Observations of Container. Hot Same Size. Hot Same Size. Hot Same Size. Observations of Container. Cold Expanded. Cold Expanded.

Transcription

1 Chapter 9: Phenomena Phenomena: Below is data from three different reactions carried out with three different amounts of reactants. All reactants were carried out in expandable/contractable containers at 25. What patterns do you see in the data. You do not need to know what work, heat, eternal energy, or enthalpy are to find the patterns in the data. C(s) + O 2 (g) CO 2 (g) CO 2 (s) + 2H 2 O(l) CH 4 (g) + 2O 2 (g) N 2 (g) + 2O 2 (g) 2NO 2 (g) Starting Amount of C Starting Amount of O mol 1.00 mol 1.00 mol 2.00 mol 2.00 mol 2.00 mol Starting Amount of CO 2 Starting Amount of H 2 O 1.00 mol 1.00 mol 1.00 mol 2.00 mol 2.00 mol 2.00 mol Starting Amount of N 2 Starting Amount of O mol 1.00 mol 2.00 mol 1.00 mol Observations of Container Work Heat Enteral Energy Enthalpy Hot Same Size 0.0 kj -394 kj -394 kj -394 kj Hot Same Size 0.0 kj -394 kj -394 kj -394 kj Hot Same Size 0.0 kj -788 kj -788 kj -788 kj Observations of Container Work Heat Enteral Energy Enthalpy Cold Expanded -3.7 kj 485 kj 841 kj 485 kj Cold Expanded -7.4 kj 970 kj 963 kj 970 kj Cold Expanded -7.4 kj 970 kj 963 kj 970 kj Observations of Container Work Heat Enteral Energy Enthalpy Cold Contract 1.2 kj 34 kj 35 kj 34 kj Cold Contract 1.2 kj 34 kj 35 kj 34 kj 2.00 mol 2.00 mol Cold Contract 2.5 kj 68 kj 71 kj 68 kj

2 Chapter 9 Energy, Enthalpy, & Thermochemistry Big Idea: Heat and work are equivalent ways of changing the energy of a system. The total energy of an isolated system is constant. The change in enthalpy of a reaction indicates whether a reaction is endothermic or exothermic. o Energy Facts o Internal Energy o Enthalpy o Hess s Law o Calorimetry o H 2 Fuel 2

3 Energy Facts Natural Gas Source of Percent Energy Petroleum 32% Coal 30% Natural Gas 24% Renewable 10% Nuclear 4% 3

4 Energy Facts Country Energy Consumption (Quadrillion Btu) China United States 98.0 Russia 30.7 India 24.3 Japan 18.9 Canada 14.5 Brazil 12.8 Germany 12.7 Korea, South 11.1 Iran Country Energy Consumption/person (MBtu) Qatar 1160 United Arad Emirates 800 Netherlands 750 Iceland 687 Kuwait 635 Singapore 633 Bahrain 614 Canada 427 Norway 407 Saudi Arabia 398 United States (#14)

5 US Energy US Sources of Energy 2014 (Quadrillion Btu) 5 Source of Energy % World 2014 % US 2014 % US 2009 Petroleum 32% 35% 37% Natural Gas 24% 28% 25% Coal 30% 18% 21% Renewable 10% 10% 8% Nuclear 4% 8% 9%

6 Energy Facts How do we generate electricity? 6

7 Internal Energy System: The object of study. Surrounding: The region outside the system. Internal Energy (E): The capacity to do work or to produce heat. Temperature (T): How hot or cold an object is. Heat (q): The energy that is transferred as a result of a temperature difference between a system and its surroundings. Note: If heat enters the system q is positive (endothermic reaction). If heat leaves the system q is negative (exothermic reaction). Work (w): The energy expended during the act of moving an object against an opposing force. Note: If the system expands, w is negative. If the system contracts, w is positive. 7

8 Internal Energy Student Question Which of these changes results in an increase in the internal energy of the system? a) The system absorbs heat and does work on the surroundings. b) The system releases heat and does work on the surroundings. c) The system absorbs heat and has work done on it by the surroundings. d) The system releases heat and has work done on it by the surroundings. 8

9 Internal Energy Student Question Helium gas, at a pressure of 2 Pa, is placed in a container with a movable piston. On the other side of the piston is a vacuum. The He gas is allowed to expand such that the volume of the helium goes from 2 m 3 to 4 m 3. How much work does the helium gas do? a) 8 J b) 4J c) 0 J d) -4 J e) None of the above 9

10 Internal Energy Heat Capacity (C): The ratio of heat supplied to the temperature rise produced (units ) Molar Heat Capacity: The heat capacity per mole of substance (units ) Note: Many books use C m for molar heat capacity. Specific Heat Capacity: The heat capacity per gram of substance (units ) Note: Many books use C S for specific heat capacity. Note: If the subscript V is added to any of the heat capacities it is the heat capacity at constant volume. If the subscript P is added to any of the heat capacities it is the heat capacity at constant pressure. 10

11 Internal Energy Molar Heat Capacities at 298 K Gas C V C P C P -C V He, Ne, Ar H N N 2 O CO C 2 H q nc T Specific Heat Capacities Substances C H 2 O(l) 4.18 H 2 O(s) 2.03 Al(s) 0.89 Fe(s) 0.45 Hg(l) 0.14 C(s) 0.71 q mc T Note: The smaller the heat capacity, the faster the transfer of heat. 11

12 Internal Energy C V (Monatomic Ideal Gas) Know ΔE=q+w=ΔPE+ΔKE Potential Energy Putting it together Kinetic Energy Work Only true for ideal monatomic gases Heat 12

13 Internal Energy C P (Monatomic Ideal Gas) Know ΔE=q+w=ΔPE+ΔKE Potential Energy Putting it together Kinetic Energy Work Only true for ideal monatomic gases Heat 13

14 Internal Energy State Function: A property of a substance that is independent of how a substance was prepared. Are ΔE, q, and w state functions? 1 mol of monatomic ideal gas Path A 1.0 atm, 1.0 L 1.0 atm, 2.0 L 2.0 atm, 2.0 L Path B 1.0 atm, 1.0 L 2.0 atm, 1.0 L 2.0 atm, 2.0 L 14

15 Internal Energy Path B (Monatomic Ideal Gas) 1.0 atm, 1.0 L 2.0 atm, 1.0 L 2.0 atm, 2.0 L Step 1 (Constant Volume) Heat Work (For monatomic ideal gas) Problem: We do not know T 0.0 (ΔV=0) Internal Energy

16 Internal Energy 16 Path B (Monatomic Ideal Gas) 1.0 atm, 1.0 L 2.0 atm, 1.0 L 2.0 atm, 2.0 L Step 2 (Constant Pressure) Heat Work (For monatomic ideal gas) Problem: We do not know T... w w Internal Energy

17 Internal Energy Path A Path B 1.0 atm, 1.0 L 1.0 atm, 2.0 L 2.0 atm, 2.0 L 1.0 atm, 1.0 L 2.0 atm, 1.0 L 2.0 atm, 2.0 L Path B (totals) q w Path A (totals) q 550J

18 Enthalpy Path A (Monatomic ideal gas) 1.0 atm, 1.0 L 1.0 atm, 2.0 L 2.0 atm, 2.0 L Step 1: q = 250 J w = J ΔE=150 J Step 2: q = J w = 0.0J ΔE= J Path B (Monatomic ideal gas) 1.0 atm, 1.0 L 2.0 atm, 1.0 L 2.0 atm, 2.0 L Step 1: q = 150 J w = 0.0J ΔE=150 J Step 2: q = Jw = JΔE= J These values were calculated in class. 18

19 Enthalpy Path B (Monatomic Ideal Gas) 1.0 atm, 1.0 L 2.0 atm, 1.0 L 2.0 atm, 2.0 L Step 1 (Constant Volume) Step 2 (Constant Pressure) Total ΔH

20 Enthalpy Heating Curve of H 2 O Heat of Fusion (ΔH fus ): The amount of heat that needs to be supplied to turn a solid into a liquid. Heat of Vaporization (ΔH vap ): The amount of heat that needs to be supplied to turn a liquid into a gas. 20

21 Enthalpy Student Question (ADDITION) At constant pressure which of the following is true for: 2NO(g) + O 2 (g) 2NO 2 (g) a) ΔH > ΔE b) ΔH = ΔE c) ΔH < ΔE 21

22 Enthalpy Standard State ( ) For a gas, the standard state is exactly 1 atm. For a substance in solution, the standard state is exactly 1 M. For a pure substance in a condensed state (liquid or solid), the standard state is the pure liquid or solid. For an element, the standard state is the form in which the element exists (is most stable) under conditions of 1 atm and the temperature of interest. Note: The temperature of the system is usually noted as a subscript. If no temperature is stated, assume 25 C. Note: Only the change in enthalpy is important, therefore, the standard state of elements are set to 0. 22

23 Enthalpy Standard Enthalpy of formation (ΔH f ): The standard reaction enthalpy per mole of compound for the compound s synthesis from its elements in their most stable form at 1 atm and the specified temperature. Note: The reaction must be written such that only 1 mole of product forms. 23

24 Enthalpy Student Question For which reaction is =? a) 2H 2 (g) + O 2 (g) 2H 2 O(l) b) 2C(graphite) + H 2 (g) C 2 H 2 (g) c) C(diamond) + O 2 (g) CO 2 (g) d) NO(g) + ½O 2 (g) NO 2 (g) e) None of the above 24

25 Enthalpy Standard Enthalpy of Combustion ( ): The change of enthalpy per mole of substance when it burns (reacts with oxygen) completely under standard conditions. Note: The reaction is written such that 1 mole of substance combusts. Note: When carbon is in the reactants, CO 2 forms, and when H is in the reactants, H 2 O forms. For standard enthalpies of combustion water is always assumed to be in the liquid phase. Example: C 3 H 8 (g) + 5O 2 (g) 3CO 2 (g) + 4H 2 O(l) 25

26 Hess s Law Hess s Law: A reaction s enthalpy is the sum of the enthalpies of any sequence of reactions (at the same temperature and pressure) into which the overall reaction can be divided. Things to remember If you add reactions together, add ΔH s. If you flip a reaction, flip the sign of ΔH. If you multiply a reaction by a constant, multiply ΔH by the same constant. 26

27 Hess s Law 1. CCl 4 (g) + 4HCl(g) CH 4 (g) + 4Cl 2 (g) ΔH 1 = kj 2. ½H 2 (g) + ½Cl 2 (g) HCl(g) ΔH 2 = kj 3. C(graphite) + 2H 2 (g) CH 4 (g) ΔH 3 = kj Calculate the standard enthalpy of formation of CCl 4 (g) using the thermochemical equations. 27

28 Hess s Law Given 2 What is ΔH rxn of 2NaHCO 3 (s) Na 2 CO 3 (s) + CO 2 (g) + H 2 O(l) NaHCO 3 (s) Na 2 CO 3 (s) + CO 2 (g) + H 2 O(l) 28 H rxn H f ( Na2CO3 ) H f ( CO2 ) H f ( H 2O) 2H f ( NaHCO3 )

29 Hess s Law Thermodynamic Data at 298 K Substance ΔH f ΔG f ΔS C 2 H 4 (g) CH 4 (g) CO 2 (g) C 2 H 6 (g) O(g) CH 3 CO 2 H(l) CH 3 OH(g) CH 3 CH 2 OH(l) C 6 H 12 O 6 (s) HCl(g) H 2 (g) H 2 O(l) H 2 O(g) Fe(s) Fe 2 O 3 (s) Thermodynamic Data at 298 K Substance ΔH f ΔG f ΔS N 2 (g) NO 2 (g) NO(g) N 2 O 4 (g) NH 3 (g) HNO 3 (l) NH 4 Cl(s) O 2 (g) P 4 O 10 (s) H 3 PO 4 (s) S rhombic (s) H 2 S(g) SO 2 (g) SO 3 (g) * Other ΔH f can be found in appendix 4 in the back of your book.

30 Hess s Law Student Question Calculate (kj) for the following reaction from the listed standard enthalpies of formation: 4NH 3 (g) + 5O 2 (g) 4NO(g) + 6H 2 O(g) a) -862 kj b) -908 kj c) kj d) Not enough information e) None of the above Substance ΔH f ( NH 3 (g) -46 NO(g) 90. H 2 O(g)

31 Calorimetry Constant Volume Calorimetry (Bomb) Constant Pressure Calorimetry (Coffee Cup) Used to find ΔE com No matter is exchanged No heat is exchanged Used to find ΔH (ΔH fus and ΔH rxn ) and C No heat is exchanged 31

32 Calorimetry Student Question A mol sample of hexane, C 6 H 14, undergoes complete combustion with excess O 2 in a bomb calorimeter. The change in temperature of the water (1500. g) surrounding the bomb is 6.66 C. The heat capacity of the bomb is What is energy of combustion for the reaction of C 6 H 14 in. Helpful Information: a) c) e) None of the above b) d)

33 Calorimetry 100. ml of M HCl ( mol HCl) was mixed with 75.0 ml of M NaOH ( mol NaOH) in a constant-pressure calorimeter of negligible heat capacity. The initial temperature of the HCl and NaOH solution was the same, C, and the final temperature of the mixed solution was C. Calculate the heat change for the neutralization reaction on a molar basis. Assume that the densities and specific heats of the solutions are the same as for water (1.00 and 4.184, respectively) 33

34 H 2 Fuel Energy Released from Different Fuel Sources Natural Gas CH 4 (l) + 2O 2 (g) CO 2 (g) + 2H 2 O(g) 51 Coal C(s) + O 2 (g) CO 2 (g) Depending on impurities Gasoline C x H y (l) + O 2 (g) CO 2 (g) + H 2 O(g)unbalanced 48 Hydrogen H 2 (g) + O 2 (g) H 2 O(l)

35 H 2 Fuel Although there is an abundance of hydrogen containing compounds on earth, very little is free hydrogen. Where do we get H 2 from? Natural Gas: CH 4 (g) + H 2 O(g) 3H 2 (g) + CO(g) Problem: Uses fossil fuels, need heat (energy), more economical to burn the CH 4. Electrolysis of Water: If a current is passed through H 2 O the water breaks down into its elements. Problem: The price of electricity is too high to make this competitive as a fuel. Most electricity is generated from fossil fuels. 35

36 H 2 Fuel Thermal Decomposition: H 2 O H 2 + ½O 2 (T=3000 C ~ ½H 2 O dissociates) 2HI I 2 + H 2 (435 C) 2H 2 O + SO 2 + I 2 H 2 SO 4 (90 C) H 2 SO 4 SO 2 + H 2 O + ½O 2 (825 C) H 2 O H 2 + ½O 2 Problem: Still needs very high temperatures Biological Hydrogen Production: If certain algae are deprived of sulfur they will switch from producing O 2 during photosynthesis to H 2. Problem: Currently the yields are to small to be a viable commercial option. 36

37 H 2 Fuel Other Problems: How to transport H 2? H 2 decomposes on metal surfaces H atoms are so small they can migrate into metal weakening it. How to store H 2 in cars? At STP, to produce the same amount of energy in a 20 gallon (76 liters) tank of gasoline you would need to have a 240,000L tank of H 2 H 2 (l) Insulated Metal hydrides 37

38 Take Away from Chapter 9 Big Idea: Heat and work are equivalent ways of changing the energy of a system. The total energy of an isolated system is constant. The change in enthalpy of a reaction indicates whether a reaction is endothermic or exothermic. Energy Facts Internal Energy Know the 1 st law of thermodynamics and its implications (1) The energy of the universe is constant Be able to calculate internal energy (E) (24&25) ΔE=q+w =ΔKE+ΔPE Be able to calculate work (w) (10,20&103) w=-p ex ΔV Be able to calculate heat (q) (11) q=cδt 38 Numbers correspond to end of chapter questions.

39 Take Away from Chapter 9 Enthalpy Be able to calculate change in enthalpy (ΔH) (37,38,39,42&125) ΔH = ΔE+ Δ(PV) ΔH = q (At constant pressure) Know implications of sign of ΔH (16,32) ΔH is -, exothermic reaction ΔH is +, endothermic reaction Be able to draw heating curves Know the implications/meaning of and Know how to use change in enthalpy as a conversion factor (35,36&85) Hess s Law Know how to get from other known (66,67,68,69&70) Know how to get from (71,76,77,81,82,88&107) 39 Numbers correspond to end of chapter questions.

40 Take Away from Chapter 9 Calorimetry Understand the differences/limitations between constant volume and constant pressure calorimetry (43) Be able to solve bomb calorimeter (constant volume) problems (57,58) Be able to solve coffee cup calorimeter (constant pressure)problems Solve for C of non soluble substance (44,46&48) (if needed add heat to ) Solve for ΔH fus (if needed add heat to ) = 40 Numbers correspond to end of chapter questions.

41 Take Away from Chapter 9 Calorimetry (Continued) Be able to solve coffee cup calorimeter problems Solve for ΔH rxn (51,54,55&56) (if needed add heat to ) H 2 Fuel (Most times need to divide by number of moles. Be careful many of these are limiting reagent problems.) 41 Numbers correspond to end of chapter questions.