Slide 1 / 118 Slide 2 / 118 Thermochemistry Slide 3 / 118 Slide 4 / 118 Table of Contents · The Nature of Energy The Nature of Energy · State Functions** Click on the topic to go to that section · Enthalpy Measuring Enthalpy Changes: Calorimetry · Energy Associated with Changes of State · Return to Enthalpies of Reaction · Table of Hess's Law Contents · · Enthalpies of Formation · Energy in Foods and Fuels Slide 5 / 118 Slide 6 / 118 Pr A Review of Energy from Physics es Thermochemistry en at Potential Energy is the energy that objects have energy due to their position. Gravitational Potential Energy GPE = mgh Elastic Potential Energy EPE = 1/2 kx 2 We know chemical and physical processes release and absorb energy. We use these thermochemical principles to design air Electric Potential Energy conditioners and refrigerators as well as foot warmers that allow us to stay comfortable as we "go big" on the hill! U E = kQ 1 Q 2 r 2
Slide 7 / 118 Slide 8 / 118 A Review of Energy from Physics A Review of Energy from Physics An outside force can change The total energy of Kinetic Energy is the energy that an object has by the energy of a system by an isolated system is virtue of its motion: doing work on it. constant. KE = 1/2 mv 2 work Algebraically, these two statements combine to become: Work is defined by the formula E 0 + W = E f W = Fd parallel Since E f - E o = ∆ E, this can also be written as ∆ E = W Slide 9 / 118 Slide 10 / 118 1 A reaction produces 3.8 cal of energy. How many joules Units of Energy of energy is produced? The SI unit of energy is the Joule (J). Another unit of energy is the calorie (cal). 1 cal = 4.184 J The energy of food is measured in Calories (C). [note the capital "C"] 1 Calorie = 1000 calories = 4184 Joules Slide 10 (Answer) / 118 Slide 11 / 118 1 A reaction produces 3.8 cal of energy. How many joules 2 A reaction uses 235 J of energy. How many calories of energy is produced? have been burned? Answer 15.9 J [This object is a pull tab]
Slide 11 (Answer) / 118 Slide 12 / 118 2 A reaction uses 235 J of energy. How many calories 3 A 20 ounce coke contains 240 Calories. How many kilojoules of energy are present in a 20 ounce Coke? have been burned? Answer 56 cal [This object is a pull tab] Slide 12 (Answer) / 118 Slide 13 / 118 Energy & Heat 3 A 20 ounce coke contains 240 Calories. How many kilojoules of energy are present in a 20 ounce Coke? From last year, we know that ∆ E = W. This year, we extend that by adding another way to change the energy of a system; by the flow of Heat (q). Answer 1.0 x 10 3 kJ When two objects of different temperature are in contact, heat flow results in an increase of the energy of the cooler object and an identical decrease of the energy of the hotter object. [This object is a pull tab] A B T = 20 ℃ T = 10 ℃ heat flow ∆E = w + q *Note, we use a lower case "w" in chemistry. Slide 14 / 118 Slide 15 / 118 The First Law of Thermodynamics System and Surroundings ∆E = w + q When considering energy changes, we need to focus on a well- defined, limited part of the universe. The portion we focus on is Energy is neither created nor destroyed. called the system and everything else is called the surroundings. In other words, the total energy of the universe is a constant; if the system loses energy, it must be gained by the surroundings, Consider the following reaction occurring within a metal cylinder. and vice versa. 2H 2 (g) + O 2 (g) --> 2H 2 O(g) Initial Final E 0 Internal energy, E Internal energy, E state E state Surroundings Energy The system includes the reactants and lost to E < E 0 E > E 0 products (here, the hydrogen, oxygen surroundings ∆ E < 0 ( - ) ∆ E > 0 ( + ) and water molecules). Energy gained system Final Initial from The surroundings are everything state state surroundings else (here, the cylinder and piston). E E 0 E of system decreases E of system increases
Slide 16 / 118 Slide 17 / 118 4 Ten grams of table salt in dissolved in water in a 250 mL Changes in Internal Energy beaker. Which of the following is a component of the system? If ∆ E > 0, E final > E initial H 2 (g) + O 2 (g) A NaCl The system absorbed energy from the surroundings. E A and B B water Internal energy, E ∆ E < 0 ∆ E >0 F A, B, and C (negative) (positive) C Na+ If ∆ E < 0, E final < E initial G A, B, and D D beaker The system released energy from the surroundings. H 2 O(l) Slide 17 (Answer) / 118 Slide 18 / 118 4 Ten grams of table salt in dissolved in water in a 250 mL 5 When a strong acid is added to a flask containing water beaker. Which of the following is a component of the the flask becomes warm to the touch. This is because... system? A the reaction performed work on the flask A NaCl Answer F B the system absorbed heat from the surroundings E A and B B water C the system released heat to the surroundings F A, B, and C C Na+ [This object is a pull D the surroundings released heat to the system tab] G A, B, and D D beaker Slide 18 (Answer) / 118 Slide 19 / 118 5 When a strong acid is added to a flask containing water 6 When a strong acid is added to a flask containing water the flask becomes warm to the touch. This is because... the flask becomes warm to the touch. Which correctly describes the change in energy? A the reaction performed work on the flask A ∆ E sys is positive and ∆ E sur is negative Answer C B the system absorbed heat from the surroundings B ∆ E sys is positive and ∆E sur is positive C the system released heat to the surroundings C ∆ E sys is negative and ∆ E sur is positive [This object is a pull tab] D the surroundings released heat to the system D ∆ E sys is negative and ∆ E sur is negative
Slide 19 (Answer) / 118 Slide 20 / 118 6 When a strong acid is added to a flask containing water Changes in Internal Energy the flask becomes warm to the touch. Which correctly describes the change in energy? A ∆ E sys is positive and ∆ E sur is negative System When energy is exchanged Answer between the system and the C B ∆ E sys is positive and ∆E sur is positive Heat q > 0 surroundings, it is either exchanged as either heat ( q ) C ∆ E sys is negative and ∆ E sur is positive or work ( w ). Surroundings [This object is a pull D ∆ E sys is negative and ∆ E sur is negative tab] ∆ E = q + w Work w > 0 ∆ E>0 Slide 21 / 118 Slide 22 / 118 7 The ∆E of a system that gains 50 kJ of heat and q , w , ∆ E, and Their Signs performs 24 kJ of work on the surroundings is ________ kJ. A -74 Sign Conventions for q, w and ∆ E B -26 q + system gains heat - system loses heat C 0 D +26 + w work done on system - work done by system E +74 ∆ E + net gain of energy by system - net loss of energy by system Slide 22 (Answer) / 118 Slide 23 / 118 7 The ∆E of a system that gains 50 kJ of heat and 8 The ∆ E of a system that releases 120 J of heat performs 24 kJ of work on the surroundings is and does 40 J of work on the surroundings is ________ kJ. ________ J. A -74 A -80 Answer B -26 D B -160 C 0 0 C D +26 [This object is a pull +80 tab] D E +74 +160 E
Slide 23 (Answer) / 118 Slide 24 / 118 9 The ∆ E of a system that absorbs 120 J of heat and 8 The ∆ E of a system that releases 120 J of heat does 120 J of work on the surroundings is and does 40 J of work on the surroundings is ________ J. ________ J. A -240 A -80 B -120 B -160 Answer C 0 B C 0 D +120 +80 D E +240 [This object is a pull tab] E +160 Slide 24 (Answer) / 118 Slide 25 / 118 10 The ∆ E of a system that absorbs 12,000 J of heat 9 The ∆ E of a system that absorbs 120 J of heat and and the surrounding does 12,000 J of work on the does 120 J of work on the surroundings is system is _______ J. ________ J. -240 A A -24000 Answer B -120 C B -12000 C 0 C 0 D +12000 D +120 [This object is a pull tab] +24000 E E +240 Slide 25 (Answer) / 118 Slide 26 / 118 10 The ∆ E of a system that absorbs 12,000 J of heat Exchange of Heat between and the surrounding does 12,000 J of work on the System and Surroundings system is _______ J. Recall, when heat is absorbed Surroundings A -24000 by the system from the surroundings, the process is Answer System E endothermic . B -12000 Surroundings C 0 Heat -q D System +12000 [This object is a pull tab] E +24000 +q Heat When heat is released by the system into the surroundings, the process is exothermic.
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