Enthelpy = internal energy+pressure*volume

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In summary, the conversation is discussing the concept of enthalpy and its relationship to internal energy, pressure, and volume. The chem book states that enthalpy for a particular state cannot be calculated, only the enthalpy change can be calculated. This is because enthalpy is used to calculate the energy involved in a reaction where bonds are broken, but it also depends on the temperature. Therefore, there is only an equation for the change in enthalpy, not for the absolute enthalpy. The speaker also mentions that the change in PV is equivalent to the enthalpy change.
  • #1
sid_galt
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My chem book says that

Enthalpy = internal energy+pressure*volume

However it says that enthalpy for a particular state cannot be calculated. Only the enthalpy change can be calculated. This has me confused.
Aren't internal energy, pressure and volume calculabe for a particular state? Then why can't we calculate absolute enthalpy?
 
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  • #2
Enthaply is used to calculate how much energy is taken in or released by a reaction where bonds have been broken (well all reactions really need bonds to brake but still...).

Enthaply, however, also depends on the temperature (if I am correct) and ,thus, it is only possible to have an equation for the change.

This might help or checking the rest of these might as well.

The Bob (2004 ©)
 
  • #3
It should be change in PV, or [tex] \Delta PV[/tex], the work in expansion against the atmosphere
 

1. What is enthalpy and how is it calculated?

Enthalpy is a thermodynamic property that represents the total energy of a system. It is calculated by adding the internal energy of the system to the product of pressure and volume.

2. What is the significance of enthalpy in thermodynamics?

Enthalpy is a useful concept in thermodynamics because it takes into account both the internal energy of a system and the work done by or on the system. It is often used to describe changes in a system's energy during a process.

3. How does pressure and volume affect enthalpy?

According to the equation for enthalpy, an increase in pressure or volume will result in an increase in enthalpy. This is because an increase in pressure or volume means more work is being done on or by the system, which contributes to the total energy of the system.

4. Can enthalpy be negative?

Yes, enthalpy can be negative. A negative enthalpy indicates that the system has lost energy, either through a decrease in internal energy, a decrease in pressure, or a decrease in volume.

5. How is enthalpy used in chemical reactions?

In chemical reactions, enthalpy is often used to calculate the heat of reaction, which is the amount of energy released or absorbed during a chemical reaction. Enthalpy can also be used to predict the direction of a chemical reaction, as reactions tend to occur in the direction that results in a decrease in enthalpy.

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