Q3. By how much do the C–X bond length and bond enthalpy change from CH3–Cl to CH3–I?
Length: 214 − 178 = 36 pm longer. Enthalpy: 351 − 234 = 117 kJ mol−1 lower (NCERT Table 6.2, page 164).
Q4. Treat the dipole moments of CH3F and CH3Cl (NCERT Table 6.2) as charges ±Q one bond length apart. Find Q for each as a fraction of e. (1 D = 3.336 × 10−30 C m; e = 1.602 × 10−19 C.)
Q = μ/r. CH3F: 1.847 × 3.336 × 10−30/(139 × 10−12) = 4.43 × 10−20 C, about 0.28 e. CH3Cl: 1.860 × 3.336 × 10−30/(178 × 10−12) = 3.49 × 10−20 C, about 0.22 e. The charges on C–F are larger, but the C–Cl bond is 39 pm longer, so CH3Cl has the larger dipole moment. This is reasoned from μ = Q × r (NCERT Class 11 Chemistry, Unit 4) and is a simplification, because the C–H bonds also contribute.