Chapter 11 - Gases

POINT > Use KMT to explain how gases exert pressure on a container POINT > Define atmospheric pressure POINT > Describe how a barometer is used to determine atmospheric pressure POINT > Identify common units of gas pressure POINT > Explain Partial Pressures (Dalton’s Law)

POINT > Use KMT to explain how gases exert pressure on a container Recall that KMT tells us that gas particles are in constant random motion So gas particles are continually hitting the inner surface of the vessel. This creates pressure

WB CHECK: What assumption of Kinetic Molecular Theory explains why gases exert pressure on their surroundings?

POINT > Define atmospheric pressure Pressure is defined as force per unit area ex. pounds per square inch (PSI) Atmospheric pressure is the pressure exerted by the gases in our atmosphere, which is ~ 18 km high The SI unit of force is the Newton (N) defined as kg∙m/s2 Atmospheric pressure exerts a force of about 10N/cm2

WB CHECK: How is pressure defined?

POINT > Describe how a barometer is used to determine atmospheric pressure Atmospheric pressure can be measured with a barometer, invented by Evangelista Torricelli in the 17th century A barometer indicates atmospheric pressure by showing how high the pressure can raise a column of liquid, traditionally mercury (Hg)

POINT > Describe how a barometer is used to determine atmospheric pressure

POINT > Describe how a barometer is used to determine atmospheric pressure At sea level and 0C (STP), the Hg column is raised 760 mm As atmospheric pressure goes up or down, the column of Hg adjusts accordingly

WB CHECK: How high does mercury go in a barometer at STP? Who invented the barometer?

POINT > Identify common units of gas pressure One common unit of pressure is mm Hg and 1 mmHg is called 1 torr 760 mm Hg (or 760 torr) = 1 atmosphere (atm) = 1 bar To complicate matters, Frenchman Blaise Pascal was also doing gas research in the 1600’s and another unit of gas pressure is the Pascal (Pa) or kPa 1 atm = 101.3kPa = 1 bar

WB CHECK: What are 4 units of gas pressure?

POINT > Explain Partial Pressures (Dalton’s Law) We often are interested in volumes that contain mixtures of gases The air we breathe: ~ 78% N2, 21% O2, <1% Ar, <1% H2O The pressure of a gas mixture is the sum of the pressure exerted by each gas by itself. This is the partial pressure of each gas

POINT > Explain Partial Pressures (Dalton’s Law) This makes sense in terms of KMT assumptions: “The particles in a gas do not interact” So, each gas has its own effect on the overall pressure Dalton’s Law: PT = P1 + P2 + P3…. Courtesy of John Dalton (who also gave us Atomic Theory)

WB CHECK: Which KMT assumption about gases explains Dalton’s Law of partial pressures?

POINT > Explain Partial Pressures (Dalton’s Law) The partial pressure exerted by water vapor is often important in gas collection experiments The pressure of the gas collected can be determined using Dalton’s Law: Patm = PH2O + Pgas The water vapor pressure (PH2O) at a given temp can be found in a Table (R-63 in your textbook)

WB CHECK: What is the water vapor pressure At 25.0 C (in mmHg)? 23.8 mmHg

Homework: Read pages 341-347 Complete FA #1-6 page 347