Chapter 12: Heat, Temperature, and Thermal Energy

Cooking Up a Science: Why is the soup at 100 °C not boiling?

Both bodies are at the same temperature, so there is no net change in heat to cause the water inside to boil; they are at thermal equilibrium.

The Theory of Heat

Heat travels from a hot surface to a colder surface…the hot surface cools down and the cold surface warms up until they both have the same temperature. We say they have reached thermal equilibrium - no more heat is transferred.

(2nd Law of Thermodynamics)

Kinetic Molecular Theory

Particles are always in motion. Particles in a hotter substance move faster…more EK

Energy is transferred (Conservation of Momentum) as heat when faster particles collide with slower particles, causing the slower ones to increase in speed and kinetic energy.

Explain how this cooking utensil can be used to keep the most “flavor” in the soup.

We can measure temperature which indicates the average kinetic energy of all of the particles in the chosen object. We use a thermometer to do this. Temperature is measured in °C or K (Kelvins). (Note: 0 °F freezing of brine; 100 °F temperature of a horse)

Thermal energy is defined as the total energy of all of the particles in the object. Thermal energy is measured in J.

Determining Absolute Zero

An ideal gas will not have any volume at absolute zero. By plotting a graph of Volume against Time for the gas, we can extrapolate (ie. extend) the linear relationship to experimentally derive

a value for absolute zero…−273.15 °C.

Heat (Q or EH)

Heat is the transfer of some of the thermal energy. Heat moves from a hot object to a colder object. The amount of heat transferred, Q, depends on

1. The type of material…indicated with the specific heat of the material, c, in J/kgK or J/kg°C (water c = 4200 J/kgK).

2. The amount of material…indicated by the object’s mass, m, in kg.3. How much the object gets hotter of colder by…indicated by the temperature change, ∆T, in

K or °C.Q=mc(∆T )whereQ∨EH is∈J .

Question: How much energy is needed to heat a piece of copper of mass 5.0 g to increase its temperature 20 °C? (ccopper = 385 J/kg°C…p.248)

Q=mc (∆ T )=(5.0×10−3 kg ) (385J /kg ∙℃ ) (20℃ )=38.5 J …39J (2 sf )

Hot Cold

Q

(Note: ∆°C = ∆K 385 J/kg°C = 385 J/kgK)

Thermal Equilibrium

When two objects of different temperatures are placed in contact with each other, heat will be transferred and

their temperatures will change until they reach the same temperature - equilibrium temperature, Teq. (Tfinal)

QH=−QC : (Heat givenoffby hot object )=−(Heat given off

bycold object )(mc ∆T )H=−(mc ∆T )C where ∆ T=T F−T I

Question: A 5.0 g piece of copper coin is placed in 100 mL of water. The coin’s initial temperature is 85 °C and the water’s initial temperature is 19 °C. What is their equilibrium temperature? (cwater = 4200 J/kg°C; ccopper = 385 J/kg°C)

(mc ∆T )H=−(mc ∆T )C (Note : ρwater=1 g/ml)

(5.0×10−3 kg) (385J /kg ∙℃ ) (T F−85℃ )=−(100×10−3kg ) (4200 J /kg ∙℃ ) (T F−19℃ )

1.925 (T F−85℃ )=−420 (T F−19℃ )

421.925T F=8 143.625

T F=EquilibriumTemperature ,T eq=19.3℃

Latent Heat

Heat is needed during phase changes such as liquid to gas (vaporization) because bonds are broken and the molecules are moved further apart. The amount of heat per unit mass to do this is called the latent heat of the material:

Latent heat of fusion, HF (J/kg): solid ↔ liquid Latent heat of vaporization, HV (J/kg): liquid ↔ gas…p. 254

Q=mH …during phase changes only

Question: Calculate the heat needed to turn 150 g of ice at −5.0 °C into water, then increase its temperature to boiling point and boil all of it off.

Data: cice = 2060 J/kg°C; HF = 3.34 × 105 J/kg; HV = 2.26 × 106 J/kg

Ice :−5℃→0℃ Q1=(mc ∆ T )ice

Ice→Water Q2=m HF

Water :0℃→100℃ Q3=(mc ∆ T )water

Water →Steam Q4=m HV

QTotal=Q1+Q2+Q3+Q4=4.5×105 J