Chapter 5 Thermochemistry Lecture Presentation Chapter 5 Thermochemistry John D. Bookstaver St. Charles Community College Cottleville, MO © 2012 Pearson Education, Inc.

© 2012 Pearson Education, Inc. Energy Energy is the ability to do work or transfer heat. Energy used to cause an object that has mass to move is called work. Energy used to cause the temperature of an object to rise is called heat. © 2012 Pearson Education, Inc.

© 2012 Pearson Education, Inc. Kinetic Energy Kinetic energy is energy an object possesses by virtue of its motion: 1 2 Ek =  mv2 © 2012 Pearson Education, Inc.

© 2012 Pearson Education, Inc. Potential Energy Potential energy is energy an object possesses by virtue of its position or chemical composition. The most important form of potential energy in molecules is electrostatic potential energy, Eel: KQ1Q2 d Eel = © 2012 Pearson Education, Inc.

© 2012 Pearson Education, Inc. 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 = 4.184 J 1 J = 1  kg m2 s2 © 2012 Pearson Education, Inc.

w = F  d = m  g  d = (5.4 kg)(9.8 m/s2)(1.6 m) = 85 kg-m2/s2 = 85 J Sample Exercise 5.1 Describing and Calculating Energy Changes A bowler lifts a 5.4-kg (12-lb) bowling ball from ground level to a height of 1.6 m (5.2 ft) and then drops it. (a) What happens to the potential energy of the ball as it is raised? (b) What quantity of work, in J, is used to raise the ball? (c) After the ball is dropped, it gains kinetic energy. If all the work done in part (b) has been converted to kinetic energy by the time the ball strikes the ground, what is the ball’s speed just before it hits the ground? (Note: The force due to gravity is F = m  g, where m is the mass of the object and g is the gravitational constant; g = 9.8 m/s2.) Solution Analyze We need to relate the potential energy of the bowling ball to its position relative to the ground. We then need to establish the relationship between work and the change in the ball’s potential energy. Finally, we need to connect the change in potential energy when the ball is dropped with the kinetic energy attained by the ball. Plan We can calculate the work done in lifting the ball by using Equation 5.3: w = F  d. The kinetic energy of the ball just before it hits the ground equals its initial potential energy. We can use the kinetic energy and Equation 5.1 to calculate the speed, v, just before impact. Solve (a) Because the ball is raised above the ground, its potential energy relative to the ground increases. (b) The ball has a mass of 5.4 kg and is lifted 1.6 m. To calculate the work performed to raise the ball, we use Equation 5.3 and F = m  g for the force that is due to gravity: w = F  d = m  g  d = (5.4 kg)(9.8 m/s2)(1.6 m) = 85 kg-m2/s2 = 85 J Thus, the bowler has done 85 J of work to lift the ball to a height of 1.6 m.

Sample Exercise 5.1 Describing and Calculating Energy Changes Continued (c) When the ball is dropped, its potential energy is converted to kinetic energy. We assume that the kinetic energy just before the ball hits the ground is equal to the work done in part (b), 85 J: We can now solve this equation for v: Check Work must be done in (b) to increase the potential energy of the ball, which is in accord with our experience. The units are appropriate in (b) and (c). The work is in units of J and the speed in units of m/s. In (c) we carry an additional digit in the intermediate calculation involving the square root, but we report the final value to only two significant figures, as appropriate. Comment A speed of 1 m/s is roughly 2 mph, so the bowling ball has a speed greater than 10 mph just before impact. Practice Exercise What is the kinetic energy, in J, of (a) an Ar atom moving at a speed of 650 m/s, (b) a mole of Ar atoms moving at 650 m/s? (Hint: 1 amu = 1.66  10–27 kg.) Answer: (a) 1.4  10–20 J, (b) 8.4  103 J 7

Definitions: System and Surroundings The system includes the molecules we want to study (here, the hydrogen and oxygen molecules). The surroundings are everything else (here, the cylinder and piston). © 2012 Pearson Education, Inc.

© 2012 Pearson Education, Inc. Definitions: Work Energy used to move an object over some distance is work: w = F  d where w is work, F is the force, and d is the distance over which the force is exerted. © 2012 Pearson Education, Inc.

© 2012 Pearson Education, Inc. Heat Energy can also be transferred as heat. Heat flows from warmer objects to cooler objects. © 2012 Pearson Education, Inc.

© 2012 Pearson Education, Inc. Conversion of Energy Energy can be converted from one type to another. For example, the cyclist in Figure 5.2 has potential energy as she sits on top of the hill. © 2012 Pearson Education, Inc.

© 2012 Pearson Education, Inc. Conversion of Energy As she coasts down the hill, her potential energy is converted to kinetic energy. At the bottom, all the potential energy she had at the top of the hill is now kinetic energy. © 2012 Pearson Education, Inc.

First Law of Thermodynamics Energy is neither created nor destroyed. In other words, the total energy of the universe is a constant; if the system loses energy, it must be gained by the surroundings, and vice versa. © 2012 Pearson Education, Inc.

© 2012 Pearson Education, Inc. Internal Energy Internal energy (E) of a system is the sum of all the kinetic and potential energies of the system. These energies include all the motions and interactions of the atoms and molecules as well as the motions and interactions within the nuclei and electron clouds – this information is generally not known. In thermodynamics, scientists are mainly concerned with the change in the system, which allows them to ignore the unknowns. Thermodynamics also takes the point of view of the system rather than that of the surroundings. In a chemical reaction, the initial state of the system refers to the reactants and the final state refers to the products © 2012 Pearson Education, Inc.

© 2012 Pearson Education, Inc. Internal Energy The internal energy of a system is the sum of all kinetic and potential energies of all components of the system; we call it E. © 2012 Pearson Education, Inc.

© 2012 Pearson Education, Inc. Internal Energy By definition, the change in internal energy, E, is the final energy of the system minus the initial energy of the system: E = Efinal − Einitial © 2012 Pearson Education, Inc.

Changes in Internal Energy If E > 0, Efinal > Einitial Therefore, the system absorbed energy from the surroundings. This energy change is called endergonic. © 2012 Pearson Education, Inc.

Changes in Internal Energy If E < 0, Efinal < Einitial Therefore, the system released energy to the surroundings. This energy change is called exergonic. © 2012 Pearson Education, Inc.

Changes in Internal Energy When energy is exchanged between the system and the surroundings, it is exchanged as either heat (q) or work (w). That is, E = q + w. © 2012 Pearson Education, Inc.

© 2012 Pearson Education, Inc. E, q, w, and Their Signs © 2012 Pearson Education, Inc.

Sample Exercise 5.2 Relating Heat and Work to Changes of Internal Energy Gases A(g) and B(g) are confined in a cylinder-and-piston arrangement like that in Figure 5.4 and react to form a solid product C(s): A(g) + B(g) C(s). As the reaction occurs, the system loses 1150 J of heat to the surroundings. The piston moves downward as the gases react to form a solid. As the volume of the gas decreases under the constant pressure of the atmosphere, the surroundings do 480 J of work on the system. What is the change in the internal energy of the system? Solution Analyze The question asks us to determine E, given information about q and w. Plan We first determine the signs of q and w (Table 5.1) and then use Equation 5.5, E = q + w, to calculate E. 21

Sample Exercise 5.2 Relating Heat and Work to Changes of Internal Energy Continued Solve Heat is transferred from the system to the surroundings, and work is done on the system by the surroundings, so q is negative and w is positive: q = –1150 J and w = 480 kJ. Thus, E = q + w = (–1150 J) + (480 J) = –670 J The negative value of E tells us that a net quantity of 670 J of energy has been transferred from the system to the surroundings. Comment You can think of this change as a decrease of 670 J in the net value of the system’s energy bank account (hence, the negative sign); 1150 J is withdrawn in the form of heat while 480 J is deposited in the form of work. Notice that as the volume of the gases decreases, work is being done on the system by the surroundings, resulting in a deposit of energy. Practice Exercise Calculate the change in the internal energy for a process in which a system absorbs 140 J of heat from the surroundings and does 85 J of work on the surroundings. Answer: +55 J 22

Exchange of Heat between System and Surroundings When heat is absorbed by the system from the surroundings, the process is endothermic. © 2012 Pearson Education, Inc.

Exchange of Heat between System and Surroundings When heat is absorbed by the system from the surroundings, the process is endothermic. When heat is released by the system into the surroundings, the process is exothermic. © 2012 Pearson Education, Inc.

© 2012 Pearson Education, Inc. State Functions Usually we have no way of knowing the internal energy of a system; finding that value is simply too complex a problem. © 2012 Pearson Education, Inc.

© 2012 Pearson Education, Inc. State Functions However, we do know that the internal energy of a system is independent of the path by which the system achieved that state. In the system depicted in Figure 5.9, the water could have reached room temperature from either direction. © 2012 Pearson Education, Inc.

© 2012 Pearson Education, Inc. State Functions Therefore, internal energy is a state function. It depends only on the present state of the system, not on the path by which the system arrived at that state. And so, E depends only on Einitial and Efinal. © 2012 Pearson Education, Inc.

© 2012 Pearson Education, Inc. State Functions However, q and w are not state functions. Whether the battery is shorted out or is discharged by running the fan, its E is the same. But q and w are different in the two cases. © 2012 Pearson Education, Inc.

© 2012 Pearson Education, Inc. Work The work involved in the expansion or compression of gases is called pressure-volume work Usually in an open container the only work done is by a gas pushing on the surroundings (or by the surroundings pushing on the gas). w = −PV 1L∙atm = 101.3 J © 2012 Pearson Education, Inc.

© 2012 Pearson Education, Inc. Work We can measure the work done by the gas if the reaction is done in a vessel that has been fitted with a piston: w = −PV © 2012 Pearson Education, Inc.

W = -0 atm x 3.8 L = 0 L•atm = 0 joules A sample of nitrogen gas expands in volume from 1.6 L to 5.4 L at constant temperature. What is the work done in joules if the gas expands (a) against a vacuum and (b) against a constant pressure of 3.7 atm? w = -P DV (a) DV = 5.4 L – 1.6 L = 3.8 L P = 0 atm W = -0 atm x 3.8 L = 0 L•atm = 0 joules (b) DV = 5.4 L – 1.6 L = 3.8 L P = 3.7 atm w = -3.7 atm x 3.8 L = -14.1 L•atm w = -14.1 L•atm x 101.3 J 1L•atm = -1430 J 6.3

© 2012 Pearson Education, Inc. Enthalpy If a process takes place at constant pressure (as the majority of processes we study do) and the only work done is this pressure–volume work, we can account for heat flow during the process by measuring the enthalpy of the system. Enthalpy is the internal energy plus the product of pressure and volume: H = E + PV © 2012 Pearson Education, Inc.

© 2012 Pearson Education, Inc. Enthalpy When the system changes at constant pressure, the change in enthalpy, H, is H = (E + PV) This can be written H = E + PV © 2012 Pearson Education, Inc.

© 2012 Pearson Education, Inc. Enthalpy Since E = q + w and w = −PV, we can substitute these into the enthalpy expression: H = E + PV H = (q + w) − w H = q So, at constant pressure, the change in enthalpy is the heat gained or lost. © 2012 Pearson Education, Inc.

Endothermicity and Exothermicity A process is endothermic when H is positive. © 2012 Pearson Education, Inc.

Endothermicity and Exothermicity A process is endothermic when H is positive. A process is exothermic when H is negative. © 2012 Pearson Education, Inc.

© 2012 Pearson Education, Inc. Enthalpy of Reaction The change in enthalpy, H, is the enthalpy of the products minus the enthalpy of the reactants: H = Hproducts − Hreactants © 2012 Pearson Education, Inc.

© 2012 Pearson Education, Inc. Enthalpy of Reaction This quantity, H, is called the enthalpy of reaction, or the heat of reaction. © 2012 Pearson Education, Inc.

The Truth about Enthalpy Enthalpy is an extensive property. H for a reaction in the forward direction is equal in size, but opposite in sign, to H for the reverse reaction. H for a reaction depends on the state of the products and the state of the reactants. © 2012 Pearson Education, Inc.

Sample Exercise 5.4 Relating H to Quantities of Reactants and Products How much heat is released when 4.50 g of methane gas is burned in a constant-pressure system? (Use the information given in Equation 5.18.) Solution Analyze Our goal is to use a thermochemical equation to calculate the heat produced when a specific amount of methane gas is combusted. According to Equation 5.18, 890 kJ is released by the system when 1 mol CH4 is burned at constant pressure. Plan Equation 5.18 provides us with a stoichiometric conversion factor: (1mol CH4 = 890 kJ). Thus, we can convert moles of CH4 to kJ of energy. First, however, we must convert grams of CH4 to moles of CH4. Thus, the conversion sequence is grams CH4 (given) → moles CH4 → kJ (unknown to be found). Solve By adding the atomic weights of C and 4 H, we have 1 mol CH4 = 16.0 CH4. We can use the appropriate conversion factors to convert grams of CH4 to moles of CH4 to kilojoules: The negative sign indicates that the system released 250 kJ into the surroundings. 40

2 H2O2(l) 2 H2O(l) + O2(g) H = –196 kJ Sample Exercise 5.4 Relating H to Quantities of Reactants and Products Continued Practice Exercise Hydrogen peroxide can decompose to water and oxygen by the reaction 2 H2O2(l) 2 H2O(l) + O2(g) H = –196 kJ Calculate the quantity of heat released when 5.00 g of H2O2(l) decomposes at constant pressure. Answer: –14.4 kJ 41

© 2012 Pearson Education, Inc. Calorimetry Since we cannot know the exact enthalpy of the reactants and products, we measure H through calorimetry, the measurement of heat flow. © 2012 Pearson Education, Inc.

Heat Capacity and Specific Heat The amount of energy required to raise the temperature of a substance by 1 K (1C) is its heat capacity. © 2012 Pearson Education, Inc.

Heat Capacity and Specific Heat We define specific heat capacity (or simply specific heat) as the amount of energy required to raise the temperature of 1 g of a substance by 1 K (or 1 C). © 2012 Pearson Education, Inc.

Heat Capacity and Specific Heat Specific heat, then, is Specific heat = heat transferred mass  temperature change s = q m  T © 2012 Pearson Education, Inc.

Constant Pressure Calorimetry By carrying out a reaction in aqueous solution in a simple calorimeter such as this one, one can indirectly measure the heat change for the system by measuring the heat change for the water in the calorimeter. © 2012 Pearson Education, Inc.

Constant Pressure Calorimetry Because the specific heat for water is well known (4.184 J/g-K), we can measure H for the reaction with this equation: q = m  s  T © 2012 Pearson Education, Inc.

HCl(aq) + NaOH(aq) H2O(l) + NaCl(aq) Sample Exercise 5.6 Measuring H Using a Coffee-Cup Calorimeter When a student mixes 50 mL of 1.0 M HCl and 50 mL of 1.0 M NaOH in a coffee-cup calorimeter, the temperature of the resultant solution increases from 21.0 C to 27.5 C. Calculate the enthalpy change for the reaction in kj/mol HCl, assuming that the calorimeter loses only a negligible quantity of heat, that the total volume of the solution is 100 mL, that its density is 1.0 g/mL, and that its specific heat is 4.18 J/g–K. Solution Analyze Mixing solutions of HCl and NaOH results in an acid–base reaction: HCl(aq) + NaOH(aq) H2O(l) + NaCl(aq) We need to calculate the heat produced per mole of HCl, given the temperature increase of the solution, the number of moles of HCl and NaOH involved, and the density and specific heat of the solution. Plan The total heat produced can be calculated using Equation 5.23. The number of moles of HCl consumed in the reaction must be calculated from the volume and molarity of this substance, and this amount is then used to determine the heat produced per mol HCl. Solve Because the total volume of the solution is 100 mL, its mass is The temperature change is Using Equation 5.23, we have (100 mL)(1.0 g/mL) = 100 g T = 27.5 C – 21.0 C = 6.5 C = 6.5 K qrxn = –Cs  m  T = –(4.18 J/g–K)(100 g)(6.5 K) = –2.7  103 J = –2.7 kJ 48

AgNO3(aq) + HCl(aq) AgCl(s) + HNO3(aq) Sample Exercise 5.6 Measuring H Using a Coffee-Cup Calorimeter Continued H = qP = –2.7 kJ Because the process occurs at constant pressure, To express the enthalpy change on a molar basis, we use the fact that the number of moles of HCl is given by the product of the volume (50mL = 0.050 L) and concentration (1.0 M = 1.0 mol/L) of the HCl solution: Thus, the enthalpy change per mole of HCl is (0.050 L)(1.0 mol/L) = 0.050 mol H = –2.7 kJ/0.050 mol = –54 kJ/mol Check H is negative (exothermic), which is expected for the reaction of an acid with a base and evidenced by the fact that the reaction causes the temperature of the solution to increase. The magnitude of the molar enthalpy change seems reasonable. Practice Exercise When 50.0 mL of 0.100 MAgNO3 and 50.0 mL of 0.100 M HCl are mixed in a constant-pressure calorimeter, the temperature of the mixture increases from 22.30 C to 23.11 C. The temperature increase is caused by the following reaction: AgNO3(aq) + HCl(aq) AgCl(s) + HNO3(aq) Calculate H for this reaction in AgNO3, assuming that the combined solution has a mass of 100.0 g and a specific heat of 4.18 J/g C. Answer: –68,000 J/mol = –68 kJ/mol 49

© 2012 Pearson Education, Inc. Bomb Calorimetry Reactions can be carried out in a sealed “bomb” such as this one. The heat absorbed (or released) by the water is a very good approximation of the enthalpy change for the reaction. © 2012 Pearson Education, Inc.

© 2012 Pearson Education, Inc. Bomb Calorimetry Because the volume in the bomb calorimeter is constant, what is measured is really the change in internal energy, E, not H. For most reactions, the difference is very small. © 2012 Pearson Education, Inc.

2 CH6N2(l) + 5 O2(g) 2 N2(g) + 2 CO2(g) + 6 H2O(l) Sample Exercise 5.7 Measuring qrxn Using a Bomb Calorimeter The combustion of methylhydrazine (CH6N2), a liquid rocket fuel, produces N2(g), CO2(g), and H2O(l): 2 CH6N2(l) + 5 O2(g) 2 N2(g) + 2 CO2(g) + 6 H2O(l) When 4.00 g of methylhydrazine is combusted in a bomb calorimeter, the temperature of the calorimeter increases from 25.00 C to 39.50 C. In a separate experiment the heat capacity of the calorimeter is measured to be 7.794 kJ/ C. Calculate the heat of reaction for the combustion of a mole of CH6N2. Solution Analyze We are given a temperature change and the total heat capacity of the calorimeter. We are also given the amount of reactant combusted. Our goal is to calculate the enthalpy change per mole for combustion of the reactant. Plan We will first calculate the heat evolved for the combustion of the 4.00-g sample. We will then convert this heat to a molar quantity. Solve For combustion of the 4.00-g sample of methylhydrazine, the temperature change of the calorimeter is We can use T and the value for Ccal to calculate the heat of reaction (Equation 5.24): We can readily convert this value to the heat of reaction for a mole of CH6N2: T = (39.50 C – 25.00 C) = 14.50 C qrxn = –Ccal  T = –(7.794 kJ/C)(14.50 C) = –113.0 kJ 52

Sample Exercise 5.7 Measuring qrxn Using a Bomb Calorimeter Continued Check The units cancel properly, and the sign of the answer is negative as it should be for an exothermic reaction. The magnitude of the answer seems reasonable. Practice Exercise A 0.5865-g sample of lactic acid (HC3H5O3) is burned in a calorimeter whose heat capacity is 4.812 kJ/C. The temperature increases from 23.10 C to 24.95 C. Calculate the heat of combustion of lactic acid (a) per gram and (b) per mole. Answer: (a) –15.2 kJ/g, (b) –1370 kJ/mol 53

© 2012 Pearson Education, Inc. Hess’s Law H is well known for many reactions, and it is inconvenient to measure H for every reaction in which we are interested. However, we can estimate H using published H values and the properties of enthalpy. © 2012 Pearson Education, Inc.

© 2012 Pearson Education, Inc. Hess’s Law Hess’s law states that “[i]f a reaction is carried out in a series of steps, H for the overall reaction will be equal to the sum of the enthalpy changes for the individual steps.” © 2012 Pearson Education, Inc.

© 2012 Pearson Education, Inc. Hess’s Law Because H is a state function, the total enthalpy change depends only on the initial state of the reactants and the final state of the products. © 2012 Pearson Education, Inc.

Enthalpies of Formation An enthalpy of formation, Hf, is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms. © 2012 Pearson Education, Inc.

Standard Enthalpies of Formation Standard enthalpies of formation, Hf°, are measured under standard conditions (25 °C and 1.00 atm pressure). © 2012 Pearson Education, Inc.

© 2012 Pearson Education, Inc. Calculation of H C3H8(g) + 5O2(g)  3CO2(g) + 4H2O(l) Imagine this as occurring in three steps: C3H8(g)  3C(graphite) + 4H2(g) © 2012 Pearson Education, Inc.

© 2012 Pearson Education, Inc. Calculation of H C3H8(g) + 5O2(g)  3CO2(g) + 4H2O(l) Imagine this as occurring in three steps: C3H8(g)  3C(graphite) + 4H2(g) 3C(graphite) + 3O2(g) 3CO2(g) © 2012 Pearson Education, Inc.

© 2012 Pearson Education, Inc. Calculation of H C3H8(g) + 5O2(g)  3CO2(g) + 4H2O(l) Imagine this as occurring in three steps: C3H8(g)  3C(graphite) + 4H2(g) 3C(graphite) + 3O2(g) 3CO2(g) 4H2(g) + 2O2(g)  4H2O(l) © 2012 Pearson Education, Inc.

© 2012 Pearson Education, Inc. Calculation of H C3H8(g) + 5O2(g)  3CO2(g) + 4H2O(l) The sum of these equations is C3H8(g)  3C(graphite) + 4H2(g) 3C(graphite) + 3O2(g) 3CO2(g) 4H2(g) + 2O2(g)  4H2O(l) C3H8(g) + 5O2(g)  3CO2(g) + 4H2O(l) © 2012 Pearson Education, Inc.

© 2012 Pearson Education, Inc. Calculation of H We can use Hess’s law in this way: H = nHf,products – mHf°,reactants where n and m are the stoichiometric coefficients. © 2012 Pearson Education, Inc.

© 2012 Pearson Education, Inc. Calculation of H C3H8(g) + 5O2(g)  3CO2(g) + 4H2O(l) H = [3(−393.5 kJ) + 4(−285.8 kJ)] – [1(−103.85 kJ) + 5(0 kJ)] = [(−1180.5 kJ) + (−1143.2 kJ)] – [(−103.85 kJ) + (0 kJ)] = (−2323.7 kJ) – (−103.85 kJ) = −2219.9 kJ © 2012 Pearson Education, Inc.

© 2012 Pearson Education, Inc. Energy in Foods Most of the fuel in the food we eat comes from carbohydrates and fats. © 2012 Pearson Education, Inc.

© 2012 Pearson Education, Inc. Energy in Fuels The vast majority of the energy consumed in this country comes from fossil fuels. © 2012 Pearson Education, Inc.