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  1. [1]
    الصورة الرمزية حمدي الزعيم
    حمدي الزعيم
    حمدي الزعيم غير متواجد حالياً

    عضو فعال

    تاريخ التسجيل: Oct 2007
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    I need Help Plz

    What is the difference between the Entropy And Enthalpy
    plz i need an clear answer for this question
    thx for every one

  2. [2]
    احمد ميكانيك
    احمد ميكانيك غير متواجد حالياً
    عضو فعال جداً


    تاريخ التسجيل: Mar 2008
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    والله اعلي واعلم

    والله انا لم اتذكر ولكن انا جبتلك الترجمة بتاعتهم يمكن تفيدك

    درجة التعادل الحراري Entropy

    المحتوي الحراري في وحدة كتله Enthalpy

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  3. [3]
    مهندس سورجي
    مهندس سورجي غير متواجد حالياً
    عضو فعال


    تاريخ التسجيل: Jul 2008
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    IN the Name of God more Merciful

    1- Enthalpy is the transfer of energy (symbolized by delta H) Entropy is disorder and is symbolized by s or delta s, these two things together give you gibb's free energy, which allows you to determine whether a reaction will be spontaneous, and under what conditions.

    2- entropy is heat absorbed in a reversible system and enthalpy includes the work done... not just the the internal heat.

    3- entropy is randomness in a system. enthalpy is the energy expended in a system.

    4- Enthalpy is the heat ******* in a system. Entropy is the amount of disorder in a system.

    5- They have different units for one.

    dQ = T dS This equation tells us that the heat input to

    "the system" is dQ. This is a change in energy or energy per unit mass. T is the temperature (absolute, e.g. Kelvin). dS is the change in entropy. So dS has units of energy/temp.

    The physical meaning of entropy is that it is a measure of disorder in the system.

    Enthalpy is one of many measures of the internal energy of the system (or internal energy per unit mass). (Other internal energies are Free energy and Gibbs Free energy.) The issue of the specific meaning of this internal energy is rather complex.


    The Difference Between Enthalpy of Reaction and Entropy of Reaction Calculations
    At first glance, tables of thermodynamic data seem inconsistent. Consider the data in the table below, for example.
    Thermodynamic Data for Aluminum and Its Compounds
    SubstanceHFo(kJ/mol)So (J/mol-K)Al(s)028.33Al(g) 326.4 164.54Al2O3(s) -1675.7 50.92AlCl3(s)-704.2 110.67
    The enthalpy data in this table are given in terms of the standard-state enthalpy of formation of each substance, Hfo. This quantity is the heat given off or absorbed when the substance is made from its elements in their most thermodynamically stable state at 0.1 MPa. The enthalpy of formation of AlCl3, for example, is the heat given off in the following reaction.
    2 Al(s) + 3 Cl2(g) 2 AlCl3(s) Hfo = -704.2 kJ/mol
    The enthalpy data in this table are therefore relative numbers, which compare each compound with its elements.
    Enthalpy data are listed as relative measurements because there is no absolute zero on the enthalpy scale. All we can measure is the heat given off or absorbed by a reaction. Thus, all we can determine is the difference between the enthalpies of the reactants and the products of a reaction. We therefore define the enthalpy of formation of the elements in their most thermodynamically stable states as zero and report all compounds as either more or less stable than their elements.
    Entropy data are different. The third law defines absolute zero on the entropy scale. As a result, the absolute entropy of any element or compound can be measured by comparing it with a perfect crystal at absolute zero. The entropy data are therefore given as absolute numbers, So, not entropies of formation, Sof.
    AlCl3(s) So = 110.67 J/mol-K
    Engineer:
    Hakar Surchi

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  4. [4]
    NAK
    NAK غير متواجد حالياً
    عضو متميز
    الصورة الرمزية NAK


    تاريخ التسجيل: Apr 2006
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    In thermodynamics and molecular chemistry, the enthalpy or heat ******* (denoted as H, h, or rarely as χ) is a quotient or description of thermodynamic potential of a system, which can be used to calculate the "useful" work obtainable from a closed thermodynamic system under constant pressure and entropy.

    The term enthalpy was composed of the prefix en-, meaning to "put into" and the Greek word -thalpein, meaning "to heat", although the original definition is thought to have stemmed from the word "enthalpos" (ἐνθάλπος).



    In thermodynamics (a branch of physics), entropy is a measure of the unavailability of a system’s energy to do work

    It is a measure of the randomness of molecules in a system and is central to the second law of thermodynamics and the fundamental thermodynamic relation, which deal with physical processes and whether they occur spontaneously. Spontaneous changes, in isolated systems, occur with an increase in entropy. Spontaneous changes tend to smooth out differences in temperature, pressure, density, and chemical potential that may exist in a system, and entropy is thus a measure of how far this smoothing-out process has progressed.

    The word "entropy" is derived from the Greek εντροπία "a turning toward" (εν- "in" + τροπή "a turning"), and is symbolized by S in physics

    another Definitions:
    In the analysis of certain types of processes,particularly in power gwneration and refrigeration,we frequently encounter the combination of properties U+PV.where U is internal energy P is the pressure and V is the volume of the substance.For the sake of simplicity and convenience,this combination is defined as a new prperty,enthalpy,and given the symbol H:
    H=U+PV
    From a statistical point of view,entropy is a measure of molecular randomness,that is,the uncertainty about the positions of molecules at any instant.
    in macroscopic approach entropy is atool that we can find that how much a process has been done irriversibly

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  5. [5]
    حمدي الزعيم
    حمدي الزعيم غير متواجد حالياً
    عضو فعال
    الصورة الرمزية حمدي الزعيم


    تاريخ التسجيل: Oct 2007
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    Thx u mr.NAK but this answer is the same as wikipedia
    thx any way

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  6. [6]
    NAK
    NAK غير متواجد حالياً
    عضو متميز
    الصورة الرمزية NAK


    تاريخ التسجيل: Apr 2006
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    Yes, Mr. Hamdee it the accuret answer from wikipedia.

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