Enthalpy



  • The  quantity of heat supplied to a system or given out from a system under the condition of constant pressure is called Enthalpy change.

  • This a  thermodynamic property and a state function.




  • Enthalpy change is given by following formula,






  • According to the enthalpy change reactions can be   classified as                                                                                     
  1. Exothermic
  2. Endothermic.


Exothermic reactions :



Exothermic reactions are the reactions which release heat to the environment . On these types of reactions enthalpy value of the reactants is higher-than the enthalpy value of the products.The enthalpy value of  a an exothermic reaction is negative.


.


Endothermic reactions :

Endothermic reactions are the the reactions that occur by absorbing the heat of the environment.If an endothermic  reaction happens temperature  of the system will decrease.On this type of reaction enthalpy value of  the reactants is less than the enthalpy of products.The enthalpy value of  a an endothermic reaction is positive





There are different types of  enthalpies which a compound or an atom can undergo. Now let me introduce them.

  1. Standard enthalpy of formation.
  2. Standard enthalpy of combustion
  3. Standard enthalpy of bond dissociation.
  4. Standard enthalpy of  neutralization.
  5. Standard enthalpy of solvation
  6. Standard enthalpy of  hydration
  7. Standard enthalpy of dissolution.
  8. Standard enthalpy of sublimation.
  9. Standard enthalpy of evaporation
  10. Standard enthalpy of fusion.
  11. Standard enthalpy of atomization.
  12. Standard enthalpy of first ionization energy
  13. Standard enthalpy of electron affinity.
  14. Standard  lattice enthalpy of an ionic compound.


Standard enthalpy of formation :

                                                       The enthalpy change that occurs when one mole of a compound is formed  in standard state from consequent elements in standard state.


Standard enthalpy of combustion : 
                                                       The enthalpy change that occurs when one mole  of an element or a compound  in the standard state undergoes complete combustion in an excess of oxygen.


Standard enthalpy of bond dissociation:
                                                       The enthalpy change that occurs when a gaseous species in standard state is converted into gaseous components by breaking one mole of the bond.

Standard enthalpy of neutralization :
                                                       The enthalpy change that occurs when a mole of  an cation of hydrogen ions in an aqueous solution reacts with a mole of hydroxyl ion in an aqueous medium under the standard state to form a mole of water


                                        
Standard enthalpy of solvation:
                                                  The enthalpy change that occurs when a  mole of gaseous ions under the standard condition change into  the solution form in the presence of excess solvent.


Standard enthalpy of hydration
                                                The enthalpy change that occurs when a  mole of gaseous ions under the standard condition change into  the solution form in the presence of excess water.(This is a type of enthalpy of solvation)



Standard enthalpy of dissolution
                                               The enthalpy change that occurs when a  mole of substance under stand condition is dissolved in a solvent to form 1000 ml  of solution.


Standard enthalpy of sublimation :
                                                    The enthalpy change that occurs when a  mole of  solid element or a mole of a solid compound at a standard state is converted completely into a gas at it's standard state.


Standard enthalpy of evaporation :
                                                     The enthalpy change that occurs when a  mole of  solid element or a mole of a liquid compound or a element at the standard state is converted into a  mole of gaseous compounds or elements  at it's standard state.


Standard enthalpy of fusion :
                                                     The enthalpy change that occurs when a  mole of  solid  compound or a element under the standard condition is converted into a  mole of a liquid compounds or elements  at standard state.


Standard enthalpy of   atomization:
                                                     The enthalpy change that occurs when a  mole of  an element under the standard condition is converted into a  mole of a gaseous   at  standard state.


Standard enthalpy of  first ionization energy :
                                                     The enthalpy change that occurs when a  mole of uni-positive ions in the gaseous state are formed by removing an electron each that is  most weakly bonded to the nucleus from a mole of  gaseous  atom  of an element   at standard state.

Standard enthalpy of  electron affinity:
                                                     The enthalpy change that occurs when a  mole of uni-negative ions  are formed in the gaseous state  when electrons are given to a mole of  an  atom  in a gaseous state  under standard state.

Standard enthalpy of   ionic  compound:
                                                     The enthalpy change that takes place when a  mole of an ionic compound in the solid state is formed from  gaseous positive and negative ions at  the standard state.



                                   

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2 comments:

  1. In a 4 stroke diesel engine, when piston takes the air into cylinder (sucks the air into cylinder) i.e. performing the first or inlet stroke while moving from top to bottom, it fully fills the cylinder with air then comes the turn of of compressing this air and this compression is too much (about the ratio of 1:20) as compared with a 4 stroke petrol engine; in this process Entropy is decreased i.e. size of air is decreased without removing heat from the air (while Enthalpy remains same), this decrease in Entropy without decreasing Enthalpy i.e. without dissipating heat somewhere in the surrounding or nearby matter or environment causes a useful rise in the temperature of air about 600 degree centigrade which is more than enough to burn the diesel sprayed at that very time.

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