Understanding Thermodynamic Functions: H, G, and A in Simple Terms

In summary: The Thermodynamic functions, such as H (enthalpy), G (Gibbs function), and A (Helmholtz function), allow for simplified calculations and measurements related to real processes. They take into account the effect of the environment on the system.
  • #1
manimaran1605
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What are the purposes of defining the Thermodynamic functions such as H (enthalpy), G (Gibbs function), A (Helmholtz function) in Thermodynamics. I just know the expressions for this functions but unable to understand the physical significance of these functions and also haven't understand what these functions tells about thermodynamic systems (please don't explain with rigorous mathematics) try to explain it as intuitive as possible.
 
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  • #2
They allow to simplify calculations/measurements related to real processes, as they take into account the effect of the environment.

Take for instance enthalpy, ##H \equiv U + PV## or its change at constant pressure ##\Delta H = \Delta U + P \Delta V##. Imagine you have a process that will lead to an increase in the internal energy (##\Delta U > 0##) and expands (say a gas phase reaction where two molecules of reactant give 3 molecules of products). ##\Delta H## tells you the energy needed for the process by considering not only the increase in internal energy but also the work that has to be done against the environment.

Likewise, for instance, the Gibss free energy takes into account both the exchange of volume and of heat with the environment. For a chemical reaction ##A \rightleftharpoons B##, in an open container, the values of ##\Delta G## tell you immediately which way the reaction will go by itself. Simply knowing the internal energy ##U## is not sufficient.

Another example: why is water liquid at 99°C, and not a gas? Surely the gas has more entropy than a liquid, and therefore should be favored? Check the Gibbs free energy: it is lower for liquid water at 99°C than steam at the same temperature.
 
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To expand further on what Dr. Claude said, H, A, and G are often very convenient to use in process calculations. H is useful in quantifying the heat effects in continuous open flow processes. A and G are very convenient to use in quantifying interphase equilibrium and chemical reaction equilibrium of systems. A is useful for constant volume systems, and G is useful for constant pressure and open flow systems.

Chet
 
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1. What are thermodynamic functions?

Thermodynamic functions are mathematical functions used to describe the behavior of a thermodynamic system. These functions include internal energy, enthalpy, entropy, and Gibbs free energy, and provide information about a system's energy and its tendency to undergo spontaneous change.

2. How do thermodynamic functions relate to each other?

Thermodynamic functions are related through various equations, such as the first and second laws of thermodynamics. For example, the change in internal energy is equal to the heat added to the system minus the work done by the system, and the change in entropy is the heat added divided by the temperature.

3. What is the significance of thermodynamic functions?

Thermodynamic functions are crucial in understanding and predicting the behavior of physical and chemical systems. They can help determine the amount of work a system can do, the direction of spontaneous reactions, and the stability of a system.

4. How are thermodynamic functions measured?

Thermodynamic functions are typically measured through experiments, such as calorimetry for measuring heat changes and thermogravimetry for measuring changes in mass. These measurements can then be used to calculate the values of the different thermodynamic functions.

5. Can thermodynamic functions change?

Yes, thermodynamic functions can change depending on the conditions of the system. For example, the internal energy of a system can change if heat is added or removed, and the entropy can change if the temperature or pressure of the system changes. These changes can then affect the other thermodynamic functions as well.

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