Kinetic Molecular Theory Chemistry

Kinetic Molecular Theory The theory that the behavior of substances can be explained by the motion of the molecules that make up the substance. Originally developed to explain the behavior of gases. It was so successful it was expanded to explain solids and liquids.

Assumptions KMT makes about gas molecules. 1. The molecules of a gas are so far apart that the volume of the molecules is negligible. This explains why gases have a very low density and are transparent. It also explains why gases can be compressed, because the molecules can be pushed closer together.

2. The molecules of a gas move in rapid straight line motion. That is they move in a straight line until they bump into something, then bounce off and move in a straight line in another direction. This explains why we can move through gases, and why gases diffuse, or spread out into each other. Rapid Straight Line Motion

3. The molecules of a gas have no attraction or repulsion for each other. This explains why a gas does not change volume if left alone. A balloon does not blow up by itself, or contract if nothing is changed.

4. The collisions between the molecules are perfectly elastic. An elastic collision is a collision where no energy is lost. The objects bounce off each other with the same energy they collided with. A car crash is an inelastic collision. Some of the energy is used to do work in deforming the cars. The collisions in air hockey are close to being elastic. Very little energy is lost due to friction. This explains why a gas does not continually cool down. If the collisions were inelastic the molecules would lose kinetic energy and slow down. This would cause the sample of gas to decrease in temperature.

5. The molecules of gas have a distribution of kinetic energies. At any temperature the molecules have the same average kinetic energy. However, not all of the molecules have the same kinetic energy. Some are moving slowly, and others faster.(blue line) As temperature increases the average kinetic energy increases and the distribution of kinetic energies gets broader. (red line)

Solids 1. The molecules of a solid are in a close packed arrangement. This explains why solids have a very high density, and are incompressible. This also explains the crystalline structure of solids

2. The molecules have a strong attraction for each other. This explains why solids have a fixed shape and volume.

3. The molecules of a solid vibrate around a fixed point. This explains why solids have a fixed shape. It also explains why we cannot go through a solid.

Liquids 1. The molecules of a liquid are closer together than a gas, but farther apart than a solid. This explains why liquids have a density between solids and gases. Liquids are slightly compressible.

2. The molecules of a liquid have slight attractions for each other. This explains why a liquid has a fixed volume.

3. The molecules are free to move through out the liquid. The molecules move by sliding over each other. This explains why liquids take the shape of the container. It also is why liquids can be poured, and we can move through liquids. Liquids will also diffuse into each other.

Review Entropy – a measure of disorder The less organized, the higher the entropy.

Summary 1. Kinetic molecular theory explains the behavior of a substance in terms of the motion of the particles that make up the substance. 2. Gases have low density, can be compressed and are transparent because the molecules are very far apart. 3. A sample of gases does not change volume or temperature because the molecules have no attraction or repulsion, and collisions between molecules are elastic. 4. The molecules of a liquid are farther apart then a solid. The attraction between liquid molecules is lower so that liquid molecules are free to move throughout the liquid. 5. Molecules of a solid vibrate around a fixed point. This gives solids a fixed shape, and high density. The molecules are held in this fixed position by strong attractions. This is why solids have a fixed volume.