Thermal energy

Chapter 10

Heat

Thermal Energy

A. Temperature & Heat

1. Temperature is the measure of the average kinetic energy of the particles in a substance.

2. SI unit for temp. is the Kelvin

(but you will see Celsius used)

a. K = C + 273 (10C = 283K)

b. C = K – 273 (10K = -263C)3. Thermal Energy – the total of all the kinetic and potential energy of all the particles in a substance.

Quantity of Heat

• It is measured in joules

• Common usage is a heat unit called the calorie (the energy needed to raise the temperature of 1 gram of water by 1o C)

• Chemists use the kilocalorie (1000 calories=4200 Joules)

• Nutritionists call it a food Calorie

4. Thermal energy relationships

a. As temperature increases, so does thermal energy (because the kinetic energy of the particles increased).

b. Even if the temperature doesn’t change, the thermal energy in a more massive substance is higher (because it is a total measure of energy).

5. Heat

a. The flow of thermal energy from one object to another.

b. Heat always flows from warmer to cooler objects. Ice gets

warmer while hand gets

cooler

Cup gets cooler while hand gets

warmer

Heat Transfer• Heat flows from hot to cold.

– If you hold something cold, heat flows from hand to object.

– If you hold something hot, heat flows from object to hand

• Conduction- transfer of thermal energy through matter by the direct contact of particles – Occurs because particles are in constant motion– KE transferred as particles collide

• http://www.physchem.co.za/Heat/Transfer.htm#Conduction

Conduction• Heating of metal pan-– Particles in handle of pan move slowly– Fast moving particles from the bottom bump into

slower particles and speed them up– Occurs until all particles move the same speed

• Conduction works best in solids- especially metals- because particles are close together

Conduction and Convection• Metals- good conductors-because electrons move

easily & transfer KE to nearby particles• Fluid- any materials that flows• Convection- transfer of energy in a fluid by the

movement of heated particles • Convection currents transfer heat from warmer to

cooler parts of a fluid.• Convection vs. Conduction-

– Conduction involves collisions and transfers of energy. – Convection involves movement of the energetic particles

from one location to another

Convection• Convection- results in changes in density

– As particles move faster, they get farther apart– Fluid expands as temperature increases– Larger volume = smaller density– Decreasing density results in the rise of the warmer

fluid

• Lava Lamp- – Cool oil = dense = sits on the bottom– Warmer oil = less dense than alcohol & rises– As it rises, it loses energy through conduction

• Causes decrease in density = sinking

• When oil is cool

Oil is Oil is warm, so warm, so it risesit rises

Oil starts to lose heat by

conduction and falls

Convection Currents• Currents in which warm portions of the fluid

move through the substance- convection

• The warm portions transfer energy to the cool section through conduction

Heat Transfer on Earth• At equator- earth experiences the most heat from the

sun. – Result: evaporation of water and large accumulations of

clouds.

– As the water vapor rises, it cools and condenses, forming rain

• After the rain = dry air– Dry air causes moisture to evaporate, drying out the ground

– causes desert

• Convection currents create deserts and rain forests over different regions of Earth

Radiation• Transfer of heat to the earth – occurs through

radiation

• Radiation- the transfer of energy by electromagnetic waves. The waves travel through space even without matter

Controlling the Flow of Heat• To control the flow of heat: Use clothing,

blankets, layers of fat, fur, etc.

• Insulator- material that does not allow heat to flow through easily

• Gases – like air- are good insulators because: – Gas particles are very far apart & can’t transmit E

through conduction. – If the gas is also held in place, particles can’t move

around and warm up the rest of the gas

Insulation• Insulation is made of fluffy materials

containing pockets of trapped air – prevents heat loss

• Thermos- vacuum layer between 2 layers of glass– Vacuum contains few particles so conduction &

convection don’t occur.

• Thermos- coated in aluminum – Reflects electromagnetic waves that would either

heat the substance or allow the substance to cool

• Picture altered from How stuff works.com

6. Specific Heat

a. Some things heat up or cool down faster than others.

Land heats up and cools down faster than water

b. Specific heat is the amount of thermal energy required to raise the temperature of 1 kg of a substance one degree (C or K).

1) C water = 4184 J / kg C

2) C sand = 664 J / kg C

This is why land heats up quickly during the day and

cools quickly at night and why water takes longer.

Specific Heat

• The higher the specific heat, the more energy is required to cause a change in temperature.

• Substances with higher specific heats must lose more thermal energy to lower their temperature than do substances with a low specific heat.

• Water is slower to heat but is also slower to lose heat

Why does water have such a high specific heat?

Water molecules form strong bonds with each other; therefore it takes more heat energy to break them. Metals have weak bonds

and do not need as much energy to break them.

water metal

c. A calorimeter is used to help measure the specific heat of a substance.

First, mass and temperature of

water are measured

Then heated sample is put

inside and heat flows into

water

T is measured for water to help get its heat gain

This gives the heat lost by

the substance

Expansion of Water

• Remarkably interesting case

Expansion of Water

Expansion of Water

• This is why lakes and ponds and rivers freeze with the ice on top

• If they didn’t, no aquatic life would be possible

Thermostat

•A thermostat is a device that controls the temperature

•The switch of a thermostat is a bimetallic strip

Bimetallic Strip•Two different metals that are bound together

•They expand at different rates when heated

•Used as a switch in a thermostat

Copyright 1999, PRENTICE HALL

Chapter 11 30

Phase ChangesPhase Changes• Sublimation: solid gas.• Vaporization: liquid gas.• Melting or fusion: solid liquid.• Deposition: gas solid.• Condensation: gas liquid.• Freezing: liquid solid.

Energy Changes Accompanying Energy Changes Accompanying Phase ChangesPhase Changes

• Energy change of the system for the above processes are:


Chapter 11 31

Phase ChangesPhase ChangesEnergy Changes Accompanying Energy Changes Accompanying

Phase ChangesPhase Changes– Sublimation: (endothermic). – Vaporization: (endothermic).– Melting or Fusion: (endothermic).– Deposition: (exothermic). – Condensation: (exothermic).– Freezing: (exothermic).

• Generally heat of fusion (enthalpy of fusion) is less than heat of vaporization:– it takes more energy to completely separate

molecules, than partially separate them.


Chapter 11 32


Phase ChangesPhase Changes• All phase changes are possible under

the right conditions (e.g. water sublimes when snow disappears without forming puddles).

• The sequence heat solid melt heat liquid boil heat gas is endothermic.

• The sequence cool gas condense cool liquid freeze cool solid is exothermic.


Chapter 11 33


Phase ChangesPhase Changes


Chapter 11 34

Phase ChangesPhase ChangesHeating CurvesHeating Curves• Plot of temperature change versus heat

added is a heating curve.• During a phase change, adding heat

causes no temperature change.

–These points are used to calculate Hfus and Hvap.

• Supercooling: When a liquid is cooled below its melting point and it still remains a liquid.

• Achieved by keeping the temperature low and increasing kinetic energy to break intermolecular forces.


Chapter 11 35

Phase ChangesPhase ChangesHeating CurvesHeating Curves


Chapter 11 36

Heating Curve Illustrated


Chapter 11 37

Phase ChangesPhase ChangesCritical Temperature and PressureCritical Temperature and Pressure• Gases liquefied by increasing pressure

at some temperature.• Critical temperature: the minimum

temperature for liquefaction of a gas using pressure.

• Critical pressure: pressure required for liquefaction.


Chapter 11 38

Critical Temperature, Tc

Phase Diagrams


Chapter 11 39

A phase diagrams show what phases exist at equilibrium and what phase transformations we can expect when we change one of the parameters of the system (T, P,composition).


Chapter10 40

Phase DiagramsPhase Diagrams• Phase diagram: plot of pressure vs.

Temperature summarizing all equilibrium between phases.

• Given a temperature and pressure, phase diagrams tell us which phase will exist.

• Features of a phase diagram:– Triple point: temperature and pressure at which all three

phases are in equilibrium. – Vapor-pressure curve: generally as pressure increases,

temperature increases.– Critical point: critical temperature and pressure for the gas.– Melting point curve: as pressure increases, the solid phase

is favored if the solid is more dense than the liquid.– Normal melting point: melting point at 1 atm.


41

Phase DiagramsPhase Diagrams• Any temperature and pressure

combination not on a curve represents a single phase.


Chapter 11 42

Phase DiagramsPhase DiagramsThe Phase Diagrams of HThe Phase Diagrams of H22O and COO and CO22

• Water:– The melting point curve slopes to the left

because ice is less dense than water.– Triple point occurs at 0.0098C and 4.58

mmHg.– Normal melting (freezing) point is 0C.– Normal boiling point is 100C.– Critical point is 374C and 218 atm.


Chapter 11 43

Phase DiagramsPhase DiagramsThe Phase Diagrams of HThe Phase Diagrams of H22O and COO and CO22

• Carbon Dioxide:– Triple point

occurs at -56.4C and 5.11 atm.

– Normal sublimation point is -78.5C. (At 1 atm CO2 sublimes it does not melt.)

– Critical point occurs at 31.1C and 73 atm.

Thermal energy

Technology

heat energy

temperature heat

heat transfer heat flows

flow of heat

heat unit

quantity of heat

transfer of energy

radiation transfer of