New Way Chemistry for Hong Kong A-Level Book 11 Intermediate Type of Bonding 9.1Incomplete Electron...

New Way Chemistry for Hong Kong A-Level Book 11

Intermediate Type Intermediate Type of Bondingof Bonding

9.19.1 Incomplete Electron Transfer in Ionic Incomplete Electron Transfer in Ionic CCompoundsompounds

9.29.2 Electronegativity of ElementsElectronegativity of Elements

9.39.3 Polarity of Covalent BondsPolarity of Covalent Bonds

99


9.9.11 Incomplete Incomplete

Electron Electron Transfer in Ionic Transfer in Ionic

CompoundsCompounds


Born-Haber cycle for the formation of sodium chloride

Hlattice can be f

ound from the Born-Haber Cycle

Exptal value of Hlattice

Theoretical values of lattice Theoretical values of lattice enthalpyenthalpy

9.1 Incomplete electron transfer in ionic compounds (SB p.247)


Another way of getting the value of Another way of getting the value of HHlatticelattice

In the ionic model, assume:

1. Ions are perfect spheres with uniform charge distribution.

2. The cations and anions are in contact with each other and with electrostatic interactions in between.



• Lattice enthalpy is calculated in terms of electrostatic interactions within the lattice

• The potential energy (P) required to bring two ions with charges q1 and q2 from an infinite distance to a distance r is:

R _

q2

q1

+


r2

q1

q P

Another way of getting the value of Another way of getting the value of HHlatticelattice


9.1 Incomplete electron transfer in ionic compounds (SB p.248)Comparison of theoretical and Comparison of theoretical and experimental values of lattice experimental values of lattice enthalpyenthalpyTheoretical value of lattice enthalpy

• Compound is purely ionic

Lattice enthalpy obtained from the Born- Haber cycle

• True value

• Based on the experimentally derived enthalpy changes



Comparison of theoretical and Comparison of theoretical and experimental values of lattice experimental values of lattice enthalpyenthalpy

• Reveal the nature of the bond in the compound


9.1 Incomplete electron transfer in ionic compounds (SB p.248)Comparison of theoretical and Comparison of theoretical and experimental values of lattice experimental values of lattice enthalpyenthalpyCompound Lattice enthalpy (kJ mol-1)

Theoretical Experimental Difference

NaCl -766 -771 5

NaBr -732 -733 1

NaI -686 -684 -2

KCl -690 -701 11

KBr -665 -670 5

KI -632 -629 -3

AgCl -770 -905 135

AgBr -758 -890 132

AgI -736 -876 140

ZnS -3430 -3739 309


Increasing polarization of a Increasing polarization of a negative ion by a positive ionnegative ion by a positive ion

Polarization / Distortion of electron

cloud

Polarization / Distortion of electron

cloud=

Covalent character in ionic bond

Covalent character in ionic bond

Polarizing power of cation

polarizability of anion



Polarizing power of a cation increases as its charge density(charge/volume ratio) increases.

polarizing power:Al3+ > Mg2+ > Na+ andLi+ > Na+ > K+

Polarizing power of cation


Polarization of ionPolarization of ion


Polarizability of an anion increases as its size increases.

polarizability:I- > Br- > Cl- > F- andS2- > O2-

polarizability of anion


Polarization of ionPolarization of ion

Example 9-1Example 9-1

Check Point 9-1Check Point 9-1


9.9.22ElectronegatiElectronegati

vity of vity of ElementsElements


ElectronegativityElectronegativity

9.2 Electronegativity of elements (SB p.250)

Electronegativity is defined as the relative tendency of an atom to attract a bond pair(s) of electrons towards itself in a covalent bond.

Electronegativity is defined as the relative tendency of an atom to attract a bond pair(s) of electrons towards itself in a covalent bond.


XY

X : Y

Electronegativity of some Electronegativity of some representative elementsrepresentative elements

9.2 Electronegativity of elements (SB p.251)


9.9.33 Polarity of Polarity of

Covalent Covalent BondsBonds


Bond Bond polarizationpolarization

a pure covalent bond a polar covalent

bond

9.3 Polarity of covalent bonds (SB p.251)


Relationship between the type of bond Relationship between the type of bond and difference in electronegativity and difference in electronegativity between two bonded atomsbetween two bonded atoms

Bond Difference in electronegativity between

the two bonded atoms

Type of bond

Cl Cl

Cl C

Cl H

F Li

3.0 - 3.0 = 0

3.0 - 2.5 = 0.5

3.0 - 2.1 = 0.9

4.0 - 1.0 = 3.0

Covalent

Polar covalent

Polar covalent

Ionic




Relationship between the type of bond Relationship between the type of bond and difference in electronegativity and difference in electronegativity between two bonded atomsbetween two bonded atoms

Check Point 9-3ACheck Point 9-3A


Polarity of Polarity of moleculesmolecules

(Debye, D) = q d (Debye, D) = q d

How polar is the molecule?How polar is the molecule?

represented by

dipole moment ()



Effect of an electric field on Effect of an electric field on polar moleculespolar molecules




Effect of an electric field on non-Effect of an electric field on non-polar moleculespolar molecules


Non-polar Non-polar moleculesmolecules


Tetrahedral

Trigonal planar

Linear

Cancelling out of dipole moments

MoleculeShape



Non-polar Non-polar moleculesmolecules

Octahedral

Trigonal bipyramidal

Cancelling out of dipole moments

MoleculeShape



Polar Polar moleculesmolecules

Tetrahedral

Trigonal pyramidal

V-shaped

( or bent)

Net resultant dipole

moment

Dipole moment of individual

polar bonds

MoleculeShape


Use of dipole momentsUse of dipole moments


• Provide important structural information about molecules


Effect of a non-uniform electric field Effect of a non-uniform electric field on polar and non-polar solventson polar and non-polar solvents

Tetrachloromethane, CCl4Cyclohexane

Benzene

Trichloromethane, CHCl3Ethanol,CH3CH2OH

Propanone,

Water, H2O

Solvents showing no deflection

Solvents showing a marked deflection




Check Point 9-3BCheck Point 9-3BExample 9-3Example 9-3


The END



The following gives the theoretical and experimental values of the lattice enthalpies of two metal bromides. X+Br- and Y+Br-.

(a)There is a high degree of agreement between the theoretical and experimental values in the case of X+Br-(s) but a large discrepancy in the case of Y+Br-(s). What can you tell about the bond type of the two compounds? Answer

Compound Theoretical lattice

enthalpy (kJ mol-1)

Experimental lattice

enthalpy (kJ mol-1)

X+Br-(s) -665 -670Y+Br-(s) -758 -890



(a) Since the theoretical value of the lattice enthalpy is calculated based on a simple ionic model, the good agreement for X+Br-(s) suggests that the compound is nearly purely ionic. The ions are nearly spherical with nearly uniform distribution of charges. The bond type in the compound is thus nearly purely ionic.

For Y+Br-(s), the large discrepancy suggests that the simple ionic model does not hold due to the distortion of the electron cloud of the anion. Thus the bond type in this compound has a certain degree of covalent character.



(b)To which group in the Periodic Table does metal X belong? Explain your answer.

Answer(b) As X+ ion must have a low polarizing power, its

charge to size ratio should be small. X is a Group I metal.

Back


Apart from those compounds mentioned in Table 9-1, list three other compounds with a large difference between the theoretical and experimentally derived lattice enthalpies.

They are the compounds with a cation of high polarizing power and an anion of high polarizability.

Examples are AlCl3, MgI2, Al2S3, CuO and Ag2S.

Back

Answer




Pure ionic bond and pure covalent bond are two extreme bond types. Why?

In pure ionic bonding, the bonded atoms are so different that one or more electrons are transferred to form oppositely charged ions. Two identical atoms share electrons equally in pure covalent bonding. This type of bonding results from the mutual attraction of the two nuclei for the shared electrons. Between these extremes are intermediate cases in which the atoms are not so different that electrons are incompletely transferred and unequal sharing results, forming polar covalent bond.

Answer

Back


How do you expect the bond type to change form the chlorides of the third period elements, NaCl, MgCl2, AlCl3, SiCl4, PCl5, SCl2 and Cl2, going from left to right?

Explain the change in the bond type.

Moving along the chlorides of the third period elements, the bond type changes from pure ionic bond, ionic bond with covalent character, polar covalent bond to pure covalent bond. This is because the difference in electronegativity between the atoms of the third period elements and the chlorine atom decreases across the period.

Back

Answer



Explain the variation in dipole moment of the following molecules.

Answer


Molecule Dipole moment (D)

CH4 0

NH3 0.35H2O 0.65HF 1.07



The dipole moment of a molecule is based on two factors:

1. Bond polarity

This depends on the electronegativity of the atoms involved in a bond. A bond is said to be polar if there is a difference in electronegativity between two bonded atoms. The larger the difference, the more polar is the bond.

H C N OF

2.1 2.5 3.0 3.54.0



2. The geometry

If the molecule have symmetrical arrangements of polar bonds, the dipole moments of the bonds will cancel out each other.

CH4 NH3

No net dipole moment Net dipole moment resulted


Back


H2O HF

Net dipole moment resulted Net dipole moment resulted

(Note: Lone pair(s) is/are not shown in the above diagrams)

Hence, zero dipole moment is only observed in CH4. HF has the largest dipole moment since the difference in electronegativity between the hydrogen atom and the fluorine atom is the largest. H2O comes the second, followed by NH3.



Give the shapes and structural formulae of the following molecules. State whether each molecule is polar or non-polar.

(a) BCl3

(b) NH3

(c) CHCl3

Answer



Back

PolarTetrahedral(c) CHCl3

PolarTrigonal pyramidal

(b) NH3

Non-polarTrigonal planar(a) BCl3

Polar or non-polar

Structural formula

ShapeMolecule

New Way Chemistry for Hong Kong A-Level Book 11 Intermediate Type of Bonding 9.1Incomplete Electron...

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