chemical bonding.....aim 4

how can molecular polarity be determined?

.....aim 3

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aim 5.....

 

MU #1 Molecular polarity can be determined by the shape of the molecule and the distribution of charge.

The electronegativity is used to determine the distribution of charge. When 2 elements are covalently bonded, the one with the highest electronegativity will carry an overall negative charge and the other an overall positive charge.

Example

HCl

Note: δ = partial charge

 

           

MU #2 Symmetrical (nonpolar) molecules include CO2, CH4, and diatomic elements.

Example 

In CH4the C has a higher electronegativity than H and pulls the electrons in the chemical bond towards itself in all four directions. Therefore, the distribution of charge is equal and the molecule is symmetrical. The shape of the molecule is a tetrahedron.

In  CO2 the O's have a higher electronegativity than C and pull the electrons in the chemical bond towards themselves in both directions. Therefore, the distribution of charge is equal and the molecule is symmetric. The shape of the molecule is linear.

 

           

Example 

In H2 ,O2, and N2 both atoms have the same electronegativity, therefore the distribution of charge is equal and the molecules are symmetrical.

 

       

       

MU #3 Asymmetrical (polar) molecules include HCl, NH3, and H2O. 

Example

In all these molecules the distribution of charge is unequal and the molecule is asymmetrical.

 

             

The shape of the water molecule is said to be "bent"

 

MU #4 Intermolecular forces created by the unequal distribution of charge result in varying degrees of attraction between molecules. Hydrogen bonding is an example of a strong intermolecular force.

 

Example

Hydrogen bonding in H2O

 

                                   

Hydrogen bonding in H2S

 

                                   

Since the difference in electronegativity between O and H is higher than in S and H it follows that the hydrogen bonding is stronger for H2O than H2S. Therefore, H2O has a higher boiling point than H2S.

 

TEST YOUR UNDERSTANDING

6/07

62 Explain, in terms of electronegativity, why a P–Cl bond in a molecule of PCl5 is more polar than a P–S bond in a molecule of P2S5.

1/07

29 Which compound has hydrogen bonding between its molecules?

65 Explain, in terms of molecular structure or distribution of charge, why a molecule of methane (CH4) is nonpolar.

8/06

10 Which formula represents a nonpolar molecule containing polar covalent bonds?    (1) H2O (2) CCl4 (3) NH3 (4) H2

11 The degree of polarity of a chemical bond in a molecule of a compound can be predicted by determining the difference in the

        (1) melting points of the elements in the compound      (2) densities of the elements in the compound

        (3) electronegativities of the bonded atoms in a molecule of the compound      (4) atomic masses of the bonded atoms in a molecule of the compound

6/06

11 Which formula represents a nonpolar molecule?     (1) CH4     (2) HCl     (3) H2O     (4) NH3

6/02

The strongest forces of attraction occur between molecules of   (1) HCl              (2) HF         (3) HBr               (4) HI

8/02

The equation for the saturated solution equilibrium of potassium nitrate (KNO3) is shown below.

a) In the space provided in your answer booklet, diagram the products. Use the key provided in your answer booklet. Indicate the exact arrangement of the particles you diagram.  

b) Compare the rate of dissolving KNO3 with the rate of recrystallization of KNO3 for the saturated solution.

1/03

Each molecule listed below is formed by sharing electrons between atoms when the atoms within the molecule are bonded together.

Molecule A: Cl2         Molecule B: CCl4         Molecule C: NH3

        1. Draw the electron-dot (Lewis) structure for the NH3 molecule.

        2. Explain why CC14 is classified as a nonpolar molecule.

        3. Explain why NH3, has stronger intermolecular forces of attraction than Cl2

        4. Explain how the bonding in KCl is different from the bonding in molecules A, B, and C.

8/03

15 Which of these substances has the strongest intermolecular forces? (1) H2O 3) H2Se  (2) H2S ( (4) H2Te

1/04

10 Which molecule is nonpolar? (1) H2O (2) NH3 (3) CO (4) CO2

20 Which of the following compounds has the highest boiling point? (1) H2O (2) H2S (3) H2Se (4) H2Te

6/04

10 Which type of molecule is CF4?
(1) polar, with a symmetrical distribution of charge (3) nonpolar, with a symmetrical distribution of charge

(2) polar, with an asymmetrical distribution of charge (4) nonpolar, with an asymmetrical distribution of charge

8/04

16 Molecules in a sample of NH3(l) are held closely together by intermolecular forces

                (1) existing between ions                                                    (2) existing between electrons

                (3) caused by different numbers of neutrons                  (4) caused by unequal charge distribution

41 Based on intermolecular forces, which of these substances would have the highest boiling point?  (1) He (2) O2 (3) CH4 (4) NH3

1/05

12 Which formula represents a nonpolar molecule? (1) HCl (2) H2O (3) NH3 (4) CF4

Base your answers to questions 51 through 53 on your knowledge of chemical bonding and on the Lewis electron-dot diagrams of H2S, CO2, and F2 below.

51 Which atom, when bonded as shown, has the same electron configuration as an atom of argon?

52 Explain, in terms of structure and/or distribution of charge, why CO2 is a nonpolar molecule.

53 Explain, in terms of electronegativity, why a C–O bond in CO2 is more polar than the F–F bond in F2.

6/05

40 Which molecule contains a nonpolar covalent bond?

Base your answers to questions 55 and 56 on the balanced equation 2Na(s) + Cl2(g) ---> 2NaCl(s)

55 In the box in your answer booklet, draw a Lewis electron-dot diagram for a molecule of chlorine, Cl2.

56 Explain, in terms of electrons, why the bonding in NaCl is ionic.

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