Thermodynamics and Kinetic Molecular Theory

Thermodynamics

• The First Law, or law of conservation of energy, states energy cannot be created or destroyed
• The increase in internal energy of a thermodynamic system equals the heat energy added to the system minus the work done by the system on its surroundings
• The Second Law states energy moves toward greater entropy or randomness
• Entropy of an isolated system not in equilibrium tends to increase over time, approaching a maximum value at equilibrium
• The Third Law says absolute zero (0° K or -273.15 °C/-460 °F) is void of all energy and absolute zero is theoretical
• Energy can be defined as the exertion of force (kinetic) or the capacity (potential) to do work
• The unit of measurement for energy is the joule
• A joule is the force of 1 N that moves its point of application 1 meter in the direction of that force
• Potential and kinetic energy are the two types of energy
• Kinetic molecular theory builds on Newtonian physics and thermodynamics
• Kinetic molecular theory focuses on molecular movement (energy) and forces between these molecules

Entropy

• Entropy is the universal trend toward equilibration
• Entropy involves everything from ice melting to gas expansion
• Sleep and the induction of general anesthesia have been proposed to be entropic processes
• Entropy is unidirectional, it is the movement of energy from a higher concentration to a lower concentration; thus energy moves because of a gradient
• Ice does not make lemonade colder; instead, lemonade makes ice warmer
• Entropy ends when all energy ​​is equally distributed

Kinetic Molecular Theory Revisited

• The sum of the kinetic and potential energies of the molecules in a gas sample is the internal energy of the sample
• The temperature of the sample is directly proportional to the average KE
• The average KE for a gas depends on the temperature and not the identity of the gas
• Different gases have the same average KE if their temperatures are the same

When Kinetic Molecular Theory Fails

• Gases consist of small particles whose volume is negligible compared to the volume of the gas
• The volume occupied by a gas is much larger than the volume of the individual molecules, but the volume of the gas molecules themselves is not zero
• This tenet starts to fail with real gases as the pressures get large, because at higher pressures, more gas molecules are crowded into the same sample volume
• Gas molecules are in constant, random motion, which is correct for real gases
• Molecules in a sample show a range of kinetic energies, but the average KE depends only on the temperature, an excellent model for real gases
• There are no attractive or repulsive forces between the gas particles, so all collisions are elastic
• This tenet starts to fail for real gases at low temperatures, when the energy needed to overcome the attractive forces between molecules becomes a greater fraction of the molecular KE

Temperature

• Celsius to Kelvin calculation: K = C + 273
• Celsius to Fahrenheit calculation: °F = 1.8(°C) + 32
• Fahrenheit to Celsius calculation: °C = (°F – 32)/1.8
• Matter can change form with the addition of greater heat energy
• Temperature is the measurement of the thermal state of an object
• Heat is thermal energy and temperature is the quantitative measurement of that energy
• Standard temperature is 273.15 K or 0 °C
• Heat and energy are the same, with heat loss (energy loss) of a system is unidirectional from the higher concentration to the lower concentration, from hotter to less hot

Redistribution

• Vasoconstriction of peripheral vessels slows heat loss from the human body
• Core temperature redistribution is the process of increased heat loss from the body resulting from the vasodilating effects of volatile and regional anesthetics
• Vasodilating effects of volatile and regional anesthetics cause greater blood flow, therefore heat flow, to the body's surface from the core
• A patient's core temperature can drop quickly due to the vasodilating actions of anesthetics, with the greatest decrease occurring in the first hour after anesthetic administration
• Anesthesia administrations that result in a drop in temperature are: Bair, hotline, HotDog, and cap
• Hypothermia is anything below 36 degrees C or 96.8 degrees
• Blood flow to the body's surface encourages heat loss by four primary processes
• In decreasing order those processes are: Radiation, Convection, Conduction and Evaporation
• Radiation is most significant at ~60% and involves infrared electromagnetic wavelength energy transfer from us to environment (head – cap!)
• Convection involves currents at ~30%
• Conduction involves direct touch at ~5, it is very important in peds anesthesia!
• Evaporation involves heat loss from evaporation ~20%
• Heat loss from evaporation includes moisture evaporation from the patient's skin, as well as exhaled water vapor

Atmospheric Pressure

• The cumulative effect of gravity on atmospheric gases gives rise to atmospheric pressure
• Atmospheric gases are less concentrated at altitude and more concentrated at sea level
• Atmospheric pressure is the gravitational force on gases in a given area
• Standard pressure in the International System of Units: (SI) is 100 kPa or kilopascal
• Memorize: 1 torr equals 1 mmHg
• Memorize: 1 kPa equals 10.2 cm H2O equals 7.5 mmHg
• Memorize: 1 atm (atmosphere) equals 760 mmHg equals 760 torr equals 1 bar equals 100 kPa equals 1020 cm H2O equals 14.7 lb./ in2
• Bourdon gauges are used in anesthesia to measure high pressures, such as in gas cylinders, zero referenced to atmospheric pressure
• Its really 760, not zero, referencing atmospheric pressure

Vaporization

• Vaporization converts liquids or solids into vapors
• Evaporation is the specific process of vaporizing liquids and it requires energy
• All liquids that have high vapor pressures at room temperatures are known as volatile liquids
• Vapor pressure and boiling points are inversely related
• The temperature of a liquid will not rise above its boiling point
• Vapor pressures of the volatile anesthetics at standard temperature and pressure: (STP)
  • Isoflurane: 238-240 mmHg
  • Sevoflurane: 160-170 mmHg
  • Desflurane: 660-669 mmHg, will boil at higher altitudes because VP is greater than barometric pressure
• Different liquids exert different vapor pressures at a given temperature
• Vaporizers must be calibrated for each specific agent, as different volatile anesthetics have different vapor pressures
• Placing the wrong agent into a vaporizer will deliver a greater or lower concentration than dialed
• If a high vapor pressure volatile anesthetic agent is placed inside a vaporizer calibrated for a lower vapor pressure volatile anesthetic, the output of that vaporizer will be higher than indicated on the control dial (HLH).
• If a volatile anesthetic agent with a lower vapor pressure is placed inside a vaporizer calibrated for a higher vapor pressure anesthetic, the output of that vaporizer will be lower
• The rate at which a liquid evaporates relies on only three factors
  • Vapor pressure of the substance
  • Partial pressure of the vapor in the container above the liquid
  • Temperature
• As the temperature or the vapor pressure increases, so does the rate of vaporization
• As the partial pressure of the vapor above the substance increases, the rate of vaporization will slow
• Boiling point is decreased as altitude increases, since atmospheric pressure depends on altitude

Vapor Pressure

• Vapor pressure is a function of temperature, not pressure
• This is the pressure at which a vapor is at equilibrium with its liquid (condensed) state
• Vapor pressure is the rate at which evaporation equals the rate of condensation
• Higher VP results in faster evaporation
• Substances that have greater intermolecular forces have lower vapor pressures, like water
• The most energetic molecules in a liquid have sufficient kinetic energy to overcome the intermolecular forces binding them into the liquid state and escape into the gas phase
• Once molecules are in the gas phase, they move around and collide with each other, the surface of the liquid, and the walls of the container
• The forces from these collisions, averaged over the area of the walls of the container, result in a pressure which is the vapor pressure of the liquid
• As temperature increases, the vapor pressure of a liquid increases, as does the volatility or tendency of a liquid to evaporate
• Water has a lower vapor pressure, and is more tightly bound in the liquid state
• The heat of vaporization (ΔΗvap) is the amount of energy necessary to liberate one mole of liquid at its boiling point into the gas/vapor phase (Calories/Energy).
• Latent heat of vaporization is the number of calories required to change 1 gram of liquid into vapor without a temperature change
• The energy withdrawn from the environment to convert 1 g water into vapor is 2500 joules, or approximately 600 calorie
• The vapor pressure of a volatile liquid determines the maximum mole fraction of that substance in a gas phase in contact with that volatile liquid
• If oxygen gas is sent through a closed container, the oxygen and gaseous anesthetic gas will mix and exit the vaporizer
• The composition of the effluent (outflow) gas will depend on the temperature of the vaporizer (which determines the vapor pressure of an anesthetic gas) and the pressure of the oxygen
• The amount of oxygen that is diverted into the vaporization chamber also influences the composition of the effluent
• Amount of oxygen diverted is a kinetic (rate determined) effect, not a thermodynamic (what's energetically possible) effect

Dalton’s Law of Partial Pressures

• The total pressure in a mixture of gases is equal to the sum of the pressures of the individual gases (Pt = P1 + P2 + P3 ...........)
• The partial pressure of a gas is caculated by multiplying the percent gas times the atmospheric pressure
• In the atmosphere at sea level, the partial pressures are:
  • Oxygen: 160 mmHg
  • Nitrogen: 600 mmHg
• For a mask induction with 3 L oxygen and 7 L nitrous oxide, the equation would be:
  • 30% x 760 = 228 mmHg oxygen
  • 70% x 760 = 532 mmHg nitrous oxide
• High altitude pressure is 550 mmHg: -O2 Equation: 21% x 550 = 116 mmHg

Humidification

• Absolute humidity is the mass of water vapor in a given volume of air
• Relative humidity is a ratio of the actual amount of water vapor in the air at a given temperature to the maximum amount of water vapor that the air can hold at that temperature (saturation of water in air)
• Relative humidity is equal to actual/saturated x 100% or is equal to concentration of water in air/solubility of water in air x 100%
• The solubility of water in the air is the maximum amount of water that a given volume of air can accommodate
• Relative humidity increases as temperature decreases because saturated vapor pressure decreases
• Inspired gas is quickly humidified to 100% (relative humidity) when breathing through the nose
• The large mucous surface area allows efficient transfer of ​​heat and moisture
• Saturated water vapor partial pressure is 44 mg/L at 37 degrees C (room air at 20 degrees C and 17 mg/L)
• Tracheal intubation and high fresh gas flows bypass the normal humidification system and expose the lower airways to dry (< 10 mg H2O/L), room temperature gases

Boiling Point

• Compounds with more intermolecular forces have higher boiling points
• Liquids boil faster, and at lower temperatures at higher altitude (lower atmospheric pressure)
• The temperature at which the vapor pressure is equal to the ambient pressure
• Normal boiling point is the temperature at which the vapor pressure of a liquid ​​is equal to 1 atm of pressure or 760 mmHg
• As the temperature increases, the average kinetic energy of the molecules in the sample increases, eventually reaching the point where they no longer remain in contact with the other molecules
• Vaporization occurs as the molecules escape from the liquid to gas state
• Because gases respond strongly to pressure, the boiling point of a liquid is highly dependent on pressure

Heat of Fusion & Heat of Vaporization

• The molar enthalpy of fusion (∆Η°fus) is the heat necessary to convert 1 mole of a solid into a liquid at its normal melting point
• Ice to water represents the molar enthaply of fusion
• The molar enthalpy of vaporization (ΔΗ°vap) is the heat required to convert 1 mole of a liquid to a gas at its normal boiling point
• Water to stream represents the molar enthalpy of vaporization
• Heat of fusion represents the amount of energy necessary to overcome the intermolecular forces enough so the that molecules can start flowing around each other
• Heat of vaporization represents the amount of energy necessary to overcome all intermolecular forces so that the molecules can escape into the gas phase
• Heat of vaporization exhibits a larger measure of vaporization than the heat of fusion
• Ex: Water - Heat of vaporization is 41 kJ/mol, Heat of fusion is 6 kJ/mol

Cooling Effect

• Evaporation has a cooling effect because vaporization is an endothermic process. The endothermic process absorbs heat from its surroundings
• Endothermic processes cause a decrease in temperature because they absorb heat from the surroundings
• If vaporization has a cooling effect and absorbs energy, then condensation must have a heating effect, because it releases energy (steam) - exothermic
• Desflurane heat of vaporization of is 45 cal/g

Phase Diagrams

• The lines represent phase equilibrium boundaries
• Crossing one of the phase equilibrium boundary lines, by changing pressure or temperature, results in a phase transition (or a change of state)
• The triple point of the substance is the single temperature and pressure combination where all three phases can exist in equilibrium with each other
• The solid-liquid equilibrium line defines the melting point of the solid
• The volumes of solids and liquids usually don’t change much with increasing pressure, meaning the melting point doesn't change much with pressure
• The liquid-gas equilibrium line defines the boiling point of the liquid at various pressures
• The liquid-gas equilibrium line terminates at a point known as the critical point
• The temperature and pressure that define the critical point are known as the critical temperature and the critical pressure
• A gas cannot be liquefied if the temperature is above the critical temperature/point
• N2O has a critical temperature of 36.5 degrees C and O2 is -119 degrees C
• The N2O pressure gauge will read 750 psi until approximately 3/4 of the volume (400 L) has exhausted

Joule-Thompson Effect

• Isothermal defines that the volume change in the absense of termperature change
• If a cylinder is opened in a closed space, the cylinder's pressure and temperature can rise rapidly. The pressure and temperature will be too rapid to dissipate, so it’s an adiabatic process or recompression
• Rapid expansion or compression of a gas without equilibration of energy with the surrounding environment
• An adiabatic process entails no increase or decrease in a system's energy
• Joule-Thompson effect determines why allowing nitrous oxide to escape freely (at high flows) results in the cylinder accumulating frost
• When a gas is quickly allowed to exit the cylinder the tank pressure drops
• If pressure drops, then the amount of energy per area decreases as the exits
• This energy decrease results in lowered termperature Reffered to as adibatic expansion

Latent Heat of Vaporization and the Vaporizer

• Anesthetic liquid in the vaporizer exerts a vapor pressure inside the vaporizing chamber with a liquid and gas existing
• Fresh gas flows over the anesthetic, carrying away some of the agent in the gas state, cooling the remaining liquid, which reduces the vapor pressure
• Vaporizers compensate using metals with a high thermal conductivity, temperature compensation valves to modulate FGF (Sevo, Iso), and applying direct heat to the anesthetic liquid (Des)

Desflurane (Tec 6) Vaporizer

• Desflurane has vapor pressure close to 1 atm at 20 C (68 F), making it very volatile
• Desflurane needs to be heated and pressurized
• Desflurane is hearted to 39 C to creat a vapor pressure of about 2 atm (669x2)
• Pure desflurane vapor joins the fresh gas as it exits the vaporizer
• No fresh has flows through the sump
• External heating compensatin for het loss dring variation
• At high altitude Desflurane leaves vaporizer for patient to which is set % decrased ambient pressure- dial a higher conceentration
• Des vaporizer/ elevation , sevo & iso compensators for elevation

Cylinders

• (N20) Cylinders
  • Gas ese from the e, liquid in the cylinder varpoizes & latent hea
• (02) Cylaners
  • 15q0 liters = full cylinder

Test your knowledge of thermodynamics and the kinetic molecular theory. Questions cover the laws of thermodynamics, entropy, absolute zero, and gas behavior. Explore the significance of these concepts in physiological processes and ideal conditions.