AP Chem Unit 6

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.

50 kJ of heat is transferred from the surroundings.

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

K(s)+ ½ Cl2(g) → KCl(s) ΔH° = -437kJ/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 ∆Ho for a process in the table is (are) less than zero (i.e., indicate(s) an exothermic process)?

y and z only

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?

−77.2kJ

The following questions refer to the graph below, which shows the heating curve for methane, CH4.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

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?

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 statement(s) characterize(s) a chemical system at equilibrium? I. The rate of the forward reaction is equal to the rate of the reverse reaction. II. The concentrations of the reactants and products are equal.

I only

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.

Which of the following best helps to explain why the value of ΔH° for the dissolving of CaF2 in water is positive?

Ca2+ ions have very strong ion-ion interactions with F- ions in the crystal lattice.

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?

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.

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−2°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 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.

The heating curve for a sample of pure ethanol is provided above. The temperature was recorded as a 50.0 g sample of solid ethanol was heated at a constant rate. Which of the following explains why the slope of segment T is greater than the slope of segment R?

The specific heat capacity of the gaseous ethanol is less than the specific heat capacity of liquid ethanol.

Consider the following equilibrium: N2O4(g) + heat ⇄ 2NO2(g) Initially, a 1.0 L container is filled with 2.0 mole of NO2. As the system approaches equilibrium, the rate of reaction of NO2 ___.

decreases and [N2O4] increases

Consider the following equilibrium: N2(g) + O2(g) ⇄ 2NO(g) Nitrogen gas and oxygen gas react when placed in a closed container. As the reaction proceeds towards equilibrium, the rate of the reverse reaction ___.

increases as the concentration of products increases

K(s) + 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

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

When 2.0 mol of O2 and 3.0 mol of N2 were placed in a 10.0 L container at 25 °C, the value of K = 0.90. If the same number of moles of reactant were placed in a 5.0 L container at 25 °C, the equilibrium constant would be ___.

0.90

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?

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.

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).

23.4°C

At a certain temperature, the value of the equilibrium constant, K, for the reaction written below is 2.0×105. What is the value of K for the reverse reaction at the same temperature? H2(g) + Br2(g) ⇄ 2HBr(g)

5.0×10^-6

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°C

A hot iron ball is dropped into a 200. g sample of water initially at 50°C. If 8.4 kJ of heat is transferred from the ball to the water, what is the final temperature of the water? (The specific heat of water is 4.2 J/(g·°C).)

60°C