Chapter 5. Thermochemistry

Transcription

1 Chapter 5 Thermochemistry

2 Energy Thermodynamics Study of the relationship between heat, work, and other forms of energy Thermochemistry A branch of thermodynamics Focuses on the study of heat given off or absorbed in a chemical reaction 2

3 Energy Energy is the ability to do work or transfer heat Work is the energy used that causes an object that has mass to move 3

4 Energy Energy is the ability to do work or transfer heat Work is the energy used that causes an object that has mass to move Heat is the energy used to raise the temperature of an object 4

5 Heat Energy can be transferred as heat Heat flows from warmer objects to cooler objects Heat lost by warmer body is equal to heat gained by colder body 5

6 Example: Energy When a piece of iron at 356 K is placed in water at 298 K, what happens? a. Energy flows from iron to water. b. Energy flows from water to iron. c. Energy does not flow. d. Energy is not conserved. 6

7 Example: Energy When a piece of iron at 356 K is placed in water at 298 K, what happens? a. Energy flows from iron to water. b. Energy flows from water to iron. c. Energy does not flow. d. Energy is not conserved. 7

8 Energy Kinetic energy is energy an object possesses due to its motion Potential energy is that energy which an object has because of its position Called potential energy because it has the potential to be converted into other forms of energy 2015 Pearson Education, Inc. 8

9 Units of Energy The SI unit of energy is the joule (J): An older, non-si unit is still in widespread use, the calorie (cal): 1 cal = J (Note: this is not the same as the calorie of foods; the food calorie is 1 kcal) 9

10 FIRST LAW OF THERMODYNAMICS 10

11 System and Surroundings System includes the molecules we want to study Surroundings are everything else 2015 Pearson Education, Inc. 11

12 First Law of Thermodynamics First Law of Thermodynamics Law of conservation of energy Energy of the universe is constant Energy can be transferred and transformed Energy is neither created nor destroyed

13 Internal Energy Internal energy (E)- the sum of all kinetic and potential energies of all components of the system 13

14 Internal Energy Change in internal energy, E Final energy of the system minus the initial energy of the system: E = E final E initial 2015 Pearson Education, Inc. 14

15 Changes in Internal Energy If E < 0, E final < E initial System released energy to the surroundings Energy change is called exergonic 2015 Pearson Education, Inc. 15

16 Changes in Internal Energy If E > 0, E final > E initial System absorbed energy from the surroundings Energy change is called endergonic 2015 Pearson Education, Inc. 16

17 Changes in Internal Energy Energy can be transferred First Law of Thermodynamics Internal Energy Energy may be exchanged between the system and the surroundings Exchanged as either heat (q) or work (w) E = q + w 2015 Pearson Education, Inc. 17

18 E, q, w, and Their Signs 2015 Pearson Education, Inc. 18

19 Exchange of Heat between System and Surroundings Endothermic reaction Heat is absorbed by the system from the surroundings 19

20 Exchange of Heat between System and Surroundings Exothermic reaction Heat is released by the system into the surroundings 20

21 State Functions State function Property whose value does not depend on the path taken to reach that specific value Internal energy is a state function In the system below, the water could have reached room temperature from either direction E depends only on E initial and E final 2015 Pearson Education, Inc. 21

22 State Functions Heat and work are not state functions Discharging a battery E is the same Battery is shorted out Battery is discharged by running the fan q and w are different in the two cases Battery is shorted out Battery is discharged by running the fan 2015 Pearson Education, Inc. 22

23 ENTHALPY 23

24 Enthalpy Enthalpy Internal energy plus the product of pressure and volume H = E + PV A state function 24

25 Enthalpy Enthalpy H = E + PV If system changes at constant pressure, the change in enthalpy, H, is This can be written H = (E + PV) H = E + P V 25

26 Work Measure the work done by the gas w = P V Perform reaction in a vessel that has been fitted with a piston 2015 Pearson Education, Inc. 26

27 Enthalpy Since E = q + w and w = P V Substitute these into the enthalpy expression: H = E + P V H = (q + w) w H = q At constant pressure, the change in enthalpy is the heat gained or lost 27

28 Enthalpy H = q at constant pressure If H is Positive the process is endothermic Negative the process is exothermic 2015 Pearson Education, Inc. 28

29 Enthalpy Change of enthalpy ( H) is called the Enthalpy of reaction Heat of reaction H = H products H reactants 2015 Pearson Education, Inc. 29

30 Enthalpy Facts 1. Enthalpy is an extensive property Depend on the amount of matter present 2. H rxn in the forward direction is equal in size, but opposite in sign, to H rxn for the reverse reaction 3. H rxn depends on the state of the products and the state of the reactants A state function 2015 Pearson Education, Inc. 30

31 Remember 1. Energy can be converted from one type to another 2. Energy can be transferred as heat How is the transfer of heat measured? I. Heat Capacity 31

32 Heat Capacity Energy required to raise the temperature of a substance by 1 K (1 C) is its heat capacity 32

33 Heat Capacity Heat capacity is the amount of energy required to raise the temperature of a substance by 1 K (1 C) x x 33

34 Example: Heat Capacity Two solid objects, A and B, are placed in boiling water and allowed to come to temperature there. Each is then lifted out and placed in separate beakers containing 1000 g water of 10.0 o C. Object A increases the water temperature by 3.50 o C; B increases the water temperature by 2.60 o C. Which object has the larger heat capacity? Analyze the question (Information given): Both objects are heated to 100 o C. The two hot objects are placed in the same amount of cold water at the same temperature. Object A raises the water temperature more than object B. 34

35 Example: Heat Capacity Plan: Apply the definition of heat capacity to heating the water and and heating the the objects to to determine which object has has the the greater greater heat heat capacity. Solve: Both beakers of water contain the same mass of water, so they they both both have have the the same same heat heat capacity. capacity. Object Object A raises raises the the temperature of its water more than object B, so more heat was transferred temperature from of its object water A more than from than object object B, B. so Since more both heat was objects transferred were from heated object to the A same than from temperature object B. initially, Since both object A must objects have were absorbed heated more to the heat same to temperature reach the 100o initially, o C temperature. object A The must greater have absorbed the heat capacity more of an object, the greater the heat required to produce a given rise in temperature. Thus object A has the greater heat capacity. 35

36 Remember 1. Energy can be converted from one type to another 2. Energy can be transferred as heat How is the transfer of heat measured? I. Heat capacity II. Specific heat capacity 36

37 Specific Heat Specific heat capacity (specific heat) amount of energy required to raise the temperature of 1 g of a substance by 1 K (or 1 C) 2015 Pearson Education, Inc. 37

38 Specific Heat Formula for specific heat J g o C g J K q = heat energy change m = mass T = temperature change T = T final T initial 38

39 Heat Capacity Heat capacity formula is Heat capacity grams specific heat C m C s J o C J K C = heat capacity m = mass C s = specific heat 39

40 Remember 1. Energy can be converted from one type to another 2. Energy can be transferred as heat How is the transfer of heat measured? I. Heat capacity II. Specific heat capacity III. Molar specific heat capacity 40

41 Molar Specific Heat Molar Specific heat amount of energy required to raise the temperature of 1 mole of a substance by 1 K (or 1 C) heat transferred Molar specific heat mole temperature change Molar specific heat specific heat molar mass C n or q n T J o n C n J K 41

42 Example: Specific Heat Capacity How much heat is needed to warm 250 g of water (about 1 cup) from 22 o C (about room temperature) to near its boiling point, 98 o C? The specific heat of water is 4.18 J/g-K. What is the molar heat capacity of water? Analyze the question (information given): You must find the total quantity of heat needed to warm the sample of water, given the mass of water (m), its temperature change ( T) and its specific heat C s. 42

43 Example: Specific Heat Capacity Plan: Given m, C s and T, you can calculate the quantity of heat, q using q = C s x m x T. Solve: 1. Calculate temperature change T = 98 o C 22 o C = 76 o C = 76 K 2. Solve for heat q = C s x m x T = (4.18 J/g-K)(250 g)(76 K) = 7.9 x 10 4 J 43

44 Example: Specific Heat Capacity What is the molar heat capacity of water? Analyze: Calculate molar heat capacity of water from its specific heat. Plan: Use molar mass of water and dimensional analysis to convert from heat capacity per gram to heat capacity per mole. Solve: Use the molar mass of hydrogen and oxygen to calculate water s molar mass. Molar mass= 18.0 g/mol H 2 O Use the specific heat calculated earlier to calculate the molar specific heat. C n 18.0 g mol J J g K 75. mol K 44

45 Calorimetry Calorimeter Instrument used to measure heat flow Calorimetry Measures heat change from chemical reactions or physical changes Useful because we cannot know the exact enthalpy of the reactants and products 2015 Pearson Education, Inc. 45

46 Constant Pressure Calorimetry Constant pressure calorimetry Carry out reaction in aqueous solution Heat change for the system equals the heat change for the water in the calorimeter H calculated by q = m C s T Specific heat for water is J/g K 2015 Pearson Education, Inc. 46

47 Constant Volume Calorimetry Bomb calorimetry Reactions carried out in a sealed bomb Heat absorbed (or released) by water is a very good approximation of the enthalpy change for the reaction q rxn = C cal T Measuring the change in internal energy ( E) not H E = q rxn For most reactions, the difference is very small 2015 Pearson Education, Inc. 47

48 HESS S LAW 48

49 Hess s Law Hess s law: the energy change in an overall chemical reaction is equal to the sum of the energy changes in the individual reactions comprising it Remember enthalpy is a state function 2015 Pearson Education, Inc. 49

50 Calculation of H C 3 H 8 (g) + 5 O 2 (g) 3 CO 2 (g) + 4 H 2 O(l) Imagine this as occurring in three steps: 1) Decomposition of propane to the elements: C 3 H 8 (g) 3 C (graphite) + 4 H 2 (g) 2) Formation of CO 2 : 3 C (graphite) + 3 O 2 (g) 3 CO 2 (g) 3) Formation of H 2 O: 4 H 2 (g) + 2 O 2 (g) 4 H 2 O(l) 2015 Pearson Education, Inc. 50

51 Calculation of H C 3 H 8 (g) + 5 O 2 (g) 3 CO 2 (g) + 4 H 2 O(l) So, all steps look like this: C 3 H 8 (g) 3 C (graphite) + 4 H 2 (g) 3 C (graphite) + 3 O 2 (g) 3 CO 2 (g) 4 H 2 (g) + 2 O 2 (g) 4 H 2 O(l) 2015 Pearson Education, Inc. 51

52 Calculation of H C 3 H 8 (g) + 5 O 2 (g) 3 CO 2 (g) + 4 H 2 O(l) The sum of these equations is the overall equation! C 3 H 8 (g) 3 C (graphite) + 4 H 2 (g) H = kj 3 C (graphite) + 3 O 2 (g) 3 CO 2 (g) H = kj 4 H 2 (g) + 2 O 2 (g) 4 H 2 O(l) H = kj 2015 Pearson Education, Inc. 52

53 Calculation of H C 3 H 8 (g) + 5 O 2 (g) 3 CO 2 (g) + 4 H 2 O(l) The sum of these equations is the overall equation! + C 3 H 8 (g) 3 C (graphite) + 4 H 2 (g) H = kj 3 C (graphite) + 3 O 2 (g) 3 CO 2 (g) H = kj 4 H 2 (g) + 2 O 2 (g) 4 H 2 O(l) H = kj C 3 H 8 (g) + 5 O 2 (g) 3 CO 2 (g) + 4 H 2 O(l) 2015 Pearson Education, Inc. H = kj 53

54 ENTHALPIES OF FORMATION 54

55 Enthalpy of Formation Enthalpy of formation ( H f ) Enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms Standard enthalpies of formation ( H f ) Measured under standard conditions (25 ºC and 1.00 atm) C ( 2 o s) O2( g) CO ( g) H f 394 kj / mol 55

56 Calculation of H Simpler way to use Hess s law H = n H f,products m H f,reactants where n and m are the stoichiometric coefficients 56

57 Calculation of H using Values from the Standard Enthalpy Table C 3 H 8 (g) + 5 O 2 (g) 3 CO 2 (g) + 4 H 2 O(l) H = n H f,products m H f,reactants H = [3( kj) + 4( kj)] [1( kj) + 5(0 kj)] = [( kj) + ( kj)] [( kj) + (0 kj)] = ( kj) ( kj) = kj 2015 Pearson Education, Inc. 57

58 58

59 EXAMPLE EXOTHERMIC AND ENDOTHERMIC PROCESSES Classify each change as exothermic or endothermic. (a) wood burning in a fire (b) ice melting SOLUTION 59

60 EXAMPLE EXOTHERMIC AND ENDOTHERMIC PROCESSES SKILLBUILDER Exothermic and Endothermic Processes Classify each change as exothermic or endothermic. (a) (b) water freezing into ice natural gas burning Answers: 60

61 EXAMPLE RELATING HEAT ENERGY TO TEMPERATURE CHANGES Gallium is a solid metal at room temperature but melts at 29.9 C. If you hold gallium in your hand, it melts from your body heat. How much heat must 2.5 g of gallium absorb from your hand to raise the temperature of the gallium from 25.0 C to 29.9 C? The specific heat capacity of gallium is J/g C. SORT You are given the mass of gallium, its initial and final temperatures, and its specific heat capacity, and are asked to find the amount of heat absorbed by the gallium. STRATEGIZE The equation that relates the given and find quantities is the specific heat capacity equation. The solution map indicates that this equation takes you from the given quantities to the quantity you are asked to find. 61

62 EXAMPLE RELATING HEAT ENERGY TO TEMPERATURE CHANGES Continued SOLVE Before solving the problem, you must gather the necessary quantities C, m, and ΔT in the correct units. SOLUTION Substitute C, m, and ΔT into the equation, canceling units, and calculate the answer to the correct number of significant figures. 62

63 EXAMPLE RELATING HEAT CAPACITY TO TEMPERATURE CHANGES A chemistry student finds a shiny rock that she suspects is gold. She weighs the rock on a balance and determines that its mass is 14.3 g. She then finds that the temperature of the rock rises from 25 C to 52 C upon absorption of 174 J of heat. Find the heat capacity of the rock and determine whether the value is consistent with the heat capacity of gold (which is listed in Table 3.4). SORT You are given the mass of the gold rock, the amount of heat absorbed, and the initial and final temperatures. You are asked to find the heat capacity of the rock. 63

64 EXAMPLE RELATING HEAT CAPACITY TO TEMPERATURE CHANGES Continued STRATEGIZE The solution map shows how the heat capacity equation relates the given and find quantities. SOLVE First, gather the necessary quantities m, q, and ΔT in the correct units. Then solve the equation for C and substitute the correct variables into the equation. Finally, calculate the answer to the right number of significant figures. 64

65 How many kj of energy are in a fast food hamburger containing 560. Calories? (Note: 1 cal = J) A kj B kj C kj D kj E kj 65

66 How many joules are required to raise the temperature of g of iron from 25.1 C to C? (The specific heat of iron is J/g C.) A J B J C J D J E J 66

67 A kg bar of aluminum at 23.8 C has 15.5 KJ of heat added. What is the final temperature of the aluminum? (The specific heat of aluminum is J/g C.) A C B C C C D C E C 67

68 What is the mass (in kg) of gold that increases in temperature by 45.1 C when Cal of heat are added? (The specific heat of gold is J/g C and 1 cal = J.) A kg B kg C kg D kg E kg 68

69 Which of the following is true of exothermic reactions? A. The reactants possess greater energy than the products. B. Energy is released as the reaction occurs. C. Energy is absorbed as the reaction occurs. D. Both a and b are true. E. Both a and c are true. 69

70 Which of the following changes is endothermic? A. Water evaporates. B. Water freezes. C. Butane gas burns. D. Steam condenses. E. None of the above 70

71 Using the following information, indicate which of the choices below is correct: The temperature of iron increases more quickly than water when it is heated. A. Iron has a greater heat capacity than water. B. Water has a greater heat capacity than iron. C. Iron and water have the same heat capacity. D. The boiling point of water is greater than that of iron. E. None of the above 71