Question

please help me...with explaination..45 qn

Answer 1

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There are two tendencies in nature: things tend to go to the lowest possible energy (enthalph) level (delta H = negative) and the highest possible state of disorder (entropy)

Delta G is a way of weighing the importance of the two trends:
                  delta G = delta H  - T delta S
At low temperatures the enthalpy change is more important, while at high temperatures the entropy change becomes more important.  Frequently these tendencies work in opposition to one another so delta G determines whether the favorable or unfavorable tendency wins out.

For instance,
ice melting at 20oC.    
            ice melting is endothermic and requires an INCREASE in energy (delta H=+).  This is UNFAVORABLE
            ice melting causes an increase in disorder (entropy) as liquids are more disordered than solids (delta S = +).  This is FAVORABLE.
            At 20 C, ice does melt spontaneously (delta G= -) so the -Tdelta S is larger (and thus more important) than the smaller delta H term.
          Note, at -20C, ice does not melt (delta G=+) because the -Tdelta is smaller (and thus less important) than the delta H term.

Na reacting with water
          Exothermic, so delta H is - (FAVORABLE)
          In terms of entropy, a solid and a liquid are going to a solution (a mixture) and a gas, so delta S should be + (favorable).
          Since both delta H is -  and delta S is positive, delta G will be negative (spontaneous) at all temperatures.