Why Does Enthalpy Decrease on Isothermal Compression?

• sero2000
In summary, for a non-ideal gas such as oxygen, increasing pressure in an isothermal system will result in a decrease in enthalpy due to the relationship between enthalpy and pressure being dependent on the gas's critical properties, as described by the corresponding states principle. This is different from an ideal gas, where enthalpy is independent of pressure.
sero2000

Homework Statement

Basically I just want to know why if I increase the pressure from 0 to 10MPa in a isothermal system containing oxygen, the enthalpy will decrease?

Homework Equations

Delta H = Delta U + delta PV

PV = nRT

PV = ZRT

The Attempt at a Solution

During a tutorial, I was actually told that because of the compression, the compressed molecules will have less space to move around which results in a decrease in internal energy. Honestly I don't get it.

If I have a compression taking place, doesn't that mean Work is done on the system? Wouldnt that mean in order to keep the system isothermal, energy would have to be removed from the system? which results in enthalpy being reduced?

In that case and also because at 10MPa the gas is not ideal, I used PV = ZRT but I can't find the link between this and Delta H = Delta U + delta PV.

Any help in clearing this doubt is really appreciated :D

sero2000 said:

Homework Statement

Basically I just want to know why if I increase the pressure from 0 to 10MPa in a isothermal system containing oxygen, the enthalpy will decrease?

Homework Equations

Delta H = Delta U + delta PV

PV = nRT

PV = ZRT

The Attempt at a Solution

During a tutorial, I was actually told that because of the compression, the compressed molecules will have less space to move around which results in a decrease in internal energy. Honestly I don't get it.

If I have a compression taking place, doesn't that mean Work is done on the system? Wouldnt that mean in order to keep the system isothermal, energy would have to be removed from the system? which results in enthalpy being reduced?

In that case and also because at 10MPa the gas is not ideal, I used PV = ZRT but I can't find the link between this and Delta H = Delta U + delta PV.

Any help in clearing this doubt is really appreciated :D
Enthalpy is independent of pressure only for an ideal gas. For a real gas beyond the ideal gas region,
$$\frac{∂H}{∂p}=\left(V-T\frac{\partial V}{\partial T}\right)$$
Note, for an ideal gas, this is zero. There are generalized dimensionless graphs and tabulations in thermo books of the integral of this expression based on the corresponding states principal. You can use these to estimate the change in enthalpy for oxygen, from knowledge its critical properties. See Introduction to Chemical Engineering Thermodynamics by Smith and Van Ness.

Chet

1. Why is enthalpy important in thermodynamics?

Enthalpy is an important concept in thermodynamics because it is a measure of the total energy of a system, including both its internal energy and the work it can do. This makes it a useful tool in analyzing and predicting changes in systems, such as during chemical reactions or phase transitions.

2. What is isothermal compression?

Isothermal compression is a process in thermodynamics where a gas is compressed at a constant temperature. This means that as the gas is compressed, its pressure increases while its temperature remains constant.

3. Why does enthalpy decrease during isothermal compression?

Enthalpy decreases during isothermal compression because the temperature of the system remains constant. According to the ideal gas law, when temperature is constant, the product of pressure and volume must also remain constant. As the volume decreases during compression, the pressure increases, resulting in a decrease in enthalpy.

4. How does isothermal compression affect the internal energy of a system?

During isothermal compression, the internal energy of a system remains constant. This is because, as stated before, the temperature remains constant and internal energy is directly related to temperature. However, the work done on the system during compression is converted into increased potential energy, which is reflected in the decrease in enthalpy.

5. What are some real-world examples of isothermal compression?

One common example of isothermal compression is the compression of air in a bicycle pump. When the handle is pushed down, the volume of air inside the pump decreases, causing an increase in pressure. However, due to the design of the pump, the temperature of the air remains constant, resulting in an isothermal compression process. Another example is the compression of natural gas in pipelines, where the temperature is kept constant to prevent the gas from expanding and losing pressure.

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