Physical Chemistry

Order of Boiling Points Based on Intermolecular Forces

To determine the increasing order of boiling points of the given compounds, we analyze the nature and strength of their intermolecular forces:

1. Dimethylamine :
   - A secondary amine that exhibits weak hydrogen bonding.
   - Possesses dipole-dipole interactions and van der Waals forces.
   - However, hydrogen bonding is not as effective as in alcohols.
   - Thus, it has the lowest boiling point among the three.
2. Ethylamine :
   - A primary amine capable of forming stronger hydrogen bonds than dimethylamine.
   - Also exhibits dipole-dipole interactions.
   - Slightly higher boiling point due to a larger ethyl group, which increases van der Waals forces.
3. Ethanol :
   - An alcohol with a hydroxyl group capable of forming strong hydrogen bonds.
   - Hydrogen bonding in alcohols is typically stronger than in amines.
   - Therefore, ethanol has the highest boiling point.

Conclusion:

The order of increasing boiling points is:

Boiling Point Order of Alkyl Halides

Step 1: Understanding the boiling point order. The boiling points of alkyl halides decrease in the order: because the iodine atom is the largest halogen, leading to stronger dispersion forces and, consequently, higher boiling points.

Step 2: Reason for the boiling point order. While Reason (R) is true in stating that alkyl halides have higher boiling points compared to hydrocarbons of comparable molecular mass, it does not directly explain the order in which boiling points decrease. The reason for this specific order lies more in the halogen size and the resultant intermolecular forces.

Step 3: Conclusion. Thus, both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of Assertion (A). The correct answer is option (B).

Step 1: Understanding boiling points. Boiling points are affected by molecular size and the type of intermolecular forces. The more carbon atoms and longer the chain, the higher the boiling point.

Step 2: Analysis of the compounds.

- Compound (i) CH_3CH_2Br has a relatively small chain, resulting in a lower boiling point.
- Compound (ii) CH_3CH_2CH_2Br has a longer chain, leading to higher boiling point.
- Compound (iii) CH_3CH_2CH_2Br, with the largest size, has the highest boiling point.

Step 3: Conclusion. Thus, the correct order of boiling points is (i)<(ii)<(iii), corresponding to option (B).

Give Reasons:

(I) in aqueous solution is a good oxidising agent.

Reason: is a strong oxidising agent because it has a high tendency to accept an electron and reduce to . The high charge on (a +4 oxidation state) and the relatively small ionic radius of compared to its reduced form give it a high electron affinity, making it an effective oxidant in aqueous solutions.

(II) The actinoid contraction is greater from element to element than lanthanoid contraction.

Reason: The actinoid contraction is greater than the lanthanoid contraction because the 5f-electrons in actinides experience poor shielding by other f-electrons, resulting in a greater increase in effective nuclear charge as the atomic number increases. In contrast, lanthanides with 4f-electrons experience better shielding; therefore, the contraction is less significant. This explains why the atomic sizes in the actinide series decrease more significantly across the series than in the lanthanide series.

(III) value is more negative than expected, whereas is positive.

Reason: The value is more negative than expected due to the large increase in ionization energy as we remove electrons from the second ionization state, leading to a stronger pull by the nucleus. On the other hand, is positive because copper has a stable electronic configuration with a filled 3d¹⁰ subshell, making it more resistant to oxidation and thus having a positive electrode potential. Copper, being a transition metal, has a more favorable reduction potential than other elements like iron, leading to a positive standard electrode potential for the Cu²⁺/Cu system.