Chapter 11:Chapter 11:GasesGases

Coach KelsoeCoach Kelsoe

ChemistryChemistry

Pages 360–389Pages 360–389

Section 11–1:Section 11–1:Gases and PressureGases and Pressure

Coach KelsoeCoach Kelsoe

ChemistryChemistry

Pages 361–367Pages 361–367

Section 11–1 ObjectivesSection 11–1 Objectives Define Define pressurepressure and relate it to force. and relate it to force. Describe how pressure is measured.Describe how pressure is measured. Convert units of pressure.Convert units of pressure. State the standard conditions of State the standard conditions of

temperature and pressure.temperature and pressure.

PressurePressure

If you have a one-liter bottle of air, If you have a one-liter bottle of air, there’s really no way to know how much there’s really no way to know how much air you have in there unless you know air you have in there unless you know the conditions inside the bottle.the conditions inside the bottle.

To describe a gas fully, you need to To describe a gas fully, you need to state four measurable quantities:state four measurable quantities:• VolumeVolume• TemperatureTemperature• Number of moleculesNumber of molecules• PressurePressure

Pressure and ForcePressure and Force

If you blow air into a rubber balloon, it If you blow air into a rubber balloon, it will increase in size. The increase in will increase in size. The increase in volume is caused by the collisions of volume is caused by the collisions of molecules of air with the inside walls of molecules of air with the inside walls of the balloon. The collisions cause an the balloon. The collisions cause an outward push, or force, against the outward push, or force, against the inside walls.inside walls.

PressurePressure (P) is defined as the force per (P) is defined as the force per area on a surface, or P = F/A.area on a surface, or P = F/A.

Pressure and ForcePressure and Force

The SI unit for force is the The SI unit for force is the newtonnewton, , abbreviated N. It is the force that will abbreviated N. It is the force that will increase the speed of a one kilogram increase the speed of a one kilogram mass by one meter per second each mass by one meter per second each second it is applied.second it is applied.

At Earth’s surface, each kilogram of At Earth’s surface, each kilogram of mass exerts 9.8 N of force, due to mass exerts 9.8 N of force, due to gravity.gravity.

Force on Earth can be found by Force on Earth can be found by multiplying an object’s mass by 9.8.multiplying an object’s mass by 9.8.

Pressure and ForcePressure and Force

For example, if a person’s mass is 51 For example, if a person’s mass is 51 kg, then the amount of force they apply kg, then the amount of force they apply on the Earth’s surface is 500 N. No on the Earth’s surface is 500 N. No matter how a person stands, he or she matter how a person stands, he or she will exert 500 N of force on Earth.will exert 500 N of force on Earth.

However, a person can change the However, a person can change the amount of amount of pressurepressure they apply to the they apply to the earth’s surface by changing the area in earth’s surface by changing the area in contact with the ground – by standing contact with the ground – by standing on their tiptoes.on their tiptoes.

Pressure and ForcePressure and Force

Let’s say this 500 N person stands flat-Let’s say this 500 N person stands flat-footed. The area of his feet is equal to footed. The area of his feet is equal to 100 cm100 cm22. The pressure he applies to the . The pressure he applies to the ground is 5 N/cmground is 5 N/cm22..

If this person stood on his tiptoes, and If this person stood on his tiptoes, and the area of his tiptoes was 10 cmthe area of his tiptoes was 10 cm22, then , then the pressure he applies to the ground is the pressure he applies to the ground is 50 N/cm50 N/cm22..

This is how the “bed of nails” trick This is how the “bed of nails” trick works, and the secret of karate.works, and the secret of karate.

Pressure and ForcePressure and Force

Gas molecules exert pressure on any Gas molecules exert pressure on any surface with which they collide.surface with which they collide.

The pressure exerted by a gas depends The pressure exerted by a gas depends on volume, temperature, and the on volume, temperature, and the number of molecules present.number of molecules present.

Even the atmosphere exerts pressure. Even the atmosphere exerts pressure. At sea level, the pressure is 10.1 N/cmAt sea level, the pressure is 10.1 N/cm22. . This pressure can be thought of as This pressure can be thought of as caused by the weight of the gases that caused by the weight of the gases that compose the atmosphere.compose the atmosphere.

Pressure and ForcePressure and Force

To understand gas pressure and it’s To understand gas pressure and it’s magnitude, let’s think about an empty magnitude, let’s think about an empty can. The can does contain a small can. The can does contain a small amount of air. Let’s say the can has a amount of air. Let’s say the can has a total area of 1700 cmtotal area of 1700 cm22. If the can is not . If the can is not expanding or contracting, then the expanding or contracting, then the pressure inside must be the same as pressure inside must be the same as the outside. If Earth’s pressure is 10.1 the outside. If Earth’s pressure is 10.1 N/cmN/cm22, then the can’s pressure is 10.1 , then the can’s pressure is 10.1 N/cmN/cm22. You can find the force of the air . You can find the force of the air inside the can mathematically.inside the can mathematically.

Measuring PressureMeasuring Pressure

A A barometerbarometer is a device used to is a device used to measure atmospheric pressure.measure atmospheric pressure.

The first barometer was introduced by The first barometer was introduced by Evangelista Torricelli in the early 17Evangelista Torricelli in the early 17thth century.century.

Through Torricelli’s experiments, it is Through Torricelli’s experiments, it is known that at sea level at 0°C, the known that at sea level at 0°C, the height of mercury in a barometer will be height of mercury in a barometer will be 760 millimeters high.760 millimeters high.

Units of PressureUnits of Pressure

A number of different units are used to A number of different units are used to measure pressure:measure pressure:• PascalPascal• mm Hgmm Hg

Since atmospheric pressure is often Since atmospheric pressure is often measured by a mercury barometer, a measured by a mercury barometer, a common unit of measure is common unit of measure is millimeters millimeters of mercuryof mercury (mm Hg). (mm Hg).

•TorrTorr

•AtmosphereAtmosphere

Units of MeasureUnits of Measure

In honor of Torricelli, 1 mm Hg is now In honor of Torricelli, 1 mm Hg is now called 1 called 1 torrtorr. The average atmospheric . The average atmospheric pressure at sea level at 0°C is 760 torr.pressure at sea level at 0°C is 760 torr.

Pressures are often measured in units of Pressures are often measured in units of atmospheres. One atmospheres. One atmosphere of atmosphere of pressurepressure (atm) is defined as being (atm) is defined as being exactly equivalent to 760 torr.exactly equivalent to 760 torr.

Units of MeasureUnits of Measure

In SI, pressure is expressed in derived units In SI, pressure is expressed in derived units called pascals, after Blaise Pascal, a French called pascals, after Blaise Pascal, a French mathematician and philosopher who studied mathematician and philosopher who studied pressure during the 1600s.pressure during the 1600s.

One One pascalpascal (Pa) is defined as the pressure (Pa) is defined as the pressure exerted by a force of one newton acting on an exerted by a force of one newton acting on an area of one square meter. It is often more area of one square meter. It is often more convenient to express pressure in terms of convenient to express pressure in terms of kilopascals.kilopascals.

The standard atmosphere is equal to 1.01325 The standard atmosphere is equal to 1.01325 x 10x 1055 Pa, or 101.325 kPa. Pa, or 101.325 kPa.

Units of MeasureUnits of Measure

UnitUnit SymbolSymbol Definition/Definition/RelationshipRelationship

PascalPascal PaPa SI pressure unit;SI pressure unit;

1 Pa = 1N/m1 Pa = 1N/m22

Millimeter of Millimeter of

MercuryMercurymm Hgmm Hg Pressure that Pressure that

supports a 1 mm supports a 1 mm mercury columnmercury column

TorrTorr torrtorr 1 torr = 1 mm Hg1 torr = 1 mm Hg

AtmosphereAtmosphere atmatm Avg. atmospheric Avg. atmospheric pressure at sea pressure at sea level; 1 atm = 760 level; 1 atm = 760 torr, 101.325 kPatorr, 101.325 kPa

Standard Temperature and PressureStandard Temperature and Pressure

To compare volumes of gases, it is To compare volumes of gases, it is necessary to know the temperature and necessary to know the temperature and pressure at which the volumes are pressure at which the volumes are measured.measured.

For purposes of comparison, scientists For purposes of comparison, scientists have agreed on standard conditions of have agreed on standard conditions of exactly 1 atm pressure and 0°C. These exactly 1 atm pressure and 0°C. These conditions are called conditions are called standard standard temperature and pressuretemperature and pressure and and commonly abbreviated as STP.commonly abbreviated as STP.

Sample Problem 10-1Sample Problem 10-1

The average atmospheric pressure in The average atmospheric pressure in Denver, Colorado, is 0.830 atm. Denver, Colorado, is 0.830 atm. Express this pressure (a) in mm Hg Express this pressure (a) in mm Hg and (b) in kPa.and (b) in kPa.

• Given: P of atmosphere = 0.830 atm, 760 Given: P of atmosphere = 0.830 atm, 760 mm Hg = 1 atm, 101.325 kPa = 1 atmmm Hg = 1 atm, 101.325 kPa = 1 atm

a)a) 0.830 atm x 760 mm Hg/atm = 631 mm 0.830 atm x 760 mm Hg/atm = 631 mm HgHg

b)b) 0.830 atm x 101.325 kPa/atm = 84.1 kPa0.830 atm x 101.325 kPa/atm = 84.1 kPa

VocabularyVocabulary

Atmosphere of pressureAtmosphere of pressure BarometerBarometer Millimeters of mercuryMillimeters of mercury NewtonNewton PascalPascal PressurePressure Standard temperature and pressureStandard temperature and pressure TorrTorr

Conversion FactorsConversion Factors

1 torr = 0.1333 kPa1 torr = 0.1333 kPa 1 kPa = 7.5 torr1 kPa = 7.5 torr 1 atm = 101.325 kPa1 atm = 101.325 kPa 1 kPa = 0.00987 atm1 kPa = 0.00987 atm 1 atm = 760 torr1 atm = 760 torr 1 torr = 0.001316 atm1 torr = 0.001316 atm