CHEM 1441 1st Edition Exam 3 Study Guide Lectures 20 26 Lecture 20 March 16 Kinetic Molecular Theory at a certain temperature each gas has the same average avg kinetic energy Kinetic Energy avg 1 2 mass velocity avg 2 KE 1 2 mv2 Each gas has to have a different velocity in order to calculate the average velocity Number of mols O2 gas is heavier and slower He gas is lighter and faster For 1 gas low temp slower High temp faster Velocity KE avg 3 2 RT R is a gas constant As KE increases temperature increases direct relationship Graham s Law of Effusion The lighter the gas the faster the rate of effusion As the rate of effusion increases the molar mass decreases inverse relationship As the time of effusion increases the molar mass increases r1 M 2 r is rate M is molar mass r2 M1 Thermodynamics Thermochemistry Energy is the capacity to do work Heat energy is temperature difference Work energy is a force acting through a distance Work Pressure Volume Work is negative if it takes energy out of the system Work is positive if it adds energy to the system Energy is possed by everything and relates to both heat and work Heat q Energy E Work w Chemical Energy Have to define the system and surroundings thermometer Reaction As reaction occurs the temperature goes up The thermometer ALWAYS measures the heat of the SURROUNDINGS Surroundings 1st Law of Thermodynamics The total energy of the universe is constant it can be neither created or destroyed Can t directly measure E but can measure E Energy final Energy initial Anytime you see delta in front Temperature Enthalpy Volume it indicates a state function Units to memorize 1 cal 4 184 J 1 Cal 1000 cal 1kcal How heat work and energy are related E q w E is total energy of the system q is the heat that is transferred w is the amount of work done on system Lecture 21 March 18 Enthalpy H measues how much energy is in the system altogether It is the sum of internal energy and the product of a system s volume constant pressure q mC T J g J g oC oC q is heat m is mass in g C is specific heat capacity in J g oC T is change in temp in oC CONCEPT CHECK How much heat is absorbed by a 1 gram penny Cs specific heat capacity 0 385 J g oC to warm it from 8 oC to 37 oC q mC T 1g 0 385 J g oC 37 8 17 325 J q is positive because the object is absorbing the heat it gained heat endothermic If q is transferred from System Surroundings q is exothermic system loses heat System Surroundings q is endothermic system gains heat Always determine q by looking at it from the systems perspective CONCEPT CHECK A system releases 622 kJ of heat and does 105 kJ of work on the surroundings What is the change in internal energy for the system E q w 622 kJ 105 kJ 727 kJ heat is released so q is negative work is done ON surrdngs energy lost so w is neg Enthalpy H H is the change in amount of heat energy per mol H q mol CONCEPT CHECK A student dissolved 1 gram NaCl in 50 grams of water at 25 oC The temperature drops to 23oC What is the H per mol of NaCl for this process NaCl s NaCl aq H q mol q mC T H mC T mol 51g 4 184 J g oC 23 25 oC 1g NaCl 58 45g mol 51g comes from 1g NaCl 50g water 24944 5 J 24 9 kJ temp of surrounding decreased rxn is positive Lecture 22 March 20 CONCEPT CHECK heat transfer problem A block of copper of unknown mass has an initial temperature of 65 4 oC The copper is immersed in a beaker containing 95 7g of water at 22 7 oC When the substances reach thermal equilibrium the final temperature is 24 2 oC What is the mass of the copper block Cs 0 385 J g oC q lost q gained q lost by Cu block q gained by H2O o Cs water 4 18 J g C mC T mC T m 0 385 24 2 65 4 95 7 4 18 24 2 22 7 m Cu 600 61 15 86 37 86 g Pressure Volume Work W P V USE EQUATION ONLY IN COSTANT PRESSURE Units P atm V liters W Joules To convert atm multiply it by 8 31446 J 0 0802574 L or 101 3 J CONCEPT CHECK If you inflate a balloon from vol of 0 100 L to 1 85 L against an external pressure of 1 atm constant pressure how much work is done W P V 1 x 101 3 1 85 1 86 1 J CONCEPT CHECK Mg metal reacts with Hydrochloric acid according to the following reation Mg s 2HCl aq MgCl2 aq H2 g In an experiment to determine the enthalpy change H We combine 0 158g Mg with enough HCl to make a 100mL solution in a coffee cup calorimeter Temperature rises from 25 6 oC to 32 8oC Use the density of the solution at 1 00g mL and C 4 184 J g oC Find the change in enthalpy per mol of Mg H q mol q mC T q is negative because the temp went up q 100g 4 184 32 8 25 6 3012 48 J heat went out to surroundings exothermic H 3012 48 0 158g 24 3g mol 463311 79 J 4 63x10 5 J mol Mg Enthalpy H E PV H E PV P V Coffee cup equation when pressure is constant H E P V Bomb calorimeter when volume is constant H E PV E q C heat capacity of calorimeter x T CONCEPT CHECK When 1 010g of sucrose C12H22O11 undergoes combustion in a bomb calorimeter the temperature rises from 24 92oC to 28 33oC Find E per mol of sucrose C 4 9 kJ oC E mol q mol C T mol temp rises loses heat to surroundings negative 4 9 28 33 24 92 16 709 kJ 1 01g 342g mol 5 66x10 3 kJ mol sucrose CONCEPT CHECK stoichiometry An LP gas tank in a home bbq contains 13 2 kg of propane Calculate heat in kJ associated with complete combustion of all propane in the tank C3H8 g 5O2 g 3CO2 g 4H2O g H 2044 kJ Mass of propane mol of propane q 13200g C3H8 x 1 mol C3H8 x 2044 kJ 6 13x10 5 kJ 44g C3H8 1mol C3H8 155 g Ammonia reacts with oxygen according to the following 4 NH4 g 5 O2 g 4 NO g 6 H2O g H 906 kJ Mass of ammonia mol ammonia q 155g NH3 x 1 mol NH3 x 906 kJ 2065 15 kJ 2 07x10 3 kJ 17g NH3 4 mol NH3 Hof formation enthalpy change Standard conditions 1 atm 25oC 1 …