3, Answer BOTH parts a) and b). a) Bond Bond enthalpy (A) F-F CI-CI Br-Br kJ mol

Question

3, Answer BOTH parts a) and b). a) Bond Bond enthalpy (A) F-F CI-CI Br-Br kJ mol1 158 242 193 151 1-1 i) Rationalize the group 17 homonuclear bond strengths g table above identifying any trends and anomalies. iven in the i) The heteronuclear CI-F bond strength is 251 kJ mol1, Why is this stronger than the homonuclear bond strengths? i) Use valence shell electron pair repulsion (VSEPR) theory to predict the shapes of the following species: b) i) PCl2F3 and PCl3F2 are fluxional in solution. What is meant by fluxional and what mechanism best describes the fluxionality of this 13] system? ii) Predict both the high temperature and low temperature 19F NMR spectra of PCI:F3 and PCI:F2. (19F and 31P are 100% abundant with [41

Explanation / Answer

3. (A) (i) The order of homonuclear bond strengths of group 17 elements from the given table is :

Cl-Cl>Br-Br>F-F>I-I

As seen, the bond strength order doesn't follow a regular trend and shows two abnormalities

that are explained as follows :

a. Very less bond strength of F-F molecule.

Explanation: Fluorine is a highly electronegative element. The fluorine molecule is very small as compared to its group members. Its atomic number is 9 and from its electronic configuration i.e. 1s22s22p5, we can see that it has seven electrons in its outermost shell and a very small atomic radius. When two fluorine atoms come together to share a pair and complete an octet, these many electrons concentrated on two small nuclei and separated over comparatively a very small distance strongly repel each other.

Hence the F-F bond is weakened.

b. An abruptly large bond strength of Cl-Cl molecule.

Explanation: The repulsion between homonuclear elements increases with an increase in the number of electrons and decreases with increase in size. The electronic configuration of Chlorine is [Ne] 3s2 3p5 , and it has 17 electrons in its shell. The Van der Waals radius of Chlorine is 175pm while that of Fluorine is 147pm. Thus, the volume as well as the number of electrons increases on going from Fluorine to Chlorine. But, the increase in the number of electrons dominates resulting in an increased attraction between the nucleus of one atom and electrons of the other atom as compared to Fluorine resulting in greater bond strength as compared to Fluorine. On moving down the group, the size factor dominates and the bond strength decreases as the distance between nucleus is increased.

(ii) From the table, it can be seen that the bond strength of Cl-F is even greater than the Cl-Cl bond strength (which has the highest homonuclear bond strength among all group 17 elements ).

This is due to the partial ionic character of the Cl?F bond. The Pauling electronegativity value of fluorine (?=3.98) is appreciably higher than that of chlorine (?=3.16). Thus, there are significant partial positive and negative charges on the fluorine and chlorine atoms in the ClF molecule. Although the difference in orbital energies and size would mean that the covalent interaction between the chlorine 3p and fluorine 2p orbital would be less strong compared to that between 2 chlorine 3p orbitals, the ionic character of the bond overcompensates for this and in fact, results in the Cl?F bond being stronger than the Cl?Cl bond.

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