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3/15/2015 Thermochemistry deals with changes in heat during chemical reactions. A main goal of the study of thermochemistry is to determine the quantity of heat exchanged between a system and its surroundings. The THERMODYNAMICS system is the part of the universe being studied, while the Thermochemistry surroundings are the rest of the universe that interacts with the system. Definitions 2 System and surroundings An open system is a system that freely exchanges energy and matter with its surroundings. Open system 3 4 1 3/15/2015 Open systems Closed system exchanges energy but not matter with an outside system. Though it is typically portion of larger system, it is not in complete contact. Closed system WM_ THERMODYNAMICS 5 WM_ THERMODYNAMICS 6 Isolated system can exchange neither energy nor matter with an outside system. While it may be portion of larger system, it does not communicate with the outside in any way. The physical universe is an isolated system; a closed thermos bottle is essentially an isolated system (though its isolation is not perfect). Isolated system WM_ THERMODYNAMICS 7 WM_ THERMODYNAMICS 8 2 3/15/2015 9 Comparison of systems 10 Q. Identify system type (open, closed or isolated) from descrition below and fill the enpty space in the table below System description System type Coffee in perfectly closed Thermos® flask •Q, •A closed system contains 2g of ice. Another 2g of ice are added to the system. What is the final mass of the system? •Q •An isolated system has an initial temperature of 30oC. It is then placed on top of a bunsen burner for an hour. What is the final temperature? Combustion of gasoline in car engine Mercury in thermometer Living plant Quiz Electric battery 11 WM_ THERMODYNAMICS 12 3 3/15/2015 Q. Which type of thermodynamic system is: 1. an ocean? 2. an aquarium? 3. a pizza delivery bag? 4. a greenhouse? Matter and energy ENERGY 13 WM_ THERMODYNAMICS 14 Matter is anything that has mass and occupies space. Mass is a measure of the quantity of matter in a sample of any material. The more massive an object is, the more force is required to put it in motion. All bodies consist of matter. MATTER AND ENERGY Energy is measure of the ability of a body or system to do work or produce any change. No activity is possible without energy. Energy 15 16 4 3/15/2015 Our senses of sight and touch usually tell us that an object occupies space. In the case of colorless, odorless, tasteless gases (such as air), our senses may fail us. Additionally, we cannot say anything about total energy which the object possesses. Chemical changes always involve energy changes, some energy transformations do not involve chemical changes at all. For example, heat energy may be converted into electrical energy or into mechanical energy without any simultaneous chemical changes. 17 18 Many experiments have demonstrated that all of the energy involved in any chemical or physical change appears in some form after the change. These observations are summarized in the Law of Conservation of Energy: Energy cannot be created or destroyed in a chemical reaction or in a physical change. It can only be converted from one form to another. Interceptor transforms energy of sun into internal water energy Warn water Cold water Warm air Use of sun energy for house heating Law of Conservation of Energy WM_ THERMODYNAMICS 19 20 5 3/15/2015 With the dawn of the nuclear age in the 1940s, scientists, and then the world, became aware that matter can be converted into energy. In nuclear reactions, matter is transformed into energy. The relationship between matter and energy is given by Albert Einstein’s now famous equation: At the present time, we have not (knowingly) observed the transformation of energy into matter on a large scale. It does, however, happen on an extremely small scale in “atom smashers,” or particle accelerators, used to induce nuclear reactions (like in Large Hadron Collider in CERN). E = m c2 The Law of Conservation of Matter and Energy The Law of Conservation of Matter and Energy 21 Now that the equivalence of matter and energy is recognized, the Law of Conservation of Matter and Energy can be stated in a single sentence: 22 Energy is very important in every aspect of our daily lives. The food we eat supplies the energy to sustain life with all of its activities and concerns. The availability of relatively inexpensive energy is an important factor in our technological society. The combined amount of matter and energy in the universe is fixed. MEANING OF THE ENERGY IN THE LIFE The Law of Conservation of Matter and Energy WM_ THERMODYNAMICS WM_ THERMODYNAMICS 23 24 6 3/15/2015 • • • • The concept of energy is at the every heart of science. All physical and chemical processes are accompanied by the transfer of energy. Energy cannot be created or destroyed. We must understand how to do the “accounting” of energy transfers from one body or one substance to another or from one form of energy to another. In thermodynamics we study the energy changes that accompany physical and chemical processes. Usually these energy changes involve heat—hence the “thermo-” part of the term. There are the two main aspects of thermodynamics. OBJECT OF THE THERMODYNAMICS „ACCOUNTING OF ENERGY’ 29 30 • The first aspect is thermochemistry. This practical subject is concerned with how we observe, measure, and predict energy changes for both physical changes and chemical reactions. • • • The second aspect is addressed to a more fundamental aspect of thermodynamics. How to use energy changes to tell us whether or not a given process can occur under specified conditions to give predominantly products (or reactants) how to make a process more (or less) favourable. FUNDAMMENTAL ASPECT OF THERMODYNAMICS THERMOCHEMISTRY 31 32 7 3/15/2015 We can define energy as follows. Energy is the capacity to do work or to transfer heat. ENERGY - DEFINITION Energy can take many forms: electrical energy, radiant energy (light), nuclear energy, chemical energy. ENERGY FORMS 33 More common we classify energy into two general types: kinetic and potential. Q. The kinetics energy of solid body with the mass of 5 kg which moved with speed 8 m s-1 is equal: a) 40 kg m s-1; b) 320 J; c) 160 J d) 160 kg m2s-2 Kinetic energy is the energy of motion. The kinetic energy of an object is equal to one half its mass, m, times the square of its velocity, v. Ek 34 1 m v2 2 The heavier a hammer is and the more rapidly it moves, the greater its kinetic energy and the more work it can accomplish. KINETIC ENERGY Kinetics energy in questions 35 WM_ THERMODYNAMICS 36 8 3/15/2015 Potential energy (EP) is the energy that a system possesses by virtue of its position or composition. The work that we do to lift an object is stored in the object as energy. Q. When a bucket with 10 kg of water is picked up at the height of 1 m the potential energy is as follows: a) Ep= m g h 10 kg m; b) 100 J; c) 100 kg m2s-2; d) 100 kg Where: m – mass (kg); h – body movement (change of height, m); g – gravitational acceleration, (10 m s-2 ) POTENTIAL ENERGY Potential energy in question 37 38 Q. What type of energy does a stationary pencil contain? falling pencil? If we drop a hammer, its potential energy is converted into kinetic energy as it falls, and it could do work on something it hits—for example, drive a nail or break a piece of glass. EXAMPLE: EpEk 39 WM_ THERMODYNAMICS 40 9 3/15/2015 Similarly, an electron in an atom has potential energy because of the electrostatic force on it that is due to the positively charged nucleus and the other electrons in that atom and surrounding atoms. Ep in ATOM The atomic or molecular level, we can think of each of these as either kinetic or potential energy. The chemical energy in a fuel or food comes from potential energy stored in atoms due to their arrangements in the molecules. ATOMIC LEVEL 41 Many forms of energy can be interconverted and that in chemical processes, chemical energy is converted to heat energy or vice versa. The amount of heat a process uses (endothermic) or gives off (exothermic) can tell us a great deal about that process. For this reason it is important for us to be able to measure the intensity of heat. 42 This stored chemical energy can be released when compounds undergo chemical changes, such as those that occur in combustion and metabolism. Reactions that release energy in the form of heat are called exothermic reactions. INTENSITY OF HEAT EXOTHERMIC REACTIONS 43 44 10 3/15/2015 Hydrocarbons—including methane, the main component of natural gas, and octane, a minor component of gasoline undergo combustion with an excess of O2 to yield CO2 and H2O. CH4 + 2O2 This reaction releases heat energy, Q. Temperature measures the intensity of heat, the “hotness” or “coldness” of a body. A piece of metal at 100°C feels hot to the touch, whereas an ice cube at 0°C feels cold. Why? Because the temperature of the metal is higher, and that of the ice cube lower, than body temperature. Heat is a form of energy that always flows spontaneously from a hotter body to a colder body—never in the reverse direction. CO2 + 2H2O + Q EXOTHERMIC REACTIONS WM_ THERMODYNAMICS TEMPERATURE vs. HEAT 45 EQUATION OF REACTION WM_ THERMODYNAMICS In such reactions, the total energy of the products is lower than that of the reactants by the amount of energy released, most of which is heat. Some initial activation (e.g., by heat)is needed to get these reactions started. This amount of energy is called activation energy. 46 ACTIVATION Energy 52 WM_ THERMODYNAMICS 53 11 3/15/2015 Fig. 1 The amount of heat shown in such an equation always refers to the reaction for the number of moles of reactants and products specified by the coefficients. THERMOTERMIC REACTION WM_ THERMODYNAMICS 54 The difference between the potential energy of the reactants—one mole of CH4(g) and two moles of O2(g)—and that of the products—one mole of CO2(g) and two moles of H2O(l)—is the amount of heat evolved in this exothermic reaction at constant pressure. For this reaction, it is 890 kJ/mol. WM_ THERMODYNAMICS 55 A process that absorbs energy from its surroundings is called endothermic. For this type of reaction, the final level of energy is higher than the initial level. ENDOTHERMIC REACTION 56 WM_ THERMODYNAMICS 57 12 3/15/2015 When solid hydrated barium hydroxide, Ba(OH)2 8H2O, and excess solid ammonium nitrate, NH4NO3, are mixed, a reaction occurs. Fig. An endothermic process. When solid hydrated barium hydroxide, Ba(OH)2 ∙ 8H2O, and excess solid ammonium nitrate, NH4NO3, are mixed, a reaction occurs. Profile of endothermic reaction WM_ THERMODYNAMICS Energy absorbed or released Relative Energy of reactants & products Sign of H Exothermic Reaction Endothermic Reaction Energy is released. It is a product of the reaction. Reaction vessel becomes warmer. Temperature inside reaction vessel increases. Energy is absorbed. It is a reactant of the reaction. Reaction vessel becomes cooler. Temperature inside reaction vessel decreases. Energy of the reactants is greater than the energy of the products H(reactants) > H(products) H = H(products) H(reactants) = negative (-ve) Energy of the reactants is less than the energy of the products H(reactants) < H(products) H = H(products) H(reactants) = positive (+ve) The dissolution process is very endothermic. If the flask is placed on wet a wooden block, the water freezes and attaches the block to the flask 58 WM_ THERMODYNAMICS Endothermic Energy Profile 60 59 Exothermic Energy of reactants Energy of reactants (NH3) (N2 & H2) is greater is less than the energy of than the energy of the products (N2 & H2). the products (NH3). Energy is absorbed Energy is released . . 61 13 3/15/2015 Exothermic processes Endothermic processes making ice cubes melting ice cubes formation of snow in clouds conversion of frost to water vapour condensation of rain from water vapor evaporation of water mixing sodium sulfite and bleach baking bread rusting iron cooking an egg burning sugar producing sugar by photosynthesis mixing water and strong acids mixing water and ammonium nitrate mixing water with an anhydrous salt making an anhydrous salt from a hydrate crystallizing liquid salts (as in sodium acetate in chemical handwarmers) melting solid salts There are two ways of looking at what happens to the enthalpy: If the reaction is exothermic the products have minimum enthalpy and the formation of products (move toward the right) is favourable If the reaction is endothermic the reactants have minimum enthalpy and the formation of products (move toward the right) is unfavourable. In this case the formation of reactants (move toward the left) is favourable. Author: Fred Senese [email protected]; http://antoine.frostburg.edu/chem/senese/101/thermo/faq/exothermic-endothermicexamples.shtml 62 63 Q. Which of the following is an endothermic Q. Chemical reactions that absorb heat energy are called __________ . a. exothermic b. eltothermic c. endothermic reaction? a) Burning propane in a gas grill b) Photosynthesis c) Baking bread Q. Electrolysis requires energy to make it work. This means it is... a) an endothermic reaction b) an exothermic reaction c) an eltothermic reaction d) a chemical reaction d) The mixture bubbled vigorously and the temperature dropped by 40°C e) A red glow spread through the mixture and the temperature increased by 10°C f) Electrolysis of water WM_ THERMODYNAMICS 64 WM_ THERMODYNAMICS 65 14 3/15/2015 Some important ideas about energy are summarized in the First Law of Thermodynamics. Energy is neither created nor destroyed in ordinary chemical reactions and physical changes. The substances involved in the chemical and physical changes that we are studying are called the system. Everything in the system’s environment constitutes its surroundings. The universe is the system plus its surroundings. THE UNIVERSE, SYSTEM, SURROUNDINGS FIRST LAW OF THERMODYNAMICS WM_ THERMODYNAMICS 66 The system may be thought of as the part of the universe under investigation. WM_ THERMODYNAMICS 67 Q. The first law of thermodynamics states that energy is The First Law of Thermodynamics tells us that energy is neither created nor destroyed. a. increased during any process b. decreased during any process c. conserved during any process Energy is only transferred between the system and its surroundings. FIRST LAW OF THERMODYNAMICS WM_ THERMODYNAMICS 68 WM_ THERMODYNAMICS 69 15 3/15/2015 The thermodynamic state of a system is defined by a set of conditions that completely specifies all the properties of the system. This set commonly includes: the temperature, pressure, composition (identity and number of moles of each component), physical state (gas, liquid, or solid) of each part of the system. THERMODYNAMIC STATE WM_ THERMODYNAMICS THERMODYNAMIC STATE 70 WM_ THERMODYNAMICS 71 The properties of a system—such as P, V, T—are called state functions. The value of a state function depends only on the state of the system and not on the way in which the system came to be in that state. A change in a state function describes a difference between the two states. Ice liquid water Changes in physical state of water due to temperature changes STATE FUNCTIONS Steam WM_ THERMODYNAMICS 72 73 16 3/15/2015 Indirect It is independent of the process or pathway by which the change occurs. 1 3 Indirect Direct 2 74 75 For instance, consider a sample of one mole of pure liquid water at 30°C and 1 atm pressure. It does not matter whether: (1) the cooling took place directly (either slowly or rapidly) from 30°C to 22°C, If at some later time the temperature of the sample is 22°C at the same pressure, then it is in a different thermodynamic state. or (2) the sample was first heated to 36°C, then cooled to 10°C, and finally warmed to 30°C, then cooled to 22oC We can tell that the net temperature change is 8°C. or (3) any other conceivable path was followed from the initial state to the final state. STATE FUNCTIONS STATE FUNCTION WM_ THERMODYNAMICS 76 WM_ THERMODYNAMICS 77 17 3/15/2015 The change in other properties (e.g., the pressure) of the sample is likewise independent of path. The most important use of state functions in thermodynamics is to describe changes. We describe the difference in any quantity, X, as X X FINAL X INITIAL STATE FUNCTION When X increases, the final value is greater than the initial value, so ΔX is positive; a decrease in X makes ΔX a negative value. STATE FUNCTIONS WM_ THERMODYNAMICS You can consider a state function as analogous to a bank account. With a bank account, at any time you can measure the amount of money in your account (your balance) in convenient terms—dollars and cents, euros and eurocents, złoty and grosz. 78 COMPARISON WM_ THERMODYNAMICS Changes in this balance can occur for several reasons, such as deposit of your paycheck, using of VISA cart, or service charges assessed by the bank. In our analogy these transactions are not state functions, but they do cause changes in the state function (the balance in the account). 79 ACCOUNT BALANCE WM_ THERMODYNAMICS 80 WM_ THERMODYNAMICS 81 18 3/15/2015 - 25 $ -50 $ Δ = -150 $ Similarly, we shall see that the energy of a system is a state function that can be changed— for instance, by an energy “deposit” of heat absorbed or work done on the system, or by an energy “withdrawal” of heat given off or work done by the system. -75 $ ENERGY OF THE SYSTEM WM_ THERMODYNAMICS We can describe differences between levels of a state function, regardless of where the zero level is located. In the case of a bank balance, the “natural” zero level is obviously the point at which we open the account, before any deposits or withdrawals. 82 ZERO LEVEL WM_ THERMODYNAMICS In contrast, the zero levels on most temperature scales are set arbitrarily. When we say that the temperature of an ice–water mixture is “zero degrees Celsius,” we are not saying that the mixture contains no temperature! 85 ZERO LEVEL WM_ THERMODYNAMICS 86 WM_ THERMODYNAMICS 87 19 3/15/2015 We have simply chosen to describe this point on the temperature scale by the number zero; conditions of higher temperature are described by positive temperature values, and those of lower temperature have negative values, “below zero.” ARBITRARY SCALES The phrase “15 degrees cooler” has the same meaning anywhere on the scale. Many of the scales that we use in thermodynamics are arbitrarily defined in this way. Arbitrary scales are useful when we are interested only in changes in the quantity being described. ARBITRARY SCALES WM_ THERMODYNAMICS 88 Q. What is the change energy of the WM_ THERMODYNAMICS Any property of a system that depends only on the values of its state functions is also a state function. For instance, the volume of a given sample of water depends only on temperature, pressure, and physical state; volume is a state function. We shall encounter other thermodynamic state functions. sausage after heating, if original energy is 4 kJ and 20 kJ is added to it? a) 16 kJ; b) 4 kJ; c) 20 kJ; d) 24 kJ 89 STATE FUNCTION WM_ THERMODYNAMICS 90 WM_ THERMODYNAMICS 91 20 3/15/2015 Most chemical reactions and physical changes occur at constant (usually atmospheric) pressure. The quantity of heat transferred into or out of a system as it undergoes a chemical or physical change at constant pressure, qp, is defined as the enthalpy change, H, of the process. ENTHALPY An enthalpy change is sometimes loosely referred to as a heat change or a heat of reaction. The enthalpy change is equal to the enthalpy or “heat content,” H, of the substances produced minus the enthalpy of the substances consumed. ENTHALPY - HEAT WM_ THERMODYNAMICS 92 WM_ THERMODYNAMICS 93 Calculate the reaction of decomposition of ammonium nitrate. The reaction is Δ H = Hfinal - Hinitial and the enthalpies of the three compounds are given in Table 1. Δ H = Hsubstances produced – Hsubstance consumed CHANGE OF ENTHALPY WM_ THERMODYNAMICS 94 WM_ THERMODYNAMICS 95 21 3/15/2015 If you reverse the previous reaction, the sign of the enthalpy of the reaction is reversed: Δ H = +36 kJ http://www.cliffsnotes.com/sciences/chemistry/chemistry/thermodynamics/e nthalpy 96 97 WM_ THERMODYNAMICS 99 Standard enthalpies of formation •Calculate the enthalpy change for the following reaction and classify it as exothermic or endothermic. WM_ THERMODYNAMICS WM_ THERMODYNAMICS 98 Compound ΔH0 MgCl2 (S) -642 kJ/mol H2O (l) -286 kJ/mol MgO (S) -602 kJ/mol HCl (g) -92 kJ/mol 22 3/15/2015 It is impossible to know the absolute enthalpy (heat content) of a system. Enthalpy is a state function, however, and it is the change in enthalpy in which we are interested; this can be measured for many processes. We can determine the energy change associated with a chemical or physical process by using an experimental technique called calorimetry. This technique is based on observing the temperature change when a system absorbs or releases energy in the form of heat. The temperature change is caused by the absorption or release of heat by the chemical or physical process under study. CALORIMETRY EHTHALPY AS STATE FUNCTION WM_ THERMODYNAMICS 100 The experiment is carried out in a device called a calorimeter, in which the temperature change of a known amount of substance (often water) of known specific heat is measured. The specific heat is the amount of heat per unit mass required to raise the temperature by one degree Celsius. Specific heat WM_ THERMODYNAMICS Chemical reactions and physical changes occur with either the simultaneous evolution of heat (exothermic processes) or the absorption of heat (endothermic processes). The amount of heat transferred in a process is usually expressed in joules or in calories. The SI unit of energy and work is the joule (J), which is defined as 1 kg m2/s2. 101 SPECIFIC HEAT WM_ THERMODYNAMICS 102 WM_ THERMODYNAMICS 103 23 3/15/2015 You may find it more convenient to think in terms of the amount of heat required to raise the temperature of one gram of water from 14.5°C to 15.5°C, which is 4.184 J. One calorie is defined as exactly 4.184 J. The so-called “large calorie,” used to indicate the energy content of foods, is really one kilocalorie, that is, 1000 calories or 1 kcal. The specific heat of a substance is the amount of heat required to raise the temperature of one gram of the substance one degree Celsius (also one kelvin) with no change in phase. Changes in phase (physical state) absorb or liberate relatively large amounts of energy (see Figure –NEXT SLIDE). We shall do most calculations in joules. SPECIFIC HEAT SPECIFIC HEAT WM_ THERMODYNAMICS 104 WM_ THERMODYNAMICS The specific heat of each substance, a physical property, is different for the solid, liquid, and gaseous phases of the substance. For example, the specific heat of: ice is 2.09 J/g °C near 0°C; liquid water it is 4.18 J/g °C; steam it is 2.03 J/g °C near 100°C. 105 SPECIFIC HEAT 106 WM_ THERMODYNAMICS 107 24 3/15/2015 Specific heats and molar heat capacities for various substances at 293 K (20oC) SPECIFIC HEAT [c] WM_ THERMODYNAMICS 108 Q. How many heat is needed to heat up 10 g of liquid Substance c in J/gm K Aluminum Copper Gold Lead Silver Zinc Mercury Alcohol(ethyl) Water Ice (-10 C) 0.900 0.386 0.126 0.128 0.233 0.387 0.140 2.4 4.186 2.05 Molar C J/mol K 24.3 24.5 25.6 26.4 24.9 25.2 28.3 111 75.2 36.9 SPECIFIC HEAT WM_ THERMODYNAMICS 109 Answer: Q = m c ΔT water from 10oC to 40oC. [c of liquid water is 4.18 J g-1 oC-1]. Q = (10 g) . (4.18 J g-1 oC-1) ( 40oC – 10oC) a) 418 J; b) 1254 J; c) 1672 J Q = 10 . 4.18 . 30 = 1254 J WM_ THERMODYNAMICS 110 WM_ THERMODYNAMICS 111 25 3/15/2015 Q. A 385 grams chunk of iron is heated to 97.5oC. Then it is immersed in 247 gram of water originally at 20.7oC. When thermal equilibrium has been reached, the water and iron are both at 31.6oC. Calculate the specific heat of iron. WM_ THERMODYNAMICS 112 A hot object, such as a heated piece of metal (a), is placed into cooler water. Heat is transferred from the hotter metal bar to the cooler water until the two reach the same temperature (b). WM_ THERMODYNAMICS 113 Solution: The number of heat gained by water from temperature 20.7oC to 31.6o C =the amount of heat which is lost by the iron. Qwater = (247 g). (4.18 J g-1 oC-1) (31.6-20.7oC) We say that they are then at thermal equilibrium. Qwater = (247) . (4.18). (10.9) = 11253.8 J THERMAL EQUILIBRIUM WM_ THERMODYNAMICS 114 WM_ THERMODYNAMICS 115 26 3/15/2015 End of part 1 WM_ THERMODYNAMICS 116 27