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THE STATES OF GASES. Chapter 1. Bulk Variables. Volume - m 3 Pressure - Pa Temperature - K Composition - moles. Volume . length 3 units m 3 or cm 3 liter = 1000 cm 3 molar volume V m = V/n. 1 m. 1 m. 1 m. Pressure. Force/Area units Pascal = Newton/m 2 = Joule/m 3
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THE STATES OF GASES Chapter 1
Bulk Variables • Volume - m3 • Pressure - Pa • Temperature - K • Composition - moles
Volume • length3 • units • m3 or cm3 • liter = 1000 cm3 • molar volume Vm= V/n 1 m 1 m 1 m
Pressure • Force/Area • units • Pascal = Newton/m2 = Joule/m3 • atmosphere = 101325 Pa • bar = 100000 Pa – Standard Pressure • mm Hg • torr h
Mechanical Equilibrium High P Low P • High pressure gas will tend to compress a low pressure gas Movable wall or “piston” Equal P
Temperature • Thermometry- relate temperature to other properties. • Fahrenheit (°F) • Celsius (°C) • Kelvin (K)
Thermal Equilibrium High T Low T • Heat or energy will flow from the high T gas to the low T gas Rigid diathermic wall Equal T
Zeroth Law of Thermodynamics • Two systems that are separately in thermal equilibrium with a third system are also in thermal equilibrium with one another. • The zeroth law is the basis for thermometry, i.e. the use of a third body (the thermometer) to measure an equilibrium property (the temperature) of other systems.
Zeroth Law of Thermodynamics A Thermal Equilibrium Thermal Equilibrium B C Thermal Equilibrium
Absolute zero = -273.15 °C T/K = θ/°C + 273.15 CHARLES’S LAW
Composition • moles: niS ni = n • mole fraction: xiS xi = 1 • partial pressure: piS pi = p
Gas Laws • Boyle’s Law: pV = constant at constant n, T
Equations of State • P,V,T, and n are not independent. • Any three will determine the fourth. • An equation of state is an equation that relates P,V,T, and n for a given substance. • Gases have the simplest equations of state. • The simplest equation of state is the ideal gas law, pV = nRT
Ideal Gas Law • p = pressure • V = volume • n = moles • T = temperature • R = universal gas constant = 0.08206 L-atm/mol-K (Table 1.2)
Ideal Gas Model • Molecules may be treated as point masses relative to the volume of the system. • Molecular collisions are elastic, i.e. kinetic energy is conserved. • Intermolecular forces of attraction and repulsion have negligible on the molecular motion. • Real gases approximately behave as ideal gases at higher temperatures and low pressures.
Ideal Gas Model http://intro.chem.okstate.edu/1314F00/Laboratory/GLP.htm
Ideal Gas Problem Gas in a vessel of constant volume is heated to 600 K. If it initially entered the vessel at 300 K and 1 atm, what is the pressure after heating?
Ideal Gas Problem Gas in a vessel of constant volume is heated to 600 K. If it initially entered the vessel at 300 K and 1 atm, what is the pressure after heating?
Ideal Gas Problem Gas in a vessel of constant volume is heated to 600 K. If it initially entered the vessel at 300 K and 1 atm, what is the pressure after heating?
Partial Pressure • pA = partial pressure of gas A • V = total volume • nA = moles of gas A • T = temperature • R = universal gas contant = 0.08206 L-atm/mol-K
Dalton’s Law • The total pressure is the sum of all the partial pressure.
Dalton’s Law Problem • Earth’s atmosphere is ~ 75.5% N2, 23.2 % O2, and 1.3 % Ar by mass. What is the partial pressure of each component when the total pressure is 1.00 atm?
Dalton’s Law Problem • Earth’s atmosphere is ~ 75.5% N2, 23.2 % O2, and 1.3 % Ar by mass. What is the partial pressure of each component when the total pressure is 1.00 atm?
Real Gases • Real gases do not obey the ideal gas law. • Deviations at low temperature and high pressure. • Especially near point of condensation.
Molecular Interactions • Real gases are not point masses – they have volume. • Real gases do interact. • Two forces of interaction • Attractive forces • Repulsive forces.
Molecular Interactions • Attractive forces are “long” range forces • Long = several molecular diameters • Beyond this attractive forces are not significant • “Low” temperature • Low = T near condensation point.
Molecular Interactions • Repulsive forces are “short” range forces • Short = ~ less than 1 molecular diameter • At low T, attractive forces overcomes repulsive forces.
Summary • At low pressures and large volumes, there is little interaction ~ Ideal Gas. • At moderate pressure, attractive forces start to dominate – gas more compressible. • At high pressure, repulsive forces dominate – gas less compressible.
Compressibility • The compressibility of a gas is defined by:
Compressibility • If the gas behaves ideally, then Z=1 at all pressures and temperatures. • For real gases, however, Z varies with pressure, and deviates from its ideal value.
