Study GuidesExam QuestionsPast Papers
AP®ChemistryCollege BoardExam QuestionsUnit 9: Thermodynamics & ElectrochemistryEntropy

Entropy (College Board AP® Chemistry): Exam Questions

51 mins15 questions

Select a question set to download for
Entropy

Select an answer set to view for
Entropy

Medium
Hard
1a1 mark
q1-june-2019---ap-chemistry

The compound urea, H2NCONH2 , is widely used in chemical fertilizers. The complete Lewis electron-dot diagram for the urea molecule is shown above.

Identify the hybridization of the valence orbitals of the carbon atom in the urea molecule.

How did you do?

1b1 mark

Urea has a high solubility in water, due in part to its ability to form hydrogen bonds. A urea molecule and four water molecules are represented in the box below. Draw ONE dashed line ( ---- ) to indicate a possible location of a hydrogen bond between a water molecule and the urea molecule.

q1b1-june-2019---ap-chemistry

How did you do?

1c2 marks
q1b-june-2019---ap-chemistry

The dissolution of urea is represented by the equation above. A student determines that 5.39 grams of H2NCONH2 (molar mass 60.06 g/mol) can dissolve in water to make 5.00 mL of a saturated solution at 20°C.

Calculate the concentration of urea, in mol/L, in the saturated solution at 20.°C.

How did you do?

1d1 mark

The student also determines that the concentration of urea in a saturated solution at 25 oC is 19.8 M. Based on this information, is the dissolution of urea endothermic or exothermic? Justify your answer in terms of Le Chatelier’s principle.

How did you do?

1e2 marks
ma1d-june-2019---ap-chemistry

The equipment shown above is provided so that the student can determine the value of the molar heat of solution for urea. Knowing that the specific heat of the solution is 4.18 J/(g⋅°C), list the specific measurements that are required to be made during the experiment.

How did you do?

1f1 mark

 

S° (J/(mol⋅K))

H2NCONH2 (s)

104.6

H2NCONH2 (aq)

?

The entropy change for the dissolution of urea, ΔS°soln, is 70.1 J/(mol⋅K) at 25°C. Using the information in the table above, calculate the absolute molar entropy, S°, of aqueous urea.

How did you do?

1g1 mark

Using particle-level reasoning, explain why ΔS°soln is positive for the dissolution of urea in water.

How did you do?

1h1 mark

The student claims that ΔS° for the process contributes to the thermodynamic favorability of the dissolution of urea at 25°C. Use the thermodynamic information above to support the student’s claim.

How did you do?

Did this page help you?

2a2 marks

Answer the following questions about the element Si and some of its compounds.

The mass spectrum of a pure sample of Si is shown below.

q2a-june-2021---ap-chemistry

i) How many protons and how many neutrons are in the nucleus of an atom of the most abundant isotope of Si ?

ii) Write the ground-state electron configuration of Si.

How did you do?

2b1 mark

Two compounds that contain Si are SiO2 and SiH4.

At 161 K, SiH4  boils but SiO2 remains as a solid. Using principles of interparticle forces, explain the difference in boiling points.

How did you do?

2c1 mark

At high temperatures, SiH4  decomposes to form solid silicon and hydrogen gas.

Write a balanced equation for the reaction.

How did you do?

2d1 mark

A table of absolute entropies of some substances is given below.

Substance

S° (J/(mol·K))

H2 (g)

131

Si (s)  

18

SiH4 (g)

205

Explain why the absolute molar entropy of Si (s) is less than that of H2 (g).

How did you do?

2e1 mark

Calculate the value, in J/(mol·K), of ∆S° for the reaction.

How did you do?

2f1 mark

The reaction is thermodynamically favorable at all temperatures.

Explain why the reaction occurs only at high temperatures.

How did you do?

2g1 mark

A partial photoelectron spectrum of pure Si is shown below. On the spectrum, draw the missing peak that corresponds to the electrons in the 3p sublevel.

q2g-june-2021---ap-chemistry

How did you do?

2h1 mark

Using principles of atomic structure, explain why the first ionization energy of Ge is lower than that of Si.

How did you do?

2i1 mark

A single photon with a wavelength of 4.00 × 10−7 m is absorbed by the Si sample. Calculate the energy of the photon in joules.

How did you do?

Did this page help you?

3a2 marks

Consider the reaction:

CH4 (g) + 2O2 (g) → CO2 (g) + 2H2O (g)

Predict whether the entropy of the system increases, decreases, or remains constant. Justify your answer by comparing the number of moles of gaseous reactants and products.

How did you do?

3b1 mark

If water is produced as a liquid instead of a gas, how does this affect the entropy change? Explain your answer by referring to molecular motion and phase differences.

How did you do?

3c1 mark

Describe the entropy of a gaseous reaction system at equilibrium. Justify your answer by referring to energy distribution and molecular arrangement.

How did you do?

Did this page help you?

4a1 mark

The following reaction occurs at standard conditions:

2NO2 (g) → N2O4 (g)

Explain whether the reaction results in an increase or decrease in the number of possible molecular arrangements.

How did you do?

4b2 marks

The standard entropies (S∘S^\circS∘) at 298 K are provided for the species involved:

Species

S∘ (J mol−1 K−1)

NO2

240.1

N2​O4​

304.3

Calculate ΔS∘ for the reaction. Show your work.

How did you do?

4c1 mark

Predict how the entropy of the system changes if the temperature is increased. Justify your answer

How did you do?

Did this page help you?

1a2 marks

Consider the reaction:

CH4 (g) + 2O2 (g) → CO2 (g) + 2H2O (g)

Predict whether the entropy of the system increases, decreases, or remains constant. Justify your answer by comparing the symmetry, molecular motion, and energy-level distribution of the reactants and products.

How did you do?

1b1 mark

Discuss how the entropy change of this reaction would differ if one of the reactants were a liquid instead of a gas. Justify your answer by considering the initial states of matter.

How did you do?

1c1 mark

Explain how entropy changes if the system is compressed to half its original volume while remaining in the gaseous state.

How did you do?

Did this page help you?

2a2 marks

Is the entropy of O3 (g) higher, lower, or the same as the entropy of O2 (g) at the same temperature? Justify your answer

How did you do?

2b2 marks

Explain how the entropy of a system containing a mixture of O2 ​(g) and O3 ​(g) changes as the temperature increases.

How did you do?

Did this page help you?

3a2 marks

A student investigates the following reactions:

  1. H2O (l) → H2O (g)

  2. H2 (g) + I2 (s) → 2HI (g)

Predict the sign of ΔS for each reaction. Justify your predictions in terms of phase changes, molecular motion, and particle arrangement.

How did you do?

3b2 marks

Compare the magnitude of entropy change for Reaction 1 and Reaction 2.

i) Explain why the entropy change for Reaction 1 might be larger than for Reaction 2.

ii) Explain why the entropy change for Reaction 2 might be larger than for Reaction 1.

How did you do?

Did this page help you?

4a1 mark

Two ideal gases, He (g) and Ne (g), are mixed in a closed container.

Predict the sign of ΔS∘ for the mixing process. Justify your answer.

How did you do?

4b1 mark

Describe how the entropy change for the mixing process depends on the relative amounts of He (g) and Ne (g).

How did you do?

4c2 marks

Compare the entropy change when mixing He (g) and Ne (g) to the entropy change when expanding He (g) into a larger container. Explain your answer.

How did you do?

Did this page help you?