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USC CHEM 105aLg - Chem 105a Lecture 5&6 notes

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Chapter 5: Ideal GasesProperties of Gases- No fixed volume (expands to fill)- No fixed shape (take the shape of container)- Highly compressible- Very low density- Mix completely with other gases- Exert pressure on surroundings- Fluid- moves from high to low pressurePressure: force/unit area. Pa (pascal)-Us mostly atm and sometimes torrAt sea level 760 mm Hg stood tall, PV=k PV=PVAt constant temperature, pressure increases and volume decreases: The gas particles collide with the container more (measure pressure by how many times particles hit a container)Ideal gas strictly obeys the ideal gas law. Real gases best obey at low pressure and high temperature.Gases deviate from the ideal gas law at higher pressures because: the volume ofthe particles themselves become significant, the particles interact with each othermore.At constant pressure: V=bT or V1/T1=V2/T2 (Kelvins)Avogadro’s Law: V=an, V1/n1=V2/n2Ex) inflating a balloon, pressure is constant, injecting more moles into balloon volume increases. R=0.08206 L-atm/K-molPV=nRT**what ever is constant: can set equal without constants.Ex) Constant V: P/nT=R/V equals P1/n1T1=P2/n2T2STP= 0 degree Celsius, 1 atm, 1 mol of gas at STP= 22.42 L*only for density of a gas: Molar mass= densityRT/PWhen gas is collected by displacement of water, it always contains a small amount of water vapor.Vapor pressureKMT:-Gas particle collide and frequently exchange KE.-Particles have a wide range of velocities-At higher temperature, the ranges become broad.KEavg=3/2RTFrom the mean, how fast is the particle going?urms= m/s = root 3RT/MM in kg/molR=8.3145 J/Kmol J=kg m2/s2KE= 1/2(MM)v2What is the average speec of a hydrogen molecule at 300K?urns=root [3(8.3145)(300K)]/2.16*10-3 = 1930 m/sEffusion: movement of a gas through a pinhole into an evacuated chamber.Diffusion: mixing of gasesGrahams Law of Effusion (M is molar mass)Effusion rat for gas 1/ effusion rate for gas 2 = root mass 2/root mass 1-Lower molar mass, lighter particles, higher velocities, and faster effusionIdeal Gas Particles:1. Are volumeless (extremely small)2. Strike each other with elastic collisions (like pool balls)3. Have no attraction to one another.*Real gases act like ideal gases at low P, and high T.Van der Waals equation (more closely fits real gases)[P+a(n/v)2][V-nb]=nRTa=constant for given gas, accounts for fact that particles may be attracted to eachother.b=subtracts volume of gas molecules themselves, generally increases with size of gas molecule. Thermochemistry - the relationship between matter and energy State functions only depend on initial and final conditions, not the pathway between them.positive value negative valueqsystem gains heat System releases heatwsystem gains energyfrom work done on it System releases energy by doing workEsystem gains energy System releases energy Enthalpy of reaction or heat of reaction Units are kJ.  Calculated by Hess’ Law or measured by calorimetry. Standard enthalpy of formation  1 mol of compound forms from its elements in their standard states; elements may have fraction coefficients Units are kJ/mol. Elements in the standard state: = 0 Standard enthalpy of reaction Hypothetical decomposition of reactants into their elements and reconstitution of elements into products. Units are kJ. = S np [products] - S nr [reactants] exothermic into system: ΔEsys negativeendothermic into system: ΔEsys positiveEnergy exchanged between system and surroundings through either heat exchange or work. Cs water= 4.18W=-PΔVWhen a fluid expands +ΔV, work is negativeWhen a fluid compresses -ΔV, work is positiveH=E+PVAt constant p:q= ΔE+PΔVΔH= ΔE+PΔV (change in enthalpy)ΔH=Hproducts - HreactantsA systems enthalpy change equals the energy flow as heat: ΔH=q +ΔH, the reaction is endothermic-ΔH, the reaction is exothermic (-ΔEsys= ΔEsurroundings)Standard state: conditions for calculating thermodynamicStandard enthalpy or formation (ΔHf) the ΔH when 1 mol of a compound forms from its elements in their standard states.For pure element in its standard state ΔHf=0Degree symbol indicates standard states. The enthalpy change for a given reaction__Vaporization: To change any substance from the liquid to gas state, requires heat into the substance - endothermic the freezing of water EXOthe combustion of propane EXO (burns to give off heat)a hot cup of coffee (system) cools on a countertop EXOthe chemical reaction in a "hot pack" often used to treat sore muscles EXOthe vaporization of rubbing alcohol ENDOthe formation of dew in the morning EXOthe melting of ice ENDOThe chemical reaction in a "cold pack" often used to treat injuries ENDOthe vaporization of water


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