What is the Definition of Enthalpy and When Does It Change?

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In summary, the conversation discusses the concept of enthalpy and how it relates to changes in temperature and energy. It is clarified that a change in enthalpy only occurs during a phase change or chemical change at a constant temperature. The formula for calculating energy change in a system is also mentioned, as well as the formula for calculating change in enthalpy during a phase change. The conversation concludes with a question about the definition of enthalpy and its relationship to potential and kinetic energy.
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dontdisturbmycircles
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Homework Statement


I am having a bit of trouble grasping the idea and definition of enthalpy. I was wondering if someone could help.

Am I correct in saying that a change in enthalpy only occurs if there is no temperature change? i.e. during a phase change/chemical change at constant temp?

Thus since there is no change in temperature, there is no change in kinetic energy. The change in enthalpy is equivalent to a change in potential energy for the molecules involved as they separate/bond.

For some reason I was confused because I thought that enthalpy was "the total kinetic and potential energy of the system" as defined in my textbook, but if temperature cannot change, by definition kinetic energy cannot change right?

Can someone let me know whether or not my assumptions about temperature change are correct?
 
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  • #2
Let me expand my explanation a little. When calculating the energy change of a system, we use the forumla q=mc[tex]\Delta[/tex]t if there are no phase changes, which makes perfect sense. We only use the formula [tex]\Delta[/tex]H=nH where [tex]\Delta[/tex](H is the change in enthalpy and H is the molar enthalpy value for whatever phase change, and n is the amount of substance in molar units) during a phase change. It seems that a change in enthalpy is a change in potential energy, since the temp does not change in the calculation, but then my textbook says its the total value of the potential and kinetic energy of the molecules in the substance, so I am confused.
 
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  • #3
Enthalpy is heat. If heat can change temperature, then a temperature change is observed. If heat is used to change the phase of a material, temperature will not change.
 

FAQ: What is the Definition of Enthalpy and When Does It Change?

What is enthalpy?

Enthalpy is a thermodynamic quantity that measures the total energy of a system, including both its internal energy and the energy required to overcome the pressure and volume constraints of the system.

How is enthalpy calculated?

Enthalpy is calculated by adding the internal energy of a system to the product of its pressure and volume.

What is the significance of enthalpy in chemistry?

In chemistry, enthalpy is important because it helps us understand the energy changes that occur during chemical reactions. It also allows us to predict the direction and extent of a reaction.

What is the difference between enthalpy and internal energy?

Enthalpy and internal energy are closely related, but they differ in that enthalpy takes into account the pressure and volume of a system, while internal energy only considers the energy contained within the system.

How is enthalpy used in practical applications?

Enthalpy is used in a variety of practical applications, such as in power plants to calculate the energy produced or consumed during a reaction, in refrigeration systems to determine the amount of heat absorbed or released, and in food packaging to control the temperature and moisture levels during storage and transportation.

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