1. Chapter 5: Gases CHE 123: General Chemistry I Dr. Jerome Williams, Ph.D. Saint Leo University

2. Overview Gases & Pressure

3. Fundamental Principles – Gases do not have a specific shape or fixed volume. – Gases assume shape of their container. – Gas molecules are constantly in motion. – As they move and strike a surface, they push on that surface – push = force

5. Gases & Pressure Fundamental Principles – If we could measure the total amount of force exerted by gas molecules hitting the entire surface at any one instant, we would know the pressure the gas is exerting. – Pressure = force per unit area

6. Gases & Pressure Pressure of a gas is result of the constant movement of the gas molecules and their collisions with the surfaces around them. Gas pressure depends on several factors. – number of gas particles in a given volume – volume of the container – average speed of the gas particles

7. Gases & Pressure Pressure: force per unit area. – SI system Units: Pressure expressed in newtons per square meter (N/m 2 ) Force: mass x acceleration (F = ma) – SI System Units: Force is measured in newtons.(1 N = 1 kg m / s 2 ) Pressure is measured in pascals.(1 Pa = 1 N / m 2 )

8. Gases & Pressure Barometer: device used to measure pressure. – Atmospheric pressure measure of air being influenced by force of gravity. Atmospheric pressure varies with altitude. – Common Pressure Units: 1 atm. = 760 mm Hg = 760 Torr 1 atm = 29.92 in Hg = 14.7 lb/in 2 [SI system]1 N / m 2 = 1 Pa 1 atm = 101,325 Pa

12. Example 5.1 – A high-performance bicycle tire has a pressure of 132 psi. What is the pressure in mmHg? since mmHg are smaller than psi, the answer makes sense 1 atm = 14.7 psi, 1 atm = 760 mmHg 132 psi mmHg Check: Solution: Concept Plan: Relationships: Given:Find: psiatmmmHg

13. 1.The height of the column increases because atmospheric pressure decreases with increasing altitude 2.The height of the column decreases because atmospheric pressure decreases with increasing altitude 3.The height of the column decreases because atmospheric pressure increases with increasing altitude 4.The height of the column increases because atmospheric pressure increases with increasing altitude 1.The height of the column increases because atmospheric pressure decreases with increasing altitude 2.The height of the column decreases because atmospheric pressure decreases with increasing altitude 3.The height of the column decreases because atmospheric pressure increases with increasing altitude 4.The height of the column increases because atmospheric pressure increases with increasing altitude Practice – What happens to the height of the column of mercury in a mercury barometer as you climb to the top of a mountain? Tro: Chemistry: A Molecular Approach, 2/e13

14. Practice—Convert 45.5 psi into kPa because kPa are smaller than psi, the answer makes sense 1 atm = 14.7 psi, 1 atm = 101.325 kPa 645.5 psi kPa Check: Solution: Conceptual Plan: Relationships: Given: Find: psiatmkPa 14 Tro: Chemistry: A Molecular Approach, 2/e