Question

For each of the following pairs of substances, determine which has the larger molar entropy at 298 K: (A) Br₂(l) or (B) Cl₂(g) (C) Fe(s) or (D) Ni(s) (E) C₂H₆(g) or (F) C₂H₄(g) (G) CCl₄(g) or (H) CH₄(g) (I) HgO(s) or (J) MgO(s) (K) NaCl(aq) or (L) MgCl₂(aq)

Answer 1

Explanation:

Entropy is defined as the degree of randomness present in a substance. Therefore, more is the irregularity present in a compound more will be its molar entropy.

Hence, decreasing order to molar entropy in state of matter is as follows.

                     Gases > Liquids > Solids

• In the first pair, we are given $Br_{2}(l)$ or $Cl_{2}(g)$. Since, molar entropy of liquids is less than the molar entropy of gases.

Hence, $Cl_{2}(g)$ will have larger molar entropy as compared to $Br_{2}(l)$.

• In the second pair, we are given Fe(s) or Ni(s). More is the molar mass of a compound more will its molar entropy. Molar mass of Fe is 55.84 g/mol and molar mass of Ni is 58.69 g/mol.

Hence, molar entropy of Ni(s) is more than the molar entropy of Fe(s).

• In the third pair, we are given $C_{2}H_{6}(g)$ or $C_{2}H_{4}(g)$. As both the given species are gaseous in nature. So, more is the molar mass of specie more will be its molar entropy.

Molar mass of $C_{2}H_{6}(g)$ is 30.07 g/mol and molar mass of $C_{2}H_{4}(g)$ is 28.05 g/mol. Therefore, molar entropy of $C_{2}H_{6}(g)$ is more than the molar entropy of $C_{2}H_{4}(g)$.

• In the fourth pair, we are given $CCl_{4}(g)$ or $CH_{4}(g)$. Molar mass of $CCl_{4}(g)$ is 153.82 g/mol and molar mass of $CH_{4}(g)$ is 16.04 g/mol.

Therefore, molar entropy of $CCl_{4}(g)$ is more than the molar entropy of $CH_{4}(g)$.

• In the fifth pair, we are given HgO(s) or MgO(s). Molar mass of HgO is 216.59 g/mol and molar mass of MgO is 40.30 g/mol.

Hence, molar entropy of HgO(s) is more than the molar entropy of MgO.

• In the fifth pair, we are given NaCl(aq) or $MgCl_{2}(aq)$. Molar mass of NaCl 58.44 g/mol and molar mass of $MgCl_{2}(aq)$ is 95.21 g/mol.

Hence, the molar entropy of $MgCl_{2}(aq)$ is more than the molar entropy of NaCl(aq).