Basic Concepts of Thermodynamics (1)

1.
System and environment

In thermodynamics, the research object is called the system (or system), and the related parts outside the system are called the environment
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Thermodynamics adds the system and environment together as the universe

CH ₄ +2O ₂ =CO ₂ +2H ₂ O

Then the reactants O ₂ , CH ₄ and the products CO ₂ , H ₂ O in the container are the system, and the airtight container, the experimental platform, the air around the experimental platform, and the ground in contact with the experimental platform are the environment

There can be an interface or no interface between the system and the environment
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In the above example, if N _{2 is} regarded as a system, there is an interface between the system and the environment; if N _{2 is} regarded as an environment, there is no interface between the system and the environment, but an imaginary interface can be designed from the perspective of volume Starting from the concept of, it is believed that the sub-volume of N ₂ belongs to the environment, so it is equivalent to having an interface between the system and the environment

According to the material and energy exchange relationship between the system and the environment, the system is usually divided into three categories
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(1) Open system: There is both energy exchange and material exchange between the system and the environment, such as the reaction between dilute sulfuric acid and zinc particles in a beaker
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(2) Closed system: There is energy exchange but no material exchange between the system and the environment, such as the reaction of oxygen and methane in a closed metal container
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(3) Isolated system: There is neither material exchange nor energy exchange between the system and the environment, such as the reaction of oxygen and methane in a closed, insulated metal container
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2.
State and state functions

The existence form of a system determined by a series of physical quantities that characterize the properties of the system is called the state of the system
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These physical quantities that determine the state of the system are called the state function of the system

For example, the standard state of ideal gas often mentioned in middle school is a state of ideal gas, and the physical quantity of this state is determined, such as n=1mol, T=273K, p=1.
013×10 ⁵ Pa, V=22.
4 dm ^{3 is} the state function
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The state before the system change is the initial state, and the state after the change is the final state
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The initial state and final state of the system are determined, and the amount of change in the state function has a definite value

△n=n ₂ -n ₁ ,△T=T ₂ -T ₁ ,△p=p ₂ -p ₁ ,△V=V ₂ -V ₁

The state of the system is certain, the state function is certain (has a certain value); if one or several state functions of the system change, the state of the system must change
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However, the properties of the system represented by some state functions are non-additive.
These state functions are called strength properties, such as physical quantities such as temperature T, pressure p, and density ρ
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3.
Process and approach

When the state of the system changes, from the initial state to the final state, the system undergoes a thermodynamic process, referred to as process
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Common thermodynamic processes include constant temperature process, constant pressure process, constant volume process and adiabatic process

The constant temperature process refers to the process in which the temperature of the system remains constant during the change; the constant pressure process refers to the process in which the pressure of the system remains constant during the change; the constant volume process refers to the process in which the volume of the system remains constant during the change; the adiabatic process is Refers to the process in which there is no heat exchange between the system and the environment in the process of change
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The state function is only related to the state of the system.
The initial state and the final state of the system are fixed, and the amount of change in the state function that determines the state of the system is determined
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Based on this, the isothermal process can be defined as the process in which the initial state and the final state of the system have the same temperature (△T=0), and the isostatic process is the process in which the initial and final states of the system have the same pressure (△p=0) , The isovolumetric process is the process in which the initial state and the final state of the system have the same volume (△V=0)

There are different ways to complete a thermodynamic process, and these specific ways are called pathways
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Figure 2-1 Three different approaches to the same process

Taking the process of an ideal gas expanding from p ₁ =16×10 ⁵ Pa, V ₁ =1dm ³ to p ₂ =1×10 ⁵ Pa, V ₂ =16dm ³ under constant temperature conditions as an example, to complete such a thermodynamic process can be taken There are many different ways.
Figure 2-1 shows three of them
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Obviously, the process and the approach are essentially different.
The process only cares about the initial state and the final state of the system, while the approach focuses on the specific way of realizing the process
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