Unit 6 Topic Q's

A student mixes 50mL of 1.0MHCl and 50mL of 1.0MNaOH in a coffee-cup calorimeter and observes the change in temperature until the mixture reaches thermal equilibrium. The initial and final temperatures (°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

Reaction 1: N2O4(g)→2NO2(g)ΔH1=+57.9kJReaction 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) A ΔH1+ΔH2 B ΔH1+(−ΔH2) C (−ΔH1)+ΔH2 D ΔH1+(2×ΔH2)

C. (−ΔH1)+ΔH2

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 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 3.01kJ of heat was absorbed to decrease the average speed of the water molecules in the liquid, which decreases the distance between molecules. B 6.02kJ of heat was absorbed to increase the number of hydrogen bonds between water molecules in the liquid compared to the solid. C 12.0kJ of heat was absorbed to decrease the polarity of the water molecules, which increases the density of the liquid compared to the solid. D 12.0kJ of heat was absorbed to overcome some of the hydrogen bonding forces holding the water molecules in fixed positions in the crystalline structure.

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

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, 57.1kJ of heat is released. If the experiment is repeated with 2.00MHCl(aq), how much heat would be released?

57.1 kJ

A student adds 50.0g of liquid water at 25.0°C to an insulated container fitted with a temperature probe. The student then adds 10.0g of ice at 0.0°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 H2O molecules initially in the ice and the molecules initially in the liquid will have the same average kinetic energy.

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

-820 kJ/mol

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°=−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°=−1075kJ2H2O(l)+2SO2(g)→2H2S(g)+3O2(g)ΔH°=+1136kJ

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-203 kJ

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

B. (−ΔH°1)+12(−ΔH°2)+(ΔH°3)+(ΔH°4)

In 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 q absorbed by the water reported with the appropriate number of significant figures? A 1600J B 1640J C 1642J D 1642.3J

B. 1640 J

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

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(1072kJmol)+(498kJmol)]−4(799kJmol).

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) 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)]

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

2H2S(g)+3O2(g)→2H2O(l)+2SO2(g)ΔH°=−1120kJ/molrxn Based 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. -2240 kJ

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

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. -77.2 kJ

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

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?

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) at 25°C is placed into H2O(l) at 75°C in an insulated container. A student predicts that when thermal equilibrium is reached, the Fe atoms, being more massive than the H2O molecules, will have a higher average kinetic energy than the H2O molecules. Which of the following best explains why the student's prediction is incorrect?

D. At thermal equilibrium, the average kinetic energy of the Fe atoms cannot be greater than that of the H2O 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?

D. NH3(l)→NH3(g), because NH3 molecules in the liquid phase must absorb energy in order to overcome their intermolecular attractions and become free gas molecules.

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?

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.

The graphs above show Maxwell-Boltzmann distributions for one-mole samples of Ar(g). Graph 1 shows the distribution of particle energies at 300K and graph 2 shows the distribution of particle energies at 600K. 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?

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.