richa sharma said:
hi,
can someone explain Entropy.and enthalpy??
More precisely, in any process where the system gives up energy ΔE, and its entropy falls by ΔS, a quantity at least TR ΔS of that energy must be given up to the system's surroundings as unusable heat (TR is the temperature of the system's external surroundings). Otherwise the process will not go forward.
most importantly it is applied to a closed system or isolated system, not to universe or conditions like weather , which are infinite and never a closed or isolated system.
some other examples:
Rocks and balls fall if you drop them, tires blow out if there's a hole in them, hot pans cool down in cool rooms, iron rusts spontaneously in air, paper and wood in air will burn and make carbon dioxide and water (if you give them a little boost over their activation energy barrier .You think these are all completely different? Sure, they are, but they're all due to the same cause -- some kind of energy in them spreads out!
Entropy is merely the way to measure the energy that disperses or spreads out in a process (at a specific temperature). What's complicated about that? Entropy change, S, measures how much energy is dispersed in a system, or how widely spread out the energy of a system becomes (both always involving T, temperature).
entropy follows the second law of thermodynamics
Enthalpy: it is based on first law of thermodynamics
In thermodynamics, the quantity enthalpy, symbolized by H, also called heat content, is the sum of the internal energy of a thermodynamic system plus the energy associated with work done by the system on the atmosphere which is the product of the pressure times the volume.
For an exothermic reaction at constant pressure, the system's change in enthalpy is equal to the energy released in the reaction, including the energy retained in the system and lost through expansion against its surroundings. In a similar manner, for an endothermic reaction, the system's change in enthalpy is equal to the energy absorbed in the reaction, including the energy lost by the system and gained from compression from its surroundings. A relatively easy way to determine whether or not a reaction is exothermic or endothermic is to determine the sign of ΔH . If ΔH is positive, the reaction is endothermic, that is heat is absorbed by the system due to the products of the reaction having a greater enthlapy than the reactants. The product of an endothermic reaction will be cold to the touch. On the other hand if ΔH is negative, the reaction is exothermic, that is the overall decrease in enthalpy is achieved by the generation of heat. The product of an exothermic reaction will be warm to the touch
hope this would help you in understanding the basics about entropy and enthalpy and its application in biochemistry,
In simple words (if not over simplified) enthalpy talks about total heat content , where as entropy talks about heat change or transferred .
so if a biomoleclue is stable with and yet it has to change inot its products here , it has to be broken down by overcoming its internal energy , thus it is a endothermic reaction (like atp or heat or work is done or used on it )
so use this info for understanding biochemical kinetics, what happens ina chemical reaction like oxidative phosphorylations,etc. As some reactions produce ATP where as some reactions consumes ATP , so here the enthalpy comes into a role.
Finally what is useful for your exam is :
Energy of reactants (N2 & H2) is greater than the energy of the products (NH3). N2 + 3H2............) 2NH3
Energy is released.
H is negative. THEN IT IS A EXOTHERMIC REACTION (ENTHALPY IS NEGATIVE)
Energy of reactants (NH3) is less than the energy of the products (N2 & H2).
NH3 ........) N2 + 3H2 enrgy is used to break this moleclue or to proceed this reaction , where Energy is absorbed.
H is positive (ENTHALPY IS POSITIVE)
