Enthalpy | Thermodynamics

Summary

This video explains the concepts of internal energy and enthalpy in thermodynamics. Internal energy is defined as the total energy of a system, consisting of kinetic and potential energy. It can only be measured in terms of change, rather than absolute values. The video then introduces enthalpy, which is the sum of internal energy and the work done by the system. Key points include the relationship between enthalpy and heat at constant pressure, as well as practical applications in calculating enthalpy changes in thermodynamic processes.

Takeaways

• 😀 Internal energy is the total energy of a system, comprising both kinetic and potential energy.
• 😀 The absolute value of internal energy cannot be determined; only changes in internal energy can be measured.
• 😀 Internal energy can be altered by changing the system's kinetic or potential energy.
• 😀 For an ideal gas in an isothermal process, internal energy is constant, as potential energy is zero and kinetic energy remains unchanged.
• 😀 Enthalpy is defined as the amount of heat absorbed or released by a system during a change.
• 😀 The formula for enthalpy change is ΔH = ΔU + PΔV, where P is pressure and V is volume.
• 😀 A positive ΔH indicates an endothermic process, while a negative ΔH signifies an exothermic process.
• 😀 Enthalpy is a state function, meaning it depends only on the initial and final states, not the path taken.
• 😀 Enthalpy is an extensive property, meaning it varies with the amount of substance present.
• 😀 According to the first law of thermodynamics, the relationship between enthalpy and heat at constant pressure is ΔH = Q.

Q & A

What is internal energy?

Internal energy is the total energy of a system, defined as the sum of kinetic energy and potential energy of its particles.

Can we measure the absolute value of internal energy?

No, we cannot measure the absolute value of internal energy; we can only determine the change in internal energy.

How can the internal energy of a system be changed?

The internal energy can be changed by altering the kinetic energy or the potential energy of the system.

What happens to the internal energy of an ideal gas in an isothermal process?

For an ideal gas in an isothermal process, the internal energy remains constant because there are no interactions between particles.

What is enthalpy?

Enthalpy is defined as the amount of heat absorbed or released by a system to cause a change, which includes both internal energy and the work done by the system.

What is the relationship between enthalpy and heat?

The change in enthalpy (ΔH) is equal to the amount of heat added to or removed from the system at constant pressure.

What does a positive ΔH indicate?

A positive ΔH indicates that the process is endothermic, meaning heat is absorbed from the surroundings.

What are the two main equations for enthalpy change?

At constant volume, ΔH = ΔU + VΔP; at constant pressure, ΔH = ΔU + PΔV.

Is enthalpy a state function?

Yes, enthalpy is a state function, which means it depends only on the initial and final states, not on the path taken.

How is the change in enthalpy calculated in a specific problem?

In a problem involving a change of state, the change in enthalpy can be calculated using ΔH = ΔU + (P2V2 - P1V1), where P and V are the initial and final pressures and volumes.