AP Chemistry Exam Unit 6

6.9 CS2(l)+2H2O(l)→CO2(g)+2H2S(g)ΔH°rxn=?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°ΔH° for the overall reaction represented by the chemical equation above? A)CS2(l)+3O2(g)→CO2(g)+2SO2(g) ΔH°=−1075kJ2H2O(l)+2SO2(g)→2H2S(g)+3O2(g)ΔH°=+1136kJ B)CS2(l)+3O2(g)→CO2(g)+2SO2(g)ΔH°=−1075kJH2O(l)+SO2(g)→H2S(g)+32O2(g)ΔH°=+568kJ C)CS2(l)+3O2(g)→CO2(g)+2SO2(g)ΔH°=−1075kJH2S(g)+32O2(g)→H2O(l)+SO2(g)ΔH°=−568kJ D)CS2(l)+3O2(g)→CO2(g)+2SO2(g)ΔH°=−1075kJ2H2S(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

6.6 2H2S(g)+3O2(g)→2H2O(l)+2SO2(g)ΔH°=−1120kJ/molrxnBased on the reaction represented by the chemical equation shown above, what is the amount of heat released when 4.00mol of H2S(g) reacts with 9.00mol of O2(g)? A)−560kJ B)−1120kJ C)−2240kJ D)−3360kJ

C −2240kJ

6.8 The enthalpy change for the reaction 2Al(s)+Fe2O3(s)→2Fe(s)+Al2O3(s)2Al(s)+Fe2O3(s)→2Fe(s)+Al2O3(s) is −860kJ/mol−860kJ/mol. Based on the standard enthalpies of formation ΔH∘fΔHf° provided in the table, what is the approximate ΔH∘fΔHf° for Fe2O3(s)Fe2O3(s) ? A)+2540kJ/mol+2540kJ/mol B)−2540kJ/mol−2540kJ/mol C)+820kJ/mol+820kJ/mol D)−820kJ/mol−820kJ/mol

C)+820kJ/mol+820kJ/mol

6.3 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? 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.

6.4 Mass of water 50.003gTemperature of water 24.95°CSpecific heat capacity for water 4.184J/g°CMass of metal 63.546gTemperature of metal 99.95°CSpecific heat capacity for metal??Final temperature 32.80°CIn an experiment to determine the specific heat of a metal, a student transferred a sample of the metal that was heated in boiling water into room-temperature water in an insulated cup. The student recorded the temperature of the water after thermal equilibrium was reached. The data are shown in the table above. Based on the data, what is the calculated heat qq absorbed by the water reported with the appropriate number of significant figures? A)1600J B)1640J C)1642J D)1642.3J

B 1640J

6.2 The following questions relate to the below information.XY2 → X + Y2The equation above represents the decomposition of a compound XY2. The diagram below shows two reaction profiles (path one and path two) for the decomposition of XY2. Which of the following best describes the flow of heat when 1.0 mol of XY2 decomposes? A)50 kJ of heat is transferred to the surroundings. B)50 kJ of heat is transferred from the surroundings. C)100 kJ of heat is transferred to the surroundings. D)100 kJ of heat is transferred from the surroundings.

B 50 kJ of heat is transferred from the surroundings.

6.6 HCl(aq)+NaOH(aq)→NaCl(aq)+H2O(l)ΔH°=−57.1kJ/molrxnThe 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, 57.1kJ of 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

6.7 The oxidation of carbon monoxide can be represented by the chemical equation 2 CO(g)+O2(g)→2 CO2(g). The table above provides the average bond enthalpies for different bond types. Based on the information in the table, which of the following mathematical expressions is correct for the estimated enthalpy change for the reaction? A)ΔHrxn=[2(1072kJmol)+(498kJmol)]−2(799kJmol) B)ΔHrxn=[2(1072kJmol)+(498kJmol)]−4(799kJmol) C)ΔHrxn=[2(799kJmol)+(142kJmol)]−4(360kJmol) D)ΔHrxn=[2(799kJmol)+(142kJmol)]−2(360kJmol)

B ΔHrxn=[2(1072kJ/mol)+(498kJ/mol)]−4(799kJ/mol)

6.8 Based on the information in the table above, which of the following expressions gives the approximate ΔH° for the reaction represented by the following balanced chemical equation?Fe2O3(s)+3CO(g)→2Fe(s)+3CO2(g)Fe2O3(s)+3CO(g)→2Fe(s)+3CO2(g) A)ΔH°rxn=[(0kJ/mol)+(−394kJ/mol)]−[(−826kJ/mol)+(−111kJ/mol)] B)ΔH°rxn=[2(0kJ/mol)+3(−394kJ/mol)]−[(−826kJ/mol)+3(−111kJ/mol)] C)ΔH°rxn=[(−826kJ/mol)+3(−111kJ/mol)]−[2(0kJ/mol)+3(−394kJ/mol)] D)ΔH°rxn=[(−826kJ/mol)+(−111kJ/mol)]−[(0kJ/mol)+(−394kJ/mol)]ΔH°rxn=[(−826kJ/mol)+(−111kJ/mol)]−[(0kJ/mol)+(−394kJ/mol)]

B ΔH°rxn=[2(0kJ/mol)+3(−394kJ/mol)]−[(−826kJ/mol)+3(−111kJ/mol)]

6.8 C(s)+H2O(g)→CO(g)+H2(g)ΔH°=+131kJ/molrxnC(s)+H2O(g)→CO(g)+H2(g)ΔH°=+131kJ/molrxnThe reaction between C(s)C(s) and H2O(g)H2O(g) is represented by the balanced chemical equation above. Based on the enthalpy change of the reaction (ΔH°ΔH°) and the standard heats of formation (ΔH°fΔHf°) given in the table below, what is the approximate ΔH°fΔHf° for CO(g)CO(g) ?Substance ΔH° (kJ/mol)C(s) 0H2O(g) −242CO(g) ??H2(g) 0 A−373kJ/mol−373kJ/mol B)−111kJ/mol−111kJ/mol C)+111kJ/mol+111kJ/mol D)+373kJ/mol

B −111kJ/mol−111kJ/mol

6.7 Shown above are the equation representing the decomposition of H2O2(l) and a table of bond enthalpies. On the basis of the information, which of the following is the enthalpy of decomposition of 2 mol of H2O2(l) ? A)−349kJ B)−203kJ C)203kJ D)349kJ

B −203kJ

6.9 Reaction 1: N2O4(g)→2NO2(g)N2O4(g)→2NO2(g)ΔH1=+57.9kJ Reaction 2: 2NO(g)+O2(g)→2NO2(g)2NO(g)+O2(g)→2NO2(g)ΔH2=−113.1kJBased 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)2NO(g)+O2(g)→N2O4(g) A)ΔH1+ΔH2 B)ΔH1+(−ΔH2) C)(−ΔH1)+ΔH2 D)ΔH1+(2×ΔH2)

C (−ΔH1)+ΔH2

6.4 For an experiment, 50.0g50.0g of H2OH2O was added to a coffee-cup calorimeter, as shown in the diagram above. The initial temperature of the H2OH2O was 22.0°C22.0°C, and it absorbed 300.J300.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 H2OH2O after thermal equilibrium was reached? Assume that the specific heat capacity of H2OH2O is 4.2J/(g⋅K)4.2J/(g⋅K). A)21.3°C21.3°C B)22.0°C22.0°C C)22.7°C22.7°C D)23.4°C

D 23.4°C

6.3 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.

6.5 A sample of CHCl3(s) was exposed to a constant source of heat for a period of time. The graph above shows the change in the temperature of the sample as heat is added. Which of the following best describes what occurs at the particle level that makes segment D longer than segment B? A)The specific heat capacity of the liquid is significantly higher than that of the solid, because the particles in the liquid state need to absorb more thermal energy to increase their average speed. B)The specific heat capacity of the solid is significantly higher than that of the gas, because the particles in the solid state need to absorb more thermal energy to increase their average speed. C)The enthalpy of fusion is greater than the enthalpy of vaporization, because separating molecules from their bound crystalline state requires more energy than separating molecules completely from the liquid state. D)The enthalpy of vaporization is greater than the enthalpy of fusion, because separating molecules completely from the liquid to form a gas requires more energy than separating molecules from their bound crystalline state to a liquid state.

D The enthalpy of vaporization is greater than the enthalpy of fusion, because separating molecules completely from the liquid to form a gas requires more energy than separating molecules from their bound crystalline state to a liquid state.

6.3 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 300K300⁢K 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. 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 300K and 600K.

6.5 A 2.00mol sample of C2H5OH undergoes the phase transition illustrated in the diagram above. The molar enthalpy of vaporization, ΔHvap, of C2H5OH is +38.6kJ/mol. Which of the following best identifies the change in enthalpy in the phase transition shown in the diagram? A)+19.3kJ B)+77.2kJ C)−19.3kJ D)−77.2kJ

D −77.2kJ

6.1 In the spring, blossoms on cherry trees can be damaged when temperatures fall below −2°C. When the forecast calls for air temperatures to be below −5°C for a few hours one night, a farmer sprays his blossoming cherry trees with water, claiming that the blossoms will be protected by the water as it freezes. Which of the following is a correct scientific justification for spraying water on the blossoms to protect them from temperatures below −2°C? A)Water on the blossoms will not freeze unless the air temperature falls significantly below −5°C−5°C. B)Water is a good thermal conductor that transfers heat from the cold air to the blossoms, keeping the blossoms from going below −2°C−2°C. C)The freezing of water is an endothermic process; thus, water that freezes on the blossoms absorbs heat from the atmosphere, which in turn keeps the blossoms above 0°C0°C. D) The freezing of water is an exothermic process; thus, water that freezes on the blossoms releases heat to keep the blossoms at or above −2°C−2°C.

D) The freezing of water is an exothermic process; thus, water that freezes on the blossoms releases heat to keep the blossoms at or above −2°C−2°C.

6.1 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.

6.2 e) Based on the data, the student claims that the catalyzed reaction has zeroth-order kinetics. Do you agree with the student's claim? Justify your answer.

The response does meet both of the following criteria. The response indicates "disagree". The response indicates that if the reaction were zeroth-order, the rate of mass loss (which corresponds to the rate of reaction) would be constant until the reactant was consumed, but the data show that the rate of mass loss from beaker 2 is not constant.

6.2 f) A second student did the experiment using larger volumes of 9.77MH2O2(aq)9.77MH2O2(aq) . The student noticed that the reaction in beaker 2 proceeded extremely rapidly, causing some of the liquid to splash out of the beaker onto the lab table. The student claims that as a result of the loss of the liquid from the beaker, the calculated number of moles of O2(g)O2(g) produced is greater than the actual number of moles of O2(g)O2(g) produced during the first 60 seconds. Do you agree with the student? Justify your answer.

The response does meet both of the following criteria. The response indicates agreement. The response indicates that the calculated amount of O2 produced is based on the change in mass, and in this case some of the change in mass is due to loss of solution rather than production of O2

6.2 g) The hydrogen peroxide used in this experiment can be prepared by the reaction of solid ammonium peroxydisulfate, (NH4)2S2O8(NH4)2S2O8, with water. The products are hydrogen peroxide (H2O2)(H2O2) and ammonium bisulfate (NH4HSO4)(NH4HSO4). Write the balanced equation for the reaction.

The response does not give one of the following equations. (NH4)2S2O8+2 H2O→H2O2+2 NH4HSO4 (NH4)2S2O8+2 H2O→H2O2+2 NH4++2 HSO4− S2O82−+2 H2O→H2O2+2 HSO4−

6.3 A student did an experiment to determine the specific heat capacity of a metal alloy. The student put a sample of the alloy in boiling water for several minutes, then quickly transferred the alloy into a calorimeter containing water originally at 25°C25°C. The temperature of the water was monitored over time. The data are given in the graph above. a) What is the value of ΔTΔT that the student should use to calculate the value of qq, the heat gained by the water?

The response does not indicate that ΔT is 1.4C

6.2 (ii) Manganese dioxide, MnO2(s)MnO2(s), is an insoluble substance that acts as a catalyst for the decomposition reaction. On the diagram above, draw a curve to represent the reaction as it occurs in the presence of MnO2(s)MnO2(s).

The response does show a curve for the catalyzed reaction that has the reactants and products at the same energy levels as the original curve and that has a lower activation energy than the original curve.

6.2 A student investigates the decomposition reaction in the laboratory. The student prepares two small beakers, adding 20.0mL20.0mL of 9.77MH2O2(aq)9.77MH2O2(aq) to each one. Each beaker is placed on an electronic balance. The student adds 0.10g0.10g of MnO2(s)MnO2(s) to the second beaker and records the mass of each beaker and its contents at 10-second intervals for one minute. The beakers and the data are shown below. (c) For beaker 2 during the 60-second period, calculate the following. (ii) The mass of H2O2(aq)H2O2(aq) that decomposed

The response gives a calculation equivalent to the following. mass H2O2=0.019 mol O2×2 mol H2O21 mol O2×34.02 g H2O21 mol H2O2=1.3 g H2O2

6.2 A student investigates the decomposition reaction in the laboratory. The student prepares two small beakers, adding 20.0mL20.0mL of 9.77MH2O2(aq)9.77MH2O2(aq) to each one. Each beaker is placed on an electronic balance. The student adds 0.10g0.10g of MnO2(s)MnO2(s) to the second beaker and records the mass of each beaker and its contents at 10-second intervals for one minute. The beakers and the data are shown below. (c) For beaker 2 during the 60-second period, calculate the following. (i) The number of moles of O2(g)O2(g) that was produced

The response gives calculations equivalent to the following. mass O2=43.19 g−42.58 g=0.61 g moles O2=0.61 g O2×1 mol O232.00 g=0.019 mol O2

6.3 A student did an experiment to determine the specific heat capacity of a metal alloy. The student put a sample of the alloy in boiling water for several minutes, then quickly transferred the alloy into a calorimeter containing water originally at 25°C25°C. The temperature of the water was monitored over time. The data are given in the graph above. c) The student claims that thermal equilibrium is reached at time tt. Justify the student's claim. In your justification, include a description of what occurs at the particulate level when the alloy and the water have reached thermal equilibrium.

The response indicates both of the following criteria: The claim is correct because the temperature levels off at that time. At equilibrium the average kinetic energy of the atoms in the alloy is equal to the average kinetic energy of the water molecules.

6.4 A student did an experiment to determine the specific heat capacity of a metal alloy. The student put a sample of the alloy in boiling water for several minutes, then quickly transferred the alloy into a calorimeter containing water originally at 25°C25°C. The temperature of the water was monitored over time. The data are given in the graph above. (c) The student claims that thermal equilibrium is reached at time tt. Justify the student's claim. In your justification, include a description of what occurs at the particulate level when the alloy and the water have reached thermal equilibrium.

The response indicates both of the following criteria: The claim is correct because the temperature levels off at that time. At equilibrium the average kinetic energy of the atoms in the alloy is equal to the average kinetic energy of the water molecules.

6.3 A student did an experiment to determine the specific heat capacity of a metal alloy. The student put a sample of the alloy in boiling water for several minutes, then quickly transferred the alloy into a calorimeter containing water originally at 25°C25°C. The temperature of the water was monitored over time. The data are given in the graph above. (b) In terms of what occurs at the particulate level, explain how the temperature of the water increases after the alloy sample is added.

The response indicates that the atoms in the alloy transfer kinetic energy to the molecules in the water.

6.4 A student did an experiment to determine the specific heat capacity of a metal alloy. The student put a sample of the alloy in boiling water for several minutes, then quickly transferred the alloy into a calorimeter containing water originally at 25°C25°C. The temperature of the water was monitored over time. The data are given in the graph above. (b) In terms of what occurs at the particulate level, explain how the temperature of the water increases after the alloy sample is added.

The response indicates that the atoms in the alloy transfer kinetic energy to the molecules in the water.

6.8 C2H5OH(l)+3O2(g)→2CO2(g)+3 H2O(l)ΔH∘2=−1370kJ/molrxnC2H5OH(l)+3⁢O2(g)→2CO2(g)+3 H2O(l)ΔH2∘=−1370kJ/molrxn (c) Based on the information above, identify the ΔH∘ΔH∘ values that should be added together to determine the value of ΔH∘rxnΔHrxn∘ for the following reaction.

The response indicates that the number calculated in part (a) and 1370 kJ/molrxn should be added together.

6.3 A student did an experiment to determine the specific heat capacity of a metal alloy. The student put a sample of the alloy in boiling water for several minutes, then quickly transferred the alloy into a calorimeter containing water originally at 25°C25°C. The temperature of the water was monitored over time. The data are given in the graph above.(a) What is the value of ΔTΔT that the student should use to calculate the value of qq, the heat gained by the water?

The response indicates that ΔT = 1.4 C

6.2 Shown below is a potential energy diagram for the uncatalyzed decomposition of H2O2(aq)H2O2(aq). ) According to the diagram, is the decomposition reaction exothermic or endothermic? Justify your answer. ( Diagram goes high to low )

The response meets both of the following criteria. The response indicates that the reaction is exothermic. The response gives a justification such as the following. The products have lower potential energy than the reactants. Thus, the system has lost potential energy, releasing heat.

6.2 2H2O2(aq)→2H2O(l)+O2(g)2H2O2(aq)→2H2O(l)+O2(g)Hydrogen peroxide, H2O2H2O2, decomposes according to the equation above. This reaction is thermodynamically favorable at room temperature. (a) A particulate representation of the reactants is shown below in the box on the left. In the box below on the right, draw the particulate representation of all the molecules that would be produced from these four reactant molecules.

The response meets both of the following criteria. The diagram shows H2O and O moleculesThe diagram shows atom balance.

6.2 d) The student continues the experiment for an additional minute. For beaker 2, will the mass of H2O2(aq)H2O2(aq) consumed during the second minute be greater than, less than, or equal to the mass of H2O2(aq)H2O2(aq) consumed during the first minute? Explain your answer referring to the data in the table.

The response meets both of the following criteria. The response indicates that the mass of H2O2(aq)consumed during the second minute would be less than the mass of H2O2(aq) consumed during the first minute. The response indicates that the decrease in mass becomes smaller over every subsequent 10-second interval and this trend would continue in the second minute.

6.8 (b) The energy of the reactants is shown on the energy diagram above. On the right side of the energy diagram, draw a horizontal line segment to indicate the energy of the products. Draw a vertical line segment to indicate ΔHΔH for the reaction, and label it with the correct value.

The response meets both of the following criteria: There is a horizontal line on the right side of the diagram that is lower than the horizontal line given on the left [but it should be higher to earn the point if a positive number was calculated in part (a)]. There is a vertical line with one end at the level of the horizontal line given on the left and one end at the level of the horizontal line on the right, and it is labeled with the number calculated in part (a) (or its absolute value).

6.8 The combustion of C2H4(g)C2H4(g) is represented by the equation above. (a) Use the enthalpies of formation in the table below to calculate the value of ΔH for the reaction.

ΔH∘=ΣΔH∘f products−ΣΔH∘f reactants ΔH∘=2(−394 kJ/mol)+2(−286 kJ/mol)−(52 kJ/mol)=−1412 kJ/molrxn

6.4 A student mixes 50mL50mL of 1.0MHCl1.0MHCl and 50mL50mL of 1.0MNaOH1.0MNaOH in a coffee-cup calorimeter and observes the change in temperature until the mixture reaches thermal equilibrium. The initial and final temperatures (°C)(°C) of the mixture are shown in the diagram above of the laboratory setup.Based on the results, what is the change in temperature reported with the correct number of significant figures? A)5.5°C B)5.50°C C)5.800°C D)6°C

A 5.5°C

6.6 Mg(s)+2HCl(aq)→MgCl2(aq)+H2(g)The chemical equation shown above represents the reaction between Mg(s) and HCl(aq). When 12.15g of Mg(s) is added to 500.0mL of 4.0MHCl(aq), 95kJ of heat is released. The experiment is repeated with 24.30g of Mg(s) and 500.0mL of 4.0MHCl(aq). Which of the following gives the correct value for the amount of heat released by the reaction? A)380kJ B)190kJ C)95kJ D)48KJ

B 190kJ

6.9 (1)2H(g)→H2(g)2H(g)→H2(g)ΔH°1=−436kJΔH°1=−436kJ(2)2O(g)→O2(g)2O(g)→O2(g)ΔH°2=−249kJΔH°2=−249kJ(3)2H(g)+O(g)→H2O(g)2H(g)+O(g)→H2O(g)ΔH°3=−803kJΔH°3=−803kJ(4)H2O(g)→H2O(l)H2O(g)→H2O(l)ΔH°4=−44kJΔH°4=−44kJ(5)H2(g)+12O2(g)→H2O(l)H2(g)+12O2(g)→H2O(l)ΔH°f=?ΔHf°=? Based on the chemical equations and their associated enthalpy changes shown above, which of the following identifies the quantities needed to calculate ΔH°fΔHf°, the standard enthalpy of formation of H2O(l)H2O(l), in kJ/molkJ/mol? A)2(−ΔH°1)+(−ΔH°2)+2(ΔH°3)+2(ΔH°4)2(−ΔH°1)+(−ΔH°2)+2(ΔH°3)+2(ΔH°4) B)(−ΔH°1)+12(−ΔH°2)+(ΔH°3)+(ΔH°4)(−ΔH°1)+12(−ΔH°2)+(ΔH°3)+(ΔH°4) C)(−ΔH°1)+1/2(−ΔH°2)+(ΔH°3)+(ΔH°4) D)(ΔH°1)+12(ΔH°2)+(ΔH°3)+(ΔH°4)

C (−ΔH°1)+1/2(−ΔH°2)+(ΔH°3)+(ΔH°4)

6.7 An equation representing the dissociation of O2(g) and a table of bond enthalpies are shown above. Based on the information, which of the following is the enthalpy of dissociation for O2(g) ? A)−641kJ/mol B)−495kJ/mol C)495kJ/mol D)641kJ/mol

C 495kJ/moL

6.5 The diagram above represents the melting of H2O(s). A 2.00mole sample of H2O(s) at 0°C melted, producing H2O(l) at 0°C. Based on the diagram, which of the following best describes the amount of heat required for this process and the changes that took place at the molecular level? A)3.01kJ3.01kJ of heat was absorbed to decrease the average speed of the water molecules in the liquid, which decreases the distance between molecules. B)6.02kJ6.02kJ of heat was absorbed to increase the number of hydrogen bonds between water molecules in the liquid compared to the solid. C)12.0kJ12.0kJ of heat was absorbed to decrease the polarity of the water molecules, which increases the density of the liquid compared to the solid. D)12.0kJ12.0kJ of heat was absorbed to overcome some of the hydrogen bonding forces holding the water molecules in fixed positions in the crystalline structure.

D 12.0kJ12.0kJ of heat was absorbed to overcome some of the hydrogen bonding forces holding the water molecules in fixed positions in the crystalline structure.

6.1 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), 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), because NH3NH3 molecules in the liquid phase must absorb energy in order to overcome their intermolecular attractions and become free gas molecules.

6.4 A student did an experiment to determine the specific heat capacity of a metal alloy. The student put a sample of the alloy in boiling water for several minutes, then quickly transferred the alloy into a calorimeter containing water originally at 25°C25°C. The temperature of the water was monitored over time. The data are given in the graph above. (a) What is the value of ΔTΔT that the student should use to calculate the value of qq, the heat gained by the water?

The response indicates that ΔT is 1.4C