5 Laws of Chemical Combination.

5 LAWS OF CHEMICAL COMBINATION.

1. Laws  of  Chemical  Combination  :  The elements  combine  with  each  other  and  form  compounds.  This  process  is  governed  by  five  basic laws discovered before the knowledge of molecular formulae.

1.1 Law of conservation of mass : Antoine Lavoisier  (1743-1794)  a  French  scientist  is  often  referred  to  as  the  father  of    modern    chemistry.    He  carefully  performed  many             combustion             experiments,       namely,       burning  of  phosphorus  and  mercury,  both  in  the presence of air. Both resulted in an increase in weight. After several experiments he found that the weight gained by the phosphorus was exactly the same as the weight lost by the air. He observed that,Total mass of reactants = Total mass of productsWhen  hydrogen  gas  burns  and  combines  with  oxygen  to  yield  water,  the  mass  of  the  water  formed  is  equal  to  the  mass  of  the  hydrogen  and  oxygen  consumed.  Thus,  the  law of conservation of  mass states that 'mass can neither be created nor destroyed.'
1.2 Law of Definite Proportions :French chemist, Joseph Proust performed experiments    on    two    samples    of    cupric    carbonate. One of the samples was natural in origin  and  the  other  was  a  synthetic  one.  He  found that the composition of elements present in it was same for both the samples as shown
This  led  Joseph  Proust  to  state  the  law  of  definite proportion as follows :'A given compound always contains exactly the same proportion of elements by weight.'Irrespective  of  the  source,  a  given  compound  always  contains  same  elements  in  the  same  proportion.  The  validity  of  this  law  has  been  confirmed  by  various  experiments.  This  law  is  sometimes  referred  to  as  Law  of  definite  composition.
1.3 Law of multiple proportions :This law was proposed by John Dalton in 1803.  It  has  been  observed  that  two  or  more  elements may form more than one compound. Law   of   multiple   proportions   summarizes   many experiments on such compounds.When two  elements  A  and  B  form  more  than  one  compounds,  the  masses  of  element  B  that  combine with a given mass of A are always in  the  ratio  of  small  whole  numbers.  For example,
 i.  Hydrogen  combines  with  oxygen  to  form  two  compounds,  namely  water  and
hydrogen peroxide.
Hydrogen(2g) + Oxygen(16g)= Water(18g) 
Hydrogen(2g)+ Oxygen(32g)= Hydrogen Peroxide(34g)
Here,  it  is  found  that,  the  two  masses  of  oxygen i.e. 16 g and 32 g which combine with a fixed mass of hydrogen (2g) are in the ratio of small whole numbers, i. e. 16:32 or 1:2.
ii. Nitrogen and oxygen combine to form two compounds, nitric oxide and nitrogen dioxide.
Nitrogen(14g) + Oxygen(16g)= Nitric Oxide(30g)
Nitrogen(14g) + Oxygen(32g)= Nitrogen Dioxide(46g)
Here, you find that the two masses of oxygen i.e. 16 g and 32 g when combine with a fixed mass  of  Nitrogen  (14  g)  are  in  the  ratio  of  small whole numbers i.e. 16:32 or 1:2.
(Similar  examples  such  as  CO  and  CO2(1:2  ratio),  SO2  and  SO3  (2:3  ratio),  can  be  found.) 
1.4  Gay  Lussac  Law  of  Gaseous  Volume  : This  law  was  put  forth  by  Gay  Lussac  in  1808.  The  law  states  that  when  gases  combine  or  are  produced  in  a  chemical  reaction  they  do  so  in  a  simple ratio  by volume, provided all gases are at same   temperature   and   pressure. Illustration   :
 i.   Under   the   same   conditions   of   temperature and pressure, 100    mL    of    hydrogen    combines  with  50  mL  of  oxygen  to  give  100  mL of water vapour.
Hydrogen (g)(100ml)(2vol) + Oxygen (g)(50ml)(1vol)= Water(g)(100ml)(2vol)
Thus, the volumes of hydrogen gas and oxygen gas  which  combine  together  i.e.  100  mL  and  50 mL producing two volumes of water vapour which amounts to 100 mL bear a simple ratio of 2:1:2
ii.  Under  the  same  condition  of  temperature  and pressure,1  L  of  nitrogen  gas  combines  with  3  L  of  hydrogen gas to produce 2 L of ammonia gas.
Nitrogen (g) + Hydrogen (g)=Ammonia(g)
 1L (1vol)          3L (3vol)         2L (2vol)  
Thus, the volume of nitrogen gas and hydrogen gas  which  combine  together  i.e.  1  L  and  3  L  and volume of ammonia gas produced i. e. 2 L bear a simple ratio of 1:3:2.
1.5    Avogadro  Law  :  In 1811,   Avogadro   proposed   that equal    volumes    of    all    gases    at    the    same    temperature  and  pressure  contain  equal  number  of  molecules. If    we    consider    the    reaction  of  hydrogen  and  oxygen  to  produce  water vapour.
Hydrogen (g) + Oxygen (g) =  Water (g)
100ml(2vol)      50ml(1vol)     100ml(2vol)
(Gay Lussac Law)
2n molecules    n molecules       2n molecules
(Avogadro law)
2 molecules      1 molecule      2 molecules
We   see   that   2   volumes   of   hydrogen   combine  with  1  volume  of  oxygen  to  give  2  volumes of water vapour, without leaving any unreacted  oxygen.  According  to  Avogadro  law,  if  1  volume  contains  n  molecules,  then  2n  molecules  of  hydrogen  combine  with  n  molecules  of  oxygen  to  give  2n  molecules  of  water.
Therefore,   2   molecules   of   hydrogen   gas  combine  with  1  molecule  of  oxygen  to  give  2  molecules  of  water  vapour.  Avogadro  could explain the above result by considering the  molecules  to  be  polyatomic.  If  hydrogen  and  oxygen  were  considered  as  diatomic,  as  recognized  now,  then  the  above  results  are  easily understandable.





 
           two volume of hydrogen react with one 
volume of oxygen to give two volumes of water vapour