What is Enthalpy?
Enthalpy, often denoted as H, is a state function in thermodynamics that describes the total energy of a system. It is defined as the internal energy of a system plus the product of the pressure and volume of the system. Enthalpy is a measure of the energy of a system that includes the effects of pressure and volume.
It is often used in thermodynamics to describe the heat of a reaction, which is the amount of heat absorbed or released when a chemical reaction takes place. The change in enthalpy is equal to the heat added or removed from the system at constant pressure.
Enthalpy is a state function, which means that its value depends only on the current state of the system and not on how the system got to that state. Enthalpy can be increased or decreased by adding or removing heat to or from the system at constant pressure. It is also used in thermochemistry to calculate the heat of formation of compounds, which is the heat absorbed or released when a compound is formed from its component elements under standard conditions.
In summary, Enthalpy is a term in thermodynamics, which is defined as the internal energy of a system plus the product of the pressure and volume. It is a state function, and it is usually used to measure the heat of a reaction, or the heat absorbed or released by the system at constant pressure, and it can be used to calculate the heat of formation.
How do you calculate the Enthalpy?
Enthalpy, often denoted as H, can be calculated using the following equation:
H = U + PV
Where:
H = Enthalpy (in joules or other units of energy) U = Internal energy (in joules or other units of energy) P = Pressure (in pascals or other units of pressure) V = Volume (in cubic meters or other units of volume)
The internal energy, U, can be calculated using the first law of thermodynamics, which states that the change in internal energy of a system is equal to the heat added to the system minus the work done by the system.
U = Q – W
Where:
Q = Heat added to the system (in joules or other units of energy) W = Work done by the system (in joules or other units of energy)
Therefore, to calculate the enthalpy of a system, you need to know the internal energy, U, the pressure, P, and the volume, V, of the system. If the pressure and volume are constant, the enthalpy change can also be calculated by measuring the heat absorbed or released by the system in a calorimetry experiment, then adding it to the initial enthalpy of the system.
In some cases, the enthalpy is given in a thermodynamic tables, which are tabulated values of thermodynamic properties of compounds, including enthalpy. These tables are based on experimental measurements of thermodynamic properties of compounds at specific temperatures and pressures.
In summary, to calculate the enthalpy of a system, you need to know the internal energy, pressure and volume of the system, and you can use the equation H = U + PV. Enthalpy change can also be calculated by measuring the heat absorbed or released by the system in a calorimetry experiment and adding it to the initial enthalpy of the system. In some cases, the enthalpy is given in thermodynamic tables.
What is Internal energy in thermodynamics?
Internal energy, in thermodynamics, is the total energy of a thermodynamic system that is available to do work. It is the sum of the kinetic and potential energies of all the particles in the system and is represented by the symbol U. It is a state function, meaning its value depends only on the current state of the system and not on how that state was reached. The internal energy of a system can change due to heat or work being added to or removed from the system.
Internal Energy Formula in Thermodynamics?
In thermodynamics, the internal energy of a system, often denoted as U, can be calculated using the first law of thermodynamics, which states that the change in internal energy of a system is equal to the heat added to the system minus the work done by the system. The formula for internal energy can be written as:
U = Q – W
Where:
U = internal energy (in joules or other units of energy) Q = heat added to the system (in joules or other units of energy) W = work done by the system (in joules or other units of energy)
This equation is valid for both closed and open systems. For a closed system, the heat added to the system is equal to the heat absorbed by the system, while for an open system, the heat added to the system is equal to the heat absorbed by the system plus the heat lost by the system.
The internal energy of a system can be increased or decreased by adding or removing heat from the system or by doing work on or by the system. The internal energy of a system can also be changed by changes in the kinetic and potential energies of the particles in the system.
In some cases, the internal energy is given in thermodynamic tables, which are tabulated values of thermodynamic properties of compounds, including internal energy. These tables are based on experimental measurements of thermodynamic properties of compounds at specific temperatures and pressures.
In summary, the internal energy of a system is a state function that describes the total energy of a system. It can be calculated using the first law of thermodynamics, which states that the change in internal energy of a system is equal to the heat added to the system minus the work done by the system. The internal energy of a system can be increased or decreased by adding or removing heat from the system or by doing work on or by the system. It can also be found in thermodynamic tables, which are tabulated values of thermodynamic properties of compounds, including internal energy.
Difference between Enthalpy and Internal energy
Enthalpy and internal energy are thermodynamic properties used to describe the state of a system, particularly in the context of heat transfer and energy changes. While they are related, there are distinct differences between enthalpy (H) and internal energy (U). Here’s a breakdown of their differences:
- Definition:
- Enthalpy (H): Enthalpy is defined as the sum of the internal energy of a system and the product of pressure and volume. Mathematically, it is expressed as H=U+PV, where U is internal energy, P is pressure, and V is volume.
- Internal Energy (U): Internal energy is the total energy contained within a system. It includes the kinetic and potential energy of particles within the system, such as atoms and molecules.
- Expression:
- Enthalpy (H): H=U+PV
- Internal Energy (U): U=kinetic energy + potential energy
- Physical Meaning:
- Enthalpy (H): Enthalpy is often associated with heat transfer at constant pressure. It represents the total energy content of a system, including the energy associated with pressure and volume changes.
- Internal Energy (U): Internal energy represents the microscopic energy content of a system, including the kinetic and potential energy of particles.
- Pressure-Volume Work:
- Enthalpy (H): Enthalpy accounts for pressure-volume work (PV) and is particularly useful in constant-pressure processes.
- Internal Energy (U): Internal energy does not explicitly account for pressure-volume work in its definition.
- Applications:
- Enthalpy (H): Enthalpy is commonly used in thermodynamics, especially in chemical reactions and processes involving gases, where pressure-volume work is significant.
- Internal Energy (U): Internal energy is used to analyze the energy changes within a system, regardless of pressure-volume work.
- Mathematical Relationships:
- Enthalpy (H): ΔH=ΔU+PΔV, where ΔH is the change in enthalpy.
- Internal Energy (U): ΔU=q+w, where ΔU is the change in internal energy, q is heat added to the system, and w is work done by the system.
In summary, while both enthalpy and internal energy are measures of the total energy content of a system, enthalpy explicitly includes the effects of pressure-volume work and is particularly useful in constant-pressure processes. Internal energy, on the other hand, focuses on the microscopic energy content of a system without explicitly considering pressure-volume work. The choice between using enthalpy or internal energy depends on the specific conditions and processes under consideration.
Here’s a table outlining the key differences between enthalpy and internal energy in table format:
| Aspect | Enthalpy (H) | Internal Energy (U) |
|---|---|---|
| Definition | A thermodynamic property that accounts for both internal energy and the energy associated with pressure and volume. | The total energy contained within a system, excluding the energy related to pressure and volume. |
| Representation | H=U+PV | U=Q−W |
| Pressure-Volume Work | Includes work done against external pressure. | Reflects only the work done on or by the system. |
| Application | Commonly used in open systems where work is done at constant pressure (e.g., chemical reactions in a reactor). | Used in closed systems where pressure-volume work is negligible or held constant. |
| Symbol | H | U |
| Mathematical Expression | H=U+PV | U=Q−W |
| Heat at Constant Pressure | Q=ΔH | Q=ΔU+W |
| Heat at Constant Volume | Q=ΔU | Q=ΔU+W |
These differences highlight that enthalpy incorporates both internal energy and the energy associated with pressure and volume changes, while internal energy specifically considers the energy associated with changes in temperature and excludes pressure-volume work. The choice between enthalpy and internal energy depends on the specific aspects of a thermodynamic system being analyzed.
Frequently Asked Questions – FAQs
Enthalpy FAQs:
1. What is enthalpy, and how is it different from internal energy?
Enthalpy is a thermodynamic property that accounts for both the internal energy of a system and the energy associated with pressure and volume. Internal energy, on the other hand, only considers the total energy within the system excluding pressure-volume work.
2. When is enthalpy commonly used in thermodynamics?
Enthalpy is commonly used in open systems where work is done at constant pressure, such as chemical reactions in a reactor.
3. How is enthalpy mathematically expressed?
Enthalpy (H) is mathematically expressed as H=U+PV, where U is internal energy, P is pressure, and V is volume.
4. What role does enthalpy play in chemical reactions?
Enthalpy change in a chemical reaction, often denoted as Δ�ΔH, represents the heat absorbed or released at constant pressure, providing insights into the reaction’s thermodynamics.
Internal Energy FAQs:
5. What is internal energy in thermodynamics?
Internal energy (U) is the total energy contained within a system, excluding the energy related to pressure and volume.
6. When is internal energy commonly used in thermodynamics?
Internal energy is commonly used in closed systems where pressure-volume work is negligible or held constant.
7. How is internal energy mathematically expressed?
Internal energy (U) is mathematically expressed as U=Q−W, where Q is heat added to the system, and W is work done on the system.
8. How is internal energy related to heat and work?
Internal energy change in a process is related to heat (Q) and work (W) through the equation U=Q−W
Comparison FAQs:
9. What is the difference between enthalpy and internal energy?
Enthalpy includes the energy associated with pressure and volume (external work), while internal energy only considers the total energy within the system.
10. When should I use enthalpy or internal energy in thermodynamic calculations?
Use enthalpy when dealing with open systems at constant pressure, and use internal energy for closed systems or when pressure-volume work is negligible.
