Kinetic Molecular Theory

Kinetic Molecular Theory

Kinetic Molecular Theory – is the idea that matter is made from moving invisible particles.

The following are the principals of Kinetic Molecular Theory:

All matter is made up of tiny particles

Different substances have different particles

The particles are in constant motion

The more energy the particles have, the faster they move.

The attraction between particles decreases with an increase in distance.

SolidSolids

Distance Particles are close together

Type of Motion Particles can only vibrate in their place in the structure

Attractive Forces

High, decreasing as vibration gets higher

Energy Increasing energy causes an increase of vibration

LiquidsLiquids

Distance Particles are kind of close together

Type of Motion Particles can still vibrate, but can now move past one another; can bump into each other and the sides of the container it is in

Attractive Forces

Still quite high, but less than solids and decreasing as distance apart increases

Energy Increase in energy causes an increase in vibration and movement

GasGas

Distance Particles are very far apart

Type of motion

Particles vibrate, rotate, move past each other, and bump into each other in a very rapid straight line of motion

Attractive forces

No attractive forces; particles are too far apart and are moving to fast.

Energy Increase in energy causes an increase in pressure due to the increase in the speed and number of particles hitting the sides of the container.

Change of StateThe change of state only happens when you add or remove energy.

What we experience as heat, molecules experience as vibration.

An increase in movement of particles results in the melting or sublimation of a solid, and evaporation of a liquid.

The reduction in movement of particles results in the condensation or deposition in gases and solidification in liquids.

A word on sublimation and

depositionSublimation, and deposition, is the ability to turn a solid into gas or a gas into a solid without passing through the liquid stage.

In order for this to occur you need to have an object reach a specific temperature and a specific pressure.

The point in which sublimation and deposition is called the triple point. The point on a pressure vs temperature graph where solid, liquid and gas meet.

Triple point of Water and Carbon

DioxideWater’s triple point – where a solid piece of water, liquid and gas can all co-exist - is at 273.16 K (0.01º C) and a pressure of 611.73 pascals (.006 atm). It is at this point where water in a solid form and has a gas coming off of it.

Dry ice’s (Carbon dioxide’s) triple point is 518 kPa (kilopascals) and -56.6ºC or 216.55 Kelvin.

Phase Diagram of H2O

p (

atm

)

T (oC)

SL

G

.01 374

218

0

.006

1

100

Triple Point

S L

L G

S G

Phase Diagram of CO2

p (

atm

)

T (oC)

L G

S G

S L

SL

G

Triple Point

Critical Point

-57

5.1

31

73

1

-78

WaterLets take water for a moment:

When it is in a solid form and we add energy (heat) we notice that the molecules vibrate a lot and turns into a liquid.

When it is in a liquid form and we add energy (heat) we notice that the molecules vibration increases and turns into a gas.

But if we cool the molecule at any stage, we slow down the vibration of the molecules and can reverse the process.

Explaining Dissolving

The kinetic molecular theory perfectly describes how compounds, like sugar, dissolve in water.

When water is in a liquid state, the molecules are vibrating and have space between them for sugar to enter into the water and dissolve.

If you increase the heat of the water, the vibration of the water molecule increases, thus more space is created between the water molecules.

Thus sugar has more space to enter in between the water molecules and thus the sugar will dissolve a lot faster.

Explaining Density

Problem, why is it that the density of water in solid form has a different density when it is in liquid form and a different density when it is in a gas form?

Answer, the kinetic molecular theory.

When you add energy to a particle and increase the vibration of the particle the particles in the compound expands and thus the volume changes.

If the volume increases and the mass stays the same, the density decreases and viola, we now know exactly why ice floats in water.

Finally….Ever notice on sidewalks and on train tracks there are gaps between the concrete sidewalk and concrete road?

Well kinetic molecular theory explains why we need this.

If a sidewalk was one solid strip without any gaps, during cooler months the sidewalk contracts and during warmer months the sidewalk expands.

This constant motion of contraction and expansion leads to the sidewalk cracking or buckling.

If you put in gap when laying the concrete, you mitigate and minimize the amount of cracks that will be created.