Pressure Pressure- force applied to one unit of surface area Pressure = force / area Metric force unit = newton (N) Metric pressure unit = pascals (Pa) So to calculate pressure exerted on object: force in newtons (N) area in square meters (m 2 ) = Pressure (Pa)

Think About It Two Chemistry books, one lying down on the table and one standing on end. Which would exert more pressure? The book lying down covers more surface area than the one standing up. The book standing up exerting more pressure because the force is over a smaller area

Chemquandry At various times in the history of fashion, spike-heeled shoes become popular. Unfortunately, during these times floors become scarred with tiny dents. When tennis shoes or shoes with broader heels are worn, no such problem develops. Why? A person who accidently walks onto thin ice on a lake can lower the risk of “falling through” by lying down on the ice or standing on a large piece of plywood. Why?

Pressure Units Atmospheres mmHg kPa Lb/in 2 Bars Torrs Standard Pressure values: 1atm = 760 mmHg = kPa= 14.7 lb/in 2 = bars = 760 torrs

Pressure Conversions The pressure of a gas is measured as 49 torr. Represent this pressure in atmospheres, Pascals, and mmHg.

Measuring Pressure A Manometer is a device to measure pressures. A common simple manometer consists of a U shaped tube of glass filled with some liquid. Typically the liquid is mercury because of its high density.

Barometer Air pressure and differences in pressure are among the most important weather makers. The centers of storms are areas of relatively low air pressure, compared to pressures around the storm. High air pressure generally brings good weather. Keeping track of how the pressure is changing is important for forecasting the weather. Differences in air pressure between places cause the winds to blow - air moves from high toward low pressure. The instruments that measure air pressure are called barometers, from Greek words for weight and measure. The U.S. National Weather Service reports air pressure at the surface in inches of mercury while air pressure aloft is reported in millibars, also known as hectopascals (hPa). Scientists, however, generally use pressures in hectopascals.

Pressure Changes Pressure changes with altitude. As there is less air pressing on you the air pressure decreases. Think about how : In the mountains the air is thinner it is because there is less air above you so less pressure In the bottom of a valley there is more pressure because there is more air above you. When you go down in the lake or ocean what happens to the pressure? It increases, this is why if you scuba dive you have to come up slowly to allow your pressure to slowly decrease. Later in the next video you will be introduced to Boyle’s law think of how this fits in here.

= Dalton’s Law of Partial Pressures & Air Pressure P O2O2 P N2N2 P CO 2 P Ar EARTH P O2O2 P N2N2 P CO 2 P Ar P Total mm Hg P Total = mm Hg 590 mm Hg 3 mm Hg 8 mm Hg P Total = 750 mm Hg

Dalton’s Law of Partial Pressure the total P of a mixture of gases is equal to the sum of partial P of component gases, no matter how many different gases P T = P 1 + P 2 + P 3 + … Partial Pressure- P of each gas in mixture This works according to the KMT because at the same temperature molecules of different gases have the same KE. It doesn’t matter if the molecules are O 2 or H 2. Both collide with the container or other molecules with the same force.

Example Oxygen gas from decomposition of KClO 3 was collected by water displacement. The barometric pressure and the temperature during the experiment were torr and 20.0°C respectively. If the partial pressure of water vapor is 17.5 torr at 20.0°C. What was the partial pressure of oxygen collected? P T = P O2 + P H 2 O torr = P O P O2 = torr

Example A mixture of oxygen, nitrogen and argon gases has a total pressure of 760.mmHg. If the partial pressure of argon is 7mmHg and that of oxygen is 230.mmHg, then what is the partial pressure of nitrogen P T = P O 2 + P N 2 + P Ar = 760.mmHg P O2 = 230mmHg; P Ar = 7mmHg P N2 = ( ) = 523mmHg