tandard enthalpy change of solution H, Standard enthalpy change of neutralization, BOND ENERGY OR BOND ENTHALPY

 (d) Standard enthalpy change of solution H  :

              H  is the enthalpy change when one mole of a solute dissolve in a solvent to give an infinitely dilute solution under standard conditions. Or

             The difference between lattice energy (positive) and the hydration energy (negative) is called the heat of solution. e.g;

             LiCL_(s)   ^water        +    = - 37.2 KJ Mole^-1

                    ^water        +   = + 17.2 KJ Mole^-1

 (e) Standard enthalpy change of neutralization :

               is the enthalpy change when one mole of water is formed as a result of reaction between an acid and an alkali under standard conditions e.g; Neutralization of strong acid with strong alkali is; +  +  +  = - 57.3 KJ mole^-1

               +                              H₂O     = - 57.3 KJ mole^-1    

             Neutralization of weak acid with strong alkali is less exothermic;

               +                   CO  +    = - 55.2 KJ mole^-1

                           +                           = - 55.2 KJ mole^-1             

 (f) Standard enthalpy change of atomization  :

             The standard enthalpy change of atomization of an element is the enthalpy change when one mole of gaseous atoms is formed from one mole of the element in its standard state or it is the energy required to convert one mole of a substance completely into gaseous atoms under standard conditions. For elements, the atomization energy refers to the formation of one one mole of gaseous atoms. eg;

                                                    = + 108 KJ mole^-1

             ½ Cl                            = + 121 KJ mole^-1

                                                = 289 KJ mole^-1

 BOND ENERGY OR BOND ENTHALPY:

             The amount of energy which is required to break a covalent bond is called bond energy or bond enthalpy. In terms of standard bond enthalpy it is related with one mole and its units are KJ mole^-1. Bond breaking is an endothermic process hence,  for bond energy is always positive. Whereas same amount of energy is released when same bond is formed as bond formation is an exothermic process.

 AVERAGE BOND ENERGIES:                      

        

            Bond energy between two atoms depends upon the nature of molecule e.g; the C-C bond has different strength in ethane as compared to propane, as it is affected by other atoms and bonds present in the molecule. Hence, the values of bond energies quoted in data-booklet are average bond energies, as they are average values from many different molecules. Some common average bond energy values are given below:

 

Bond	KJ mole-1	Bond	KJ mole-1
H-H C-C C=C C C C=C	+436 +350 +610 +840 +740	C-H O=O O-H C-O N N	+410 +496 +460 +360 +994

 

CALCULATIONS INVOLVED TO FIND OUT ENTHALPY CHANGES FROM EXPERIMENTAL RESULTS BY USING A RELATIONSHIP:

 

ENTHALPY CHANGE = MC T:

 

            In this relationship m is the 0.025 moles = - 10.3 KJ 1 mole

                 = -  x 1

= - 412 KJ mole^-1

(i)                 +                       Ca (OH)  + H  

Mole ratio ; Ca : H₂

                                      1:1         

                        n ⁼  

= 0.025 moles                                               

Same no of moles for hydrogen = 0.025 moles,

volume of hydrogen at r.t.p = 0.025 x 24000

                                                = 600 cm³

 HESS’ LAW:

             Hess’ law states that enthalpy change for a chemical reaction is determined only by the initial and final states of the system and is independent of the pathway by which the reaction takes place e.g;