5.58 A 2.5-L flask at 15oC contains a mixture of 3 gases, N2, He, and Ne, at partial pressures of 0.32 atm for N2, 0.15 atm for He, and 0.42 atm for Ne. (a) Calculate the total pressure of the mixture. (b) Calculate the volume in liters at STP occupied by He and Ne if the N2 is removed selectively. (a) 0.32 + 0.15 + 0.42 = 0.89 atm (b) 1.4 L 12/3/2018 S.A. McFarland©2006

5.62 A sample of Zn metal is allowed to react completely with an excess of HCl: Zn(s) + 2HCl(aq) -> ZnCl2(aq) + H2(g) The H2 gas produced is collected over water at 25oC. The volume of the gas is 7.8 L, and the pressure is 0.980 atm. Calculate the amount of Zn metal in grams consumed in the reaction. (Vapor pressure of water at 25oC is 23.8 mmHg.) 19.8 g Zn 12/3/2018 S.A. McFarland©2006

5.64 A sample of ammonia (NH3) gas is completely decomposed to nitrogen and hydrogen gases over heated iron wool. If the total pressure is 866 mmHg, calculate the partial pressures of N2 and H2. 2NH3 -> N2 + 3H2 1 mol N2: 3mol H2 so XN2 is 0.25 and XH2 is 0.75 PN2 = XN2 x PT = 217 mmHg PH2 = XH2 x PT = 650 mmHg 12/3/2018 S.A. McFarland©2006

5.72 The temperature of the stratosphere is -23oC. Calculate the root-mean-square speed of N2 molecules in this region. urms = sqrt(3RT/M) urms = sqrt (3x8.314x250K/0.02802) = 471 m/s *Remember: R is in J/mol.K and M is kg/mol because 1 J= 1 kg m2/s2 12/3/2018 S.A. McFarland©2006

5.80 At 27oC 10.0 moles of a gas in a 1.50-L container exert a pressure of 130 atm. Is this an ideal gas? PV = nRT P = 164 atm No. An ideal gas under these conditions would exert a pressure of 164 atm. 12/3/2018 S.A. McFarland©2006

Energy relationships in chemical reactions Chapter 6

Energy is the capacity to do work Radiant energy is earth’s primary energy source and comes from the sun Thermal energy is the energy associated with the random motion of atoms and molecules Chemical energy is the energy stored within the bonds of chemical substances Nuclear energy is the energy stored within the collection of neutrons and protons in the atom Electrical energy is the energy associated with the flow of electrons Potential energy is the energy available by virtue of an object’s position 12/3/2018 S.A. McFarland©2006

Energy changes in chemical reactions Heat is the transfer of thermal energy between two bodies that are at different temperatures. Temperature is a measure of the thermal energy. Temperature = Thermal energy 400C 900C greater thermal energy 12/3/2018 S.A. McFarland©2006

Temperature is a measure of the thermal energy. Temperature = Thermal Energy 400C 900C greater thermal energy larger volume, greater mass more H2O molecules, more molecular motion 12/3/2018 S.A. McFarland©2006

Thermochemistry is the study of heat change in chemical reactions. The system is the specific part of the universe that is of interest in the study. SURROUNDINGS SYSTEM open closed isolated Exchange: mass & energy energy nothing 12/3/2018 S.A. McFarland©2006

2H2 (g) + O2 (g) 2H2O (l) + energy Exothermic process is any process that gives off heat – transfers thermal energy from the system to the surroundings. 2H2 (g) + O2 (g) 2H2O (l) + energy H2O (g) H2O (l) + energy Endothermic process is any process in which heat has to be supplied to the system from the surroundings. energy + H2O (s) H2O (l) energy + 2HgO (s) 2Hg (l) + O2 (g) 12/3/2018 S.A. McFarland©2006

12/3/2018 S.A. McFarland©2006

Thermochemistry is part of a broader subject called thermodynamics. Thermodynamics is the study of interconversion of heat and other kinds of energy. changes in the state of a system composition energy temperature pressure volume Chapter 18 gives all of the details about thermodynamics. For now, we’re concerned with only the 1st law of thermodynamics. 12/3/2018 S.A. McFarland©2006