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Explain entropy and spontaneity of reactions. OR Explain entropy and spontaneity. |
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Answer» SOLUTION :`1/2N_(2(g)) + (3)/(2) H_(2(g)) = NH_(3(g)), Delta_(r) H^( Theta ) = -46.1` kj/mol `(1)/(2) H_(2(g)) + (1)/(2) CI_(2(g)) = HCI_((g)) , Delta_(r) H^( Theta )= -92.32` kj/mol `H_(2(g)) + (1)/(2) O_(2(g)) = H_(2) O_((g)) , Delta_(r) H^( Theta ) = -285.8` kj/mol The decrease in ENTHALPY is passing from reactants to products may be shown for any exothermic reaction on an enthalpy diagram as shown in FIG. (a).  Figure-(a) Enthalpy diagram for exothermic reactions Thus, the postulate that driving force for a chemical reaction may be DUE to decrease in energy sounds .reasonable. as the basis of evidence so far ! Endothermic reaction : `(1)/(2) N_(2(g)) + O_(2(g)) to NO_(2(g)) , Delta_(r) H^( Theta ) = + 33.2` kj/mol `C_((s)) + 2S_((l)) + CS_(2(l)) ,Delta_(r) H^( Theta ) = + 128.5` kj/mol These reactions though endothermic, are spontaneous. The increase in enthalpy may be represented on an enthalpy diagram as shown in Fig.-(b).  Figure-(b) : Enthalpy diagram for endothermic reactions Therefore, it becomes obvious that while decrease in enthalpy may be a contributory factor for spontaneity, but it is not true for all cases. |
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