CHAPTER NO 3

CHAPTER NO 3

COMPOSITION OF WATER

Water molecule composed of two Hydrogen atoms and one Oxygen atom. Its molecular formula is H2O.

Experimental verification

   Composition of water can be determined by using an apparatus called "Hofmann voltameter" .

Construction of Hofmann voltameter

   The voltameter consists of two glass tubes of equal volume.

   Each glass tube is fitted with a platinum plate .

   One platinum plate is connected to positive terminal of power supply and it is called "Anode".

   The other plate is connected to negative terminal of power supply and it is called "Cathode".

   Glass tubes of voltameter contain pure water.

Experiment

When an electric current is passed through water containing a little salt, water decomposes into    hydrogen gas and oxygen gas. Hydrogen is collected at cathode. Oxygen is collected at anode.

Observations

   The tube on left side contains two volumes of H2 while the other tube has one volume of O2 .

Conclusion

   This experiment shows that :

Composition of water by Volume

   Water composed of two volumes of hydrogen and one volume of oxygen.

Composition of water by parts

   Hydrogen =1 part by mass.

   Oxygen = 8 parts by mass.

Composition of water in percentage Composition

   Hydrogen =11.11 %

   Oxygen = 88.89 %

Molecular Formula of Water

   In the light of above facts it is concluded that the molecular formula of water is H2O.

ELECTROLYTES - ELECTROLYSIS - ELECTROLYTIC CELL

 ELECTROLYTES

   Substances, which ionize in aqueous solution into positive and negative ions and conduct electricity are    known as ELECTROLYTES . Electrolytes also conduct electricity in molten state.
   Example:
   NaCl , HCl , HNO3, H2SO4, KCl , CuSO4 , ZnSO4 etc.
TYPES OF ELECTROLYTES
   There are two types of electrolytes:
   1. Strong electrolytes.
   2.  Weak electrolytes.
STRONG ELECTROLYTES
   Compounds that ionize to greater extent in aqueous solution and consequently conduct large amount if    electric current are known as STRONG ELECTROLYTES.
   Example:
   NaCl , KCl , CuSO4 , ZnSO4 , KOH etc.
WEAK ELECTROLYTES
Compounds that ionize to very small extent in aqueous solution and consequently conduct very small    amount of electric current are known as WEAK ELECTROLYTES.
   Example:
   Acetic Acid (CH3COOH), Benzoic Acid (C6H5COOH),
NON-ELECTROLYTES
   Substances, which do not ionize in aqueous solution into positive and negative ions and hence do not    conduct electricity are known as NON- ELECTROLYTES . They are covalent compounds and mainly    organic in nature.
   Example:
   Urea, Benzene, Sugar, Ethanol, Chloroform , ether etc.
ELECTROLYSIS
   The chemical change caused by the passage of an electric current through a solution of an electrolyte is    called "Electrolysis".
OR
   A process in which an electric current is passed through the solution of an electrolyte and due to    passage of electric current, positive and negative ions move towards their respective electrodes is called    "Electrolysis".
ELECTROLYTIC CELL
The process of electrolysis is carried out in a cell which is known as "Electrolytic cell".
   An electrolytic cell consists of a container (or beaker) that contains an electrolytic solution.
   The cell contains two conducting plates called electrodes.
   These electrodes are connected to the terminals of D.C. power supply.
   The electrode connected to positive terminal of power supply is called anode.
   The electrode connected to negative terminals of power supply is called cathode.
   Electric current enters through anode and leaves from cathode.

ELECTROLYSIS OF COPPER CHLORIDE

 Copper chloride is ionized in water as follows:

CuCl2 èCu+2 + 2Cl-1

   When an electric current is passed through the aqueous solution of CuCl2, Cu+2 ions migrate towards    cathode and Cl-1 ion towards anode.

REACTION AT CATHODE

Cu+2 + 2e- èCu (Reduction)

REACTION AT ANODE

2Cl-1è Cl-1 + 2e- (Oxidation)

 OVER ALL REACTION

Cu+2 + 2e-è Cu

2Cl-1 è Cl2 + 2e- 

..........................................

Cu+2 + 2Cl-1 è Cu + Cl2

RESULT

   Copper deposited on cathode.

   Cl2 gas liberated trough anode

ELECTROLYSIS O F WATER

   Water is a weak electrolyte because it conducts very little amount of electric current. In order to    electrolyze water , a little amount of an electrolyte (salt or acid) is added to water, which makes it an    electrolyte. Generally sodium sulphate as added.

IONIZATION OF WATER

H2O è H+ + OH-1

   When an electric current is passed through water H+ ions are deposited at cathode and OH- ions are    deposited at anode.

REACTION AT CATHODE

4H+ + 4e- è 2H2 (Reduction)

REACTION AT ANODE

4OH-è 2H2O + O2 + 4e- (Oxidation)

RESULT

2H2O è 2H2 + O2

SPECIFIC HEAT

   Specific heat of a substance is defined as:

   "The amount of heat energy required to increase the temperature of 1gm of the substance through    1 degree centigrade".

   It is denoted by "C".

   Unit of specific heat is Calorie/gram.k

   Formula:

LATENT HEAT OF FUSION

   It is defined as :

    "The amount of heat energy required to melt 1 gm of a solid substance at its melting point".

   It is denoted by Hf.

   Unit of latent is calorie/gram.

   Formula:

LATENT HEAT OF VAPORIZATION

Latent heat of vaporization is defined as "The amount of heat energy required to vaporize one gram of a    liquid at its boiling point ".

   It is denoted by Hv.

   Unit of latent is calorie/gram.

   Formula:

HEAVY WATER

Water consists of deuterium (D) isotope of hydrogen is called heavy water. Formula of heavy water is    D2O. It has same chemical properties as that of ordinary water but differ slightly in physical properties

   It has high density than H2O.

   Its freezing point is 3.97 C.

   It has low vapor pressure than ordinary water.

   It has smaller refractive index than ordinary water.

   It is used as moderator in nuclear reactors to slow down neutrons.

   It is used as Tracer in chemical and biological research.

   Molecular Mass:2 x 2 + 16 = 20 amu.

SOLUBILITY
"The solubility of a solute in a solvent at a particular temperature is the number of grams of the solute necessary to saturate 100gm of the solvent at that temperature."
FACTORS AFFECTING SOLUBILITY
There are three main factors that control solubility of a solute.    (1) Temperature    (2) Nature of solute or solvent    (3) Pressure
EFFECT OF TEMPERATURE
Generally in many cases solubility increases with the rise in temperature and decreases with the fall of    temperature but it is not necessary in all cases. However we must follow two behaviors:
In endothermic process solubility increases with the increase in temperature and vice versa.
For example:solubility of potassium nitrate increases with the increase in temperature.
In exothermic process solubility decrease with the increase in temperature.
For example:solubility of calcium oxide decreases with the increase in temperature.
Gases are more soluble in cold solvent than in hot solvent.
NATURE OF SOLUTE AND SOLVENT
Solubility of a solute in a solvent purely depends on the nature of both solute and solvent.    A polar solute dissolved in polar solvent.    Solubility of a non-polar solute in a solvent is large.    A polar solute has low solubility or insoluble in a non-polar solvent.
EFFECT OF PRESSURE
The effect of pressure is observed only in the case of gases.    An increase in pressure increases of solubility of a gas in a liquid.    For example carbon di oxide is filled in cold drink bottles (such as coca cola, Pepsi 7up etc.)under    pressure. HARD WATER "Water that produces curd with soap and produces very little lather is called hard water. Hard water affects cleaning ability of soap."    Hard water contains bicarbonate, chlorides and sulphates ( any one or all ) of calcium and magnesium . SOFT WATER   "Water that produces lather with soap easily is called soft water. Soft water does not affect cleaning action of soap" TYPES OF HARD WATER      There are two types of hard water.    1. Temporary hard water    2. Permanent hard water TEMPORARY HARD WATER    "Water that contains bicarbonate of calcium and magnesium or of both is called temporary hard water."    These bicarbonate are soluble in water and produce corresponding ions. Mg(HCO3)2 è Mg+2 +2HCO3-1 Ca(HCO3)2 è Ca+2 + 2HCO3-1 PERMANENT HARD WATER      Water that contains chlorides or sulphates of calcium or magnesium or of both is called Permanent water. CaCl2è Ca+2 + 2Cl-1 MgSO4 è Mg+2 + SO4-2    These compounds are soluble in water and produce corresponding ions. REMOVAL OF HARDNESS OF WATER      Hardness of water can be removed by the following two methods: CLARK'S METHOD    In Clark’s method a calculated amount of calcium hydroxide Ca (OH)2 is added to hard water. Due to    reaction, insoluble carbonates are obtained which are separated by filtration. Ca(HCO3)2 + Ca(OH)2 è 2CaCO3 + 2H2O Mg(HCO3)2 + Ca(OH)2 è CaCO3 + MgCO3 + 2H2O ION-EXCHANGE METHOD      In this method sodium zeolite is used as ion-exchanger. Na-zeolite is passed through the pipes containing    hard water. Sodium zeolite is converted into calcium-zeolite or magnesium-zeolite. These are insoluble in    water and are separated from water by filtration. Ca+2 + Na2-zeolite è Ca-zeolite + 2Na+1 Mg+2 +Na2-zeolite è Mg-zeolite + 2Na+1 DISADVANTAGE OF HARD WATER    Domestic:    Hard water affect cleaning ability of soap.    When hard water is used for washing, large amount of soap is consumed.    Industrial:    Hard water can cause "Scaling" inside the pipes that transport water . Therefore if we use hard water in    turbines and heat exchangers, their pipes will be corroded    Health:    Hard water when used for drinking for long period can lead to stomach disorders. Especially hard water    contains magnesium sulphate can weaken the stomach permanently CRYSTALLIZATION    The process by which we prepare crystals of a substance is called crystallization.    In the process of crystallization following steps are involved.   Preparation of solution. Filtration. Crystal formation. (Cooling) Drying of crystals PURIFICATION OF SUBSTANCES    Substances can be purified by crystallization. STEPS OF PURIFICATION     Preparation of solution      A definite amount of given substance is dissolved in a specific amount of water in a beaker to prepare    aqueous solution of substance. The beaker is heated to dissolve maximum amount of solute. The solution    must be saturated. Filtration of solution    In second step, solution is filtered while hot. The insoluble impurities are separated. Crystal formation      The filtered solution is cooled to produce crystals of substance. Drying of crystals      Crystals so obtained are wet. They are dried by solar heat or by placing between the paper folds to    remove moisture. SOLUTION "A homogenous mixture of two or more than two substances is called "Solution."    There are two components of a solution.    Solute.    Solvent. SOLUTION = SOLUTE + SOLVENT SOLUTE    Component of a solution which is in small quantity is called "Solute". SOLVENT    Component of solution which is in large quantity is called "Solvent". SATURATED SOLUTION    A solution that can hold no more of the solute at a particular temperature is said to be a saturated    solution at that temperature. UNSATURATED SOLUTION    An unsaturated solution is a solution, which contains less amount of solute than is required to saturate it    at that temperature. SUPER SATURATED SOLUTION    Solution that is more concentrated than a saturated solution is known as super saturated solution. If a    crystal of solute is added to this solution, the excess of solute crystallizes. AQUEOUS SOLUTION    A solution of any substance in which solvent is water is called "Aqueous solution". CONCENTRATION OF SOLUTION    Concentration of solution is defined as the amount of solute dissolved in a specific (fixed) amount of    solvent. UNITS OF CONCENTRATION    Molarity    Molality    Normality    Percentage composition    Mole fraction MOLARITY Molarity is defined as the number of moles of solute dissolved in one litre (one dm3) of solution.    For example:    If 2 moles of a solute are present in one litre of a solution then the molarity of solution will be (2M). FORMULA Molarity = number of moles of solute /volume of solution in litre OR Molarity = mass of solute (in grams) /molecular mass x volume of solution in litre PERCENTAGE OF COMPOSITION OF SOLUTION    Concentration of a solution can be expressed in terms of percentage composition. It is based on the             weight or volume of the components of solution. Weight / Weight %    2% w / w solution means grams of solute is dissolved in 100 grams of solution. Weight / volume %    4% w / v solution means 4 grams of solute is dissolved in 100 ml of solution. Volume / weight %    3% v/ w solution means 3 ml of solute is dissolved in 100 grams of solution. Volume / volume %    5% v / v solution means 5 ml of solute is dissolved 100 ml of solution.