Thermochemistry Practice Test 2020
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?
5.5°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?
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 molecular solid coexists with its liquid phase at its melting point. The solid-liquid mixture is heated, but the temperature does not change while the solid is melting. The best explanation for this phenomenon is that the heat absorbed by the mixture
is used in overcoming the intermolecular attractions in the solid
The following questions relate to the graph below. The graph shows the temperature of a pure substance as it is heated at a constant rate in an open vessel at 1.0 atm pressure. The substance changes from the solid to the liquid to the gas phase. The substance is at its normal freezing point at time
t2
Based on the information in the table above, which of the following expressions gives the approximate ΔH°ΔH° for the reaction represented by the following balanced chemical equation? Fe2O3(s)+3CO(g)→2Fe(s)+3CO2(g)
ΔH°rxn=[2(0kJ/mol)+3(−394kJ/mol)]−[(−826kJ/mol)+3(−111kJ/mol)]ΔH°rxn=[2(0kJ/mol)+3(−394kJ/mol)]−[(−826kJ/mol)+3(−111kJ/mol)]
2H2S(g)+3O2(g)→2H2O(l)+2SO2(g)ΔH°=−1120kJ/molrxn2H2S(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 4.00mol4.00mol of H2S(g)H2S(g) reacts with 9.00mol9.00mol of O2(g)O2(g)?
−2240kJ
A 2.00mol2.00mol sample of C2H5OHC2H5OH undergoes the phase transition illustrated in the diagram above. The molar enthalpy of vaporization, ΔHvapΔHvap, of C2H5OHC2H5OH is +38.6kJ/mol+38.6kJ/mol. Which of the following best identifies the change in enthalpy in the phase transition shown in the diagram?
−77.2kJ
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) ?
−820kJ/mol−820kJ/mol
Reaction 1:N2O4(g)→2NO2(g)N2O4(g)→2NO2(g)ΔH1=+57.9kJΔH1=+57.9kJReaction 2:2NO(g)+O2(g)→2NO2(g)2NO(g)+O2(g)→2NO2(g)ΔH2=−113.1kJΔH2=−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)
(−ΔH1)+ΔH2
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 ?
-46 kJ/mol
How much energy is required to melt 64 g of methane at 90 K? (The molar mass of methane is 16 g/mol.)
3.8 kJ
HCl(aq)+NaOH(aq)→NaCl(aq)+H2O(l)ΔH°=−57.1kJ/molrxnHCl(aq)+NaOH(aq)→NaCl(aq)+H2O(l)ΔH°=−57.1kJ/molrxn The chemical equation above represents the reaction between HCl(aq)HCl(aq) and NaOH(aq)NaOH(aq). When equal volumes of 1.00MHCl(aq)1.00MHCl(aq) and 1.00MNaOH(aq)1.00MNaOH(aq) are mixed, 57.1kJ57.1kJ of heat is released. If the experiment is repeated with 2.00MHCl(aq)2.00MHCl(aq), how much heat would be released?
57.1kJ
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?
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.
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?
The H2OH2O molecules initially in the ice and the molecules initially in the liquid will have the same average kinetic energy.
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).)
The combustion of 1.0 g of cashew releases more energy than the combustion of 1.0 g of marshmallow.
A sample of CHCl3(s)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?
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.
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?
The metal temperature changed more than the water temperature did, but the metal lost the same amount of thermal energy as the water gained.
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?
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.
K(s) + 12MathType@MTEF@5@5@+= feaagKart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq=Jc9 vqaqpepm0xbba9pwe9Q8fs0=yqaqpepae9pg0FirpepeKkFr0xfr=x fr=xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaWaaSaaaeaaca aIXaaabaGaaGOmaaaaaaa@377D@ Cl2(g) → KCl(s) ΔH° = −437 kJ/molrxn The elements K and Cl react directly to form the compound KCl according to the equation above. Refer to the information above and the table below to answer the questions that follow. Which of the values of ΔH° for a process in the table is (are) less than zero (i.e., indicate(s) an exothermic process) ?
y and z only
A → X The enthalpy change for the reaction represented above is ΔHT. This reaction can be broken down into a series of steps as shown in the diagram: A relationship that must exist among the various enthalpy changes is
ΔHT - ΔH1 - ΔH2 - ΔH3 = 0
C(s)+H2O(g)→CO(g)+H2(g)ΔH°=+131kJ/molrxnC(s)+H2O(g)→CO(g)+H2(g)ΔH°=+131kJ/molrxn The 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) ? SubstanceΔH°fΔHf° (kJ/molkJ/mol)C(s)C(s)0H2O(g)H2O(g)−242−242CO(g)CO(g)??H2(g)H2(g)0
−111kJ/mol
(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?
(−ΔH°1)+12(−ΔH°2)+(ΔH°3)+(ΔH°4)(−ΔH°1)+12(−ΔH°2)+(ΔH°3)+(ΔH°4)
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;)
-607 kJ
The diagram above represents the melting of H2O(s)H2O(s). A 2.00mole2.00mole sample of H2O(s)H2O(s) at 0°C0°C melted, producing H2O(l)H2O(l) at 0°C0°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?
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.
Mass of water50.003g50.003gTemperature of water24.95°C24.95°CSpecific heat capacity for water4.184J/g°C4.184J/g°CMass of metal63.546g63.546gTemperature of metal99.95°C99.95°CSpecific heat capacity for metal??Final temperature32.80°C32.80°C 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 qq absorbed by the water reported with the appropriate number of significant figures?
1640J
Mg(s)+2HCl(aq)→MgCl2(aq)+H2(g)Mg(s)+2HCl(aq)→MgCl2(aq)+H2(g) The chemical equation shown above represents the reaction between Mg(s)Mg(s) and HCl(aq)HCl(aq). When 12.15g12.15g of Mg(s)Mg(s) is added to 500.0mL500.0mL of 4.0MHCl(aq)4.0MHCl(aq), 95kJ95kJ of heat is released. The experiment is repeated with 24.30g24.30g of Mg(s)Mg(s) and 500.0mL500.0mL of 4.0MHCl(aq)4.0MHCl(aq). Which of the following gives the correct value for the amount of heat released by the reaction?
190kJ
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).
23.4°C
In an insulated cup of negligible heat capacity, 50. g of water at 40.°C is mixed with 30. g of water at 20.°C. The final temperature of the mixture is closest to
33°C
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?
CS2(l)+3O2(g)→CO2(g)+2SO2(g)CS2(l)+3O2(g)→CO2(g)+2SO2(g)ΔH°=−1075kJΔH°=−1075kJ2H2O(l)+2SO2(g)→2H2S(g)+3O2(g)2H2O(l)+2SO2(g)→2H2S(g)+3O2(g)ΔH°=+1136kJΔH°=+1136kJ
When water is added to a mixture of Na2O2(s) and S (s) , a redox reaction occurs, as represented by the equation below. Two trials are run, using excess water. In the first trial, 7.8 g of Na2O2(s) (molar mass 78 g/mol) is mixed with 3.2 g of S(s). In the second trial, 7.8 g of Na2O2(s) is mixed with 6.4 g of S(s). The Na2O2(s) and S(s) react as completely as possible. Both trials yield the same amount of SO2(aq). Which of the following identifies the limiting reactant and the heat released, q, for the two trials at 298 K?
Limiting ReactantqNa2O2 30. kJ
Which of the following phase changes involves the transfer of heat from the surroundings to the system?
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 dissolution of an ionic solute in a polar solvent can be imagined as occurring in three steps, as shown in the figure above. In step 1, the separation between ions in the solute is greatly increased, just as will occur when the solute dissolves in the polar solvent. In step 2, the polar solvent is expanded to make spaces that the ions will occupy. In the last step, the ions are inserted into the spaces in the polar solvent. Which of the following best describes the enthalpy change, ΔH, for each step?
Steps 1 and 2 are endothermic, and the final step is exothermic.
In the spring, blossoms on cherry trees can be damaged when temperatures fall below −2°C−2°C. When the forecast calls for air temperatures to be below −5°C−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−2°C?
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.
NaOH(aq) + HCl(aq) → H2O(l) + NaCl(aq) A student is trying to determine the heat of reaction for the acid-base neutralization reaction represented above. The student uses 0.50 M NaOH and 0.50 M HCl solutions. Which of the following situations, by itself, would most likely result in the LEAST error in the calculated value of the heat of reaction?
The thermometer was incorrectly calibrated and read 0.5 Celsius degree too high during the procedure.