13.1 The Nature of Gases > 1 Copyright © Pearson Education, Inc., or its affiliates. All Rights...

Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

13.1 The Nature of Gases >13.1 The Nature of Gases >

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Chapter 13States of Matter

13.1 The Nature of Gases

13.2 The Nature of Liquids13.3 The Nature of Solids13.4 Changes of State



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What factors most strongly affect the weather?

CHEMISTRY & YOUCHEMISTRY & YOU

The atmosphere is a gas, and the factors that determine the behavior of gases—temperature and pressure—affect the weather in the atmosphere.



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Kinetic Theory and a Model Kinetic Theory and a Model for Gasesfor Gases

Kinetic Theory and a Model for Gases

What are the three assumptions of the kinetic theory as it applies to gases?



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The word kinetic refers to motion.


• The energy an object has because of its motion is called kinetic energy.



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The word kinetic refers to motion.


• The energy an object has because of its motion is called kinetic energy.

• According to the kinetic theory, all matter consists of tiny particles that are in constant motion.

• The particles in a gas are usually molecules or atoms.



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The kinetic theory as it applies to gases includes the following fundamental assumptions about gases.

The particles in a gas are considered to be small, hard spheres with an insignificant volume.

– Within a gas, the particles are relatively far apart compared with the distance between particles in a liquid or solid.

– Between the particles, there is empty space.

– No attractive or repulsive forces exist between the particles.



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Bromine molecule



The motion of particles in a gas is rapid, constant, and random.

– Gases fill their containers regardless of the shape and volume of the containers.

– An uncontained gas can spread out into space without limit.



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– The rapid, constant motion of particles in a gas causes them to collide with one another and with the walls of their container.



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– The particles travel in straight-line paths until they collide with another particle.

– The particles change direction only when they rebound from collisions.



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All collisions between particles in a gas are perfectly elastic.

– During an elastic collision, kinetic energy is transferred without loss from one particle to another.

– The total kinetic energy remains constant.



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Describe an elastic collision between gas molecules.



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Describe an elastic collision between gas molecules.

An elastic collision is one in which kinetic energy is transferred from one particle to another with no overall loss of kinetic energy.



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Gas PressureGas Pressure

Gas Pressure

How does kinetic theory explain gas pressure?



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Gas pressure results from the force exerted by a gas per unit surface area of an object.

• Moving bodies exert a force when they collide with other bodies.



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Gas pressure is the result of billions of rapidly moving particles in a gas simultaneously colliding with an object.



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• If no particles are present, no collisions can occur. Consequently, there is no pressure.

• An empty space with no particles and no pressure is called a vacuum.




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Air exerts pressure on Earth because gravity holds the particles in air within Earth’s atmosphere.

• The collisions of atoms and molecules in air with objects results in atmospheric pressure.

• Atmospheric pressure decreases as you climb a mountain because the density of Earth’s atmosphere decreases as the elevation increases.



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A barometer is a device that is used to measure atmospheric pressure.• At sea level, air exerts

enough pressure to support a 760-mm column of mercury.

• On top of Mount Everest, at 9000 m, the air exerts only enough pressure to support a 253-mm column of mercury.

Vacuum

Atmospheric pressure

760 mm Hg (barometric pressure)

253 mm Hg

Sea level On top of Mount Everest



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When weather forecasters state that a low-pressure system is moving into your region, it usually means that a storm is coming. What do you think happens to the column of mercury in a barometer as a storm approaches? Why?




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When weather forecasters state that a low-pressure system is moving into your region, it usually means that a storm is coming. What do you think happens to the column of mercury in a barometer as a storm approaches? Why?


When a storm approaches, the column of mercury goes down, indicating a decrease in atmospheric pressure.



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The SI unit of pressure is the pascal (Pa).

• Normal atmospheric pressure is about 100,000 Pa, that is, 100 kilopascals (kPa).

• Two older units of pressure are commonly used.– millimeters of mercury (mm Hg)

– atmospheres (atm)



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One standard atmosphere (atm) is the pressure required to support 760 mm of mercury in a mercury barometer at 25°C.

• The numerical relationship among the three units is

1 atm = 760 mm Hg = 101.3 kPa.

• Recall that standard temperature and pressure (STP) are defined as a temperature of 0°C and a pressure of 101.3 kPa, or 1 atm.



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A pressure gauge records a pressure of 450 kPa. Convert this measurement to

a. atmospheres

b. millimeters of mercury

Converting Between Units of Pressure

Sample Problem 13.1Sample Problem 13.1



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Analyze List the knowns and the unknowns.1


KNOWNS UNKNOWNSpressure = ? atm

pressure = ? mm hg

The given pressure is converted into the desired unit by multiplying by the proper conversion factor.

pressure = 450 kPa

1 atm = 101.3 kPa

1 atm = 760 mm Hg



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Identify the appropriate conversion factor

Calculate Solve for the unknowns.2

a. to convert kPa to atm.

b. to convert kPa to mm Hg.

1 atm101.3 kPa

101.3 kPa760 mm Hg




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Multiply the given pressure by the conversion factor.

Calculate Solve for the unknowns.2

b. 450 kPa × = 3400 mm Hg = 3.4 × 103 mm Hg

101.3 kPa760 mm Hg

1 atm101.3 kPaa. 450 kPa × = 4.4 atm




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Because the first conversion factor is much less than 1 and the second is much greater than 1, it makes sense that the values expressed in atm and mm Hg are respectively smaller and larger than the value expressed in kPa.

Evaluate Do the results make sense?3




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What is the pressure in millimeters of mercury inside a vacuum?



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What is the pressure in millimeters of mercury inside a vacuum?

0 mm Hg



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Kinetic Energy and Kinetic Energy and TemperatureTemperature

Kinetic Energy and Temperature

What is the relationship between the temperature in kelvins and the average kinetic energy of particles?



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As a substance is heated, its particles absorb energy, some of which is stored within the particles.• This stored portion of the energy, or potential

energy, does not raise the temperature of the substance.

• The remaining absorbed energy does speed up the particles, that is, increases their kinetic energy.– This increase in kinetic energy results in an

increase in temperature.



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Average Kinetic Energy

The particles in any collection of atoms or molecules at a given temperature have a wide range of kinetic energies.• Most have kinetic energies somewhere in the

middle of this range.

• We use average kinetic energy when discussing the kinetic energy of a collection of particles in a substance.



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At any given temperature, the particles of all substances, regardless of physical state, have the same average kinetic energy.



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The figure below shows the distribution of kinetic energies of water molecules at two different temperatures.

Interpret GraphsInterpret Graphs

• The green curve shows the distribution of kinetic energy in cold water.

• The purple curve shows the distribution of kinetic energy in hot water.



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The figure below shows the distribution of kinetic energies of water molecules at two different temperatures.

Interpret GraphsInterpret Graphs

• Most of the molecules have intermediate kinetic energies, close to the average value.

• Notice that the molecules at the higher temperature have a wider range of kinetic energies.



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The average kinetic energy of the particles in a substance is directly related to the substance’s temperature.• An increase in the average kinetic energy of

the particles causes the temperature of a substance to rise.

• As a substance cools, the particles tend to move more slowly, and their average kinetic energy decreases.



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Absolute zero (0 K, or –273.15oC) is the temperature at which the motion of particles theoretically ceases.• No temperature can be lower than absolute

zero.

• Absolute zero has never been produced in the laboratory.– A near-zero temperature of about 0.000 000 000 1 K,

which is 0.1 nanokelvin, has been achieved.



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The coldest temperatures recorded outside the laboratory are from space.• Astronomers used a

radio telescope to measure the temperature of the boomerang nebula.

• At about 1 K, it is the coldest known region of space.



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Average Kinetic Energy and Kelvin Temperature

The Kelvin temperature of a substance is directly proportional to the average kinetic energy of the particles of the substance.



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What is the result of increasing the temperature of a gas sample?

A. A decrease in the average kinetic energy of the sample

B. No effect on the sample

C. An increase in the average kinetic energy of the sample

D. The particles slow down.



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What is the result of increasing the temperature of a gas sample?

A. A decrease in the average kinetic energy of the sample

B. No effect on the sample

C. An increase in the average kinetic energy of the sample

D. The particles slow down.



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Key ConceptsKey Concepts

Particles in a gas are considered to be small, hard spheres with an insignificant volume. The motion of the particles in a gas is rapid, constant, and random. All collisions between particles in a gas are perfectly elastic.


The Kelvin temperature of a substance is directly proportional to the average kinetic energy of the particles of the substance.



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Glossary TermsGlossary Terms

• kinetic energy: the energy an object has because of its motion

• kinetic theory: a theory explaining the states of matter, based on the concept that all matter consists of tiny particles that are in constant motion

• gas pressure: results from the force exerted by a gas per unit surface area of an object; due to collisions of gas particles with the object



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• vacuum: a space where no particles of matter exist

• atmospheric pressure: the pressure exerted by atoms and molecules in the atmosphere surrounding Earth, resulting from collisions of these particles with objects

• barometer: an instrument used to measure atmospheric pressure



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• Pascal (Pa): the SI unit of pressure

• standard atmosphere (atm): a unit of pressure; it is the pressure required to support 760 mm of mercury in a mercury barometer at 25°C



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END OF 13.1END OF 13.1

13.1 The Nature of Gases > 1 Copyright © Pearson Education, Inc., or its affiliates. All Rights...

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