Entropy (College Board AP® Chemistry): Exam Questions

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

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

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.

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.

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.

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.

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.

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

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.

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.

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.

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.

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

Write a balanced equation for the reaction.

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

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

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

The reaction is thermodynamically favorable at all temperatures.

Explain why the reaction occurs only at high temperatures.

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.

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

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.

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.

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.

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

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.

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

Calculate ΔS^∘ for the reaction. Show your work.

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

A student investigates the entropy change for the following process:

N₂O₄ (g) ⇌ 2NO₂ (g)

The student collects the following data:

Predict the sign of ΔS^∘ for the forward reaction. Explain your answer in terms of the number of gas molecules.

Calculate the value of ΔS^∘ for the forward reaction at standard conditions. Show your work, including units.

Explain how the entropy of a gas changes as the temperature increases. Use kinetic molecular theory in your answer.

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.

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.

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

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

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

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.

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.

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.

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

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.