11 1 Properties of Gases Sample of gas assumes both shape and volume of its container Gases are compressible Densities of gases are smaller than those of liquids and solids density of a gaseous substance depends highly on temperature and pressure Gases form homogeneous mixtures solutions with one another in any proportion 11 2 The Kinetic Molecular Theory of Gases Explains how molecular nature of gases gives rise to their macroscopic properties gas is composed of particles separated by relatively large distances volume of molecules is negligible gas molecules are constantly in random motion straight paths elastic collisions energy is transferred not lost in the collisions gas molecules don t exert attractive or repulsive forces on each other average kinetic energy of gas molecules is proportional to absolute temperature individual molecules are proportional to mass and velocity Molecular Speed root mean square speed m ol urms M 3RT R 8 314 J K rms is directly proportional to rms is inversely proportional to molar mass if two gases are at same temp gas with larger molar mass will have lower rms absolute temperature u 1 rms u 2rms M 2 M 1 M 2 is heavier than M 1 Diffusion Effusion Grahams Law mixing of gases as result of random motion and frequent collisions escape of gas molecules from a container to a region of vacuum rate of effusion or diffusion is inversely proportional to molar mass lighter gases diffuse and effuse more rapidly than heavier gases 11 3 Gas Pressure Pressure force applied per unit area ressure force area p 11 4 The Gas Laws Boyle s Law pressure volume V 1 P P V 1 pressure is inversely proportional to volume 1 P 2 decreasing volume increases frequency of collisions increasing pressure at constant temperature V 2 Charles s and Gay Lussac s Law temperature volume temperature is directly proportional to volume TV V 1 T 2 V 2 T 1 faster moving molecules collide more frequently increasing pressure at constant pressure container expands volume increases decreasing collisions until pressure is same again Avogadro s Law amount volume volume is directly proportional to the number of moles V n V 2 V 1 n 2 n 1 more molecules more collisions increasing pressure at constant temperature and pressure increases volume decreases collisions until pressure is constant 11 5 Ideal Gas Equation V P n STP 1 atm 273 K RT 11 6 Real Gases Deviation from ideal behavior High pressure gas molecules relatively close to each other distances reduce volume occupied by each molecule becomes more significant Low temperature gas molecules move more slowly lower kinetic energies magnitude of IM forces btwn molecules becomes more significant Van der Waals Equation b P term V term P V 2 an 2 V n n RT 11 7 Gas Mixtures Dalton s Law of Partial Pressures eachother P total i P Mole Fractions i ni ntotal mole fraction is always less than 1 sum of mole fractions is always 1 neither gas is affected by the presence of the other don t attract repel mole fractions are dimensionless i P i P total 10 4 Calorimetry measurement of heat changes Specific Heat and Heat Capacity specific heat s heat capacity C amount of heat required to raise temp of 1g of substance by 1 C q s q C amount of heat required to raise the temp of an object by 1 C m T T q q is for endothermic processes is for exothermic processes Constant Pressure Calorimetry heat change for the process m T qsys s q is equal to enthalpy change T if temperature increases heat is released from system if temperature decreases heat is absorbed by system q q qsurr s m T if temperature increases heat is absorbed by surroundings if temperature decreases heat is released from surroundings Constant Volume Calorimetry isolated system no heat enters or leaves the system overall heat change of system is 0 qcal qrxn 12 2 Properties of Liquids Surface Tension amnt of energy required to stretch or increase the surface of a liquid by unit area liquid with strong IMF has high surface tension capillary action formation of meniscus cohesion attractions between like molecules adhesion attractions between unlike molecules if adhesion cohesion meniscus is concave if cohesion adhesion meniscus is convex Viscosity measure of a fluid s resistance to flow higher viscosity slower liquid flows decreases with increasing temperature strong IMF higher viscosity Vapor Pressure volatile high vapor pressure at room temperature evaporation vaporization molecules leave the liquid phase gas phase in space above liquid condensation gas phase molecules strike liquid surface and become trapped by IMF no of molecules in gas phase increases so does rate of condensation dynamic equilibrium vapor pressure of liquid increases until rate of condensation rate of evaporation equilibrium vapor pressure pressure exerted by molecules that escaped to gas phase 12 6 Phase Changes A B heat is absorbed to overcome the attractive forces in a solid B C heat absorbed increases KE of liquid molecules and temp raises C D increased KE used to overcome cohesive forces in a liquid Liquid Vapor Phase Transition boiling point temperature at which vapor pressure external atmospheric pressure normal boiling point vapor pressure is equal to 1 atm molar heat of vaporization H vap amnt of heat required to vaporize a mole of substance at its boiling point highest temp a substance can exist as a liquid independent of critical temperature T c pressure critical pressure P c supercritical fluid minimum pressure required to liquify a substance at its critical temp fluid at a temp and pressure that exceed no distinction between gas and liquid used as solvents T c and P c Solid Liquid Phase Transition freezing liquid to solid melting fusion solid to liquid melting point freezing point temp where solid and liquid phases coexist normal melting freezing point temp at which melts freezes at 1 atm molar heat of fusion H fus energy required to melt 1 mole of a solid smaller than H vap supercooling liquid temporarily cooled to below its freezing point unstable occurs when heat is removed rapidly Solid Vapor Phase Transition sublimation deposition molecules go directly from solid phase to vapor phase molecules go directly from vapor phase to solid phase molar enthalpy of sublimation H sub energy required to sublime 1 mole of a solid equal to the sum of molar enthalpies of fusion and vaporization H vap H sub H fus 12 7 Phase Diagrams Phase boundary line line separating any two regions indicates conditions where two phases can exist in
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