ENTHALPY CHANGE OF REACTION, STANDARD CONDITIONS & STANDARD ENTHALPY CHANGES

 ENTHALPY CHANGE OF REACTION:          

                 The total energy content of the system is called its enthalpy. It is given the symbol H. It is impossible to obtain an absolute value of energy of the system at any instant, but it is possible to measure the enthalpy change when energy is transferred from the system to the surroundings or vise versa. The enthalpy change is given the symbol H. Mathematically

             H = H                              - H

                    (Product)                       (Reactants)  

                 Enthalpy change, H is the measure of energy transferred from the reactants to the surroundings or vise versa.

 STANDARD CONDITIONS & STANDARD ENTHALPY CHANGES:

                 When we compare the enthalpy changes of various reactions, we must use standard conditions H^Ɵ.

(a)          amount of substance; 1 mole.

(b)         a temperature of 298 K (25 ⁰C)

(c)          1 atmospheric pressure i-e; 101 K Pa.

(d)         any solution have a concentration of 1.0 mole dm^-3.

(e)          In case of elements which exist as different allotropes, the most stable  form at 298 K and 1 atom Is used as the standard e.g; carbon exist as graphite and diamond. Since graphite is more stable, hence reactions involving carbon under standard conditions should  relate to graphite rather than diamond . Some terms related to standard enthalpy  changes are given below:

                 Formation; combustion; hydration; solution; neutralization; atomization;

 (a)         Standard enthalpy change of formation H :

                 H is the enthalpy change when one mole of a compound is formed from its  elements under standard conditions e.g;

                 H  +  O₂                               H₂          H  = - 286 KJ mole^-1 it is H  of water.

 (b)      Standard enthalpy change of combustion H :

              H  is the enthalpy change when one mole of an element or compound reacts completely with oxygen under standard conditions. e.g; H  of hydrogen is;

            H  +  O                             H₂          H  = - 286 KJ mole^-1

             Similarly H  of sulpher is;

                                 + O                                     SO       H  = - 296.8 KJ mole^-1

(c)       STANDARD ENTHALPY CHANGE OF HYDRATION ^Ɵ_Hyd

              H     is the enthalpy change when one mole of a gaseous ion dissolves in water to give an infinitely dilute solution under standard conditions e.g;          H   of Cl^Ө  is;

             Cl          water         Cl   H^Ө_(Hyd) = - 364 KJ mole^-1

  H^Ө_(Hyd) of K  is;         

K     water          K                             = - 319.8 KJ mole^-1