Lecture 2.5Lecture 2.5

Phase Changes and latent heat

Foundation PhysicsFoundation Physics

Temperature, Internal Energy and Heatp , gy

Most phase changes, or changes of a substance from one phase of matter to another, require large amounts of energy

d t th d d f t t hcompared to the energy needed for temperature changes. Energy must be put into a Substance to cause it to melt or boil Energy must be taken out of a Substance to cause it toboil. Energy must be taken out of a Substance to cause it to freeze or condense (gas to liquid). The energy can be heat transfer or can be due to work done on or by the system.y yEnergy used to cause a phase change does not cause a temperature change. When ice melts at OoC it becomes

t t 0oC h t b il t 100oC i t b twater at 0oC; when water boils at 100oC,i t becomes steam at 100oC.The same is true in reverse: When water at 0oC freezes it becomes ice at 0oC; when steam at 100oCfreezes, it becomes ice at 0 C; when steam at 100 C condenses it becomes water at 100oC

Atoms, Molecules, Phases of Matter, ,• Heat required for phase changes:

Vaporization: liquid vapour Melting: liquid solid Sublimation: solid vapour Sublimation: solid vapour

• Heat released by phase changes: Condensation: vapour liquid Condensation: vapour liquid Fusion: liquid solid Deposition: vapour solidepos t o apou so d

Heat of Fusion, Heat of Vaporization, p

To understand where energy goes during a phase change, first consider melting and boiling; both require energy input. There are attractive forces between molecules that must be overcome during melting and boiling. For a solid to melt, the spring-like forces that g g , p ghold molecules in place must be broken, and a certain amount of energy is required to break each "spring." For a liquid to boil, attractive forces between molecules must be overcome and workattractive forces between molecules must be overcome, and work must be done to move the molecules to the larger separations found in gases. The amount of energy required is thus proportional to the number of molecules in the object and also to the strength of

hQ

to the number of molecules in the object and also to the strength of the forces acting between molecules.

fhmQ hf

hv

vhmQ solid liquid liquid gas

f

Latent heatsLatent heats

Latent heat: Energy associated with the phase changesLatent heat: Energy associated with the phase changes

Heat of combustionHeat of combustion

• Chemical reactions such as combustion are• Chemical reactions such as combustion are analogous to phase changes in that they involve definite quantities of heat Complete combustiondefinite quantities of heat. Complete combustion of 1 gram of gasoline produces about 46000 J.

H t f b ti h f li iHeat of combustion hc of gasoline is:46000 J/g = 4.6x107 J/kg

Complete conversion to CO2 and H2O

Energy from food:C H O + 6O > 6CO + 6H O +C6H12O6 + 6O2 -> 6CO2 + 6H2O + energy

1g of glucose -> 3.81 cal/g released

EvaporationEvaporation

Humidity has a definite effect on the net evaporation t f t th hi h th h idit th l thrate of water: the higher the humidity, the lower the

evaporation rate.densityvapor 100

density vapor saturationdensityvapor humidity Relative %

Scheme of the experimental setup of the micro array sensorarray sensor

Precise %r.h. controlin bacterial growth measurements

Dynamic Detection of selective Microorganism growth

Active micro-organism growth detection on cantileverscantilevers

y (k

Hz)

32.8

uenc

y

32 2

32.4

32.6 Reference lever LBE. colimodified Gompertz fit

t Fre

q31.8

32.0

32.2

sona

n

31.2

31.4

31.6

Gfeller K et al (2005)

Res

Time (min)0 60 120 180 240 300 360 420 480

31.2

Gfeller, K., et al (2005) Biosens.Bioelectron.21 528-533.

Dynamic mode

No growthMicro-organism growth on nano mechanical systems

Micro-OrganismH2O v.p.nano-mechanical systems

Nutritive layer

CantileverGrowth w -> fStart: Cantilever ‚inked‘ with micro-organism

Micro Organism IH2O v.p.

Micro-Organism II

w -> f

Nutritive layer

Micro-Organism I Micro-Organism II

Cantilever

Dynamic mode Nugaeva. N. et al. Biosensors & Bioelectr. (2005)

Saturation density of water vapor in airy pTemperature (oC) Water Vaport Density (g/m3)

10 2 36-10 2.36

0 4.85

5 6.80

10 9.40

15 12.83

20 17 3020 17.30

25 23.0

30 30.4

37 44.0

40 51.1

60 130 560 130.5

80 293.8

95 505

100 598

200 7840

ProblemProblem

• What is the density of water vapor in• What is the density of water vapor in grams per cubic meter in the desert when

%relative humidity is 10% and air temperature is 40oC?p

1g Water heated (Temperature vs time)1g Water heated (Temperature vs. time)

100

120

°C) d e

60

80tu

re (°

20

40

mpe

rat c

-20

0Tem

a

b

0 100 200 300 400 500 600 700

Time (sec)Heat is put into the system at 1 cal/sec

Water is WeirdWater is Weird

• Density INCREASES between 0ºC and 4 ºC• Maximum density of water is 1000 kg/m3 at 4 ºC• Maximum density of water is 1000 kg/m at 4 C • Density of ice = 917 kg/m3 .... Ice floats!

Explanation for the Anomalous Behaviour of Water

ProblemProblem

• One day the relative humidity is 90% and• One day the relative humidity is 90% and the temperature is 25oC. (1) How many

f fgrams of water will condense out of each cubic meter of air if the temperature drops p pto 15oC? (2) How much energy does the condensation from each cubic metercondensation from each cubic meter release?

Gas lawsGas laws

The gas laws are a set of laws that describe the relationshipthat describe the relationship between thermodynamic temperature (T) pressure (P) andtemperature (T), pressure (P) and volume (V) of gases. They are a loose collection of rules developed between the late pRenaissance and early 19th centurycentury.

Boyle’s LawBoyle s Law

constant (constant temperature)pV

Charles’ LawCharles Law

V constant (constant pressure)VT

Gay-Lussac’s LawGay Lussac s Law

constant (constant volume)p ( )

T

Ideal Gas LawIdeal Gas LawThe ideal gas law is a special form of an equation of stateThe ideal gas law is a special form of an equation of state,i.e., an equation relating the variables that characterize a gas(pressure, volume, temperature, density, ….).The ideal gas law is applicable to low-density gases.

constant (fixed mass of gas)pV constant (fixed mass of gas)

TpV nRT

B

pVpV Nk T

RT

PV RTp RT PV nRT

pressurevolume Ideal Gas Constant

temperature

volumenumber of moles

Ideal Gas Constant One mole is NA =6.023x1023 molecules(number of 12C atoms in 12 g of 12C)

R=8.31 Nm/moleK

Phases repetitionPhases repetition• In physics and chemistry, the triple p y y, p

point of a substance is the temperature and pressure at which three phases (gas, liquid, and solid) of that substance may coexist inof that substance may coexist in thermodynamic equilibrium.

• For example the triple point• For example, the triple point temperature of mercury is at −38.8344 °C, at a pressure of 0.2 MPa.

• The triple point of water is used to define the Kelvin, the SI base unit of thermodynamic temperature. The number given for the temperature of the triple point of water is an exact definition rather than a measureddefinition rather than a measured quantity.

Next LectureNext Lecture

• To Be Covered: heat transfer• To Be Covered: heat transfer

• Reading: Chapter 5 Section 5 4 Section 5.4 Section 5.5