Unit 4 Sections A 7 -A 9 In

Unit 4 Sections A 7 -A 9 In which you will learn about: • Kinetic molecular theory • Pressure and volume relationships 1

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 howtype explanations on phenomena in the natural world. 2

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 3

• 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 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 4

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) 5

Kinetic Energy • Gas molecules move at average speeds that depend on how much kinetic energy they possess – Kinetic energy = energy of motion = 1/2 mv 2 (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 6

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 7

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 8

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. • Px. V=k • Better known as P 1 V 1 = P 2 V 2 • What would a graph of Boyle’s Law look like? 9

Example Problem • You know that initially a gas sample occupies a volume of 8. 0 m. L 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 m. L? • Use the GUESS method. • P 1 = 1. 0 atm, V 1 = 8. 0 m. L, V 2 = 10. 9 m. L, P 2 = ? • P 1 V 1 = P 2 V 2 --> P 2 = P 1 V 1/ V 2 plug and chug to get P 2 = 0. 73 atm • CHECK: Volume increased and pressure decreased which is in accordance with Boyle’s law. 10

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. 11

More Homework • 2) You buy helium gas in small pressurized cans to inflate party balloons. The can label indicates that the container delivers 7100 m. L of helium gas at 100. 0 k. Pa pressure. The volume of the gas container is 492 m. L. – A. Do you think that the initial pressure of helium gas inside the can is greater or less than 100. 0 k. Pa? Explain. – B. Calculate the initial pressure of helium gas inside the container. – C. Was your prediction in 2 a correct? 12

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. 13