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IB PHYSICS

LSN 3-2, THERMAL PROPERTIES

Questions From Reading Activity?

Assessment Statements

IB Topic 3.2., Thermal Properties of MatterSpecific Heat Capacity, Phase Changes,

and Latent Heat3.2.1.Define specific heat capacity and thermal

capacity.3.2.2.Solve problems involving specific heat

capacities and thermal capacities.3.2.3.Explain the physical differences between

the solid, liquid and gaseous phases in terms of molecular structure and particle motion.

Assessment Statements

IB Topic 3.2., Thermal Properties of Matter

Specific Heat Capacity, Phase Changes, and Latent Heat

3.2.4. Describe and explain the process of phase changes in terms of molecular behavior.

3.2.5. Explain in terms of molecular behavior why temperature does not change during a phase change.

Assessment Statements

IB Topic 3.2., Thermal Properties of Matter

Specific Heat Capacity, Phase Changes, and Latent Heat

3.2.6. Distinguish between evaporation and boiling.

3.2.7. Define specific latent heat.3.2.8. Solve problems involving

specific latent heats.

Assessment Statements

IB Topic 3.2., Thermal Properties of Matter

Kinetic Model of an Ideal Gas3.2.9. Define pressure.3.2.10. State the assumptions of

the kinetic model of an ideal gas.

Assessment Statements

IB Topic 3.2., Thermal Properties of Matter

Kinetic Model of an Ideal Gas3.2.11. State that temperature is a

measure of the average random kinetic energy of the molecules of an ideal gas.

3.2.12. Explain the macroscopic behavior of an ideal gas in terms of a molecular model.

Objectives

State the basic definitions of calorimetry, such as specific heat capacity and specific latent heats of fusion and vaporization.

Understand why temperature stays constant during a phase change.

Outline methods for determining specific and latent heats experimentally.

Objectives

Solve calorimetry problems using Q = mcΔT and Q = mL.

State the factors that affect the rate of evaporation and distinguish evaporation from boiling.

Appreciate Boltzmann’s equation, the fundamental relationship between the absolute temperature and the average kinetic energy of the molecules.

Specific Heat Capacity, c

When you add heat (thermal energy) to an object it will, most of the time, heat up

Specific heat capacity is the amount of thermal energy needed to raise the temperature of a mass of one kilogram of a substance by one Kelvin. TmcQ

Specific Heat Capacity, c

The units for specific heat capacity are J/kg•K

Does the temperature have to be in Kelvin? Can it be Celcius/centigrade? Can it be Fahrenheit?

TmcQ

Specific Heat Capacity, c

The units for specific heat capacity are J/kg•K

Does the temperature have to be in Kelvin? Can it be Celcius/centigrade? YES Can it be Fahrenheit? NO

TmcQ

Specific Heat Capacity, c

Specific heat capacity is a property of the material you are dealing with.

To be specific, specific heat capacity is specific to each specific material, specifically speaking.

TmcQ

Specific Heat Capacity, cWhat is the difference in thermal energy required to raise the temperature of 10kg of aluminum and lead by 10 K?

TmcQ

Specific Heat Capacity, cWhat is the difference in thermal energy required to raise the temperature of 10kg of aluminum and lead by 10 K?

Aluminum = 91,000 JLead = 13,000 J 78,000 JAl is 600% more than Pb

TmcQ

Specific Heat Capacity, cWhat is the difference in thermal energy required to raise the temperature of 1 mol of aluminum and lead by 10 K?

TmcQ

Specific Heat Capacity, cWhat is the difference in thermal energy required to raise the temperature of 1 mol of aluminum and lead by 10 K?

molAl = 27 g = 246 JmolPb = 207 g = 269 J = 23 JPb is 9% more than AlHow come?

TmcQ

Specific Heat Capacity, c Thermal energy

increases the kinetic energy of the molecules which causes temperature to increase.

1 kg of each substance gives a large difference in the number of molecules

1 mol of each substance gives the same number of molecules with a negligible difference in masses

TmcQ

Heat Capacity, C

Equal to mass times specific heat capacity

The amount of thermal energy required to change the temperature of a body by one Kelvin

Useful when dealing with a system of objects

If we know the heat capacity of the system,

mcC

2211 cmcmC

TCQ

Thermal Equilibrium

Thermal energy will always flow from a hot body to a colder body

As long as there is a means of conducting that energy, the energy will flow until both bodies are at the same temperature which is, thermal equilibrium

Change of State One example is golfer Phil Mickelson

moving from California to Texas to avoid paying state income taxes

What we are more concerned with is the change in the physical state of matter

There are four states of matter: Solid Liquid Gas Plasma

Change of State

In general, these states of matter are a function of temperature with solids being the coldest and plasma being the hottest

When thermal energy is added to a body, it increases in temperature

Temperature increases until the body reaches a transition point determined by its physical and chemical properties

Further energy added will cause a change of state

• Solid• Liquid• Gas• Plasma

Change of State Melting point

Point at which a substance transitions from a solid to a liquid or vice versa if thermal energy is being extracted

Once the melting point is reached, further energy is required to provide the work needed to overcome the intermolecular bonds which hold the solid together

Temperature will remain constant during this transition

Once melting is complete, additional energy added will increase the temperature of the liquid

• Solid• Liquid• Gas• Plasma

Change of State Melting point

The thermal energy required to melt a given substance is called the specific latent heat of fusion, Lf

The units for specific latent heat of fusion are, J/kg

• Solid• Liquid• Gas• Plasma

fmLQ

Change of State Boiling point

Point at which a substance transitions from a liquid to a gas or vice versa if thermal energy is being extracted

Once the boiling point is reached, further energy is required to provide the work needed to overcome the intermolecular forces which hold the solid together, i.e. to further separate them

Temperature will remain constant during this transition

Once boiling is complete, additional energy added will increase the temperature of the gas

• Solid• Liquid• Gas• Plasma

Change of State Boiling point

The thermal energy required to turn a given liquid into a gas is called the specific latent heat of vaporization, Lv

• Solid• Liquid• Gas• Plasma

vmLQ

Change of State Gas to Plasma

Not assessed by IB or discussed in Tsokos

At extremely high temperatures, electrons have so much energy that it overcomes the electromagnetic attraction of the nucleus

Electrons separate from the nucleus and we are left with a mixture of electrons and nuclei

• Solid• Liquid• Gas• Plasma

Change of State Sample

ProblemHow much heat is required to completely vaporize 10 kg of lead at 25° C?

• Solid• Liquid• Gas

Change of State Specific Latent Heats

Amount of heat needed to change the state of a unit mass of a material at its specific melting / boiling point

Latent Heats (Not Specific) The amount of heat needed to change

the state of a material irrespective of mass

• Solid• Liquid• Gas• Plasma

Measuring Specific Heats In order to determine the specific heat of

a substance, we need to measure The amount of heat added Mass of the substance The change in temperature

Tm

Qc

TmcQ

Calorimetry

Measuring Specific Heats A second method is to drop a hot

substance into a cold liquid whose specific heat is known

By obtaining an equilibrium temperature we can calculate the unknown specific heat

Tm

Qc

TmcQ

Calorimetry

Stopped Here on 1/23/2013

Measuring Specific Latent Heats A similar method can be

used to determine the latent heat of fusion

Drop a cold solid substance at its melting temperature into a warm liquid mixture of the same substance

By measuring the temperature change of the liquid we can calculate the energy given up by the liquid that was used to melt the solid

sl

s

ll

QQ

mLQ

TcmQ

Measuring Specific Latent Heats Specific heat of vaporization

can be measured using the device shown at right

Measure the amount of heat added to a liquid in a double container

As the liquid vaporizes, it leaves the inner container, condenses on the outer container, and then drips down and is collected in a beaker at the bottom

mLQ

Evaporation

The molecules of a liquid are constantly moving at various speeds

At the surface of the liquid, the fastest of these molecules may contain enough energy to break free and enter the atmosphere as vapour (vapor)

When that molecule has left, the total kinetic energy of the liquid has decreased which means its temperature will drop

Evaporation Rate of evaporation increases with

surface area and temperature of the liquid

In an enclosed system, the escaping vapour creates vapour pressure above the surface of the liquid causing some of the molecules to re-enter the liquid

Evaporation continues until equilibrium is reached where as many molecules leave the liquid as those that return

The air is then considered saturated

Kinetic Theory of Gases

Explained through a simple mechanical model

Several basic assumptions must be made

Kinetic Theory of Gases

Basic Assumptions: Gas consists of a large number of

molecules Molecules move with a range of speeds Volume of individual molecules is

negligible compared to volume of the container

Collisions between the molecules and molecules with the container are elastic

Molecules exert no forces on each other or on the container except when in contact

Kinetic Theory of Gases

Basic Assumptions: Duration of collisions (impulse) is small

compared to time between collisions Molecules follow the laws of Newtonian

mechanics

Kinetic Theory of Gases

Boltzmann Equation:

The v2 term is the average of the squares of the speeds of the molecules of the gas This is called the root mean square

(rms) speed Not the average speed, but close enough

that the terms are used interchangeably

kTvm2

3

2

1 2 n

vvvv n

222

212 ...

Kinetic Theory of Gases

Boltzmann Equation:

k is the Bolzmann constant and is equal to 1.38 x 10-23 J/K It is a ratio of the gas constant R to the

Avogadro (avocado, acevedo) number Note that temperature in this

equation must be in Kelvin

kTvm2

3

2

1 2 n

vvvv n

222

212 ...

Kinetic Theory of Gases

Boltzmann Equation:

The importance of this equation is that it shows how absolute temperature is directly proportional to the average kinetic energy of the molecules of a gas

kTvm2

3

2

1 2 n

vvvv n

222

212 ...

Molecular Explanation of Pressure Pressure in a gas is a result of

collisions of the molecules with the walls of the container

Each collision results in a momentum change in the molecule

The wall must exert a force on the molecule to effect this change in momentum

Newton’s third law says that the molecule must then exert an equal and opposite force on the wall of the container

Molecular Explanation of Pressure Pressure then is the total force

created by all colliding molecules divided by the surface area of the container

Pressure results from collisions of molecules with the container, NOT from collisions with each other Elastic collisions between molecules

result in individual changes in velocity and energy, but momentum and kinetic energy are conserved

Molecular Explanation of Pressure The two factors that affect pressure

are speed of the molecules and frequency of the collisions

When the gas is heated, speed increases and collision frequency increases as a result

When gas is heated isothermally, speed stays the same but collision frequency increases due to less separation distance

frequencyspeedP

Molecular Explanation of Pressure The two factors that affect pressure

are speed of the molecules and frequency of the collisions

So what do you think will happen if a gas is compressed rapidly with a piston?

frequencyspeedP

Σary Review

Can you state the basic definitions of calorimetry, such as specific heat capacity and specific latent heats of fusion and vaporization?

Do you understand why temperature stays constant during a phase change?

Can you outline methods for determining specific and latent heats experimentally?

Σary Review

Can you solve calorimetry problems using Q = mcΔT and Q = mL?

Can you state the factors that affect the rate of evaporation and distinguish evaporation from boiling?

Do you appreciate Boltzmann’s equation, the fundamental relationship between the absolute temperature and the average kinetic energy of the molecules?

Assessment Statements

IB Topic 3.2., Thermal Properties of MatterSpecific Heat Capacity, Phase Changes,

and Latent Heat3.2.1.Define specific heat capacity and thermal

capacity.3.2.2.Solve problems involving specific heat

capacities and thermal capacities.3.2.3.Explain the physical differences between

the solid, liquid and gaseous phases in terms of molecular structure and particle motion.

Assessment Statements

IB Topic 3.2., Thermal Properties of Matter

Specific Heat Capacity, Phase Changes, and Latent Heat

3.2.4. Describe and explain the process of phase changes in terms of molecular behavior.

3.2.5. Explain in terms of molecular behavior why temperature does not change during a phase change.

Assessment Statements

IB Topic 3.2., Thermal Properties of Matter

Specific Heat Capacity, Phase Changes, and Latent Heat

3.2.6. Distinguish between evaporation and boiling.

3.2.7. Define specific latent heat.3.2.8. Solve problems involving

specific latent heats.

Assessment Statements

IB Topic 3.2., Thermal Properties of Matter

Kinetic Model of an Ideal Gas3.2.9. Define pressure.3.2.10. State the assumptions of

the kinetic model of an ideal gas.

Assessment Statements

IB Topic 3.2., Thermal Properties of Matter

Kinetic Model of an Ideal Gas3.2.11. State that temperature is a

measure of the average random kinetic energy of the molecules of an ideal gas.

3.2.12. Explain the macroscopic behavior of an ideal gas in terms of a molecular model.

QUESTIONS?

Part A, #1-10Part B, #11-20

Homework