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gas

This is a mind map of gases in chemistry. It gives a general introduction from two aspects: the equation of state of ideal gases and the comparison between actual gases and ideal gases. Collect the picture below to learn!

Edited at 2021-09-08 21:14:09

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Outline

gas

ideal gas equation of state

formula

PV=nRT

Conditions: Only suitable for low pressure gases

Scientific abstract concepts do not exist objectively

Comparison of real gases and ideal gases

Microscopic model of an ideal gas

The volume of the molecule is negligible

There is no interaction force between molecules, and there is no potential energy between molecules.

The distance between molecules becomes larger or smaller (the volume of the gas expands or compresses), and there is no potential energy between molecules.

However, actual gas molecules have a volume, and the molecules cannot be regarded as elastic particles that move freely; there is an interaction force between the molecules, so when the gas expands and compresses, the potential energy changes.

Van der Waals equation - behavior of real gases

p=(p a/Vm2);V=(Vm-b)

a/Vm is called internal pressure, which is a correction to the intermolecular attraction. The existence of this attractive force weakens the pressure exerted by the molecules on the container wall, reducing the actual gas pressure.

b is called the occupied volume, which is a correction to the volume (a reduction in the effective total volume). Statistical thermodynamics proves that the value of b is equal to 4 times the volume of the molecule itself

p-V diagram of an ideal gas

Isotherm

Mixing ideal gases

Dalton's law of partial pressures

partial pressure definition

The pressure generated when a certain component B in the mixed gas exists alone and has the same temperature and volume as the mixed gas is called the partial pressure of component B, expressed in pB

formula

The total pressure of a mixed gas is equal to the sum of the partial pressures of each gas. p(total) = pB; pB = ptotal XB (XB is the mole fraction)

Be applicable

The law of partial pressure only applies to low pressure gases or ideal gases

Armager's law of partial volumes

Partial volume definition

The volume produced when a certain component B in the mixed gas exists alone and has the same temperature and pressure as the mixed gas is called the volume of component B, represented by VB.

formula

At a certain T, p, the volume of the mixed gas is equal to the sum of the partial volumes of the components that make up the mixed gas. V(total)=VB; VB=VtotalXB (XB is the mole fraction)

Be applicable

The partial volume law only applies to low pressure gases or ideal gases

First law of thermodynamics

Introduction to Thermodynamics

thermodynamics

definition

The science that studies the laws that should be observed in the process of mutual conversion of energy

Research object

A macroscopic system with a large number of particles (regardless of microscopic properties and the behavior of individual molecules and atoms)

Several basic concepts

System and environment

System: Research object

The object of thermodynamics research is a macroscopic system composed of a large number of particles, and the properties studied are all overall average properties, such as T and P.

Classification

open system

There can be both material and energy exchange between the system and the environment.

closed system

There cannot be material exchange between the system and the environment, but there can be energy exchange

isolated system

There is neither matter nor energy exchange between the system and the environment

environment

Parts outside the system that interact with the system

States and state functions

state

Comprehensive expression of the physical and chemical properties of the system

state function

definition

The properties of the system when it is in a certain state are the macroscopic physical quantities of the system itself, such as: T, p, V, m, U, H, S...

Classification

breadth property

The value is proportional to the amount of matter in the system and is additive

Strength properties

Numerical values are independent of the amount of matter in the system and are not additive

Notice

Divide two extent properties to get the intensity property

Features

The state function of the system can only explain the state of the system at that time, but cannot explain the previous state of the system.

The change amount of the state function is only related to the initial and final states, and has nothing to do with the change path.

Small changes in the state function can be expressed by total differential expansion

process and approach

process

When the state of a system changes, it is said that a process has occurred (the system reaches a new thermodynamic state from one thermodynamic state.)

way

definition

The specific steps in which a system changes are called pathways

Classification

Simple state change process

The aggregation state and quantity of materials in the initial and final states remain unchanged, only the PVT is different.

phase change process

The physical state of an object changes, but the quantity remains unchanged. gas liquid solid

Chemical changes

The initial state and final state of matter are different

thermodynamic equilibrium system

There is no exchange of matter or energy between the system and the environment, the state functions of the system do not change with time, and the system is in thermodynamic equilibrium.

A thermodynamic equilibrium system must be in the following four equilibria:

Thermal equilibrium

The temperature of all parts of the system is the same and equal to the environment

force balance

The pressure in the system is equal everywhere and is equal to the environment

chemical equilibrium

Chemical reactions between different substances reach equilibrium and the composition of the reaction does not change with time

phase equilibrium

The number and composition of phases do not change with time, and there is no macro-phase change

Conservation of Energy - First Law of Thermodynamics

Energy conservation law

Energy in nature cannot be created out of nothing, nor will it disappear invisible. It is conserved.

Energy can be converted from one form to another, such as heat and work. But during the conversion, energy remains conserved.

Perpetual motion machines of the first kind are impossible

Thermodynamic energy (internal energy)

concept

The total energy within the system. It includes all forms of energy in the system, excluding the overall kinetic energy and overall potential energy of the system. Internal energy forms: translational kinetic energy, rotational energy, vibrational energy, electron kinetic energy and atomic nuclear kinetic energy of molecules.

Thermodynamic energy is the state function of the system. When the system state changes, the change of U only depends on the initial state and has nothing to do with the change path.

dU is mathematically a total differential

Thermodynamic energy is a capacity property, and its value is proportional to n in the system

The absolute value of thermodynamic energy cannot be measured now, but what is important is the change in internal energy,

work and heat

concept

achievement

In addition to heat, other forms of energy transfer between a system and its environment are collectively called work.

The most common work in physical chemistry is volume work, which is the work caused by changes in the volume of a system. All work except volume work is collectively called useful work in physical chemistry.

hot

Energy transfer due to temperature differences between the system and the environment

Features

Work and heat are the only two forms of energy transfer between a system and its environment

Work and heat are related to the specific process of the system. Without the process, there would be no heat and work, and their numerical values are related to the change path.

It is not a state function of the system. It cannot say how much heat or work is in a certain state of the system.

Work and heat do not have total differentials

The changes in the state function of the forward process and the reverse process of any process are equivalent and opposite, but the process quantity does not have this rule.

mathematical expression

The increase in thermodynamic energy of a closed system ΔU is equal to the sum of the heat absorbed from the environment and the work done by the environment on the system ΔU = Q W

volume work

definition

The work exchanged between the system and the environment caused by changes in the volume of the system is called volume work.

Volume work is the work done by the system against external pressure (expansion work) or the work done by the environment on the system (compression work)

formula

W= – ∫nRT dV/V = nRT ln(V1/V2)

Reversible and impossible processes

If the state of the system changes, from the initial state to the final state, and then from the final state to the initial state, depending on whether the environment can be restored at the same time, the process can be divided into reversible and irreversible processes.

Reversible process

The system goes through a certain process from the initial state to the final state. If a path can be found to restore the system state to the initial state and the environment to its initial state at the same time, then the path of the system from the initial state to the final state is a reversible process. ("Double Recovery").

Under the same conditions, among the various paths of expansion from the initial state to the final state, the system does the greatest work to the outside world during the reversible process; if it compresses from the final state to the initial state, the environment does the least work to the system.

Features

The process of a reversible process is composed of countless infinitesimal processes. During the entire reversible process, the system is always infinitely close to the equilibrium state (reversible == equilibrium)

Reversible processes are infinitely slow

Reversible processes are most efficient

There is no reversible process in the strict sense, it is an ideal process. In nature, some processes are very close to reversible processes, such as phase change processes of substances under equilibrium conditions, reversible chemical reactions, etc.

irreversible process

In the above expression, if it is impossible to find a way to restore the system state to the initial state and the environment to its initial state at the same time, then the path from the initial state to the final state experienced by the system is an irreversible process.