "M"& read as molar.

Molarity = No. of mole of solute

               Volume of solution (dm³)

e.g. one molar H₂SO₄ solution contain 98g of  H₂SO₄ in one dm³ solution.

2.      Molal Concentration  (Molality)

The number moles of solute dissolved per kilogram of solvent is called molality. It is denoted by "m" & read as molal.

Molality = No. of moles of solute

                  Mass of solvent (Kg)     

 e.g. one molal solution of HCl solution contain 35.5g of       HCl in one kilogram of solvent  

 3.    Percentage Concentration

               

Concentration of a solution can also be determine by (%) percentage

For e.g. 10% (w/w) of KCl solution is prepared by dissolving 10g NaCl in 90g of solvent. 

THEORY OF IONIZATION:

This theory of ionization was proposed by Arrehenius in 1880 .Main postulates of this theory are given below.

i)              Electrolytes contain electrically charged particle called as ion. The positive ion is derived from metal (except H⁺) and negative ion is derived from non metal .All acids and base are believed to be electrolytes

ii)            When an electrolyte is dissolved in water , it dissociates into positive and negative ions

iii)          Electrolytic solution is as a whole “neutral” that is the net charges on the +ve and –ve ions must be equal

iv)          The appositively charge ions move freely in aqueous medium and combine during their collision , they form unionized molecules .Thus and equilibrium is established between ionized and unionized specie

v)            The degree of ionization depends upon the nature of electrolyte. Strong electrolytes ionized completely while weak electrolytes ionized partially.

vi)          On electrolysis, ions begin to migrate towards the opposite electrode, +ve ions move towards cathode and  –ve ions move towards anode where they become reduce and oxidizes to form neutral species.

vii)        The dissociation of electrolyte depends upon

                                 a) Nature of electrolyte       b) Temperature            c) Dilutions

viii)   The electrical conductivity depends upon the number of ions present in the solution and speed of ions

Application of Arrehenious Theory :

i)        Colligative properties such as lowering of vapors pressure, elevation of boiling point, depression of freezing point depends upon the no. of particles but not depends upon the nature of compound are explainable by Arrehenious theory. The ionization of an electrolyte in aqueous medium increase the no. of particles therefore colligative properties should effected

ii)      As electrolytic solution regarded as a mechanical mixture of solvent molecules and ions therefore hydration is explainable by Arrehenious theory of ionization.

HYDRATION:

The process in which water molecules surround and interact with the ions of an electrolyte is called “Hydration” and the ions is called hydrated ions.

            When an aqueous solution of a salt is evaporated the salt is crystallized with a definite no. of molecules

Is due to the attraction of positive ions for the –ve terminal of water molecules and negative ion for the +ve terminal of water molecules. In solution the number of water molecules which surround the ions is indefinite but on evaporation the salt crystallizes with précised no. of water molecules called water of crystallization.

            Some examples of hydrated salt are given below

            CuSO₄.5H₂O, MgCl₂.H₂O &  FeSO₄.7H₂O.

            A hydrate may decompose upon heating into anhydrous substance.

e.g       CuSO₄.5H₂O                                               CuSO₄    + 5H₂O

            The energy release during the hydration of one mole ions an electrolyte into water is called hydration energy and it is measured in kj/mol.

Relation Between Hydration And Charge Density

           

            The charge density is defined as

                        “Charge of ion per unit of ionic size”

                                    Charge density =  charge of ion

                                                                 size of ion

            The ability of an ion to be hydrated depends upon its charge density. ion with smaller size and greater charged would have greater charge density and strongly hydrated small +ve ions such as Cu2+ ,Al3+ possess great attraction for water molecules  because of greater charge and smaller size

HYDROLYSIS:

                       

            "The reaction of cation and anion in which water is dissociates & combined with the ions so as to change its pH is known as hydrolysis".   Hydrolysis is a chemical reaction between the ions of electrolyte and water molecules during which the pH of solution becomes change. This reaction is associated with weak electrolyte i.e. salts of weak acids and base.

Cation Hydrolysis

                        When cation of weak base interacts with water to give an acidic solution whose pH is less than 7 is called cation hydrolysis or acidic hydrolysis.

                        Ag⁺   + H₂O                               AgOH     + H⁺

                        Cu²⁺  + 2H₂O                              Cu(OH)₂ + 2H⁺

Anion  Hydrolysis

           

            When an ion of weak acid interacts with water molecules to give an alkaline solution whose PH is more than 7 is called anion hydrolysis or basic hydrolysis.

                        CH₃COO^-  +  H₂O                          CH₃COOH  +  OH^-

                        HCO₃^2-     +  2H₂O                          H₂CO₃       +  OH^-

ELECTROLYSIS:

            When electric current is passed through the electrolytic solution, movement of ions towards their respective electrodes take place where they are eventually discharged.

                        "The movement of cat ion and anion towards their respective electrodes accompying all chemical change in the electrolytic solution under the influence of electric current is known as electrolysis".

            During the process of electrolysis cation discharged on cathode by the absorption of electron and this process is called Reduction. Similarly anion discharged on anode by the electron and called Oxidation.

Importance of electrolysis

            Conduction of electric current through a solution plays important part in industrial applications.

i)                    Electrolysis of Gold, Silver, Zinc, and extensively use for metallic substance.

ii)                  Electrolysis refining of some metals like Al, and Cu, is Carried out by electrolytic conductance.

iii)                Extraction of certain metals from their areas takes place through electrolytic process.

Oxidation Number:

“The formal of apparent positive or negative charge which an atom would have in compound is known as oxidation number”.                                                                                                                                                                     

       Using the concept of oxidation number, all the compound are imagined to be completely ionic and the term formal charge means, not the actual charge of atom in the molecule or ion.

               The determination of oxidation number of an atom is followed by following general rules.

i)                    The oxidation number of any element in the Free State is zero for e.g.  H₂, Cl₂, N₂, Br₂ and O₂  have zero oxidation number

ii)                  The oxidation number of hydrogen in compound is 1+ except in hydride such as NaH where it is 1-

iii)                The oxidation number of oxygen in compound is 2- except in peroxide

iv)                The oxidation number of each element of group VIIA, halogen in binary compound is 1-

v)                  The oxidation number of each element of group IA , IIA and IIIA  is 1+ ,2+ and 3+

vi)                The algebraic sum of oxidation number of all atoms in the formula of a neutral compound equal zero

vii)              The oxidation number of any ion is equal to the charge on the ion  

OXIDATION:

                        An increase in the oxidation number of an element or ion during a chemical change or loss of electron in a chemical reaction is called oxidation

e.g                   Zn                                Zn²⁺  +  2e^-

REDUCTION:

                        A decrease in the oxidation number of an element or ion during a chemical change or gain of electron in chemical reaction is called reduction

eg.                   Cl   +1e^-                                     Cl^1-

OXIDIZING AGENTS:

                                    An atom or ion which picks up electron and show decrease in oxidation number is called an oxidizing agent or those which easily reduce (gain of electron) and oxidized other is known as oxidizing agent.

eg.   Cu  + HNO₃                                   Cu(NO₃)₂  + NO₂  +H2O  

                        Here HNO3 is oxidizing agent

REDUCING AGNETS:

                                    An atom or ion which loss electron and show increase in oxidation number is called on reducing agent

e.g.                  H₂S    +Br₂                                         2HBr   + S

                        Here H₂S is reducing agent.

Oxidation Reduction Reactions

                                    Such reaction which is brought about by loss of electrons (oxidation) and gain of electron (reduction) simultaneously is called an Oxidation Reduction reaction or Redox reaction.

                        The Redox reaction can be considered as made up of two of reaction

                                    

                        2Na  + Cl₂                                   2NaCl

here

                        2Na                                         2Na⁺               +2e^-

                        Cl₂    +2e^-                                                    2Cl^-

DEGREE  OF IONIZATION:

                                    It is the ratio of no. of dissociation particles and the total no. of particles

                                    α = No. of dissociated particle

                               Total no. of particle

            Strong electrolyte (HCl, NaOH, NaCl) have high degree of ionization and weak electrolyte have low degree of ionization. The conductance depends upon degree of ionization of electrolyte. Greater is the degree of ionization greater the ionization.

ELECTRODE POTENIAL:

                               "The difference of potential created between a metal and solution of its salt is called electrode potential of metal".

                   It is the measurement of tendency of an electrode to loose or gain electron or the measurement of chemical activity at cathode and anode in its electrolytic solution. It is denoted by E and measures in volts.

Standard Electrode Potential (^oE)

                   A metal electrode which is held in 1 molar solution of it electrolyte at 250^OC and 1 atmos pressure is called standard electrode and its potential is called as standard electrode potential.

                   Electrode potential may be taken as oxidation potential and reduction potential.

                   The ability of anode to lose electrons and undergo oxidation is called oxidation potential where as ability of cathode to gain electron and undergo reduction is called reduction potential.

                  

                   The standard oxidation and reduction potential of an electrode remains the same but with the opposite signs.       

                   E_Red(reduction) = -E_0x (oxidation)

                  

                   In 1953 standard reduction potential was decided to takes as standard electrode potential.

                  

Standard Hydrogen Electrode:

                   It is a reference electrode and uses to determine the electrode potential of other metal.

                   It consists of a platinum foil coated with a layer of platinum and connected to a platinum wire in a glass sleeve, with hydrogen gas passing through 1 atmos pressure. It is than immersed in 1M of HCl or 0.5 M H₂SO₄ solution. The reduction potential of hydrogen electrode was decided to be 0.00v.

Determination Of Electrode Potential Of Zinc:

                   The electrode potential of zinc can be determine by reference electrode (i.e. hydrogen electrode)

                   For this purpose we have to construct a voltaic cell up of Zn and Hydrogen electrode.

                   The first half cell consists of platinum foil immersed in 1M HCl solution and H₂gas passes through the solution at 1 atmosphere pressure.

                   Salt bridge which is made of KCl jelly completes the circuit between the two half cells but prevents the mixing of solution .Two electrode are connected to a potentiometer to note the emf of the cell.

Direction of Flow of Electrons

                   In external circuit it is observed that the flow of electrons take place from zinc to hydrogen electrode and therefore zinc act as anode and electrons must have originated at zinc i.e. it is oxidized.

Cell Reactions

                               Zn                                Zn²⁺  +2e^-   (anode)

                   2H⁺  +2e^-                                    H₂               (cathode)

Emf of Cell

The emf of cell as noted by potentiometer is –o.76v

                   Emf =E_R (cathode) +E_O (anode)

                   0.76=0.00     +   E_O (Zn)

                   0.76v= E_O (Zn)

Determination Of Electrode Potential Of  Copper :

                  

                   The electrode potential of can also determine by Hydrogen as reference electrode.

                   Voltaic cell which is constructed for this purpose consist of copper and hydrogen electrode.

The first half cell consists of a strip of copper immersed in “1 M” of CuSO₄ solution while the other half cell consists of platinum foil immersed in 1M of H₂SO₄ solution and H₂ gas passes through the solution of 1 atmospheric pressure.

                   The two half cells connected through KCl salt bridge and the two electrode join through potentiometer.

Direction Of Flow Of Electrons:

                   In the external circuit it is observed that the direction of flow of electrons from hydrogen to copper electrode therefore hydrogen act as anode and copper electrode therefore hydrogen and oxidation takes place.

Cell reactions:

                               H₂                          2H⁺  +2e^-     (anode)

                               Cu²⁺   + 2e^-               Cu           (cathode)

Emf of cell:

                   The emf cell as noted by potentiometer is 0.36V.

Therefore              emf = E_o (anode) +E_R (cathode)

                                           0.36 =   0.0      + E_R     (cathode)

                            036V = E_R (copper)

Electrochemical Series:

                        This is an arrangement of electrode potential values according to their increase order of reducing property. The important facts of electrochemical series are given below.

i)          The electrode potential of hydrogen is zero and it is taken as reference electrode

ii)        Metal above hydrogen in ECS undergo oxidation and act as anode in cell

iii)      Metal below hydrogen in ECS undergo reduction and act as cathode. The order of reducing strength decrease and oxidizing  strength increase from top to bottom

iv)      All metal above hydrogen –ve electrode potential bad below hydrogen +ve electrode potential.

PH:

       PH is a scale of measurement of strength of hydrogen ion concentration in a aqueous solution. It defines as.

                   “The negative logarithm of hydrogen ion concentration is called as pH”

                               PH = -log [H⁺]

                               PH =    1         

                                           Log [H⁺]         

                   The PH concept is very convention for expressing hydrogen ion concentration. It was introduce by Sorenson in 1990.it is now used as a general way of expressing other quantities also.

                   For example, Concentration of OH^- in aqueous solution of a base is expressed as POH = -log [OH^-]

                  

                   The PH value of difference solutions was computed on a scale taking water as a reference substance. This is called as pH scale.

                   PH scale ranges from 0 to14 and solution whose PH is 7 are neutral. All substance having pH less than 7 are called acids and those whose pH is more than 7 are called base.

Measurement of PH:

                   PH of a solution is determined by using universal indicator. The pH numbers related to universal indicator color are given below.

Red Orange yellow green blue deep purple                                                                            blue

  0          3         6            7              8        10         14

                   For more precise measurement, PH meter are available which gives a direct reading on meter.

                  

BUFFER SOLUTION:

“A solution which tends to resists to the change in the pH even upon the addition of small amount of acid or base is called buffer solution”

                                                       OR

“Solution which exhibiting the property of opposing a change in their pH are called buffer solution of simply buffers and their properties are known as buffer action”

                   A buffer solution resists to the change in its pH. If we add small amount of acid or base to a buffer solution the pH will change very little.

Kinds Of Buffer Solution:

                   There ere two common types of buffer solution.

 1.   Buffer solution of weak acid with its strong basic salt.

                   e.g.[CH₃COOH + CH₃COONa]

2.      Buffer solution of weak base with its strong acidic salt.        e.g. [NH₄OH + NH₄Cl]

Buffer Action:

                        Consider the equilibria of buffer e.g. acetic acid.

                        CH₃COOH                        CH₃COO^- + H⁺

                        CH₃COONa                     CH₃COO + Na⁺

Addition of HCl:

                        Upon the addition of H₂CO₃ the increase of H⁺ ion is association with the access of acetate ion to form ionize CH₃COOH.Thus the added H⁺ ions are neutralized and the pH of the buffer solution remains virtually uncharged.

Addition Of NaOH:

                        When NaOH is added to the buffer solution the addition of OH negative ion combine with H⁺ ion of the buffer to form water molecules as a result of equilibrium shifts to the right to produce more sand more H⁺ ion till practically all the excess OH^- ions are neutralized and the original PH restore.

Applications Of Buffers:

1. Human blood is the buffer of carbonic acid in conjunction with protein molecules.        

Its PH remains 7.3.

2. Buffer solutions are used in analytical researched labs.

3. These are used in pathological labs and pharmaceutical industries.

4. These are used in agricultural to maintain the PH of soil.

5. These are used in food industries.

INDICATIORS:

                       

                        “Indicators are complex organic molecules and chemically weak acids or bases”

                        indicators are generally used to determine the end  point of a reaction e.g. the neutralization of acid and base is monitored by indicator i.e. phenolphthalein and methyl orange some common indicators along with their colors changed are listed below.

Medium methyl orange, phenolphthalein & universal indicator

 Acid           red                    colorless                  red

 Base          yellow                   pink                     purple

 Water        orange                 colorless                green

Ostwald’s Theory Of Indicator:

                                    The behavior of an indicator is explained by Oswald theory. According to this theory ions and unionized molecule of an indicator passes different colours.

Methyl Orange:

                        Methyl orange is a week base and represented by MeOH.

                        MeOH                        Me⁺  +  OH

                       (yellow)                       (red)

            The un dissociated molecule is yellow and Me⁺ ion is red. Addition of acid forces the reaction towards right (as the H⁺ ion combines OH^- to give water) and the color of solution became red. On adding alkali, OH^- ion are added which shift the equilibrium to the left hence the solution becomes yellow.

PHENOLPHTHALEIN:

                        Phenolphthalein is a week acid and its ionization is given below.

                                    HpH                          H⁺ + PH

                               (Colorless)                      (Pink)

                        There is increase in the concentration of H⁺ by adding acid to the solution. Shift the equilibrium towards the colorless “HpH”. The addition of OH^- ion forced the reaction in the forward direction as OH ion combines with H+ ion to give water. Hence the solution becomes pink.

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