Entropy (College Board 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.

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°C 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.