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13.1 The Nature of Gases >
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Chapter 13 States of Matter 13.1 The Nature of Gases 13.2 The Nature of Liquids 13.3 The Nature of Solids 13.4 Changes of State
13.1 The Nature of Gases >
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Kinetic Theory and a Model for Gases
Kinetic Theory and a Model for Gases The three assumptions of the kinetic theory as it applies to gases
13.1 The Nature of Gases >
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Kinetic Theory and a Model for Gases
• kinetic energy: the energy an object has because of its motion.
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13.1 The Nature of Gases >
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Kinetic Theory and a Model for Gases
• kinetic energy: the energy an object has because of its motion.
• kinetic theory: all matter consists of tiny particles that are in constant motion.
• The representative particles in a gas are usually molecules or atoms.
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Kinetic Theory and a Model for Gases
The kinetic theory as it applies to gases includes the following fundamental assumptions.
The particles in a gas are considered to be small, hard spheres with an insignificant volume.
– Particles are relatively far apart (compared to particles in a liquid or solid).
– Between particles = empty space.
– No attractive or repulsive forces between the particles.
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Bromine molecule
Kinetic Theory and a Model for Gases
The motion of particles in a gas is rapid, constant, and random.
– Gases fill their containers
– An uncontained gas can spread out into space without limit.
– The rapid, constant motion of particles in a gas causes them to collide with one another and the walls of their container.
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13.1 The Nature of Gases >
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Kinetic Theory and a Model for Gases
The motion of particles in a gas is rapid, constant, and random.
– particles travel in straight- line paths until they collide with another particle.
– particles change direction only when they rebound from collisions.
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Kinetic Theory and a Model for Gases
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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Gas Pressure
Gas Pressure
How does kinetic theory explain gas pressure?
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13.1 The Nature of Gases >
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Gas Pressure
Gas pressure: the force exerted by a gas per unit surface area of an object.
• If no particles are present, no collisions occur, therefore, no pressure.
• An empty space with no particles and no pressure is called a vacuum.
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Gas Pressure
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. Why?
• the density of Earth’s atmosphere decreases as the elevation increases.
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Gas Pressure Barometer: device used to measure atmospheric pressure.
Vacuum
Atmospheric pressure 760 mm Hg
(barometric pressure)
253 mm Hg
Sea level On top of Mount Everest
Low atmospheric pressure is associated with bad weather.
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Gas Pressure
The SI unit of pressure is the pascal (Pa).
• Normal atmospheric pressure is about 100,000 Pa (100 kPa).
• Two other commonly used pressure units:
– millimeters of mercury (mm Hg)
– atmospheres (atm)
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Gas Pressure
One standard atmosphere (atm) is the pressure required to support 760 mm of mercury in a mercury barometer at 25°C.
1 atm = 760 mm Hg = 101.3 kPa.
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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 13.1
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13.1 The Nature of Gases >
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Multiply the given pressure by the conversion factor.
b. 450 kPa × = 3400 mm Hg = 3.4 × 103 mm Hg
101.3 kPa 760 mm Hg
1 atm 101.3 kPa a. 450 kPa × = 4.4 atm
Sample Problem 13.1
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Kinetic Energy and Temperature
Kinetic Energy and Temperature What is the relationship between the temperature in kelvins
and the
average kinetic energy of particles?
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Kinetic Energy and Temperature
As a substance is heated, some of the energy is stored and some is absorbed.
• This stored portion of the energy (potential energy) does not raise the temperature of the substance.
• The absorbed portion of the energy does speed up the particles, increasing their kinetic energy. This results in increased temperature.
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13.1 The Nature of Gases >
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Kinetic Energy and Temperature
Average Kinetic Energy The particles in any collection of atoms or molecules at a given temperature have a wide range of kinetic energies. • We use average kinetic energy when
discussing the kinetic energy of a collection of particles in a substance.
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Kinetic Energy and Temperature
Average Kinetic Energy At any given temperature, all of the particles in a substance, have the same average kinetic energy.
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Interpret Graphs
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13.1 The Nature of Gases >
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Interpret Graphs
• Most of the molecules have kinetic energies close to the average value.
• Molecules at the higher temperature have a wider range of kinetic energies.
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Kinetic Energy and Temperature
Average Kinetic Energy The average kinetic energy of the particles in a substance is directly related to the substance’s Kelvin 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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Kinetic Energy and Temperature
Average Kinetic Energy Absolute zero
• (0 K, or –273.15oC)
• the temperature at which the motion of particles theoretically ceases.
• No temperature can be lower than 0 K
• 0 K 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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Kinetic Energy and Temperature
Average Kinetic Energy
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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Kinetic Energy and Temperature
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.
