AP Chemistry Unit 6 - AP Classroom

The diagram above represents the melting of H2O(s). A 2.00mole sample of H2O(s) at 0°C0°C melted, producing 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?

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

The cooling curve above shows how the temperature of a sample varies with time as the sample goes through phase changes. The sample starts as a gas, and heat is removed at a constant rate. At which time does the sample contain the most liquid?

t4

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

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?

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

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)

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

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

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.

Which of the following is a list of the minimum amount of data needed for determining the molar enthalpy of solution of KCl(s) in pure H2O(l)? (Assume that the KCl(aq) has the same specific heat capacity as pure water and that the initial temperatures of the KCl(s) and the water are the same.)

Mass of KCl(s), mass of H2O, initial temperature of the water, and final temperature of the solution

Which of the following best explains why the combustion reactions represented in the table are exothermic?

The energy required to break the bonds in the reactants is less than the energy released in forming the bonds in the products.

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?

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.

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.

C(s)+H2O(g)→CO(g)+H2(g)ΔH°=+131kJ/molrxn The reaction between C(s) and H2O(g) is represented by the balanced chemical equation above. Based on the enthalpy change of the reaction (ΔH°) and the standard heats of formation (ΔH°f) given in the table below, what is the approximate ΔH°f for CO(g)?

-111 kJ/mol

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)

-18 kJ/g

2H2O2(l)→2H2O(l)+O2(g) 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 mol2 mol of H2O2(l)?

-203 kJ

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 4.00mol4.00mol of H2S(g) reacts with 9.00mol of O2(g)?

-2240 kJ

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

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?

-77.2 kJ

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)?

-820 kJ/mol

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?

1640 J

CH4(g) + Cl(g) → CH3(g) + HCl(g) ΔH° = -14 kJ/molrxn NH3(g) + Cl(g) → NH2(g) + HCl(g) ΔH° = -36 kJ/molrxn H2O(g) + Cl(g) → OH(g) + HCl(g) ΔH° = + 40 kJ/molrxn Based on the data above, what can be concluded regarding the strength of the C-H, N-H, and O-H bonds in the molecules shown?

The O-H bond is the strongest.

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?

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.

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.

CH3OH9(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?

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.

A 30.g sample of Al(s) is heated to 50°C and placed in a calorimeter containing 150g of water at 20°C. As the system approaches thermal equilibrium, energy is transferred

from the Al(s) to the water and the temperature of the Al(s) decreases

AgNO3(aq) + NaC1(aq)→ AgC1(s) + NaNO3(aq) In an experiment a student mixes a 50.0 mL sample of 0.100 M AgNO3(aq) with a 50.0 mL sample of 0.100 M NaCl(aq) at 20.0°C in a coffee-cup calorimeter. Which of the following is the enthalpy change of the precipitation reaction represented above if the final temperature of the mixture is 21.0°C? (Assume that the total mass of the mixture is 100. g and that the specific heat capacity of the mixture is 4.2 J/(g °C).)

−84 kJ/molrxn

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)

(-ΔH1) + ΔH2

In an experiment, 30.0g of ethane and 30.0g of propanol are placed in separate reaction vessels. Each compound undergoes complete combustion with excess O2(g). Which of the following best compares the quantity of heat released in each combustion reaction?

(q)ethane > (q) propanol

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?

5.5 oC

The following questions relate to the below information. XY2 → X + Y2 The 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?

50 kJ of heat is transferred from the surroundings.