Which entropy value is incorrectly reported at 298 K among the following species: Ca (s) S' = 202; Br2 (l) S' = 152; CO2 (g) S' = 214; AlCl3 (s) S' = 109?

Which entropy value is incorrectly reported at 298 K among the following species: Ca (s) S' = 202; Br2 (l) S' = 152; CO2 (g) S' = 214; AlCl3 (s) S' = 109?

• A. Ca (s) S' = 202
• B. Br2 (l) S' = 152
• C. CO2 (g) S' = 214
• D. AlCl3 (s) S' = 109

Answer: (A)

Standard molar entropy reflects how much disorder a substance has at 298 K. Entropy increases from solid to liquid to gas, so gases have the largest S°, liquids intermediate, and solids the smallest. The CO2 gas around 214 J/(mol·K) fits the expected high S° for a gas, Br2 being a liquid with about 152 J/(mol·K) is reasonable, and a solid like AlCl3 near 109 J/(mol·K) is plausible for a solid compound. The value for calcium in the solid state, 202 J/(mol·K), is far too large for a solid metal at 298 K, since solids typically have entropies in the tens of J/(mol·K). Therefore, the incorrectly reported value is the calcium solid entry.

Standard molar entropy reflects how much disorder a substance has at 298 K. Entropy increases from solid to liquid to gas, so gases have the largest S°, liquids intermediate, and solids the smallest. The CO2 gas around 214 J/(mol·K) fits the expected high S° for a gas, Br2 being a liquid with about 152 J/(mol·K) is reasonable, and a solid like AlCl3 near 109 J/(mol·K) is plausible for a solid compound. The value for calcium in the solid state, 202 J/(mol·K), is far too large for a solid metal at 298 K, since solids typically have entropies in the tens of J/(mol·K). Therefore, the incorrectly reported value is the calcium solid entry.