Ch 23 pp Lecture 3 – The Ideal Gas

What is temperature?

 Statistical mechanical principles allow us to calculate the properties of a complex macroscopic system from its microscopic characteristics (energy levels) A fundamental tool of statistical mechanics is the Boltzmann distribution that describes how energy levels are occupied for a system at thermodynamic equilibrium  From the partition function we can derive thermodynamic properties such as pressure, energy or entropy  By being able to relate macroscopic properties to the microscopic molecular properties of a system, we can use macroscopic measurements to obtain microscopic parameters such as energy differences between states Summary of lecture 2

In the next two lectures, we will reexamine ideal gases from the point of view of their microscopic nature The ideal gas is the simplest macroscopic system and therefore an ideal introduction to more complex macroscopic collections of molecules By doing so, we will be able to introduce molecular motion and transport, and therefore apply the statistical mechanical tools we have been describing to characterize transport properties of biological molecules (diffusion, separation, etc.) The Ideal gas revisited

We will relate molecular motion to temperature and mass We will then introduce collisions between molecules, as occurs in denser states of matter such as liquids. In this more general case, molecular motion depends not just on mass and temperature, but also on size and shape We will define concept such as sedimentation (the motion of a molecule in a gravitational or centrifugal force), viscosity (the resistance to flow; when macromolecules are added to a fluid, its viscosity depends on the size and shape of the molecule) and electrophoresis (how charged molecules move in an electric field, a key technique to measure the size and shape of macromolecules like proteins and nucleic acids) The Ideal gas revisited and biological separation techniques

By measuring transport properties, we learn about the size and shape of biological molecules in solution: are they big or small, compact or extended, rigid or flexible We will use statistical mechanical methods to relate macroscopic (transport) and microscopic (size and shape) properties of the molecules we study By doing so, we learn how to characterize, separate and purify proteins or nucleic acids, for example we are able to sequence DNA by separating it on a gel under an electric field or to measure the size of proteins or nucleic acids and how they interact with each other The Ideal gas revisited and biological separation techniques

The macroscopic description of a substance consists of an equation of state, which provides a relationship between the state variables P, V, and T The ideal gas equation PV=nRT is an example of an equation of state (n is the number of moles of gas) Does statistical mechanics allow us to reinterpret the ideal gas equation in terms of the molecular properties of the gas? The ideal gas: statistical mechanical description

Does statistical mechanics allow us to reinterpret the ideal gas equation in terms of the molecular properties of the gas? Of course it does; remember that, for an isolated system composed of N non-interacting particle: The ideal gas: statistical mechanical description We will use Boltzmann distribution to derive the equation of state for an ideal gas

What is an ideal gas from the microscopic perspective? It is a system composed of N non interacting particles of mass m confined within a certain volume V=abc, where a, b and c are the dimensions of the container In the first lecture, I have provided the expression for the energy levels for such a system: The ideal gas: partition function

We have also calculated the partition function for the 1- dimensional case under the assumption that the energy levels are spaced very close together (corresponding to a system of large mass, or a macroscopic, classical system) The ideal gas: partition function Because the expression for the energy level is a sum, we can do the same calculation independently for each dimension:

We can now use the above expressions The ideal gas: partition function To obtain, for one mole of gas:

Here we will described an ideal gas from in its mechanical properties We aim to provide insight into the microscopic meaning of temperature and on transport properties It will also introduce the subsequent analysis of transport properties of biomolecules We will do so by relating the pressure of a gas with the collisions of the gas molecules against the walls of the container The ideal gas: microscopic interpretation of temperature

An ideal gas is a collection of a very large number of particles (molecules or atoms). At a given instance in time, each particle (mass m) in the gas has a position described by (x,y,z) and a velocity (u,v,w), where u, v, and w are the x, y, and z components of the velocity, respectively. The history of a particle’s position/velocity is called a trajectory. Each point on a trajectory of a particles is described by 6 parameters (x,y,z,u,v,w). The speed of a particle c is related to the components of its x, y, and z velocity components (i.e. u, v, and w, respectively): The ideal gas: microscopic interpretation of temperature

A trajectory is obtained by applying the laws of classical mechanics, also called Newton’s laws of motion The ideal gas: microscopic interpretation of temperature Pressure is the force F exerted per unit area A of the container wall by gas molecules as they collide with the walls In principle, one should be able to obtain an expression for the pressure P by applying Newtons’ laws of motion This mechanical view of pressure is called the Kinetic Theory of Gases

The ideal gas: microscopic interpretation of temperature Newton’s First Law of Motion (Law of Inertia): Every body persists in its state of rest or of uniform motion in a straight line unless it is compelled to change that state by forces impressed on it  Newton’s Second Law of Motion: The sum of all forces (F) acting on a body with mass (m) is related to its vector acceleration (a) by the equation F=ma  Newton’s Third Law of Motion: To every action there is always opposed an equal reaction; or, the mutual actions of two bodies upon each other are always equal, and directed to contrary parts

The kinetic (i.e. mechanical) theory of gases is based on the following assumptions: The ideal gas: microscopic interpretation of temperature A gas is composed of molecules in random motion obeying Newton’s laws of motion The volume of each molecule is a negligibly small fraction of the volume V occupied by the gas No appreciable forces act on the molecules except during collisions between molecules or between molecules and the container walls Collisions between molecules and with the container walls conserve momentum and kinetic energy (elastic collisions)

The ideal gas: microscopic interpretation of temperature Consider a particle with mass m moving with velocity u (i.e. in the +x direction). It collides with a wall of unit area A

Momentum p of a particle is mass times velocity. Just prior to a wall collision the momentum of a gas particle is p=mu. Just following an elastic collision the momentum is p=-mu (the particle has the opposite direction, i.e. it is moving in the –x direction). The change in momentum  p is: The ideal gas: microscopic interpretation of temperature Suppose in an instant of time  t a particle with velocity u covers a distance d, collides with the wall and subsequently covers a distance d; then u  t=2d or  t=2d/u; the change in momentum during the time period  t is given by

From Newton’s Second Law (see above) this is the force exerted by the gas particle as it collides with the wall: The ideal gas: microscopic interpretation of temperature To get the total force F resulting from the collisions of N particles against an area A of the container wall, add up all the particle forces f i :

This equation can be rearranged to read: The ideal gas: microscopic interpretation of temperature Now just divide both sides by A to get the pressure:

This mechanical calculation states that the product of pressure P and volume V is proportional to the average squared velocity Temperature does not explicitly appear in this equation, and it cannot: temperature is not a mechanical concept, it is a thermodynamic, macroscopic concept The ideal gas: microscopic interpretation of temperature

Let us reconcile the macroscopic view of the pressure of an ideal gas PV=nRT and the microscopic (mechanical) view of the gas; let us being back the statistical mechanical formulation we have give earlier: What is temperature? PV=nRT mechanical description statistical description

We can interpret the temperature, a thermodynamic concept, in terms of the average velocity or kinetic energy of the molecules that compose the gas What is temperature? PV=nRT mechanical description statistical description

Pressure is related to the average kinetic energy per molecule Temperature is a measure of the average kinetic energy of each molecule in the gas The proportionality constant that relates T and average molecular kinetic energy is Boltzmann’s constant What is temperature?

Every atom or molecule in every molecule (protein, nucleic acid or oxygen) in any environment has an average kinetic energy proportional to its T Average speed depends on molecular mass in addition to T What is temperature?

The average kinetic energy of molecules or atoms depends only on the temperature of the system; this is true for the translational energy of solids and liquids as well The average speed is an important quantity; for example, the rate at which molecules collide (an important determinants of chemical reactivity) depends on it Of course not all gas molecules in a container have the same speed: molecular speeds are distributed (like exam grades) What is the distribution like? What is temperature

Of course not all gas molecules in a container have the same speed: molecular speeds are distributed (like exam grades); what is the distribution like? The Maxwell-Boltzmann distribution We should now be used to the idea of averaging microscopic properties using Boltzmann distribution to calculate macroscopic properties of a system For the average (or mean) squared speed:

In the weighted average equation we are averaging over the groups of molecules with equal speeds, where n i is the number of molecules with speed c i, and f i =n i /N is the fraction of molecules with velocity c i (distribution) We also know that energies associated with the motions of microscopic particles are quantized. However, the spacing between energy levels is very small for large amplitude motions such as molecular translation; thus, quantization is not an important effect in molecular speed distributions Because the energy level spacing is small for the translational motion, the sum The Maxwell-Boltzmann distribution can be replaced by:

In general, the average of any property can be calculated from the distribution as follows: The Maxwell-Boltzmann distribution

The function f(c) represents the probability of the particle having a certain speed c and is called Maxwell-Boltzmann speed distribution function What is the form of the speed distribution function? In lecture 2 we considered the general form for the Boltzmann distribution function, which in quantized systems gives the population of particles in the    energy level is proportional to The Maxwell-Boltzmann distribution

Let us consider that the only energy of the system under consideration is kinetic energy The Maxwell-Boltzmann distribution the probability of finding a molecule between c and c+dc is: since: then:

This is the Maxwell-Boltzmann speed distribution function that can be used to calculate mechanical averages: The Maxwell-Boltzmann distribution In the argument of the exponential, the energy is the molecular kinetic energy The constant term The term ensures that skews the distribution function toward higher speeds

How does the probability depends on the mass of the molecules and the absolute temperature? The Maxwell-Boltzmann distribution

How does the probability depends on the mass of the molecules and the absolute temperature? The Maxwell-Boltzmann distribution The graph shows the results of experimental measurements of the distribution of molecular speeds for nitrogen gas N 2 at T=0 o C, 1000 o C, and 2000 o C; each curve also corresponds very closely to the Maxwell-Boltzman distribution. The y axis is the fraction of molecules with a given speed (x axis)

The Maxwell-Boltzmann distribution As the temperature is increased, the distribution spreads out and the peak of the distribution is shifted to higher speed; that is why T has such an effect, for example, on reaction rates (for a given mass, molecules move faster, encounter each other more frequently)

We can use the Maxwell-Boltmann distribution to calculate the mean speed and the mean square speed : The Maxwell-Boltzmann distribution