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Reviews for Chemistry II
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Date Created: 09/19/15
Exam 1 Review Guide Kaelyn Stevick Properties of gases Microscopic Macroscopic Molecules are spaced out Compressible molecules are spaced out which Molecules are constantly moving means volume of container can decrease Molecules collide with each other with no loss Much lower density than solid or liquid of energy No such thing as an immiscible gas Molecules bounce off side of container all gases will mix freely Gases exert pressure by hitting container walls Pressure Pressure comes from gas molecules hitting vessel walls The greater the number of collisions the greater the pressure Higher speed collisions result in greater pressure To get higher speed collisions add energy 139 e increase temp Pressure is measured in atm torr or mmHg 1 atm 760mm Hg 760 torr Relations between gas properties Pressure and Volume PTVi PLVT Boyle s Law volume is inversely proportional to pressure assuming constant moles and temperature Ex Empty bottle and airplane As the plane decreased in altitude the bottle compressed The pressure increased as the plane approached the ground and subsequently forced the volume of the bottle to decrease Volume and Moles nTVT niVi Avogadro s Law number of moles of gas and volume are directly proportional assuming constant pressure and temperature Volume does not depend on type of gas just the moles of gas Volume and Temperature TTVT TLVL Charles Law temperature and volume are directly proportional assuming constant pressure and moles Ex Balloon and liquid nitrogen When liquid nitrogen was poured over a balloon the gas molecules inside the balloon were cooled resulting in less pressure on the sides of the balloon causing a smaller volume T must be an absolute temp min temp 0 gt T must be in Kelvin K 0C 27315 You get this equation when all 3 gas laws are combined PV nRT R is an experimentally determined value 0082057 L atm mol K Another useful equation may be m 22y n1T1 11sz Other examples of gas laws at work I Bicycle tires becoming atter as the weather gets colder 7 during the winter the lower temperatures cause the air molecules in the bicycle tire to constrict With the gas molecules occupying less space the tire s volume decreases Pop can crushing demo 7 when the heated pop can containing water vapor is turned over into a beaker of cold water the gas molecules cool and compress back into a liquid which decreases the pressure within the pop can This means that the external pressure is now much greater than the pressure within the can resulting in the crushing of the pop can Exploding water bottle demo 7 when liquid nitrogen is put into a water bottle and immediately capped the bottle explodes This results because liquid nitrogen at room temperature boils and the bottle cannot withstand the pressure that results from the expanding gas molecules Applying PV nRT Ex Consider sublimation of dry ice C02 s gt C02 g What is the volume at STP of 162g of dry ice in liters 1162g C02 1 1 mol 1 0368 mol C02 4401g C02 V nRTP 10368 mol10082057 L atmmolK127315K1 825 L 1 atm Then the 162g of dry ice is put into 300L ask at 25 C and sealed Ifthe pressure of the ask exceeds 250 atm the ask will break Will the ask break what is the actual pressure P nRTV 0368 mol0 082057 L atm mol K29815K1 300 atm 300 atm gt 250 atm pressure limit therefore the ask will break Ex H2g C12 7 2HCIg 0360g C12 and 0049g H2 are put into a 45L vessel at 200 C The reaction occurs to produce HCl What is the nal pressure in torr Hard Way Limiting reactant problem Final pressure Pressure of HCl Pressure of excess reagent 10360g C1211molCl212 mol HC 1 00101 mol HCl gt C12 LIMITING 71g C12 1 mol C12 10049g H2 11 mol H212 mol HC 1 00490 mol HCl gt H2 EXCESS 2gH2 1molH2 Excess 00101 mol HCl used 1 mol H2 1 000507 mol H2 used 2 mol HCl 00245 mol H2 available 000507 mol H2 used 00194 mol excess H2 Total moles of gas 00101molHCl 00194 mol H2 00296 mol gas Pressure P nRTV 00296 mol0082057 L atm mol K47315 K 0255 atm 45L 10255 atm1760 torr 193 torr latm Easier Way Must understand that the composition of a gas does not affect the pressure it exerts Since there is no change in the number of moles of gas during the reaction there will be no change in pressure Thus the initial pressure the nal pressure To nd the initial pressure add the pressure of C12 and H2 Pclz nRTV 000507 mol0082057 L atm mol K47315 K 00437 atm L PHZ nRTV 100245 mol 0082057 L atm mol K147315 K 0211 atm 45L Total Pressure 00437 0211 0255 atm convert to torr same as above Although it was not relevant for the preceding problem remember that volume like pressure is not dependant on the nature of the gas This means that we can sum up the moles of all the products and then nd P or V Hint for nding pressure in any reaction Only a gas will exert pressure which means you only have to consider the excess gaseous reactants and gaseous products of course using a limiting reactant to nd out how much gas is actually produced A solid or liquid may be your limiting reactant but that is all it counts for in these types of problems Density Density 7 the density of a gas is much lower than that of a liquid or solid D m Density given in gL RT M molar mass Density of a gas depends on molar mass pressure and temp Ex What is the density of SO gas at 720 torr and 15 C 1720 torr 1 atm 1 0947 atm 760 torr SO molar mass 6407 gmol D 1 0947 atm 6407 meol 1 256 gL 0082057 L atm mol K28815K Dalton s Law of Partial Pressure Dalton s Law the total pressure of a mixture of gases is the sum of the individual pressures P101PAPBPC Ex Mix of 2 gases with 1 atm total pressure If amount of2 gases is equal what does PA 12 atm If 2A1B what does PA 23 atm Mole Fraction Mole Fraction decimal fraction used for mixtures XAXBXC 1 EX lAlB XA05 XB05 Plot XAPtot XBPtot XcPrm f PA XAPtot Kinetic Molecular Theory Average kinetic energy of gas proportional to temperature T1 lt T2 molecules Higher temp particles move faster Energv To gure out how fast particles are actually moving you can use the equation for Root Mean Square Speed RMS V112 3RT M Solving for 13912 gives you the average speed of molecules of a certain type of gas Speed depends on temperature and molar mass Use different version of gas constant so units cancel out nicely R 8314 kg mzsz mol K EX How fast does Hz travel at room temperature 112 38314kg mzsszol K100091kg298K1 1918 ms 202 gmol To get a better idea of how fast that really is convert to mihr 1918 ms100 cm 1 in W 1 ft 1 mi 3600 sec 4290 mihr 1 m 254 cml2 in5280 ft 1 hr Following the same steps as above the speed of methane CH4 at room temp is calculated to be 1524 mihr Methane moves much slower than hydrogen gas because its molecules are bigger and heavier Gas Di usion Diffusion 7 the gradual miXing of one gas with another due to random motion Graham s Law the rate of gas diffusion at constant temp and pressure is inversely related to the square root of molar mass r M r is the rate of diffusion 1V2 M1 Lighter molecules move faster thus they diffuse more quickly than heavier molecules Gas Effusion Effusion 7 the process by which a gas escapes out of a small opening in a container works mathematically the same as Graham s Law Lighter molecules escape faster for example a helium balloon deflates faster than a balloon filled with air So far we have only dealt with ideal gases which means we have had to make a few assumptions 1 No IMF between molecules 6 this becomes an issue at low temps with slower moving particles because the IMFs may be strong enough to keep molecules together when they collide which would make the actual pressure less than predicted 2 Molecules have no volume 4 this becomes an issue when the gas is under high pressure because the molecules are forced very close together and the molecules themselves occupy a significant portion of the total gas volume The equation for the real gas law is nRT P anV2V ibn where anV2 corrects for IMF and 7 bn corrects for volume of gas Phase Diagrams Important factors when considering the phase of a molecule IMFs strength of forces holding molecules together Temp add energy to potentially overcome lMF move closer to gas Pressure push molecules closer together to allow IMFs to have impact move closer to solid Fusion curve LIQUID hpnrizarinn curve SOLID Sublimmkm cuwc i I t HZO Phase Dlagmm L39mpemmle mm to aim Only on the lines can there be 2 phases in equilibrium At equilibrium both lag gal occur at the same rate same for solids and liquids The critical point represents the T andP above which a supercritical uid exists the uid is neither a gas nor a liquid but possesses properties of bot As liquids are heated they expand and density i As gases are pressurized they compress and density T Critical Point Density 1mm liquid Density pmsudzed gas If you look at the phase diagram of water you Will see that the sl line for water has a negative slope unlike the phase diagram of most other substances which would have a positive slope This is because higher pressure favors the denser phase which for most substances is the solid phase However for water the liquid phase is denser than the solid phase as water seeks to expand Hbonding thus resulting in atypical behavior Solids 4 types of solids Solid Type Bonding Properties Examples Molecular IMFs Low melting pt Ice Non conducting Sugar Depending 0quot W176 OfIME brittle Dry ice weak to strong attraction Plastic bottles Ionic IonIon Very high melting pt NaCl Non conducting KBr Electrostatic attraction Very brittle NH4F Covalent Covalent Bonds Very high melting pt Graphite Insulator Quartz Semi conducting Glass Bond formed by electron Very hard Diamond sharing between atoms Metallic Metal Bonds Ductile Copper can be drawn out into aWire Gold Attraction of metal cations Malleable Iron to electron sea can bimmjed Categories of Solids Crystalline 7 possess regular long range order ex diamond Amorphous wo form 7 lacking regular long range arrangement of molecules ions or atoms Unit Cells ex glass A unit cell is a repeating unit that when translated gives the entire structure of the compound Atoms in a unit cell may be shared with neighboring unit cells Comer atoms only contribute 18 of the atom to each unit cell 8 comers 18 l atomunit cell Face atoms contribute 12 of the atom to each unit cell 6 faces 12 3 atomsunit cell assuming there is an atom on every face Edge atoms contribute 1 of the atom to each unit cell 12 edges 1A 3 atomsunit cell assuming there is an atom on every edge Body atoms completely contained in unit cell 3 ways to pack atoms looking only at cubic unit cells Simple cubic Bodycentered cubic Facecentered cubic 1 atom 8 comers 2 atoms 8 comers 1 body 4 atoms 8 comers 6 faces 39 A few equations may come in handy when doing calculations including unit cells r radius of atom l length of cube edge Simple l 2r Bodycentered l 4r3 Face centered l 4r2 You can then simply cube the lengths found using the equation to nd the volume of the unit cell EX Silver crystallizes in a facecentered cubic unit cell Each side of the unit cell has a length of 409 pm What is the radius of a silver atom in pm 409 4W2 r 145 pm Knowing that silver has a weight of 1078682 gmol and that the side length is 409 pm what is the density of silver answer in gcm3 1409 pm 1 cm 40910398 cm 1010 pm v 13 40910398 cm3 684quot103923 cm3unit cell 110786822 1 mol 4 atoms unit cell 105 gcm3 1 mol 60221023 atomsunit cell 684quot103923 cm3 Bonding Valence Bond Theory Good for o Qualitative picture 0 Large molecules 0 Works only for molecules in ground state Problems 0 Not good for predicting magnetism EX 0 is paramagnetic yet if you look at its Lewis structure you would think it is diamagnetic because there are no unpaired electrons 2030 remember paramagnetic magnetic diamagnetic not magnetic 0 Only good for predicting directional bonds EX Na has a facecentered cubic structure which means that it is bonded to 6 neighboring atoms Na only has 1 valence electron so according to the Valence bond theory it can only bond once 0 It cannot explain why metals conduct electricity Molecular Orbital Theory Molecular orbitals are formed by the summation of atomic orbitals The orbitals on the edges are the atomic orbitals 1 for each atom involved in the bond and the center orbitals are the molecular orbitals There are 2 molecular orbitals for each shell 1s 2 s and 1 orbital always goes up in energy and the other orbital goes down in energy Fill the lower energy orbital rst this is called the 6 orbital or the bonding orbital Add any of the remaining electrons into the high energy orbital 6 orbital or the antibonding orbital HZ Hez He 4 T at l 0 11 u T 1 L 1 Tl 41 15 15 15 is ls ls 1 7L uh m 2eiinE 2eiinE 2eiinE More stable whenbonded 2 e T in E 1 e in E will stay together No net E loss so there is E lost will bond no reason to stay together Liz has e in another shell 2s than the previous examples did but you ll the orbitals the same way orbital 1 low to high E then orbital 2 low to high E LiZ o b 4 4 25 K 25 quot11 L n 41E 4 ls n Bond Order 2 e in bonding orbital 7 e in antibonding orbital 4 e L in E 2 e Tin E E lost will bond Bond order is the number of bonds formed between a pair of atoms Hz 2211 Hez V222 0 No bond forms Hez r 1221 12 Only forms a weak bond forms LiZ 24 2 1