Unit 6 Chemistry Assignments
3 C2H2(g) → C6H6(g) What is the standard enthalphy change ΔHo, for the reaction represented above? (ΔHof of C2H2(g) is 230 kJ mol-1; (ΔHof of C6H6(g) is 83 kJ mol-1;) A -607 kJ B -147 kJ C -19 kJ D +19 kJ E +773 kJ
A -607 kJ
CH3OH(g) -> CO(g) + 2H2(g); ΔHº = +91 kJ/molrxn The reaction represented above goes essentially to completion. The reaction takes place in a rigid, insulated vessel that is initially at 600 K Which of the following statements about the bonds in the reactants and products is most accurate? A The sum of the bond enthalpies of the bonds in the reactant is greater than the sum of the bond enthalpies of the bonds in the products. B The sum of the bond enthalpies of the bonds in the reactant is less than the sum of the bond enthalpies of the bonds in the products. C The length of the bond between carbon and oxygen in CH3OH is shorter than the length of the bond between carbon and oxygen in CO. D All of the bonds in the reactant and products are polar.
A The sum of the bond enthalpies of the bonds in the reactant is greater than the sum of the bond enthalpies of the bonds in the products.
2 NH3(g) → 3 H2(g) + N2(g) ΔH°298 = 92 kJ/molrxn According to the information above, what is the standard enthalpy of formation, ΔH°f , for NH3(g) at 298 K ? A -92 kJ/mol B -46 kJ/mol C 46 kJ/mol D 92 kJ/mol E 184 kJ/mol
B -46 kJ/mol
(increasing line graph) How much energy is required to melt 64 g of methane at 90 K? (The molar mass of methane is 16 g/mol.) A 0.24 kJ B 3.8 kJ C 33 kJ D 60. kJ
B 3.8 kJ
HCl(aq)+NaOH(aq)→NaCl(aq)+H2O(l)ΔH°=−57.1kJ/molrxn The chemical equation above represents the reaction between HCl(aq) and NaOH(aq). When equal volumes of 1.00MHCl(aq) and 1.00MNaOH(aq) are mixed, 57.1kJof heat is released. If the experiment is repeated with 2.00MHCl(aq), how much heat would be released? A 28.6kJ B 57.1kJ C 85.7kJ D 114kJ
B 57.1kJ
Four identical 50mL50mL cups of coffee, originally at 95°C, were stirred with four different spoons, as listed in the table above. In which cup will the temperature of the coffee be highest at thermal equilibrium? (Assume that the heat lost to the surroundings is negligible.) A Cup A B Cup B (lowest mass of spoon and, between the two lowest masses, b has lowest specific heat capacity) C Cup C D Cup D
B Cup B
A 100 g sample of a metal was heated to 100oC and then quickly transferred to an insulated container holding 100 g of water at 22oC. The temperature of the water rose to reach a final temperature of 35oC. Which of the following can be concluded? A The metal temperature changed more than the water temperature did; therefore the metal lost more thermal energy than the water gained. B The metal temperature changed more than the water temperature did, but the metal lost the same amount of thermal energy as the water gained. C The metal temperature changed more than the water temperature did; therefore the heat capacity of the metal must be greater than the heat capacity of the water. D The final temperature is less than the average starting temperature of the metal and the water; therefore the total energy of the metal and water decreased.
B The metal temperature changed more than the water temperature did, but the metal lost the same amount of thermal energy as the water gained.
A 30.g30.g sample of Al(s)Al(s) is heated to 50°C50°C and placed in a calorimeter containing 150g150g of water at 20°C20°C. As the system approaches thermal equilibrium, energy is transferred A from the Al(s)Al(s) to the water and the temperature of the water decreases B from the Al(s)Al(s) to the water and the temperature of the Al(s)Al(s) decreases C from the water to the Al(s)Al(s) and the temperature of the water increases D from the water to the Al(s)Al(s) and the temperature of the Al(s)Al(s) increases
B from the Al(s)Al(s) to the water and the temperature of the Al(s)Al(s) decreases
Reaction 1: N2O4(g)→2NO2(g) //ΔH1=+57.9kJ Reaction 2: 2NO(g)+O2(g)→2NO2(g) ΔH2=−113.1kJ Based on the information for two different reactions given above, which of the following gives the quantities needed to calculate the enthalpy change for the reaction represented by the overall equation below? 2NO(g)+O2(g)→N2O4(g)2 A ΔH1+ΔH2 B ΔH1+(−ΔH2) C (−ΔH1)+ΔH2 D ΔH1+(2×ΔH2)
C (−ΔH1)+ΔH2
A 1.0 g sample of a cashew was burned in a calorimeter containing 1000. g of water, and the temperature of the water changed from 20.0°C to 25.0°C. In another experiment, a 3.0 g sample of a marshmallow was burned in a calorimeter containing 2000. g of water, and the temperature of the water changed from 25.0°C to 30.0°C. Based on the data, which of the following can be concluded about the energy content for 1.0 g of each of the two substances? (The specific heat of water is 4.2 J/(g⋅°C).) A The combustion of 1.0 g of cashew releases less energy than the combustion of 1.0 g of marshmallow. B The combustion of 1.0 g of cashew releases the same amount of energy as the combustion of 1.0 g of marshmallow. C The combustion of 1.0 g of cashew releases more energy than the combustion of 1.0 g of marshmallow. D No comparison can be made because the two systems started with different masses of food, different masses of water, and different initial temperatures.
C The combustion of 1.0 g of cashew releases more energy than the combustion of 1.0 g of marshmallow.
substance= Fe(s); Specific Heat= 0.46J/(g⋅C∘); Mass of sample=1.0 g // substance= Al(s); Specific Heat= 0.91J/(g⋅C∘); Mass of sample=1.0 g // Samples of FeFe and AlAl are heated from 25∘C25∘C to 75∘C75∘C. Based on the information in the table above, the change in the average kinetic energy of the atoms in the samples is A higher for the FeFe sample, because with its lower specific heat it absorbs more energy B higher for the AlAl sample, because with its higher specific heat it absorbs more energy C the same for both metal samples, because the initial and final temperatures are the same D the same for both metal samples, because the masses are equal
C the same for both metal samples, because the initial and final temperatures are the same
For an experiment, 50.0g of H2O was added to a coffee-cup calorimeter, as shown in the diagram above. The initial temperature of the H2O was 22.0°C, and it absorbed 300.J of heat from an object that was carefully placed inside the calorimeter. Assuming no heat is transferred to the surroundings, which of the following was the approximate temperature of the H2O after thermal equilibrium was reached? Assume that the specific heat capacity of H2O is 4.2J/(g⋅K). A 21.3°C B 22.0°C C 22.7°C D 23.4°C
D 23.4°C
For a classroom demonstration, a chemistry teacher puts samples of two different pure solid powders in a beaker. The teacher places the beaker on a small wooden board with a wet surface, then stirs the contents of the beaker. After a short time the students observe that the bottom of the beaker is frozen to the wood surface. The teacher asks the students to make a claim about the observation and to justify their claims. Which of the following is the best claim and justification based on the students' observation? A An exothermic chemical change occurred because heat flowed from the contents of the beaker to the room. B An exothermic physical change occurred because heat flowed from the contents of the beaker and the water on the board to the room. C An endothermic physical change occurred because the freezing of water is an endothermic process. D An endothermic chemical change occurred because the temperature of the beaker and the water on the board decreased as heat was absorbed by the reaction.
D An endothermic chemical change occurred because the temperature of the beaker and the water on the board decreased as heat was absorbed by the reaction.
A piece of Fe(s)Fe(s) at 25°C25°C is placed into H2O(l)H2O(l) at 75°C75°C in an insulated container. A student predicts that when thermal equilibrium is reached, the FeFe atoms, being more massive than the H2OH2O molecules, will have a higher average kinetic energy than the H2OH2O molecules. Which of the following best explains why the student's prediction is incorrect? A At thermal equilibrium, the less massive H2OH2O molecules would have a higher average kinetic energy than the FeFe atoms because they are more free to move than are the FeFe atoms. B At thermal equilibrium, the collisions between the FeFe atoms and the H2OH2O molecules would cease because the average kinetic energies of their particles would have become the same. C At thermal equilibrium, the movement of both the FeFe atoms and the H2OH2O molecules would cease; thus, the average kinetic energy of their particles would have to be the same. D At thermal equilibrium, the average kinetic energy of the FeFe atoms cannot be greater than that of the H2OH2O molecules; the average kinetic energies must be the same according to the definition of thermal equilibrium.
D At thermal equilibrium, the average kinetic energy of the FeFe atoms cannot be greater than that of the H2OH2O molecules; the average kinetic energies must be the same according to the definition of thermal equilibrium.
Which of the following phase changes involves the transfer of heat from the surroundings to the system? A CH4(g)→CH4(l)CH4(g)→CH4(l), because CH4CH4 molecules in the gas phase must absorb energy in order to move closer together, thereby increasing the intermolecular attractions in the solid state. B CO2(g)→CO2(s)CO2(g)→CO2(s), because CO2CO2 molecules in the gas phase must absorb energy in order to move closer together, thereby increasing the intermolecular attractions in the liquid state. C H2O(l)→H2O(s)H2O(l)→H2O(s), because H2OH2O molecules in the liquid phase must absorb energy in order to create a crystalline structure with strong intermolecular attractions in the solid state. D NH3(l)→NH3(g)NH3(l)→NH3(g), because NH3NH3 molecules in the liquid phase must absorb energy in order to overcome their intermolecular attractions and become free gas molecules.
D NH3(l)→NH3(g)NH3(l)→NH3(g), because NH3NH3 molecules in the liquid phase must absorb energy in order to overcome their intermolecular attractions and become free gas molecules.
The graphs above show Maxwell-Boltzmann distributions for one-mole samples of Ar(g)Ar(g). Graph 1 shows the distribution of particle energies at 300K300K and graph 2 shows the distribution of particle energies at 600K600K. A student predicts that if the samples are combined in an insulated container and thermal equilibrium is attained, then the most probable particle energy will be between the most probable energy shown in graph 1 and the most probable energy shown in graph 2. Which of the following is the best justification for the student's claim? A When the samples are combined, the gas particles will collide with one another, with the net effect being that the speed of the lowest energy particles decreases while the speed of the highest energy particles increases, leaving the average speed of the particles in the original samples unchanged. (graph 1 taller or more number of particles than graph 2) B When the samples are combined, the gas particles from each sample will collide with the gas particles from the other sample until every particle in the mixture has the same speed, which is between the average speed of the particles in the hotter sample and the average speed of the particles in the cooler sample. C When the samples are combined, the gas particles collide with one another until every particle in the mixture has the same kinetic energy, which is between the average kinetic energy of the particles in the hotter sample and the average kinetic energy of the particles in the cooler sample. D When the samples are combined, the gas particles will collide with one another, with the net effect being that energy will be transferred from the more energetic particles to the less energetic particles until a new distribution of energies is achieved at a temperature between 300K300K and 600K600K.
D When the samples are combined, the gas particles will collide with one another, with the net effect being that energy will be transferred from the more energetic particles to the less energetic particles until a new distribution of energies is achieved at a temperature between 300K300K and 600K600K.
(mass of solid: 1 g; mass of water:2000 g; ΔTwater: +2.1ºC) A sample of a solid organic compound is completely combusted in a calorimeter. The heat generated by combustion is transferred to the water, causing the temperature of the water to increase. Based on the data in the table above, which of the following is the best estimate of the heat of combustion, ΔHcomb, of the organic solid? (The specific heat of water is 4.2J/(g⋅°C).) A +18kJ/g B +8.4kJ/g C −8.4kJ/g D −18kJ/g
D −18kJ/g
The enthalpy change for the reaction 2Al(s)+Fe2O3(s)→2Fe(s)+Al2O3(s) is −860kJ/mol. Based on the standard enthalpies of formation ΔHf° provided in the table, what is the approximate ΔHf° for Fe2O3(s) ? A +2540kJ/mol B −2540kJ/mol C +820kJ/mol D −820kJ/mol
D −820kJ/mol
½ H2(g) + ½ I2(s) → HI(g) ∆H = 26 kJ/molrxn ½ H2(g) + ½ I2(g) → HI(g) ∆H = -5.0 kJ/molrxn Based on the information above, what is the enthalpy change for the sublimation of iodine, represented below? I2(s) → I2(g) A 15 kJ/molrxn B 21 kJ/molrxn C 31 kJ/molrxn D 42 kJ/molrxn E 62 kJ/molrxn
E 62 kJ/molrxn
CS2(l)+2H2O(l)→CO2(g)+2H2S(g)ΔH°rxn=? Which of the following combinations represents the individual reactions and the quantities needed to determine ΔH° for the overall reaction represented by the chemical equation above? A CS2(l)+3O2(g)→CO2(g)+2SO2(g) ΔH°=−1075kJ // 2H2O(l)+2SO2(g)→2H2S(g)+3O2(g) ΔH°=+1136kJ B CS2(l)+3O2(g)→CO2(g)+2SO2(g) ΔH°=−1075kJ // H2O(l)+SO2(g)→H2S(g)+3/2O2(g) ΔH°=+568kJ C CS2(l)+3O2(g)→CO2(g)+2SO2(g) ΔH°=−1075kJ //H2S(g)+3/2O2(g)→H2O(l)+SO2(g) ΔH°=−568kJ D CS2(l)+3O2(g)→CO2(g)+2SO2(g) ΔH°=−1075kJ //2H2S(g)+3O2(g)→2H2O(l)+2SO2(g) ΔH°=−1136kJ
A CS2(l)+3O2(g)→CO2(g)+2SO2(g) ΔH°=−1075kJ // 2H2O(l)+2SO2(g)→2H2S(g)+3O2(g) ΔH°=+1136kJ
A student adds 50.0g50.0g of liquid water at 25.0°C25.0°C to an insulated container fitted with a temperature probe. The student then adds 10.0g10.0g of ice at 0.0°C0.0°C, closes the container, and measures the temperature at different intervals. Part of the data is shown in the graph above. The student predicts that the temperature will continue to decrease then level out to a constant temperature. Which of the following best explains why the student's prediction is correct? (graph shows that as time goes on, temperature decreases) A The H2OH2O molecules initially in the ice and the molecules initially in the liquid will have the same average kinetic energy. B The transfer of energy between the H2OH2O molecules in the ice and liquid water stops once all the molecules are in the liquid phase. C Once all of the H2OH2O molecules are in the liquid phase, the individual molecular speeds either increase or decrease until all the particles have the same speed. D Once all of the H2OH2O molecules are in the liquid phase, collisions between them virtually stop as they reach an equilibrium distance from their neighboring molecules.
A The H2OH2O molecules initially in the ice and the molecules initially in the liquid will have the same average kinetic energy.
When a 50.mL50.mL sample of C2H5OHC2H5OH is mixed with a 50.mL50.mL sample of H2OH2O, the resulting mixture has a volume of 95mL95mL, and the container is warm to the touch. Which of the following best describes these observations? A A chemical reaction occurs, as evidenced by the volume of the resultant mixture being less than the total volume of the initial components. B A chemical change occurs, as evidenced by the formation of new covalent bonds releasing more energy than is absorbed by the breaking of the existing covalent bonds. C A physical change occurs, and the solvation process is exothermic. D A physical change occurs, and the solvation process is endothermic.
C A physical change occurs, and the solvation process is exothermic.
Which of the following best helps to explain why the value of ΔH° for the dissolving of CaF2 in water is positive? A CaF2(s) is insoluble in water. B CaF2(s) dissolves in water to form CaF2(aq) particles. C Ca2+ ions have very strong ion-ion interactions with F- ions in the crystal lattice. D Ca2+ ions have very strong ion-dipole interactions with water molecules in the solution.
C Ca2+ ions have very strong ion-ion interactions with F- ions in the crystal lattice.
Which of the following is a list of the minimum amount of data needed for determining the molar enthalpy of solution of KCl(s)KCl(s) in pure H2O(l)H2O(l) ? (Assume that the KCl(aq)KCl(aq) has the same specific heat capacity as pure water and that the initial temperatures of the KCl(s)KCl(s) and the water are the same.) A Mass of KCl(s)KCl(s), initial temperature of the water, and final temperature of the solution B Mass of H2OH2O, initial temperature of the water, and final temperature of the solution C Mass of KCl(s)KCl(s), mass of H2OH2O, initial temperature of the water, and final temperature of the solution D Mass of KCl(s)KCl(s), mass of H2OH2O, initial temperature of the water, final temperature of the solution, and atmospheric pressure
C Mass of KCl(s)KCl(s), mass of H2OH2O, initial temperature of the water, and final temperature of the solution
4 NH3(g) + 3 O2(g) → 2 N2(g) + 6 H2O(g) If the standard molar heats of formation of ammonia, NH3(g), and gaseous water, H2O(g), are -46 kJ/mol and -242 kJ/mol, respectively, what is the value of ΔH°298 for the reaction represented above? A -190 kJ/molrxn B -290 kJ/molrxn C -580 kJ/molrxn D -1,270 kJ/molrxn E -1,640 kJ/molrxn
D -1,270 kJ/molrxn
A 1.0mol sample of He(g) at 25°C is mixed with a 1.0mol sample of Xe(g) at 50°C. Which of the following correctly predicts the changes in average kinetic energy and the average speed of the Xe(g) atoms that will occur as the mixture approaches thermal equilibrium? A Average Kinetic Energy of Xe atoms will increase; Average Speed of Xe Atoms will increase B Average Kinetic Energy of Xe atoms will increase; Average Speed of Xe Atoms will decrease C Average Kinetic Energy of Xe atoms will decrease; Average Speed of Xe Atoms will increase
D Average Kinetic Energy of Xe atoms will decrease; Average Speed of Xe Atoms will decrease
