Thermochemistry Answer Key Assignments 1- 8 Page 1. Answers to Energy Assignment 1 1. Indicate whether the following scenarios illustrate matter possessing mainly potential energy, or kinetic energy. a) There are 30 liters of fuel in the tank of a stationary automobile. o Potential energy (chemical energy in fact) b) A large rock is perched at the edge of a 40 m cliff. o Potential energy (of the gravitational type) c) Energy is supplied to a beaker of water raising its temperature from 20 oC to 40 oC o The warming water is increasing its kinetic energy content d) The average speed of a molecule of a gas at room temperature is about 1000 m / s .

o Kinetic energy is possessed by moving molecules e) Air in an automobile tire exerts pressure on the walls of the tire. o Kinetic energy of the moving molecules in the tire volume. Some argue that if the tire could 'blow', it could also be considered potential energy. That's acceptable also. f) A container of dynamite sits on a storage room floor o Chemical potential energy g) Millions of gallons of water is held behind a hydroelectric power dam. o Gravitational potential energy h) Thirty grams of ice at 0 oC are melted to become liquid water at 0 oC.

o The ice experiences an increase in its potential energy content as it melts to become liquid water. There is no change in kinetic energy content because there is no increase in temperature. 2. Indicate whether the following scenarios illustrate the First law, or Second law of Thermodynamics (or both). Explain.

a) A toboggan which possesses 100 units of energy at the top of a hill converts 85 units to kinetic energy and 15 units to heat as it slides to the bottom of the hill. o First Law - all forms of energy accounted for, none lost or gained) Hot iron is plunged into a bucket of cold water which warms up. o Second Law - heat goes from hot iron to cold water) An open window in December cools the house down very quickly. o Second Law - heat transfers from hot house to cold outside air) Burning wood converts chemical energy to heat, light and sound.

o Both Laws First Law- energy is being converted to many different forms and their total should equal the total energy the wood possessed in the first place. Second Law- heat is transferred from the burning wood to the cool are) Chemical energy is converted to electrical energy by a battery. o First Law (mainly) This is a simple conversion from one form to another but in reality, a small amount of the energy is converted to heat Energy Assignment 2 - Answers 1. The chart Scenario System (syst) Surroundings (sure) Exothermic or Endothermic Direction of Energy Flow Energy Conversion a) dissolving KBr water endothermic surroundings to system Ek Surr to Ep system b) burning CH 4 air in furnace exothermic system to surroundings Ep syst to Ek sure c) dissociating H 2 SO 4 water component of solution exothermic system to surroundings Ep syst to Ek sure d) combustion of candle wax surrounding air exothermic system to surroundings Ep syst to Ek sure e) melting ice water in glass endothermic surroundings to system Ek Surr to Ep syst f) condensing water air in cloud exothermic system to surroundings Ep syst to Ek sure g) your metabolic reactions air around you exothermic system to surroundings Ep syst to Ek sure h) the chemical reaction immediate surroundings endothermic surroundings to system Ek Surr to Ep syst yours 2. The equation for the fermentation of glucose to alcohol and carbon dioxide is C 6 H 12 O 6 (s) 2 C 2 H 5 OH (l) + 2 CO 2 (g) a) The enthalpy change for the reaction is -68. 1 kJ.

Is the reaction exothermic or endothermic? Ans: Its exothermic as you can tell from the negative sign on the energy term. b) Is energy absorbed or released as the reaction proceeds? Explain how you know this. Ans: Released. All exothermic systems release energy to the surroundings. 3. The last step in the commercial production of sulfuric acid is SO 3 (g) + H 2 O (l) H 2 SO 4 (aq) DH = -250 kJ In designing a sulfuric acid plant, is it necessary to provide for heating or cooling the building in which the reaction is taking place? Explain.

Ans: You would need to cool the building as the system is constantly releasing energy into it. 4. 25. 0 grams of molten silver metal is dropped into a container of cold water. a) Write a chemical equation that describes the system. Ag (l) Ag (s) + energy b) What substance gains kinetic energy during this process? The water into which the molten silver was dropped will gain kinetic energy.

c) What substance changes its potential energy content during this process? The silver loses potential energy as it solidifies. Energy Assignment 3 - Answers Multiple choice 1. B 2. A 3. D 4. D 5.

B 6. D 7. Convert each of the following equations to DH notation. Indicate whether each reaction is exothermic or endothermic. a) Cd (s) + 1/2 O 2 (g) CdO (s) DH = -261 kJ b) Cl 2 (g) + 7/2 O 2 (g) Cl 2 O 7 (g) DH = +272 kJ c) 2 Au (s) + 3/2 O 2 (g) Au 2 O 3 (s) DH = -8. 4 kJ d) 2 B (s) + 3/2 H 2 (g) B 2 H 6 (g) DH = +31 kJ e) 1/2 Br 2 (l) + 1/2 Cl 2 (g) Br Cl (g) DH = +15 kJ 8.

Identify each of the following reactions as exothermic or endothermic. Which processes store more energy in the products than in the reactants? a) Hg (l) + 1/2 I 2 (s) Hi (g) DH = +138 kJ endothermic b) 1/2 N 2 (g) + 3/2 F 2 (g) NF 3 (g) DH = -114 kJ exothermic c) Si (s) + 2 Br 2 (l) Si Br 4 (l) DH = -398 kJ exothermic d) Na (s) Na (g) DH = +109 kJ endothermic Therefore (a) and (d) store more energy in the products than in the reactants. 9. At 100 oC, solid ammonium nitrite decomposes to form nitrogen gas and gaseous water.

At this temperature, the decomposition releases 224 kJ per mole of ammonium nitrite decomposed. The key word is 'releases' indicating that the system is exothermic, so... (a) Write the balanced equation for this reaction including the energy term in the equation. NH 4 NO 2 (s) N 2 (g) + 2 H 2 O (g) + 224 kJ (b) Write the balanced equation for the reaction and use the DH notation for the energy term. NH 4 NO 2 (s) N 2 (g) + 2 H 2 O (g) DH = -224 kJ 10. At elevated temperatures, solid lead (II) nitrate decomposes to form solid lead monoxide, oxygen gas and gaseous nitrogen dioxide.

The decomposition absorbs 292 kJ of energy per mole of lead (II) nitrate decomposed. The key word is absorbs, indicating the system is exothermic, so... (a) Write the balanced equation for the reaction including the energy term in the equation. If balanced with whole numbers, it results in 2 moles of lead (II) nitrate so... 2 Pb (NO 3) 2 (s) + 584 kJ 2 PbO (s) + O 2 (g) + 4 NO 2 (g) If balanced in order to express it in terms of one mole of lead (II) nitrate its... Pb (NO 3) 2 (s) + 292 kJ PbO (s) + 1/2 O 2 (g) + 2 NO 2 (g) (b) Write the balanced equation for the reaction using DH notation.

Either: 2 Pb (NO 3) 2 (s) 2 PbO (s) + O 2 (g) + 4 NO 2 (g) DH = +584 kJ Or: Pb (NO 3) 2 (s) PbO (s) + 1/2 O 2 (g) + 2 NO 2 (g) DH = +292 kJ Use the following diagram to answer the question which follows. 11. The reaction shown in the diagram is A. exothermic and DH is negative C. endothermic and DH is negative B. exothermic and DH is positive D.

endothermic and DH is positive Answers to Energy Assignment 4 Stoichiometry and the Energy Term 1. Consider the equation: I 2 (s) + 14. 9 J I 2 (g) Based on this data, determine the mass of I 2 (g) formed if 3. 78 J of heat are absorbed by a sample of solid iodine. mass of I 2 (g) = +3. 78 J x 1 mol I 2 (s) x 253.

80 g I 2 (g) +14. 9 J mol I 2 (g) mass of I 2 (g) = 64. 4 g 2. Given the following equation: Na 2 O (s) + 2 HI (g) 2 NaI (s) + H 2 O (l) + 503 kJ a) If 9. 00 grams of NaI (s) are formed by the process, how much heat will be involved? DH = 9. 00 g NaI (s) x 1 mol NaI (s) x -503 kJ 149.

89 g NaI (s) 2 mol NaI (s) DH = -15. 1 kj) Is the heat absorbed or released in this reaction? Explain how you arrived at this conclusion. Since he energy term is on the product side of the given equation, it's an exothermic reaction, so heat must be released. 3. When concentrated nitric acid (HNO 3 (aq) ) is added to water, heat is liberated.

This heat of dilution is 30 kJ per mole of nitric acid diluted. How many grams of nitric acid must be diluted to release 3620 kJ of heat? If you had trouble with this one it's probably because you are writing the wrong equation. Use this: HNO 3 (aq) H+ (aq) + NO 3- (aq), + 30 kJ and try again. The answer is: 7. 6 x 103 g The method of solution for the remaining questions follows the pattern set in questions 1 and 2 a which illustrate the use of dimensional analysis for solving thermochemical stoichiometry questions. Only the answers to questions 4-6 are provided below Good luck! 4.

(Ans: 3. 46 kJ absorbed) 5. a) (Ans: 78. 13 g) b) (Ans: 208.

2 g) 6. (Ans: +13. 2 kJ) Quiz 1 examines all the material to this point. Answers to Energy Assignment 5 Ek, Ep, or both or neither? 1.

Determine whether the following systems involves a change in kinetic energy, chemical potential energy, neither or both. a) A container of liquid acetone cools from 54 oC to 35 oC as it sits on a lab bench. -change in kinetic energy as only the temperature is changing. b) A bottle of concentrated nitric acid is placed on a low shelf for safety's sake. -neither.

It has experienced a change in gravitational potential energy, but that is not one of the choices c) A compound is heated in a crucible until it decomposes into its constituent elements. -both. Kinetic energy change (as the compound warms) and a potential energy change as it decomposes to its elements d) Sodium vapor is cooled to its boiling point and then liquefied. -both. Sodium vapor undergoes a kinetic energy change as it cools, and a potential energy change as it changes phase from gas to liquid e) Liquid water at 50 oC is warmed to its boiling point and then vaporized. -kinetic energy change as the water warms, and a potential energy change as it changes phase to become vapor f) Ice at -15 oC warms to 0 oC, melts, and then warms up to 45 oC.

- kinetic energy change as ice warms to 0 oC, a potential energy change as it changes phase from solid to liquid, and a further kinetic energy change as it warms to 45 oC. Energy Assignment 6 - Answers Warming/Cooling and Phase Changes (Answers are in red) 1. If energy is added to a substance and it does not result in a rise in temperature, the inference is that the substance may be changing A. from a gas to a solid B. from a liquid to a gas C. from a gas to a liquid D.

in amount of kinetic energy The answer to question 2 is C 3. The section on the graph from B to C would best be interpreted as representing an increased A. Ep and a phase change C. Ep and a loss in Ek B.

Ek and a phase change D. Ek and a loss in Ep 4. During the change represented by H 2 O (l) H 2 O (g) at water's boiling point, the molecules undergo A. an increase in kinetic energy C.

an increase in potential energy B. a decrease in potential energy D. a decrease in both potential and kinetic energy 5. When a student warms a sample of ice from -10 oC to -6 oC the best prediction is that the A. kinetic energy of the molecules decreases B. kinetic energy of the molecules increases C.

potential energy of the molecules increases D. potential and kinetic energies of the molecules increase 6. 50 g of H 2 O (g) possesses A. more kinetic energy than 50 g of H 2 O (s) B. less potential energy than 50 g of H 2 O (s) C. the same total energy as 50 g of H 2 O (l) D.

less kinetic energy than 50 g of H 2 O (l) 7. Energy added to matter during a phase change is stored as A. kinetic energy C. ionization energy B. potential energy D.

nuclear energy 8. A student listed the characteristics of a phase change: I. Kinetic energy and temperature were changing II. the new phase had a different amount of energy from that of the old phase III.

Potential energy was increased in fusion and in vaporization IV. Potential energy was decreased during freezing and condensation The correct characteristics are A. I, II and III C. I, III and IV B. I, II and IV D. II, III and IV 9.

When solid aluminum at 660 oC changes to molten aluminum at 660 oC, its A. kinetic energy decreases B. potential energy increases C. kinetic energy increases and potential energy remains constant D. potential energy decreases and kinetic energy remains constant 10. A student heated ice at -20 oC and took temperature readings every 30 seconds.

Initially the temperature increased at each reading. Then there were three subsequent identical temperature readings. The inference is that A. melting point was reached B. boiling point was reached C. kinetic energy of the water molecules was dropping D.

potential energy of the water molecules was dropping 11. Which statement describes temperature and potential energy conditions during a phase change? A. Both temperature and EP change B. Both temperature and EP remain constant C.

Temperature remains constant and EP changes D. Temperature changes and EP remains constant 12. The potential energy of a substance includes the A. momentum of its molecules C.

vibrational energy of the nuclei B. energy of its chemical bonds D. rotating motion of its molecules 13. When ice at 0 oC changes to liquid water at 0 oC there is an increase in A. kinetic energy only C. both kinetic and potential energy B.

potential energy only D. kinetic energy but a decrease in potential energy 14. When steam at 100 oC condenses, the energy released results mainly from the formation of A. ionic bonds C. intermolecular bonds B. covalent bonds D.

bonds between nucleons 15. 1. 0 g of steam at 100 oC possesses more energy than 1. 0 g of water at 100 oC because the A.

potential energy is greater in the steam B. molecules in the steam have greater bonding energy C. EP and the EK are equal in the steam D. attractive forces in the molecules are greater in the steam. 16. The potential energy of a substance is A.

due to translational, rotational, and vibrational movements of its particles of matter B. energy that is released when intermolecular or intramolecular bonds are broken and / or reformed during the formation of new and different substances C. the energy possessed by the particles of the substance as they gain momentum during an increase in temperature D. always equal to the amount of energy present in the substance 17.

Which of the following statements best describes chemical reactions and the energy changes associated with them? A. The heat added is used to do work against Van ser Waals forces resulting in a gradual spreading out of the particles B. Internal bonding forces are overcome and break resulting in the opportunity for new bonds of lower or higher energy content to form C. After a change in temperature, a temperature is reached at which no apparent temperature change occurs, but a physical change takes place D. The heat added weakens the forces between nucleons resulting in the formation of new elements and accompanied by the release of large amounts of energy.

Short Answer 1. Why on average, is so much more energy released or absorbed by chemical changes than by phase changes? Chemical changes involve the rearrangement of intramolecular bonds which are much stronger that the intermolecular bonds involved in phase changes. Stronger bonds involve larger energy values. 2. Define temperature. A relative measure of the average kinetic energy of the particles in a given sample.

3. During phase changes, what kinds of bonds are being formed and / or broken in molecular compounds? Intermolecular bonds are being rearranged in phase changes. 4. State the First Law of Thermodynamics. Energy can neither be created nor destroyed. It can be converted from one form to another.

5. In what form is energy stored when water is vaporized? Explain. Vaporization of water is a phase change, so potential energy is stored in the gaseous water molecules formed. 6. Which molecule has the greater ability to store kinetic and potential energy, CCl 4 (g) or Ar (g)? Explain, by comparing all the modes by which these two substances can store energy. The Answer is CCl 4.

The reasons are complex. You need to compare the abilities of Ar and CCl 4 to store energy in the various modes available to them. Imagine these modes as 'pockets' into which energy can be stuffed. CCl 4 clearly wins as in has more energy in more pockets. See the table below. Summary of Energy Storage Abilities for Argon and Carbon tetrachloride FORM OF ENERGY Available 'pockets' for energy storage Argon (Ar) Storage ability Carbon tetrachloride (CCl 4) Storage ability Kinetic Energy Ek in vibrational mode no yes Ek in rotational mode no yes Ek in translational mode yes yes Ek as momentum yes more that Ar (CCl 4 is heavier) Potential Energy Ep in intermolecular bonds yes (weak London Dispersion forces) yes (large London Dispersion forces) Ep in intramolecular bonds no (there are none) yes (there are four) Assignment 7 - Calculating Kinetic Energy Changes The answers are with the questions in this Assignment Assignment 8 - Successive Ek and Ep changes during warming or cooling The answers are with the questions in this Assignment - except for the following 4.

Consider the situation in which sodium metal (mp = 98 oC; bp = 883 oC) is heated from 50. 0 oC and 1 atm pressure to 1600 oC and 1 atm pressure. Draw a temp vs. time graph describing what would happen to the system as it warmed up through this temperature range. (Draw your own graph first and then go here to compare) l = liquid g = gas The starting point is 500 oC, well over the melting point of sodium metal, so the heating process begins with sodium in the liquid phase. As it is heated, it gains kinetic energy until it reaches its boiling point at 883 oC.

It then goes through a potential energy change as it vaporizes. Once it is converted to sodium in the gas phase, it continues to warm up, gaining kinetic energy until it reaches the final temperature or 1600 oC. 5. Using information from your data booklet, draw the graph that describes the cooling of chlorine (Cl 2 (g) ) from 25 oC and l atm pressure to -50 oC and 1 atm pressure. Describe in writing what you would see during the change as well as the energy changes occuring. (Draw your graph, and then go here to compare) l = liquid g = gas At 25 oC, chlorine (Cl 2) is in the gas phase.

As it cools it loses kinetic energy, until it reaches the freezing point at -35 oC at which point loses potential energy as it condenses to form liquid chlorine at -35 oC. Upon further cooling it loses more kinetic energy until it reaches its final temperature of -50 oC. Since it cools no further, it remains in the liquid state at -50 oC. The answers to Assignments 9 to 15 are on page 2.