Slide 1 / 118 Slide 2 / 118 Thermochemistry Slide 3 / 118 Table of Contents · 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 · Enthalpies of Reaction · Hess's Law · · Enthalpies of Formation · Energy in Foods and Fuels
Slide 4 / 118 The Nature of Energy Return to Table of Contents Slide 5 / 118 Pr es Thermochemistry en at We know chemical and physical processes release and absorb energy. We use these thermochemical principles to design air conditioners and refrigerators as well as foot warmers that allow us to stay comfortable as we "go big" on the hill! Slide 6 / 118 A Review of Energy from Physics 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 Electric Potential Energy U E = kQ 1 Q 2 r 2
Slide 7 / 118 A Review of Energy from Physics Kinetic Energy is the energy that an object has by virtue of its motion: KE = 1/2 mv 2 Work is defined by the formula W = Fd parallel Slide 8 / 118 A Review of Energy from Physics An outside force can change The total energy of the energy of a system by an isolated system is doing work on it. constant. work Algebraically, these two statements combine to become: E 0 + W = E f Since E f - E o = ∆ E, this can also be written as ∆ E = W Slide 9 / 118 Units of Energy 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 / 118 1 A reaction produces 3.8 cal of energy. How many joules of energy is produced? Slide 10 (Answer) / 118 1 A reaction produces 3.8 cal of energy. How many joules of energy is produced? Answer 15.9 J [This object is a pull tab] Slide 11 / 118 2 A reaction uses 235 J of energy. How many calories have been burned?
Slide 11 (Answer) / 118 2 A reaction uses 235 J of energy. How many calories have been burned? Answer 56 cal [This object is a pull tab] Slide 12 / 118 3 A 20 ounce coke contains 240 Calories. How many kilojoules of energy are present in a 20 ounce Coke? Slide 12 (Answer) / 118 3 A 20 ounce coke contains 240 Calories. How many kilojoules of energy are present in a 20 ounce Coke? Answer 1.0 x 10 3 kJ [This object is a pull tab]
Slide 13 / 118 Energy & Heat 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). 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. A B T = 20 ℃ T = 10 ℃ heat flow ∆E = w + q *Note, we use a lower case "w" in chemistry. Slide 14 / 118 The First Law of Thermodynamics ∆E = w + q 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. Initial Final Internal energy, E E 0 Internal energy, E state E state Energy lost to E < E 0 E > E 0 surroundings ∆ E < 0 ( - ) ∆ E > 0 ( + ) Energy gained Final Initial from state state surroundings E E 0 E of system decreases E of system increases Slide 15 / 118 System and Surroundings When considering energy changes, we need to focus on a well- defined, limited part of the universe. The portion we focus on is called the system and everything else is called the surroundings. Consider the following reaction occurring within a metal cylinder. 2H 2 (g) + O 2 (g) --> 2H 2 O(g) Surroundings The system includes the reactants and products (here, the hydrogen, oxygen and water molecules). system The surroundings are everything else (here, the cylinder and piston).
Slide 16 / 118 Changes in Internal Energy If ∆ E > 0, E final > E initial H 2 (g) + O 2 (g) The system absorbed energy from the surroundings. Internal energy, E ∆ E < 0 ∆ E >0 (negative) (positive) If ∆ E < 0, E final < E initial The system released energy from the surroundings. H 2 O(l) Slide 17 / 118 4 Ten grams of table salt in dissolved in water in a 250 mL beaker. Which of the following is a component of the system? A NaCl E A and B B water F A, B, and C C Na+ G A, B, and D D beaker Slide 17 (Answer) / 118 4 Ten grams of table salt in dissolved in water in a 250 mL beaker. Which of the following is a component of the system? A NaCl Answer F E A and B B water F A, B, and C C Na+ [This object is a pull tab] G A, B, and D D beaker
Slide 18 / 118 5 When a strong acid is added to a flask containing water the flask becomes warm to the touch. This is because... A the reaction performed work on the flask B the system absorbed heat from the surroundings C the system released heat to the surroundings D the surroundings released heat to the system Slide 18 (Answer) / 118 5 When a strong acid is added to a flask containing water the flask becomes warm to the touch. This is because... A the reaction performed work on the flask Answer C B the system absorbed heat from the surroundings C the system released heat to the surroundings [This object is a pull tab] D the surroundings released heat to the system Slide 19 / 118 6 When a strong acid is added to a flask containing water the flask becomes warm to the touch. Which correctly describes the change in energy? A ∆ E sys is positive and ∆ E sur is negative B ∆ E sys is positive and ∆E sur is positive C ∆ E sys is negative and ∆ E sur is positive D ∆ E sys is negative and ∆ E sur is negative
Slide 19 (Answer) / 118 6 When a strong acid is added to a flask containing water the flask becomes warm to the touch. Which correctly describes the change in energy? A ∆ E sys is positive and ∆ E sur is negative Answer C B ∆ E sys is positive and ∆E sur is positive C ∆ E sys is negative and ∆ E sur is positive [This object is a pull D ∆ E sys is negative and ∆ E sur is negative tab] Slide 20 / 118 Changes in Internal Energy System When energy is exchanged between the system and the Heat q > 0 surroundings, it is either exchanged as either heat ( q ) or work ( w ). Surroundings ∆ E = q + w Work w > 0 ∆ E>0 Slide 21 / 118 q , w , ∆ E, and Their Signs Sign Conventions for q, w and ∆ E q + system gains heat - system loses heat + w work done on system - work done by system ∆ E + net gain of energy by system - net loss of energy by system
Slide 22 / 118 7 The ∆E of a system that gains 50 kJ of heat and performs 24 kJ of work on the surroundings is ________ kJ. A -74 B -26 C 0 D +26 E +74 Slide 22 (Answer) / 118 7 The ∆E of a system that gains 50 kJ of heat and performs 24 kJ of work on the surroundings is ________ kJ. A -74 Answer B -26 D C 0 D +26 [This object is a pull tab] E +74 Slide 23 / 118 8 The ∆ E of a system that releases 120 J of heat and does 40 J of work on the surroundings is ________ J. A -80 B -160 0 C D +80 +160 E
Slide 23 (Answer) / 118 8 The ∆ E of a system that releases 120 J of heat and does 40 J of work on the surroundings is ________ J. A -80 B -160 Answer B C 0 D +80 [This object is a pull tab] E +160 Slide 24 / 118 9 The ∆ E of a system that absorbs 120 J of heat and does 120 J of work on the surroundings is ________ J. A -240 B -120 C 0 D +120 E +240 Slide 24 (Answer) / 118 9 The ∆ E of a system that absorbs 120 J of heat and does 120 J of work on the surroundings is ________ J. -240 A B Answer -120 C C 0 D +120 [This object is a pull tab] E +240
Slide 25 / 118 10 The ∆ E of a system that absorbs 12,000 J of heat and the surrounding does 12,000 J of work on the system is _______ J. A -24000 B -12000 C 0 D +12000 +24000 E Slide 25 (Answer) / 118 10 The ∆ E of a system that absorbs 12,000 J of heat and the surrounding does 12,000 J of work on the system is _______ J. A -24000 Answer E B -12000 C 0 D +12000 [This object is a pull tab] E +24000 Slide 26 / 118 Exchange of Heat between System and Surroundings Recall, when heat is absorbed Surroundings by the system from the surroundings, the process is System endothermic . Surroundings Heat -q System +q Heat When heat is released by the system into the surroundings, the process is exothermic.
Slide 27 / 118 11 The reaction that occurs inside the foot warmer packet is endothermic? True False Slide 27 (Answer) / 118 11 The reaction that occurs inside the foot warmer packet is endothermic? True Answer False False [This object is a pull tab] Slide 28 / 118 12 What will happen when a hot rock is put into cold water? A the water and rock will both gain energy B the water and rock will both lose energy C the rock will gain energy and the water will lose energy D the rock will lose energy and the water will gain energy
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