AP Chemistry Unit 6 Review

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 less massive H2O molecules would have a higher average kinetic energy than the Fe atoms because they are more free to move than the Fe atoms.

• At thermal equilibrium, the collisions between the Fe atoms and the H2O molecules would cease because the average kinetic energies of their particles would have become the same.

• At thermal equilibrium, the movement of both the Fe atoms and the H2O molecules would cease; thus, the average kinetic energy of their particles would have to be the same.

• 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.

Tags

NGSS.HS-PS3-4

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

• 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.

• 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.

• 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 300 K and 600 K.

Tags

NGSS.HS-PS1-5

NGSS.HS-PS3-2

NGSS.HS-PS3-4

A student adds 50.0 g of liquid water at 25.0°C to an insulated container fitted with a temperature probe. The student then adds 10.0 g 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?

• The H2O molecules initially in the ice and the molecules initially in the liquid will have the same average kinetic energy.

• The transfer of energy between the H2O molecules in the ice and liquid water stops once all the molecules are in the liquid phase.

• Once all of the H2O molecules are in the liquid phase, the individual molecular speeds either increase or decrease until all the particles have the same speed.

• Once all of the H2O molecules are in the liquid phase, collisions between them virtually stop as they reach an equilibrium distance from their neighboring molecules.

Tags

NGSS.HS-PS3-4

For an experiment, 50.0 g 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 4.2 J/(g · K).

• 21.3°C

• 22.0°C

• 22.7°C

• 23.4°C

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NGSS.HS-PS3-1

NGSS.HS-PS3-4

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 absorbed by the water reported with the appropriate number of significant figures?

• 1600 J

• 1640 J

• 1642 J

• 1642.3 J

Tags

NGSS.HS-PS3-2

NGSS.HS-PS3-4

A student mixes 50 mL of 1.0 M HCl and 50 mL of 1.0 M NaOH 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?

Tags

NGSS.HS-PS3-4

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 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.

• 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.

• 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.

• 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.

Tags

NGSS.HS-PS1-4

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.)

• 0.24 kJ

• 3.8 kJ

• 33 kJ

• 60 kJ

Tags

NGSS.HS-PS3-4

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

• +19.3 kJ

• +77.2 kJ

• −19.3 kJ

• -77.2 kJ

Tags

NGSS.HS-PS1-4

NGSS.HS-PS3-1

The substance is at its normal freezing point at time

• t1

• t2

• t3

• t4

• t5

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NGSS.HS-PS1-3

NGSS.HS-PS3-2