Internal Energy and Enthalpy: First Law of Thermodynamics: Internal Energy and Enthalpy The First Law of Thermodynamics states that the total energy of an i...
Internal Energy and Enthalpy:
First Law of Thermodynamics: Internal Energy and Enthalpy
The First Law of Thermodynamics states that the total energy of an isolated system remains constant over time. This means that the internal energy of a system, which encompasses the kinetic and potential energy of its molecules, cannot decrease or increase, regardless of the changes occurring within the system.
Internal Energy (U)
Internal energy refers to the total kinetic and potential energy of all the molecules within a system at a specific temperature. It represents the internal energy of the system, which can be calculated using various methods.
Enthalpy (H)
Enthalpy, on the other hand, is a thermodynamic property that represents the total energy of a system, including both internal energy and external energy (work done by the system). Enthalpy can be calculated using the following formula:
H = U + PV
where P is the pressure, V is the volume, and U is the internal energy.
Changes in Internal Energy and Enthalpy
When the internal energy of a system increases, the system's internal energy also increases. Conversely, when the internal energy decreases, the system's internal energy decreases as well.
The change in internal energy (ΔU) is equal to the negative of the change in enthalpy (ΔH). This implies that the internal energy of a system can never decrease, and that any change in energy must be accompanied by an increase in enthalpy.
Examples
When you ignite a candle, the internal energy of the candle's molecules increases due to the transfer of heat energy from the surroundings. This leads to an increase in the system's internal energy, resulting in the burning process.
When you open a door, the internal energy of the air inside the room increases, while the internal energy of the room itself decreases. This is because the energy from the door is transferred to the air, resulting in an increase in the air's internal energy.
When you dissolve a solid in water, the internal energy of the solid increases, while the internal energy of the water decreases. This is because the solid's particles have more energy due to their increased motion compared to the water molecules