Acid, Base and Buffer

  Acid, Base and Buffer

·         Acid :-

The substance called as acid usually have a sour taste. It aqueous solutions turns the litmus paper red and has pH lower than 7.

·         Base :-

Base has a bitter burning taste and aqueous solutions turns the litmus paper blue and haspH higher than 7.

·        ArrheniusTheory :-

-         According to the Arrhenius theory in acid or base on dissolution in water dissociates forming ions and established equilibrium between ionised and unionised molecule.

-         Arrhenius defined acid is a substances which dissolves in water gives H+ ions ( hydrogen ions).

-         E.g.HClrevrciblewith H+ + Cl-

-         Base defined as the substance which when dissolve in water gives OH- ions ( hydroxide ions).

-         E.g.NaoHrevrcible with Na+ + OH-

-         Acid and base undergoes a reaction known as neutrilisation.  The neutrilisationreaction result in forming water and salt.

-         E.g.HCl+ NaoH it givesNaCl+ H2O.

-         Limitations of Arrheniustheory :-

1)      The definition of acid and base are only in terms of aqueous solutions and not in terms of substances.

2)      The theory does not explain acidic and basic properties of a substances in non-aqueous solvent.

3)      The neutrilisation of acid and base in absence of solvent is not explain.

4)      The basicsubstances which does not contain hydroxyl ions is not explain by the theory.

·        Lowry&BronstedTheory :-

-          According to Bronsted and Lowry  theory acid is a substance that can donate a proton and base is a substance that can accept a proton.

-          Thus, acid is a proton donar and base is a proton acceptor.

-          As per theory,

Acid = H++ conjugate base

e.g., HClit gives H++ Cl-

-          HCl is an acid and Cl- is a base such a pair of acid and base which differs only by a proton is known as a conjugate acid, base pair.

·        LewisTheory :-

-          According to the Lewis theory acid is a molecule that can accept a lone pair of electrons and base is a molecule that can donate a lone pair of electrons

-          E.g  H+ +:NH3 it gives NH4

-          H+ accept a pair of electron from NH3 thus, according to theory H+ is Lewis acid and NH3 is thelewis base.

-          According to this concept acid base reaction involves the formation of a co-ordinate covalent bond.

E.g. a reaction between amonia& boron trifluoride is as flows

BF3 + NH3 it givesF3B : NH3

·        BufferSolution :-

-          Thesolution that resist the changes in pH values are called buffer solution.

1)      Acidic buffer solution.

2)      Basic buffer solution.

1)    Acidicbuffersolution :-

The solution containing a mixture of weak acid ( e.g. acetic acid ) and its salts ( sodium acetate ) is known as acidic buffer solution.

2)    Basicbuffersolution :-

The solution containing a mixture of weak base ( ammonia ) and its salt ( ammonium chloride ) is known as basic buffer solution.

·        Propertiesofbuffersolution :-

1)      ThepH of buffer solution is constant.

2)    The pH of solution does not change on dilution.

3)    The pH does not change even after addition of small quantities of acid and base.

4)    As the pH of solution remains constant it is useful in no. of chemical reaction.

·        DefinitionBuffercapacity :-

Buffer capacity is defined as the moles of strong acid or strong base required to change the pH of 1 litter of buffer solution by 1 unit .

1] Hydrochloric acid ( HCl ) :-

Preparation :-

-          It is preparation of hydrogen chloride in a water.

-          It is prepared by the addition of sulphuric acid and sodium chloride. Both are heated in cast iron pans and HCl gas is passed in a tower which is spread with water. The dilute HCl which is collected at the bottom is again circulated at tower to absorb more hydrogen chloride gas till saturated. The acid produced is then purified. NaHSO4 form in the process is mixed with some more contiaty of Nacl and heated strongly obtain more HCl.

-          H2so4 + Naclthen heated and gives NaHSO4 + HCl

-          NaHso4 +Nacl then heated and gives Na2SO4 + HCl

Muriatic acid :-

·        Properties :-

1)      It occures has colourless, from fuming liquid.

2)      It has pungent odourless.

3)      It is missible with water and alcohol.

·        Uses :-

1)      It is used as the pharmaceutical aids.

2)      The dilute HCl is used as acidifying agent.

3)      It is also used as a solvent in many industries.

4)      In compatibility alkalis, metal in oxides silver, mercury and lead.

·        Storage :-

It is stoaredin a glass stoppered container at a temperature not more than 30°c.

2] Storageammoniasolution (NH3) :-

It is also called as 'liquor ammonia fortis'.

·        Preparation:-

         It is prepared by passing ammonia gas into cool water until the water is saturated with ammonia.

·        Properties :-

1)      It is clear colourless liquid having strong pungent odour ( spicy odour ) with a characterised test.

2)      It is missible with water and alcohol.

3)      When ammonia gas is passed over the heated oxides of metal, nitrogen gas is evolved.

·        Uses :-

1)      The dilute solution of ammonia is common labreagent.

2)      It is used as antacids, counter erritant and stimulant.

3)      It is useful in the manufacture of nitric acid and sodium bicarbonate.

·        Incompatibility :-

It is incompatible with iodine, heavy metals, silver, Yannis or mercury.

·        Storage :-

                It is stored in well closed container in cool place.

3] Sodium Hydroxide ( NaoH ) : caustic soda :-

·        Preparation :

It is prepared by sodalime process. In this method, Na2Co3 is heated with milk of lime, Ca(OH)2 in a large tank. The mother liquor is removed, filter and evapour . The mass form is moulded into flocks palleres or sticks.

                         Na2Co3 +Ca(OH)2 it gives 2NaOH + CaCO3

·        Properties :-

1)      It occur as white mass and is odourless.

2)      It is very deliquescent.

3)      It obsorbCO2 and gets partialy converted into sodium bicarbonate.

4)      It is soluble in water, freely soluble in alcohol and glycerine with the evolution of heat.

5)      It is very strong base and it is very caustic.

·        Uses :-

1)      It is useful as a common lab reagent.

2)      In industry, it is used in the preparation of soap.

·        Storage :-

        It is stored in tightly closed container as it absorbs moisture and CO2.