Thermochemistry: Heat and Chemical Change

Transcription

1 Thermochemistry: Heat and Chemical Change 1

2 Heat or Thermal Energy (q) Heat is a form of energy Is heat the same as temperature? Heat flows between two objects at different temperatures. Hot Cold 2

3 Chemical Potential Energy Every substance stores chemical PE within it depending on: Types of atoms Bonding of the atoms Intermolecular forces between molecules State e.g. CH vs. CO 4 2 PE: high low 3

4 Example: Chemical PE Depends on Types of Bonds It takes energy to break a bond. (Endothermic) Energy is released if bonds form. (exothermic). atoms molecule 4

5 Thermochemistry: Definition Study of heat changes that occur during a physical process or chemical reaction. Exchange of : Thermal energy between substances Thermal energy and chemical PE 5

6 Thermochemistry An Example Lighting a bunsen burner: CH 4 + 2O 2 CO 2 + 2H 2 O + energy Where does the thermal energy (heat) come from? 6

7 Burning Methane CH 4 & 2O 2 reactants Difference in energy given off as heat. CO 2 & 2H 2 O products 7

8 Burning Methane CH 4 + 2O 2 CO 2 + 2H 2 O + energy Chem PE of reactants = Chem PE of products + Heat energy Energy is conserved. 8

9 The System 9

10 Heat Changes In any process (e.g. chemical reaction) heat can flow: Surroundings System or System Suroundings 10

11 Surroundings System system gains energy heat (q) is positive endothermic in 11

12 System Surroundings system loses energy heat (q) is negative exothermic exit 12

13 Endo- or Exothermic? system 2. NH 4 Cl + H 2 O demo 1. 13

14 Metric Heat Unit Energy, including thermal energy (heat) is measured in joules (J) 14

15 Specific Heat (C) (specific heat capacity) The amount of heat required to raise the temperature of 1 gram of a substance by 1 o C. Each substance has its own value for specific heat. What might it depend on? 15

16 Specific Heat (C) C = q m DT where DT = T - T f i (DT in o C or K) (DT is same number in o C and K) C has units of J/(g o C) or J/(g K) 16

17 Specific Heat (C) Substance J/(g o C) Water 4.18 Regents Table B Ethanol 2.4 Aluminum 0.90 Iron 0.46 Mercury

18 Specific Heat (C) Does it take more heat to raise the temperature of equal masses of water or iron? 18

19 Specific Heat (C) q C = m DT Regents rearranges to: q = m C DT Table T Make sure J = g x J g o C x o C units check out 19

20 Specific Heat (C) 95.5 g of Cu absorbs 849 J of heat and its T rises from 25 to 48 o C. What is the specific heat of Cu? q = m C DT 849J = (95.5g) C (23 o C) What is the C = 0.39 J/g o C unknown? 20

21 You Try It When 435 J of heat is added to 3.4 g of olive oil at 21 o C, its T increases to 85 o C. What is specific heat of olive oil? 21

22 Try Another How much heat is required to raise the temperature of 0.25 kg of H 2 O from 12 to 82 o C? 22

23 Calorimetry The precise measurement of heat changes in chemical or physical processes. There is no heat meter. 23

24 Calorimetry Heat exchange measured in calorimetry is called enthalpy (DH). q = DH = (change in heat or enthalpy) 24

25 Calorimetry Heat given off by reaction or process in the cup is absorbed by surrounding water. Thermometer Cups Stirrer 25

26 Conservation of Energy -q rxn = q water = m C DT Heat given off by process or reaction equals heat absorbed by the solution. 26

27 Calorimetry You try it. A heated iron bar is placed in 25 ml of water at 20.0 o C. The water temperature rises to 28.0 o C. How much heat was released by the iron bar? 27

28 Thermochemical Equations: Exchange of Heat and Chemical PE 2C(s) + 3H 2 (g) C 2 H 6 (g) kj Shows conservation of energy product Endothermic or exothermic? Treat the energy just like another reactant or product! 28

29 Pot. Energy Thermochemical Equations reactants products DH negative DH positive products reactants Exothermic vs. Endothermic 29

30 Thermochemical Equations 2C(s) + 3H 2 (g) C 2 H 6 (g) kj This reaction releases 84.0 kj of energy. Thus q = DH = kj (exothermic) product 2C(s) + 3H 2 (g) C 2 H 6 (g) DH = kj See Table I 30

31 2C(s) + 3H (g) C H (g) DH = kj Rules for thermochemical equations: 1. Must give states 2. All coefficients are moles, thus DH = kj / 2 mol C DH = kj / 3 mol H 2 DH = kj / 1 mol C 2 H 6 3. If reaction is reversed, sign of DH is changed. 31

32 Problem H 2 (g) + I 2 (g) 2HI(g) DH = 53 kj (on Table I) 1. Is the reaction exo- or endothermic? 2. What is the heat change when 13 g of HI is formed? 32

33 H (g) + I (g) 2 HI(g) 2 2 DH = 53 kj Since DH is +, endothermic To form 13g HI: 13g HI x 1 mol HI 128 g x 53 kj 2 mol HI = 2.7 kj molar mass of HI 33

34 Problem: You Try It 4Al(s) + 3O 2 (g) 2Al 2 O 3 (s) DH found on Table I 1. Is this process exo- or endothermic? 2. How much heat is evolved when 116 g of aluminum is burned? 34

35 Combustion In all combustion reactions, heat is given off (exothermic). What s the other reactant? 35

36 Heat of Combustion See Table I Formula DH (kj) Propane C 3 H 8 (g) (1 mol) Carbon C (s) (1 mol) Octane C 8 H 18 (l) (2 moles) 36

37 Combustion of Octane: C 8 H 18 How much energy is released when a tank-full of gasoline (62 kg) is burned in your car? (Assume gasoline is octane,c 8 H 18) 37

38 Heat of Solution Heat changes take place when a solid is dissolved in water. 38

39 Heat of Solution Sodium hydroxide dissolves in water NaOH(s) NaOH(s) H 2 O H 2 O NaOH(aq) Na + (aq) + OH - (aq) DH = kj (Regents Table I) Very exothermic 39

40 Heat of Solution Ammonium nitrate dissolves in water. Write the equation. NH NO (s) NH + (aq) + NO - (aq) DH = 25.7 kj Will the temperature of the water go up or down? 40

41 Heat of Solution You Try it. How much heat is absorbed when 150 g of NH NO is 4 3 dissolved in water? 41

42 Heat in Changes of State Ice absorbs heat from its surroundings when it melts. Why, on the molecular level? H 2 O(s) H 2 O(l) (break attractive forces between molecules) solid 42 liquid

43 Ice For ice at 0 o C Water (melting) Molar heat of fusion H 2 O(s) H 2 O(l) DH = 6.01 kj/mol endothermic 43

44 Water Ice (freezing) Likewise at 0 o C Molar heat of solidification H 2 O(l) H 2 O(s) DH = kj/mol exothermic 44

45 Water Steam (boiling) Water must absorb heat in order to turn it into steam. Why? H 2 O(l) H 2 O(g) (break attractive forces between molecules) liquid gas 45

46 Water Steam (boiling) For water at 100 o C Molar heat of vaporization H 2 O(l) H 2 O(g) DH = 40.7 kj/mol endothermic 46

47 Steam Water What is this process called? For steam at 100 o C, what is the molar heat of condensation? 47

48 Heat in Changes of State Every substance has its own DH fus and DH vap as determined by its molecular attractions. See text p.308, Table

49 Regents Chart B Heat of Fusion of water: Show that Regents chart value of 334 J/g is same as 6.01 kj/mol. Heat of Vaporization of water: Show that Regents chart value of 2260 J/g is same as 40.7 kj/mol. 49

50 Regents Chart T H f of water = 334 J/g (Table B) H v of water = 2260 J/g q = m H f J = g J g fusion (melting) Why no DT? q = m H v vaporization(boiling) 50

51 Heat in Changes of State S L G: Up in state, endothermic (+) G L S: Down in state, exothermic (-) 1. How many grams of ice at 0 o C can be melted by 4.2 kj of heat? 2. What is the heat change when 95 g of steam is condensed to water at 100 o C? Exothermic or exothermic? (Use Regents chart values.) 51

52 H Three Equations So Far 1. Heating or cooling (KE heat) q = m C DT 2. Changing state (PE heat) q = m H f q = m H v 3. Chemical reaction (PE heat) Table I (DH as conversion factor) 52

53 Heating Curve for Water Let s follow the heat required to turn 120. g of ice at 20. o C into 120. g of water at 42. o C. Identify changes in kinetic (KE) and potential (PE) energy. 53

54 Temperature 1. Warm 120 g ice from 20 to 0 o C BP KE increases MP q = m C DT C ice is 2.1 J/g o C Heat Supplied 54

55 Temperature 2. Melt 120 g ice at 0 o C BP q = m H f (no T change) MP PE increases Heat Supplied 55

56 Temperature 3. Heat 120 g water from 0 to 42 o C BP KE increases q = m C DT MP C = 4.18 J/g o C Heat Supplied 56

57 Temperature General Heating Curve Heat gas (KE) BP Boil liquid (PE) Melt solid (PE) Heat liqiud (KE) MP Heat solid (KE) Heat Supplied 57

58 Heating Curve for Water Let s follow the heat required to turn 120.g of ice at 20. o C into 120.g of steam at 110. o C. Identify changes in kinetic (KE) and potential (PE) energy. 58

59 Temperature 1. Warm 120 g ice from 20 to 0 o C BP KE increases MP q = m C DT C ice is 2.1 J/g o C Heat Supplied 59

60 Temperature 2. Melt 120 g ice at 0 o C BP q = m H f (no T change) MP PE increases Heat Supplied 60

61 Temperature 3. Heat 120 g water from 0 to 100 o C BP KE increases MP q = m C DT C = 4.18 J/g o C Heat Supplied 61

62 Temperature 4. Boil 120 g water at 100 o C BP PE increases MP q = m H v (no T change) Heat Supplied 62

63 Temperature 5. Heat steam from 100 to 110 o C BP KE increases MP q = m C DT C steam = 2.0 J/g o C Heat Supplied 63

64 Your Turn!!! Calculate heat required to turn 150 g of ice at -15. o C into water at 58 o C. Draw a graph of T vs. heat supplied. Identify steps where the KE or PE of the water changes. (C of ice is 2.1 J/g o C) 64

65 Standard Heat of Formation For many reactions, it is difficult to measure the heat of reaction. 2C(s) + O 2 (g) 2CO(g) This reaction actually gives a mix of CO and CO 2. 65

66 Standard Heat of Formation Heats of reaction can be calculated using standard heat of formation DH o f = amount of heat to form one mole of a compound from its elements. 66

67 Where do you get DH f o values? DH o = 0 for free elements in f most stable form (e.g. O 2 ) Look up DH o for compounds f (text table 11.6) 67

68 To Calculate Heat of Reaction DH o = SDH f o (prod) SDH f o (react) sum of (be careful with # moles) 68

69 Problem What is heat of reaction for: 2CO(g) + O 2 (g) 2CO 2 (g) DH f o CO = kj/mol DH f o CO 2 = kj/mol 69

70 2CO(g) + O 2 (g) 2CO 2 (g) DH o = SDH f o (prod) SDH f o (react) = 2DH o f (CO 2 ) - 2DH o f (CO) - DH o f (O 2 ) = 2(-393.5) 2(-110.5) 0 = kj 70

71 You Try It What is the heat of reaction for: 2 NO(g) + O 2 (g) 2NO 2 (g) DH o f NO = 90.4 kj/mol DH o f NO = 33.9 kj/mol 2 71

72 72

73 Warm-up When wood is burned in a fireplace, where does the heat come from? What are all the things that must balance in every chemical equation? 73

74 Warm-up If you put an ice pack on a sprained ankle, is this an endo- or exothermic process, given the ankle is the system? How many kj of heat is required to raise the temperature of 240 g water from 52 to 87 o C? 74

75 Warm-up A hot pebble is put into a cup calorimeter containing 55.0 g water. The water temp. rises. from 22.0 o C to 24.5 o C. How much heat was released by the pebble? 75

76 Calorimetry: Lab Practice 42g of aluminum at 87 o C is placed into 32g water at 28 o C. The final temperature of the mixture is 41 o C. What is the specific heat of aluminum? 76

77 Warm-up How many kj is 3400 cal? 70. g iron at 90. o C is placed into 35 g water at 22 o C in a calorimeter. The final temperature of the iron + water mixture is 34 o C. What is C Fe? 77

78 Warm-up 3.4 kj of heat is added to 61 g water at 25.5 o C. What is the final temp.? Calculate the heat change when 27g of NH is formed from its elements. 3 78

79 Warm-up 52g of NaOH is dissolved in 410 ml of water at 23 o C. What is the final temperature of the water? 79

80 Warm-up Calculate the heat change when 102 L N 2 (at STP) reacts with excess O 2 to form nitrogen dioxide. 80

81 Warm-up For the equation, q = mcdt, define each symbol and give its unit. What is H of water? What molecular v property is this related to? How many grams of water at 100 o C can be boiled with 682 kj of heat? 81

82 Warm-up N 2 + 3H 2 2NH 3 Is this reaction endo- or exothermic? Which have lower potential energy (i.e. more stable), reactants or products? What is DH for forming of 94g NH 3? 82

83 Warm-up How much heat is given off when 85g of steam at 135 o C is converted to water at 68 o C? C is 2.0 J/g o C steam 83

84 Warm-up What is the heat change when 72 g water is frozen into ice at 0 o C? 84

85 Warm-up What is the heat change when 95 g of steam is condensed into water at 100 o C? 85

86 Warm-up: Review Calculate the heat change when 42 g O 2 (at STP) reacts with excess N 2 to form nitrogen dioxide. What is the heat change when 95 g of steam is condensed into water at 100 o C? 55 g of hot zinc is put into a calorimeter containing 27.0 g water. The water temp. rises. from 21.7 o C to 26.5 o C. How much heat was released by the zinc? 86