Phase Changes and Kinetic Theory

 General Chemistry Index

Where are we going with this? Describe the characteristics of solids, liquids, and gases and changes in state at the macroscopic and microscopic levels. 

Phase Changes and Kinetic Theory

So, we have names for all the phase changes. So, that's good, right?

Now, we are going to talk about phase changes from the perspective of the Kinetic Theory of Matter.

So, all matter is made up of particles that are in constant motion. The higher the temperature, the faster they move and, consequently, the higher their kinetic energy.

Got it.

Now, we need to get an idea about what makes something into a solid. Or a liquid. Or… You get the idea.

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To super-over-simplify this, let's say that every molecule of a substance is held together bonds of some sort. So, let's get a little crazy and call the bond that holds a substance together a molecular bond.

Each bond has some amount of energy. You can think of it (metaphorically) like a spring, because that will help with the discussion about what happens that leads to phase changes. Remember, this discussion is, in part, only modelling what happens.

Modelling Phase Change

So, picture a solid. To simplify, think in 2 dimensions.

You can think of it as rows and columns of molecules connected by springs. Remember that each of the molecules are moving around (vibrating, if you will, in place) at some rate depending on temperature.

As the temperature goes up, the molecules move faster and faster. Imagine what that would look like if the bonds were springs.

The faster everything shook, the more the springs would stretch out. This is why most things usually expand as the temperature goes up.

Okay… bonds… vibration… temperature… Got it.

Solid

In a solid, the kinetic energy of the molecules is less than the energy of the bonds that holds the molecules together in a fixed arrangement.

Thus…

Solids have a definite volumes (at any given temperature)

Solids have a definite shape

Solid to Liquid

The melting phase change occurs at the point where the kinetic energy of the molecules first becomes greater than the energy of the bonds that holds the molecules together in a fixed arrangement.

At this temperature (melting/freezing point), as energy goes into the substance, instead of increasing the kinetic energy of the molecules, it breaks the bonds that hold the substance in a definite shape. The amount of energy needed to break those bonds is called the heat of fusion.

Melting is endothermic.

Liquid

In a liquid, the molecules are attracted to each other, but not as strongly as in solids.

In a liquid, the kinetic energy of the molecules is greater than the energy of the bonds that holds the molecules together in a fixed arrangement, but less than the energy that holds them into a fixed volume.

Thus…

Liquids have a definite volume (at any given temperature)

Liquids have an indefinite shape.

Liquid to Gas

Vaporization (and Evaporation) occurs when a molecule of a liquid reaches a level of kinetic energy that is greater than the energy that holds liquids in a definite volume.

At this temperature (boiling point), as energy goes into the substance, instead of increasing the kinetic energy of the molecules, it overcomes the attractive forces that give a liquid its definite shape. The amount of energy needed to overcome the attractive forces is called the heat of vaporization.

Vaporization and evaporation are endothermic.

Gas

In a gas, although the molecules are still attracted to each other some, the kinetic energy of the molecules is greater than the energy of the bonds that give them a definite shape and greater than the attractive forces that give them a definite volume.

Thus…

Gases have an indefinite volume.

Gases have an indefinite shape.

Gas to Plasma

 If the kinetic energy (temperature) keeps going up, the molecules will continue to move faster.

Ionization occurs when the kinetic energy of the molecules is so high that the electrons separate from the nucleus. 

Ionization is endothermic.

Plasma

In a plasma, the kinetic energy is so high that the electrons have separated from the nucleus. It can be thought of that the molecules are moving so fast that the electrons have been shaken off.

Plasma to Gas

Deionization or recombination occurs when molecules decrease in kinetic energy such that the electrons are rejoined to the nucleus.

Deionization / recombination is exothermic.

Gas to Liquid

Condensation occurs when the kinetic energy of molecules decreases to a point less than the energy of the attractive forces that give liquids a definite shape.

Condensation is exothermic.

Liquid to Solid

Freezing occurs when the kinetic energy of molecules decreases to a point less than than the energy of the bonds that give solids their definite volume.

Freezing is exothermic.