Thermodynamic Processes at Constant Pressure (Isobaric Process)

Definition

An isobaric process is a thermodynamic process in which the pressure of the system remains constant throughout the entire transition. In this process, the heat transferred to or from the system results in changes in both the internal energy and the volume of the system, often performing work on or by the surroundings.

Main Content

1. The Ideal Gas Law Relationship

• According to the Ideal Gas Law ($PV = nRT$), when pressure ($P$) is constant, the volume ($V$) of the gas is directly proportional to its absolute temperature ($T$).
• This relationship is known as Charles's Law, which states that for a fixed mass of gas, $V/T = \text{constant}$.

2. Work Done in Isobaric Process

• Since the pressure remains constant, the work done ($W$) by the system is calculated as the product of pressure and the change in volume: $W = P \Delta V$.
• Geometrically, this work is represented by the area under the curve on a Pressure-Volume ($PV$) diagram.

3. First Law of Thermodynamics and Enthalpy

• The First Law states $\Delta U = Q - W$. Substituting $W = P \Delta V$, we get $Q = \Delta U + P \Delta V$.
• This sum ($\Delta U + P \Delta V$) is defined as the change in Enthalpy ($\Delta H$). Therefore, in an isobaric process, the heat added to the system is equal to the change in enthalpy ($\Delta H = Q_p$).

Working / Process

1. Expansion Phase

• As heat is added to the system, the particles gain kinetic energy, causing the gas to expand against the constant external pressure.
• The volume increases, meaning the system does positive work on its surroundings.

2. P-V Diagram Representation

• The process is represented by a horizontal line on a $PV$ graph, moving from left to right as volume increases.

Pressure (P)
^
|-----------> (Expansion)
|          
|           
|____________________> Volume (V)
(Constant P)

3. Thermal Equilibrium

• For the pressure to remain constant, the system must be in contact with a heat reservoir that allows the temperature to adjust gradually as the volume expands or contracts.
• The process must be slow enough that the internal pressure of the gas is always equal to the external atmospheric pressure.

Advantages / Applications

• Heat Engines: Many heat engines, such as the Brayton cycle used in jet engines, operate with isobaric combustion phases.
• Biological Systems: Most biochemical reactions in living organisms occur at constant atmospheric pressure, making the study of enthalpy changes essential for biology.
• Laboratory Experiments: Many chemical reactions performed in open beakers are isobaric, allowing researchers to measure heat exchange by tracking enthalpy changes.

Summary

An isobaric process is a thermodynamic state change where system pressure remains constant while volume and temperature fluctuate according to Charles's Law. Heat added is directly converted into enthalpy change, making it a fundamental concept for understanding engine efficiency and chemical reactions.

• Key concept: Volume is directly proportional to temperature.
• Key formula: $W = P(V_f - V_i)$.
• Important terms to remember: Isobaric (constant pressure), Charles's Law, Enthalpy, and $PV$ Work.