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STOICHIOMETRY

         Chemistry is the science of atoms, molecules and their transformations. Chemistry deals with the composition, structure, properties of matter. It plays an important role in daily life. Matter is made of atoms, molecules and exists in three physical states. Matter can be classified into mixtures (homogeneous or heterogeneous) or pure substances (elements or compounds). The combination of elements to form compounds is governed by the following basic laws (of chemical combination).

The Law of conservation of mass:
        This law was given by Antoine Lavoisier. This law states that "matter can neither be created nor destroyed". Total mass of the products formed during a chemical reaction is exactly equal to the total mass of the reactants. This law is basis for several later developments in Chemistry.
                                  e.g.:  2 H2 + O2 2 H2O
                                            4 g      32 g       36 g


The Law of definite proportions:
       This law was proposed by Joseph Proust. He states that "A chemical compound always contains definite proportion of elements by weight (irrespective of the source). For example, CuCO3 found in nature and prepared in the laboratory contain the same percentage of composition i.e., 51.35% Cu, 9.74% O and 38.91% of C.
The law of multiple proportions:
        This law was put forth by John Dalton. According to this law "If two elements combine to form more than one compound, the masses of one element that combine with a fixed mass of the other element are in a simple multiple ratio".
                               e.g:   H2 +  O2 H2O
                                          H2  +  O2  H2O2
Here, the masses of Oxygen is 1 : 2 (16 g : 32 g) which combine with fixed mass (2g) of Hydrogen in both the H2O and H2O2.


Gay Lussac's law of gaseous volumes:
    This law was proposed by Gay Lussac. According to him "At constant pressure and temperature, gases combine (or are produced) in a Chemical reaction, are in simple ratio by volume". This law is similar to the law of definite proportions.
                             e.g.: 2H2     +    O2      2 H2O
                                     20 ml        10 ml          20 ml
         the ratio of H2 : O2 : H2O is 20 : 10 : 20 or 2 : 1 : 2.


Avogadro's law:
         This law was proposed by Avogadro in order to distinguish between atoms and molecules. It was stated as "At constant pressure and temperature, equal volumes of gases should contain equal number of molecules (or moles)".
          According to Dalton's atomic theory, atom ('a-tomio' means indivisible) is "The smallest indivisible particle of an element that can take part in chemical reaction". Today we could able to determine accurate atomic masses by using mass spectrometry. But in 19th century, due to lack of today's techniques they determined relative atomic mass by taking mass of H as a reference.

It is the ratio of mass of one atom of an element to the mass of th of C-12 atom. th part of Carbon-12 is known as atomic mass unit (a.m.u.), which is equal to 1.66 × 10-24 g. In the periodic table, average atomic masses of the elements are mentioned. If atomic weight is expressed in grams, it is called gram atomic weight or "gram atom". 1 gram atom of Sulphur means 32 grams. Atomic Weights of elements are determined with the help of Dulong and Petit's law (Atomic Weight =

) This law is applicable to solid metals.
       "The smallest particle of an element or compound that can exist freely (Can't take part in chemical reaction) is called a "molecule". The ratio of weight of one molecule of the substance to the weight of th of Carbon-12 atom is called "molecular weight". The mass of one mole of any substance in grams is called "molar mass". Quite commonly, we use the Unit dozen to represent 12 articles. Same way we use Unit mole in chemistry to represent 6.022 × 1023 entities (atoms / molecules / ions/ electrons etc).


Mole Concept:
       The term mole was introduced by Ostwald. "The amount of a substance that contains as many entities as there are atoms in exactly 12 grams of C - 12 isotope'' is called mole.

The exact value of (number of entities) in one mole is 602213670000 0000 0000 0000
(or 6.022 × 1023).
                e.g.: 1 Mole of oxygen atoms = 6.022 × 1023 oxygen atoms
                        1 Mole of oxygen molecules = 6.022 × 1023 oxygen molecules
Gram molar volume: The volume occupied by one gram mole of any gas at S.T.P
                             1 G. M.V. = 22.414 litres
                                     = gram molecular weight
                                     = Volume occupied by Avogadro number of entities


Avogadro number:
         Number of entities present in one gram mole is called Avogadro number (N).
         N = 6.022 × 1023 entities.
         Reciprocal of N, i.e.  is called Avogram
         1 Avogram =   = 1.66 × 10-24 grams.
      For example 2 grams of H2 is 1 Mole, that occupies 1 G.M.V. (22.4 lit) that consists
6.022 × 1023 molecules of H2.


Equivalent weight concept:
        "Number of parts by weight of the substance that combine (or displace) directly (or indirectly) with 1.008 parts of H or 8 parts of oxygen or 35.5 parts of Cl is called equivalent weight. Equivalent weight of an element or radical is equal to its atomic weight or formula weight divided by valency it assumes in a compound. It is a number, but not a weight. It need not necessarily be a fixed value, it is variable (Eq. wt. of Fe+2 = = 27.92, Fe+3 = = 18.61) but molecular weights or atomic weights are constant. Equivalent weights important to calculate accurate atomic weight of an element and calculations involving chemical changes. 
             As 23 g. of Na combine either with 8 g of Oxygen (to give Na2O) or 1.008 g of H (to give NaH), the equivalent weight of Na is 23. Equivalent weights can be calculated as follows.
               Equivalent weight of acid (EAcid) = 
               Basicity is the number of displaceable H+ ions.
                    




Chemical Reactions and Equations:
        A chemical equation is the short-hand notation of a chemical reaction written with the help of formulae and symbols (shorthand sign for element). This can be compared with writing a speech delivered by Prime Minister in shorthand form by a Journalist. A well balanced reaction is meaningful and gives qualitative and quantitative significance. An equation can be balanced by hit and trial method, partial equation method, Algebraic method, Oxidation number method, ion electron method (either in acid medium or in basic medium).

Chemical equation is balanced according to the law of conservation of mass. "The quantitative relationship existing between the reactants and products in a chemical reaction is called Stoichiometry." It is derived from 2 Greek words Stoicheion (means element) and metron (means measure). Stoichiometry can be compared with heart of chemistry. If you have art of learning stoichiometry, you will understand physical chemistry, inorganic chemistry topics very easily.


Oxidation Number or Oxidation State
         "The charge which an atom or ion or molecule appears to have acquired according to some rules" is called Oxidation number (O.N.). An atom can have +ve, -ve, 0, fractional O.N. It is zero for atoms of homo nuclear molecules, amalgams, +1 for H, IA, +2 for IIA, -1 for F, -2 for Oxygen. O.N. of an element can not exceed its group number (e.g.: Xe = +8 in XeO4). The sum of oxidation numbers of atoms in a neutral molecule is zero (or equal to charge of the species).
H exhibit -1 in hydrides, 'O' exhibit -1 in peroxides, - in super oxides, +1 in O2F2, +2 in OF2. Due to inert pair effect IIIA element Tl exhibit +1, IV A element Pb exhibit +2, VA element Bi exhibit +3 oxidation states.
A species acquire oxidation number either due to loss or due to gain of electrons. O.N. is important to understand formulae of compounds, type of reactions, to know whether a substance is Oxididising agent (element of a substance with maximum oxidation number
e.g.: HNO3) or reducing agent (element of a substance with minimum oxidation number
e.g.: H2S) or both oxidising as well as reducing agent (element of a substance with middle oxidation state e.g.: HNO2)
Oxidation number of Cr in CrO5, S in H2SO5, H2S2O8 are different due to peroxy bonds.
          H2SO5 = 2(+1) + x + 3(-2) + 2(-1) = 0                x = O.S. of Sulphur = +6
          CrO5 = x + 1(-2) + 4(-1) = 0                                  x = +6
          H2S2O8 = 2(+1) + 2x + 6(-2) + 2(-1) = 0            x = +6
     Species get stability due to electronic transitions like oxidation, reduction. Oxidation is a process which involves addition of Oxygen or removal of hydrogen or addition of electronegative element or removal of electropositive element or increase in oxidation number. Reduction is a process which involves addition of Hydrogen or removal of Oxygen or addition of electropositive element or removal of electronegative element or decrease in oxidation number. According to modern concept oxidation is "loss of electrons" or "increase in oxidation number".
Reduction is "gain of electrons" or "decrease in oxidation number". Remember "LEOGER' to avoid confusion. LEO = Loss of Electrons is oxidation, GER = Gain of Electrons is Reduction. "A substance, which undergoes oxidation" is called "reducing agent". "A substance which undergoes reduction" is called "oxidising agent".
For example: Zn   +    Cu+2     Zn+2   +   Cu.
                            Zn Zn+2 (Oxidation  Zn is reducing agent)
                            Cu+2 Cu (Reduction

 Cu+2 is oxidising agent)
          Reactions in which both the oxidation and reduction takes place due to transfer of electrons from one substance to another substance is called "Redox reaction". There are several types of redox reactions. Some of them are:
Combination reaction: The reaction in which two or more atoms or molecules combine to form a new molecule.
                                    e.g.: S    +     O2        SO2
Decomposition reaction: The reaction in which a molecule of a compound split into two or more simple components.  
                                   e.g.: 2 KClO-3  2 KCl + 3 O2
Displacement reaction: The reaction in which a metal atom of a compound is displaced by other metal atom of another compound.
                    e.g.: Ni + CuSO4 NiSO4 + Cu
Disproportionation reaction: The reaction in which same element undergoes oxidation as well as reduction. That element exhibits 3 different oxidation states.
                     e.g.: 3 Cl2 + 6 NaOH 5 NaCl + NaClO3 + 3 H2O
                 Cl2, Br2, I2, P, S, O undergoes disproportionation reactions.


Comproportionation reaction: The reaction in which same element with different
oxidation states give a single product with single intermediate oxidation state.
                      e.g.: Ag+2 (aq) + Ag (s) 2 Ag+ (aq)
Redox reactions are widely used in the extraction, refining of metals, dry cells, car batteries, photosynthesis, fuel cells, preparation of chemicals like NaOH, Na, Mg, Al, F2, Cl2 and in quantitative analysis. Based on the nature of oxidising agent redox titrations are classified as:
Iodometric titrations: The reactions in which I2 is liberated when KI is treated with KMnO4, K2Cr2O7, CuSO4 or I2 is titrated directly with antimonite, arsenite, sulphite or thiosulphate.

All redox reactions are exothermic. Some times ions do not take part in redox reaction are called spectator ions.
                    e.g.: AgNO3  +   NaCl AgCl + Na+ + NO3-
                              Here Na+ & NO3- are spectator ions.


Balancing of Redox reactions:

a) By Half- reaction ion - electron method:
(i) In acid medium: This method was developed by Jette and LameV in 1927. We have to balance the reaction as follows.
                     e.g.: MnO4-   +   SO3-2    Mn+2  +  SO4-2
In this reaction, oxidation state of Mn is changed from + 7 (in  MnO4-) to + 2 (in  Mn+2). So this is reduction reaction. The remaining half is oxidation reaction.




Empirical formula and molecular formula: When a compound is analysed one can get percentage of mass of each element in it. From this data Empirical and Molecular formulae can be calcualted.
Empirical formula: The formula which represents the simplest ratio of atoms of all the elements in a molecule of a compound.
              e.g.: E.F. of Benzene = CH (Ratio of C:H = 1:1)
Molecular formula: The formula which represents the actual ratio of atoms of all the elements in a molecule of a compound.
              eg.: M.F. of Benzene = C6H6 (ratio of C:H = 6:6)
                       Relation between E.F., M.F.:
                       M.F. = E.F. × n
                
e.g.: In an organic compound on analysis was found to contain 40% Carbon, 53.3% Oxygen and 6.6% Hydrogen. The vapour density is 90. Find its molecular formula.
Ans: Check whether the sum of percentages of all the elements is equal to 100 or not. If not, the remaining element is Oxygen (if not mentioned any element).
1. Divide all the elements by their atomic weights
                 C            H            O
             
2.            3.3      6.6        3.3
      Divide with lowest no. 3.3
              
                 1            2            1
     E.F.  =  C H2O       E.F. Wt. = 12 + 2 + 16  =  30
Molecular Weight = 2 × Vapour density
                              = 2 × 90 = 180                  
                       
                        M.F. = (E.F) × n
                        = (CH2O) × 6
              Molecular formula = C6H12O6


Stoichimetric Calculations 

        A balanced reaction is meaningful and gives quantitative relationship between the reactants and products. Stoichiometric calculations are classified into:
1. Weight - Weight relationship:
       This relationship is worked out with the help of Law of Conservation of mass. If the mass of reactants or products is given, then one can calculate the mass of unknown reactants or products after balancing the reaction.
e.g.: How many grams of lime stone is required to produce 4.4 g. of Carbon dioxide?
                    CaCO3 (s)  CaO (s)  +   CO2 (g)
                     100 g ------------------------- 44 g
                        ? --------------------------- 4.4 g        
   

                                           200 ml ----------- ?


                                  = 1500 ml
     Sometimes, the reactions are carried out not according to balanced chemical reaction. In such cases the reactant present in less quantity, limits the amount of product and is called "limiting reagent".
e.g.: What is the maximum volume of H2 liberated at STP when 2.4 g of
          Magnesium is treated with 28 grs of HCl.
           Mg    +    2 HCl       MgCl2     +    H2
        1 Mole      2 Moles          1 Mole         1 Mole
         24 g of Mg reacts with 2 × 36.5 grs of HCl to give 22.4 lit. of H2. But 2.4 g of Mg can reacts with 7.3 grs of HCl only. HCl is taken in excess. Mg limits the liberation of H2 gas. So Mg is limiting reagent
                      24 g of Mg ---------  22.4 lit of H2
                       2.4 g of Mg --------- ?
             

Posted Date : 04-08-2021

గమనిక : ప్రతిభ.ఈనాడు.నెట్‌లో కనిపించే వ్యాపార ప్రకటనలు వివిధ దేశాల్లోని వ్యాపారులు, సంస్థల నుంచి వస్తాయి. మరి కొన్ని ప్రకటనలు పాఠకుల అభిరుచి మేరకు కృత్రిమ మేధస్సు సాంకేతికత సాయంతో ప్రదర్శితమవుతుంటాయి. ఆ ప్రకటనల్లోని ఉత్పత్తులను లేదా సేవలను పాఠకులు స్వయంగా విచారించుకొని, జాగ్రత్తగా పరిశీలించి కొనుక్కోవాలి లేదా వినియోగించుకోవాలి. వాటి నాణ్యత లేదా లోపాలతో ఈనాడు యాజమాన్యానికి ఎలాంటి సంబంధం లేదు. ఈ విషయంలో ఉత్తర ప్రత్యుత్తరాలకు, ఈ-మెయిల్స్ కి, ఇంకా ఇతర రూపాల్లో సమాచార మార్పిడికి తావు లేదు. ఫిర్యాదులు స్వీకరించడం కుదరదు. పాఠకులు గమనించి, సహకరించాలని మనవి.

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