Chapter 11 Gases Section 1 Gases and Pressure

Objectives Define pressure, give units of pressure, and describe how pressure is measured. State the standard conditions of temperature and pressure and convert units of pressure. Use Dalton’s law of partial pressure to calculate partial pressures and total pressures.

Remember… Kinetic molecular theory  particles of matter are always in motion Temperature of a gas is related to the kinetic energy of gas molecules In this chapter… You will learn about other properties of gases  pressure, volume, amount of gas, and the relationship between these properties

Pressure and Force Pressure  def. the force per unit area on a surface Example: Pump air into tire= increase pressure bc increase the number of collisions of air molecules with the inside walls of tire

Did you know…

Pressure and Force Force SI unit Newton N = kg x m/s 2 (mass x acceleration due to gravity 9.8 m/s 2 ) Consider a ballet dancer  mass 51 kg Force = 51 kg x 9.8 m/s 2 Force = 500 N No matter how she stands = Force SAME However pressure can be different…

Dancer rests weight on soles on both feet  contact area on floor = 325 cm 2 Pressure = 500 N / 325 cm 2 = 1.5 N/cm 2 Dancer stands on her toes  contact area on floor = 13 cm 2 Pressure = 500 N / 13 cm 2 = 38 N/cm 2 Dancer stands on one toe  contact area on floor = 6.5 cm 2 Pressure = 500 N / 325 cm 2 = 77 N/cm 2

Atmosphere def. shell of air surrounding Earth that exerts pressure

Gas particles have small masses  their kinetic energies of motion override the gravitational forces  THERFORE all the molecules that make up the atmosphere don’t pile up in a thin layer at the Earth’s surface NEVERTHELESS  gravity does cause the atmosphere as a whole to press down on Earth’s surface creating atmospheric pressure

Measuring Pressure Barometer - device used to measure atmospheric pressure At sea level and 0°C  the average pressure of the atmosphere can support 760 mmHg

Units of Pressure

Pressure Unit Conversions 745 mm Hg = ______________psi 727 mm Hg = ______________kPa 52.5 kPa = ________________atm

Standard Temperature and Pressure For purposes of comparison, scientists have agreed on standard conditions of exactly 1 atm pressure and 0 C.

Dalton’s Law of Partial Pressure John Dalton  English chemist proposed atomic theory studies gas mixtures The pressure exerted by each gas in an unreactive mixture is independent. (they don’t affect other gases) Partial Pressure  pressure of each gas in a mixture Dalton’s Law of Partial Pressure  the total pressure of a gas mixture is the sum of the partial pressures of the component gases *Total pressure results from total # of collisions per unit of wall area in a given time.

What is the total pressure of the molecules in the air?

Collecting Gases Over Water Experiments often encountered in chemistry laboratories involves determining the number of moles of gas collected from chemical reaction  sometimes gases are collected over water heat Ex: 2KClO 3 (s)  2KCl (s) + 3O 2 (g) *O 2 collected in bottle filled with water that is inverted in pan *Water vapor (when evaporates at surface) exerts vapor pressure Volume of gas measured by raising or lowering the bottle until water levels inside and outside are same = pressure inside bottle is equal to atmosphere pressure outside P atm = P gas collected + P H2O

HOMEWORK Section Review pg 367 #1-6

Boyle’s Law The law that states that the volume of a gas is inversely proportional to the pressure of a gas when temperature is constant. P1V1=P2V2 Q. A balloon with a volume of 2.0 L is filled with a gas at 3 atmospheres. If the pressure is reduced to 0.5 atmospheres without a change in temperature, what would be the volume of the balloon? volume-relationship-of-gas-Boyles-law.html volume-relationship-of-gas-Boyles-law.html

Boyle’s Law Q. A 17.5 mL sample of gas is at 4.5 atm. What will be the volume if the pressure becomes 1.5 atm, with a fixed amount of gas and temperature?

Charles’s Law The law that states that the volume of a gas is directly proportional to the temperature of a gas when pressure is constant. A 600 mL sample of nitrogen is heated from 27 °C to 77 °C at constant pressure. What is the final volume? The final volume after heating will be _____ mL. temperature-relationship-of-gas-Charles-law.html temperature-relationship-of-gas-Charles-law.html

Gay-Lussac’s Law: Pressure- Temperature Relationship The pressure of a fixed mass at constant volume varies directly with the Kelvin temperature.

The Combined Gas Law Expresses the relationship between pressure, volume, and temperature of a fixed amount of gas.

2.00 L of He gas is collected at 25.0 °C and mmHg. What is the volume at STP? P 1 = P 2 = V 1 = V 2 = T 1 = T 2 =

A closed gas system initially has pressure and volume of 920torr and 9.64L with the temperature unknown. If the same closed system has values of 1.94atm, 7810mL and 770K, what was the initial temperature in K?

HOMEWORK Section 2 Review Please complete on same sheet as section 1 m/watch?v=bftkRnTcFj8 m/watch?v=bftkRnTcFj8 m/watch?v=CpfKLAVPw_ w m/watch?v=CpfKLAVPw_ w

Objectives State law of combining volumes State Avogadro’s law and explain its significance State the ideal gas law and use it to calculate temperature, pressure, volume, or amount of gas.

Measuring and Comparing the Volumes of Reacting Gases

Avogadro’s Law