The correct order of molar heat capacities measured at 298 K and 1 bar is :
- Copper(s) > Bromine(l) > Helium(g)
- Bromine(l) > Copper(s) > Helium(g)
- Helium(g) > Bromine(l) > Copper(s)
- Helium(g) > Bromine(l) = Copper(s)
Answer
(2) Bromine(l) > Copper(s) > Helium(g)
Solution
He(g): monatomic gas, only translational energy
⇒ Cp,m = 52R ≈ 20.8 J K-1 mol-1
Cu(s): atoms vibrate in the lattice (Dulong–Petit law)
⇒ Cp,m ≈ 3R ≈ 24.4 J K-1 mol-1
Br2(l): diatomic molecules in a liquid absorb energy in translation, rotation, vibration and in overcoming intermolecular forces
⇒ Cp,m ≈ 75.7 J K-1 mol-1
∴ Bromine(l) > Copper(s) > Helium(g)
Answer: (2)