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Melting and Boiling Points

a Give reason...

Question

(a) Give reasons for the following.
(i) Bond enthalpy of $F_{2}$ is lower than that of $C l_{2}$ .
(ii) $P H_{3}$ has lower boiling point than $N H_{3}$ .
(b) Draw the structures of the following molecules.
(i) $B r F_{3}$ (ii) $(H P O_{3})_{3}$ (iii) $X e F_{4}$

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Solution

a) i. Fluorine has lower bond enthalpy than chlorine. This happens because the size of fluorine is small and F-F bond length is short. Due to this the electrons lie in 2p orbital with large electron-electron repulsion among the lone pairs of $F_{2}$ molecule but they are relatively much closer to each other in $C l_{2}$ molecule.
ii. This is due to the fact that $N H_{3}$ has hydrogen bonding which increase the bond strength, thus, increases the boiling point, while in case of $P H_{3}$ Van der Waal's forces are weaker.
b. The structures of the following molecules are:

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Q. (a) Give reasons for the following:
(i) Bond enthalpy of

F_{2}

is lower than that of

C l_{2}

P H_{3}

has lower boiling point than

N H_{3}

.
(b) Draw the structures of the following molecules:
(i)

B r F_{3}

(H P O_{3})_{3}

X e F_{4}

OR
(a) Account for the following:
(i) Helium is used in diving apparatus.
(ii) Fluorine does not exhibit positive oxidation state.
(iii) Oxygen shows catenation behavior less than sulphur.
(b) Draw the structures of the following molecules:
(i)

X e F_{2}

H_{2} S_{2} O_{8}

Q. (a) Draw the molecular structures of the following compounds:
(i)

N_{2} O_{5}

X_{e} O F_{4}

(b) Explain the following observations
(i) Sulphur has a greater tendency for catenation than oxygen.
(ii)

I - C l

is more reactive than

I_{2}

.
(iii) Despite lower value of its electron gain enthalpy with negative sign, fluorine

(F_{2})

is a stronger oxidizing agent than

C l_{2}

P H_{3}

has lower boiling point than

N H_{3}

Q. (a) Draw the structures of the following molecules:
(i)

H_{3} P O_{2}

C l F_{3}

(b) Explain the following observations:
(i) Nitrogen is much less reactive than phosphorus.
(ii) Despite having greater polarity, hydrogen fluoride boils at a lower temperature than water.
(iii) Sulphur has a greater tendency for catenation than oxygen in the same group.

Q. Give the correct order of T or F for following statements. If the statement is correct then Use T and F if it is false.
1.

H F

boils at a higher temperature than

H C l

H B r

boils at lower temperature than

H I

3. Bond length of

N_{2}

N_{2}^{+}

F_{2}

has higher boiling point than

C l_{2}

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