From SC Education

Description: Developing courseware for General Chemistry

Gases

Use this section to get a feel for what the gas phase of matter looks like and how it behaves.

• Gas Law Demonstrator: This demonstrator is from the Oklahoma State University. <iframe src="http://intro.chem.okstate.edu/1314F00/Laboratory/GLP.htm" width="660" height="495" frameborder="0" scrolling="no" style="float:right">Your browser doesn't support iframe. Go to http://intro.chem.okstate.edu/1314F00/Laboratory/GLP.htm</iframe> Study the visualization of a gas on the right and write a single, short, and complete sentence to answer each of these questions. "Yes" and "No" answers are too short.
  • Set the number molecules (n He) to 1 and (n Ne) to 0. The pressure (P) button should be filled. You are going to see how pressure changes when you manipulate temperature (T), volume (V), and the number of particles (n). But first, answer some general questions about the gas.
    • Do any of the balls stick together after a collision?
    • Are all of the balls moving at the same speed?
    • Pick one ball; the red one is easiest to follow, but any one would do. Does this one always go at the same speed? If not, what causes it to change speed? Does its speed change when it hits the wall?
    • Change the temperature:
      • Write a sentence that describes what you observe about the speed of the molecules as the temperature is raised or lowered.
      • Write a sentence that describes what happens to the pressure (P) as the temperature is raised and lowered. Pressure on a wall is a measure of how often and how hard the particles hit the wall. High pressure comes from frequent hard hits. So write a sentence explaining why pressure changes in the way it does when the temperature is raised.
    • Change the volume (V): Write two sentences. 1) Describe how the volume was changed on the screen. 2) What was the effect on the pressure as the volume became smaller? What was the effect on pressure with a larger volume? Going back to the description of pressure, explain why a larger volume has the effect that it does on pressure.
  • Change the n He.
    • Write a sentence describing what happens to the pressure when the number of molecules increases or decreases.
  • Now you have determined how P, T, V, and n are related. Below you will construct the Ideal Gas Law equation based on a couple of math principles. But first the relationship of kinetic energy to mass must be established. I use it in an explanation of how equations are built.

• Kinetic Energy versus Mass:
  • Temperature is defined as the average kinetic energy (KE) of the particles in a substance. Look again at the visualization. All of the particles aren't moving at the same speed. What happens to the average speed when T is raised?
  • Mass is another factor in KE. Set both the (n Ne) and (n He) to 1. Ne has a higher mass than He. So Ne is shown as the larger molecule.
    • How does the Ne move differently than the He even though both are at the same temperature?

• Equation Builder
  • Any two factors in an equation, T and P for example, can be directly or indirectly proportional. If they are directly proportional, then they increase together or decrease together. If factors are indirectly proportional, then their values move in opposite directions: one increases while the other decreases. Direct and indirect proportion are expressed in an equation by where the factors are written around the math operator. If an equation is written all on one line (no fractions or divisions), then if two factors are directly proportional they are written on opposite sides of the math operator (math operators here means the equals sign or proportionality sign: = or &#8733;). For example, in the equation a = b x c, the factors "a" and "b" are directly proportional. If "b" increases, then so does "a", assuming that "c" is constant. Of course, the factors "a" and "c" are also directly proportional, if "b" is constant. That means that factors with indirect proportion are written on the same side of the operator. The factors "b" and "c" are indirectly proportional. If "a" is constant, then "b" increases as "c" decreases and vice versa.
  • To test this description of equations, look at the equation for kinetic energy (KE). 'Kinetic energy = mass x velocity^2'. Ignore the square on the velocity.
    • Write a sentence saying how if the mass increases the velocity should ___?___. Does this match your experience with how heavy Ne moved compared to lighter He?
  • Look at your answers about T, V, and n versus P from above and decide how these factors are proportional.
    • How is T proportional to P?
    • How is V proportional to P?
    • How is n proportional to P?
    • Start writing an equation by putting " = P" on the paper. Now fill in T, V, and n according to their proportionality with P. If they are directly proportional to P, then they go on the other side of the equal sign from P; indirectly proportional to P, same side as P.
  • One last point:constants. A proportionality often requires a constant to become an equation. A simple example is calculating a friend's age based on your own. Let's say that as a freshman you meet a senior who becomes a good friend. You are 18, and the senior guy who is 21. Years later you are 30. How old is your friend? You know that YourAge ∝HisAge. Your age has increased and so has his. But YourAge = HisAge is not true. In order to calculate HisAge from YourAge a "3" must be added to YourAge. The "3" never changes. YourAge changes, but the "3" is constant. The constant changes a proportion to an equation. So YourAge ∝HisAge is a proportionality, but YourAge + 3 = HisAge is an equation. An example from science is that the energy of light is directly proportional to the frequency of the light. So Energy ∝ Frequency. In order to make an equation, a constant is required: Energy = Planck's constant x frequency. The ideal gas law also requires a constant, the gas constant, R. The R goes on the opposite side of the equal sign from P because of the units it was assigned.
  • Check your Ideal Gas Equation against the one in the book.