Unit 4 Sections A7-A9 In which you will learn about: Kinetic molecular theory Pressure and volume relationships

A.7 Atoms & Molecules in Motion Making observations is the first step in “doing science” A more difficult task is crafting a theory that explains all observations and that also predicts the outcomes of investigations yet to be completed One such theory concerns the motion of atoms and molecules within all states of matter Remember, scientific laws describe the behavior of events in nature, but they do not provide explanations for the observed behavior. Scientific theories and models attempt to offer how-type explanations on phenomena in the natural world.

States of Matter What do you already know about the three states of matter from observing them? A solid has a definite shape and its structure is rigid A liquid flows and depends on its container to define its shape A gas does not have a definite

The molecules and ions within liquids are more mobile The atoms, molecules, or ions that make up solids are held closely to one another Particles vibrate about a fixed position Tightly packed in an orderly fashion (except for ice) and cannot move past one another The molecules and ions within liquids are more mobile Molecules can move past one another Intermolecular forces prevent them from moving too far apart Remember, IMFs are attractive forces between molecules that are NOT bonds

What We Care About Right Now: Gases Most molecules in the gaseous state are not as strongly attracted to each other In contrast to particles in solids and liquids, particles in gases are very far apart Their size is negligible compared to the great distances that separate them Gases molecules move in straight-line paths at very high speeds Change direction only when a collision occurs (remember, when the collisions are with the container walls, we measure it as pressure)

Kinetic Energy Gas molecules move at average speeds that depend on how much kinetic energy they possess Kinetic energy = energy of motion = 1/2mv2 (where m is mass and v is velocity) Traveling at the same velocity, a more massive object has a greater kinetic energy than a less massive object When comparing the speeds of gases, we can use Graham’s Law which will be explained in detail later

Kinetic Molecular Theory (KMT) KMT explains the behavior of gas molecules in motion. It is based on four postulates (statements accepted as the basis for further reasoning and study): 1) Gases consist of tiny particles whose size is negligible compared with the great distances that separate the molecules from each other 2) Gas molecules are in constant, random motion. 3) Molecular collisions are elastic (no gain or loss in total kinetic energy during collisions) 4) At a given temperature, gas molecules have a range of kinetic energies; however, the average kinetic energy is constant and depends on the temperature of the sample

Putting KMT Into Action: A.8 Pressure-Volume Behavior of Gases Earlier, when you pushed down on a sealed syringe filled with gas, you observed that a gas sample can easily be compressed The volume of a gas sample is easily changed when an external pressure is applied As you decreased the volume of the gas within the syringe, the molecules collided more often with the walls, increasing the pressure

Boyle’s Law As discussed in the previous slide, as volume decreases, pressure increases (and vice versa). An inverse relationship is one where an increase in one variable results in the decrease of the other, but the product of the two variables remains the same. P x V = k Better known as P1V1 = P2V2 What would a graph of Boyle’s Law look like?

Example Problem You know that initially a gas sample occupies a volume of 8.0 mL and exerts a pressure of 1.0 atm. How would the pressure of the gas sample changes if its volume were increased to 10.9 mL? Use the GUESS method. P1 = 1.0 atm, V1 = 8.0 mL, V2 = 10.9 mL, P2 = ? P1V1 = P2V2 --> P2 = P1V1/ V2  plug and chug to get P2 = 0.73 atm CHECK: Volume increased and pressure decreased which is in accordance with Boyle’s law.

A.9 Predicting Gas Behavior: Pressure-Volume (HOMEWORK) 1) Explain each of the following observations: A. Even if they have ample supplies of oxygen gas, airplane passengers experience discomfort when the cabin undergoes a drop in air pressure. B. New tennis balls are sold in pressurized containers. C. After descending from a high mountain, the capped, half-filled plastic water bottle from which you drank while standing at the summit now appears dented or slightly crushed.

More Homework 2) You buy helium gas in small pressurized cans to inflate party balloons. The can label indicates that the container delivers 7100 mL of helium gas at 100.0 kPa pressure. The volume of the gas container is 492 mL. A. Do you think that the initial pressure of helium gas inside the can is greater or less than 100.0 kPa? Explain. B. Calculate the initial pressure of helium gas inside the container. C. Was your prediction in 2a correct?

Last Question 3) Two glass bulbs are separated by a closed valve. The 0.50-L bulb on the left contains a gas sample at a pressure of 6.0 atm. The 1.7-L bulb on the right is evacuated; it contains no gas: A. Draw a model of gas molecules in the two glass bulbs before and after the middle valve is opened. B. Explain your model in terms of kinetic molecular theory. C. Predict, in general, what will happen to the total volume of the gas sample if you open the middle valve. Explain. D. Predict, in general, what will happen to the total pressure of the gas sample if you open the middle valve. Explain. E. Calculate the actual pressure of the gas sample after the middle valve is opened.