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Lecture 2.5 Phase Changes and latent heat Foundation Physics Temperature, p , Internal Energy gy and Heat Most phase changes, or changes of a substance from one phase of matter to another, require large amounts of energy compared d tto the th energy needed d d ffor ttemperature t changes. h 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 to boil. freeze or condense (gas to liquid). The energy can be heat transfer or can be due to work done on or byy the system. y Energy used to cause a phase change does not cause a temperature change. When ice melts at OoC it becomes water t att 0oC; C when h water t b boils il att 100oC,i Citb becomes steam t at 100oC.The same is true in reverse: When water at 0oC freezes it becomes ice at 0oC; when steam at 100oC freezes, 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 • Heat released by phase changes: Condensation: vapour liquid Fusion: liquid solid Deposition: epos t o vapour apou solid 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 g and boiling. g For a solid to melt,, the spring-like p g forces that hold 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 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 the forces acting between molecules. Q m hf hf solid liquid Q m hv hv liquid gas Latent heats Latent heat: Energy associated with the phase changes Heat of combustion • Chemical reactions such as combustion are analogous to phase changes in that they involve definite quantities of heat heat. Complete combustion of 1 gram of gasoline produces about 46000 J. H t off combustion Heat b ti hc off gasoline li iis: 46000 J/g = 4.6x107 J/kg Complete conversion to CO2 and H2O Energy from food: C6H12O6 + 6O2 -> > 6CO2 + 6H2O + energy 1g of glucose -> 3.81 cal/g released Evaporation Humidity has a definite effect on the net evaporation rate t off water: t the th higher hi h the th h humidity, idit th the llower th the evaporation rate. vapor density % Relative humidity 100 saturation vapor density Scheme of the experimental setup of the micro array sensor Precise %r.h. control in bacterial growth measurements Dynamic Detection of selective Microorganism growth Res sonant Frequency y (kHz) Active micro-organism growth detection on cantilevers 32.8 Reference lever LB E. coli modified Gompertz fit 32.6 32.4 32 2 32.2 32.0 31.8 31.6 31.4 31.2 0 60 120 180 240 300 360 420 480 Time (min) Gfeller, K Gfeller K., et al (2005) Biosens.Bioelectron. 21 528-533. Dynamic mode Micro-organism growth on nano mechanical systems nano-mechanical No growth H2O v.p. Micro-Organism Nutritive layer Cantilever Growth w -> f Start: Cantilever ‚inked‘ with micro-organism H2O v.p. Micro Organism I Micro-Organism II Micro-Organism Nutritive layer Cantilever Dynamic mode Nugaeva. N. et al. Biosensors & Bioelectr. (2005) Saturation density y of water vapor p in air Temperature (oC) Water Vaport Density (g/m3) -10 10 2 36 2.36 0 4.85 5 6.80 10 9.40 15 12.83 20 17 30 17.30 25 23.0 30 30.4 37 44.0 40 51.1 60 130 5 130.5 80 293.8 95 505 100 598 200 7840 Problem • What is the density of water vapor in grams per cubic meter in the desert when relative humidity is 10% % and air p is 40oC? temperature 1g Water heated (Temperature vsvs. time) Tem mperatture (°°C) 120 d 100 e 80 60 c 40 20 b 0 -20 a 0 100 200 300 400 500 600 700 Time (sec) Heat is put into the system at 1 cal/sec Water is Weird • Density INCREASES between 0ºC and 4 ºC • Maximum density of water is 1000 kg/m3 at 4 ºC C • Density of ice = 917 kg/m3 .... Ice floats! Explanation for the Anomalous Behaviour of Water Problem • One day the relative humidity is 90% and the temperature is 25oC. (1) How many grams off water will condense out off each p drops p cubic meter of air if the temperature to 15oC? (2) How much energy does the condensation from each cubic meter release? Gas laws The gas laws are a set of laws that describe the relationship between thermodynamic temperature (T) (T), pressure (P) and volume (V) of gases. They are a loose collection of rules p between the late developed Renaissance and early 19th century. century Boyle’s Boyle s Law pV constant (constant temperature) Charles’ Law Charles V constant T (constant pressure) Gay-Lussac’s Gay Lussac s Law p constant T ((constant volume)) Ideal Gas Law The ideal gas law is a special form of an equation of state 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. pV T pV constant (fixed mass of gas) nRT pV Nk BT p RT PV nRT RT temperature pressure volume Ideal Gas Constant One mole is NA =6.023x1023 molecules number of moles (number of 12C atoms in 12 g of 12C) R=8.31 Nm/moleK Phases repetition • In p physics y and chemistry, y, the triple p point of a substance is the temperature and pressure at which three phases (gas, liquid, and solid) of that substance may coexist in thermodynamic equilibrium. • For example 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 measured quantity. Next Lecture • To Be Covered: heat transfer • Reading: Chapter 5 Section 5 5.4 4 Section 5.5