We mentioned in the study section of Lecture 2 that hydrogen and oxygen combine in the ratio of 1 to 8, but that this is not enough information for leading to the conclusion that two hydrogen atoms combine with one of oxygen to form a water molecule. A key idea is attributed to Avagadro who said that equal volumes of gas (at the same temperature and pressure) contain equal numbers of constituent atoms or molecules. Experiments show that two liters of hydrogen gas will combine with one liter of oxygen gas to form two liters of water vapor. Each hydrogen molecule in hydrogen gas consists of two hydrogen atoms bonded together. Likewise, two oxygen atoms bind to make a oxygen molecule.
A "model" of a physical process is used to represent what one actually observes, even though this is an "ideal" model and not expected to be correct in all respects. However, it is a good enough model to explain many of the properties of gases with sufficient accuracy.
The motion of gas particles can be used to explain the pressure exerted and the temperature of a gas. The pressure on a surface is due to the force on that surface divided by its area. The force comes about from the multiple impacts of individual gas particles. Temperature, on the other hand, is DEFINED in terms of the average kinetic energy assocated with the motion of the gas particles. The greater the kinetic energy, the greater the temperature. See the apparatus shown in Figure 7.6 of the text which gives a simple way of measuring the distributions of speeds of atomic particles.
To visualize how gas particles colliding with a container create pressure, see Website II.
Gas particles move in all possible directions with differing speeds. The Kinetic Energy (KE) of a gas particle is equal to 1/2 its mass times its speeds squared. That is KE = 1/2 M x V2 , where M is the mass of the gas particle and V is its speed. The gas particles have a range of speeds, just like cars on a road, but it is the average of the speed squared times the mass, or the average kinetic energy which characterizes the temperature of a gas.
High temperature is associated with high kinetic energies and low temperatures are associated with low kinetic energies. However, keep in mind that the kinetic energy, and in this case the temperature, is proportional to the mass times the speed squared. So heavy particles moving more slowly will have the same kinetic energy as light particles moving more rapidly. Also, because the kinetic energy varies as the square of the speed, if two particles have the same mass, but one moves twice as fast as the other, it will have four times the kinetic energy (or temperature).
If temperature is associated with kinetic energy of a gas, one could ask at this point what controls the temperature of solids and liquids. It turns out that it is the kinetic energy of the constituent atoms and molecules that characterize the temperature of liquids and solids as well. We show in class a transparency picturing a solid with its atoms rigidly connected to each other. We will discuss more about liquids and solids in the next lecture, based on chapter 8. However, for now, let's keep in mind that the atoms or molecules in a solid, although bound to its neighbors in a rigid structure, can oscillate back and forth, and it is this motion that characterizes the temperature of a solid (or in a similar manner, of a liquid as well). As before, rapid oscillations mean high temperatures, and slower oscillations are lower temperatures.
4 - The Three Temperature Scales
There are three temperature scales. In the United States, we commonly use the Farenheit scale while in most other nations, the Celsius or Centigrade scale is used. Figure 7.10 shows these two scales side by side. Water boils at 212 degrees Farenheit or 100 degrees Centigrade. Water freezes at 32 degrees Farenheit or zero degrees Centigrade. However, the most important temperature scale for scientific calculations is the absolute temperature scale, or the Kelvin scale. Zero degrees Kelvin is the coldest possible temperature: it can be physically interpreted as the situation where the atoms or molecules have zero kinetic energy...so this is a very natural temperature scale. Zero degrees Kelvin is also -273 degrees Centigrade. Water freezes at +273 degrees Kelvin and zero degrees Centigrate. Hence, a difference of one degree is the same on the Centigrade and Kelvin scales, but the zero points are different.
R.S. Panvini
9/2/2002Explanation:
