KMT and Physical States

KMT and Physical States

In this portion of the course we are going to focus on the states or phases of matter (solid, liquid and gas). We are going to approach this topic from the following perspective:

The physical state that a sample is found in is directly related to the

Kinetic Energy of the molecules in that sample.

Due to the differences in how the molecules move (or don’t move) in each phase of matter, they all behave differently. We will look at each state at a molecular level, explore the properties of each and analyze how the molecular movement changes as we move from solid to liquid and finally to gas.

There will also be some discussion on the uniqueness of water, the various “liquid” samples that exist and how we can differentiate between them. To finish up our study of physical states we will study how changes in pressure, volume, temperature and molar amount can cause changes in state.

What is KMT?

KMT stands for Kinetic Molecular Theory: It is a series of assumptions that allow us to explain the observed properties and the behavior of matter in each phase on a molecular level.

For each phase we will first look at the assumptions of the KMT and then create a definition or explanation for the phase.

KMT Gases:

1.  Gases consist of tiny particles that are in random ceaseless motion.

2.  Collisions between molecules and the container walls are always 100% elastic. (Elastic Collisions are ones in which no energy is lost during a collision)

3.  The space between gas particles is very large compared to the minute size of the particles themselves

4.  There are no attractive or repulsive forces between molecules when they are in the gas phase & the volume of the particles themselves is negligible.

5.  The Kinetic Energy of the particles in the gas is directly proportional to the temperature.

So what do gas molecules look like?

Imagine a glass cube with super bounce balls inside.

The balls are bouncing off the walls and off each other,

however, they never slow down or stop moving!

Common examples:

·  Bingo machine

·  Lottery machine

·  Cash grab booth

How do we define the Gas Phase?

A state of matter where the particles show fluidity, are in random, ceaseless motion and will distribute themselves evenly throughout the entirety of the container, thus the shape and volume of a sample in this form is determined entirely by the size and shape of its container.

Properties of Gases:

Low Density: Gases have extremely low densities, a gas is approximately 1/1,000th that of a solid or liquid of the same substance.

Compressibility: because of the low density, a gas's volume can be decreased by adding pressure ... this property is called compressibility.

Expansion or Diffusion: Because of the random ceaseless motion of the molecules in a gas state the particles will reach out to all regions of the container the substance is in. There will be an even distribution of the molecules throughout the container. A more specific explanation of this property will be covered in more detail when we get to Graham's Law of Diffusion ... a law that discusses the rates at which substances will expand in the gas phase.

Fluidity: Because the molecules are far apart and collide in a fashion similar to pool balls, the molecules are said to be fluid. By fluid I mean to refer to their ability to move past each other with relatively no resistance. Another word synonymous with Fluidity is Flowing...this paints a picture of a group of particles sliding by one another smoothly.

KMT Liquids:

The Four Assumptions of the Liquid KMT:

1. Liquids consist of tiny particles that are close together and are in constant rotational and translational motion.

1. The particles in the liquid state are held together by Intermolecular Molecular Forces (IMFs) that have a greater effect on the particles because of the lower kinetic energy (KE) of the particles.

1. Due to the closeness of the particles with respect to one another along with the attraction due to the IMF combine to provide the fluid nature of liquids.

1. Just like in gases, the KE of the particles in this state is directly proportional to the Absolute Temperature.

So what do Liquid molecules look like?

In the liquid state the particles are very

close together (compared to gas particles

that are very far apart) and have

translational and rotational motion that

give this state a fluid nature.

Common Examples:

·  Lava Flow

·  White Water

Here is an example of water flowing

around the stones and boulders … this

exemplifies fluidity.

Liquid: state of matter where the particles are

in no set arrangement, but show signs of fluid

motion. The shape of the material in this state

is determined by it container since liquids

always take the shape of their container. The

volume of the substance in this state is fixed

Many samples of matter that we consider to be “liquid” are actually not. By definition a liquid is a sample of a pure substance with a set volume, whose molecules have translational and rotational motion and thus takes the shape of its container. So what are all the other “liquid samples? Read on…

Liquids, Solutions, Colloids & Suspensions…

Ø  In this section we are going to take a look at mixtures in the liquid state. There are three types of mixtures that can exist in the liquid state. All three are dependant upon the size of the particles that are distributed throughout the liquid medium.

P  The Classification of these liquid mixtures is primarily dependant on the size of the particles. The size of the particles typically tells us whether the particle will stay evenly dispersed in the liquid or if it will settle out.

P  The table below shows a summary of the properties of each of the three types…

P  Here is a molecular model for a solution…

Net Ionic Equation for Salt Water:

NaCl + H2O à Na+ + Cl- + H2O)

Mg(OH)2 à Mg+2 + 2 (OH)-1 + H2O

The purpose of the net ionic equation is to illustrate how ionic compounds break into ions in solution and to show the total number of particles in a mixture…this will become very important in later chapters…here it is just to show that ionic compounds break into individual ions in solution.

P  Here is a molecular model of a colloid…

Milk, as well as other colloids, is composed of water and high numbers of colloids that are evenly distributed throughout the sample of water. The colloids can be held in place versus the particles in a suspension because their particle size is small enough that the structure of the water is able to keep them from settling out.

If you look at the table above that summarizes the three classes of the liquid mixtures, you will see that the two that we have discussed thus far have particles that remain dispersed in the liquid phase. The primary way that we can tell the difference between these two types of liquid phase mixtures is by passing a beam of light through the sample and observing the interference of the light or lack thereof.

The Tyndall Effect: when a beam of light is passed through a sample of a liquid where the particles dispersed within cannot be seen, the beam of light is scattered. You can notice a broadening of the beam as well as noticing the path that the light follows through the sample. This Tyndall Effect is noticeable when a car’s headlights cut through fog, water vapor or dust.

When a light beam is passed through

a solution the particles (individual ions)

are too small to scatter the beam thus

the Tyndall Effect is not observed.

Here is a molecular model for a suspension…

In this model the particles of clay are temporarily held between the water molecules. After some time, the clay particles, due to their mass and lack of attractive forces, settle out and form a layer at the bottom of the glass. This is similar to what happens in rivers after big storms. The mass induction of water from the storm pulls soil from the land. While the river is moving quickly the soil is suspended in the water. When the storm subsides the soil will settle out and deposit on the riverbed. Below is a picture that show the sample after the clay has settled out.

The Particles in this type of liquid mixture are visible so the Tyndall Effect does not need to be observed. In most cases, Colloids are not transparent or even translucent, so the light would be completely absorbed by the mixture.

Information about Solutions…

As we have already learned, solutions are homogeneous mixtures in the liquid phase. These mixtures can be produced from mixing pure substances together.

There are two components in a solution:

•  The solvent is the dissolving medium (the one that makes up the majority of the mixture).

•  The solute is the substance that is dissolving in the solvent (the one that comprises the minority of the solution).

When the amounts of each substance are approximately equal, then it is not worth the energy or the time to determine which is which.

When the solution is made, the solute particles are pulled apart and then evenly distributed throughout the solvent. These particles are not visible to the naked eye.

There are, however, some occurrences when the solute cannot be broken down in the solvent. These are deemed insoluble. For a quick reference you could look at the solubility rules or follow the general rule of thumb: “like dissolves like.”

When two substances are not soluble, they may show this in a variety of ways…

Liquid Solute and Liquid Solvent Interactions.

•  If two liquid substances are able to freely dissolve in one another are deemed Miscible. When miscible liquids combine you end up with a homogeneous liquid mixture as shown in Figure A below.

•  If two liquids are not soluble in one another, they are said to

be Immiscible. When two immiscible liquids are mixed they

will separate and form a distinct interface as shown in the

figure below in Figure B.

Gas Solute and Liquid Solvent Interactions:

You don’t typically think about gases being dissolved in liquid solvents but it is a fairly common occurrence. Gases dissolve into liquids all the time…when water vapor molecules collide with the surface of a sample of liquid water they are pulled into the sample. Another common occurrence is the dissolution of carbon dioxide gas into pop. But how do they get the carbon dioxide to dissolve into the pop? Why does it immediately come out of solution when the cap is removed?

To better understand the solubility of gases in liquids we must look into some dissolution techniques used to create the solution in the first place. In the bottling industry pop and other carbonated drinks are bottled under high pressure CO2 (5 – 10 atm). When the liquid is placed into the bottle along with the CO2, some of the gas can be dissolved in the liquid. When the drink is bottled, the CO2 pressure above the liquid is very high, which allows the CO2 to remain in solution until the cap is removed, decreasing the pressure above the liquid solution. This sudden release of a gas from a liquid is known as effervescence.

William Henry studied the impact of Pressure on the Solubility of a Gas in a Liquid and developed a law to explain it.

Henry’s Law: the solubility of a gas in a liquid is directly proportional to the partial pressure of that gas on the liquid’s surface (when temperature is constant).

As the pressure of the gas above the liquid solvent increases,

the solubility of that gas also increases.

The Effects of Temperature on Solubility…

Effects on Gas Solubility:

As the temperature of a system is increased, the KE of the gas molecules in solution also increases. Due to the greater KE, more gas molecules are able to break the attractive forces of the solvent.

Effects on Liquid and Solid Solubility:

As the temperature of a system containing a solid or a liquid solute is heated, the increased KE of the solute molecules typically yields higher solubility. Although the general trend holds true for most solids and liquids, some are conversely affected. And among those whose solubility does increase with temperature, there are drastic variations in the magnitude of the effect of temperature from substance to substance.

An example temperature change on the solubility of a solid is the difference in dissolution rates in iced tea versus hot tea. The rate at which sugar can be dissolved in the hot tea is much faster than that of the iced tea.

Why does this occur? Again refer back to the effect that increased KE has on the motion and freedom of molecules. As the temperature increases, so does the KE of the molecules, therefore, the energy of the solute and solvent are increased allowing for faster diffusion of solute particles throughout the solution.

Effects of Increased Surface Area on Rate of Diffusion:

The surface area is another factor affecting the rate of solubility for solid solutes. The rate of dissolution increases with an increase in the surface area.

By crushing large crystals into smaller, finer crystals the solvent has more area to attack and therefore can remove ions or molecules from the crystal surface at a much greater rate.

Effects of Solution Agitation on the Rate of Dissolution:

Agitation refers to the mixing or stirring of a solution.

Agitation speeds up the rate of diffusion and dissociation by dispersing the solute particles from the areas of high concentration right around the solute crystal(s) to regions of lower concentration elsewhere in the solution.