Download HSEB Notes of Chemistry | Reaction Type | Extra Reference | E-Book | Class 11

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Chemical  Reactions

Chemical  reactions  are  also  known  as  chemical  changes.This  refers  to  the changes in the structure of molecules.  Such reactions can result in molecules at taching to each other to form larger molecules, molecules breaking apart to form two  or  more  smaller  molecules,  or  rearrangements  of  atoms  within  molecules. Chemical reactions usually involve the making or breaking of chemical bonds.  A chemical reaction does not involve or change the nucleus of the atom in any way. Only  the  electron  clouds  around the  atoms,  involved  in  the  reaction,  interact. A  chemical  reaction  can  result  in:

• molecules  attaching  to  each  other  to  form  larger  molecules
• molecules  breaking  apart  to  form  two  or  more  smaller  molecules
• rearrangements  of  atoms  within  molecules

A chemical reaction almost always involves a change in energy,  conveniently measured  in  terms  of  heat.    The  energy  diference  between  the  "before"  and "after" states of a chemical reaction can be calculated theoretically using tables of  data  (or  a  computer).    For  example,  consider  the  reaction  CH4  +  2O₂ ?

CO₂ + 2H₂O (combustion of methane in oxygen).  By calculating the amounts of  energy  required  to  break   all  the  bonds  on  the  left  ("before")  and  right  ("af-ter")  sides  of  the  equation,  we  can  calculate  the  energy  diference  between  the reactants  and  the  products.  This  is  referred  to  as  ∆H,  where  ∆  (Delta)  means diference,  and  H  stands  for  enthalpy,  a  measure  of  energy  which  is  equal  to the  heat transferred  at  constant  pressure.   ∆H  is  usually  given  in  units  of  kJ (thousands of joules) or in kcal (kilocalories).  If ∆H is negative for the reaction, then energy has been released.  This type of reaction is referred to as exothermic (literally,  outside  heat,  or  throwing  of  heat).   An  exothermic  reaction  is   morefavourable  and  thus  more  likely  to  occur.  Our  example  reaction  is  exothermic, which we already know from everyday experience, since burning gas in air givesof  heat. A  reaction  may  have  a  positive  ∆H.  This  means  that,  to  proceed,  the  reac- tion  requires  an  input  of  energy  from  outside.   This  type  of  reaction  is  called endothermic  (literally,  inside  heat,  or  absorbing  heat). 

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