ellanti

Chapter 6: Molecular Compounds Worksheet Answers

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1) Covalent bond

Is a bond formed as a result of attraction between the nuclei and valence electrons of different atoms. The attraction  binds the atoms together resulting in the formation of a molecular compound. 

2)  the three types of chemical boding and their description:

  • Ionic bond – bonding that results from electrical attraction between anions and cations 
  • Covalent bond- bonding that results from sharing of electrons between two atoms
  • Metallic bond– bonding that results from the attraction between the metal atoms and the surrounding sea of electrons

3)  Relationship between electronegativity and the ionic character of a chemical bond

Electronegativity of atoms determines their affinity for electrons.  If the electro negativity difference between two atoms forming a compound > 1.7  then the bond formed has ionic character where one atom with higher electronegativity gains electrons and one with lower electronegativity gives up electrons. 

4a) What is the meaning of the term polar, as applied to chemical bonding?

Read: P176

If participating atoms  in a covalent bond have different electronegativities then, since the atoms will have have uneual attraction for the shared electrons involved in the bond, the bond formed will tend to have polarity. Such  bonds are called polar-covalent bonds.

Bonds in which the electronegativity diffeernce betwen participating atoms is between 0.3 to 1.7  form polar-covalent bonds.

4b) Distinguish between polar-covalent and nonpolar-covalent bonds. 

Read: P176

Covlanet bonds in which bonded electrons are shared equally between the participating atoms are called nonpolar- covalent bonds .

Example :  Hydrogen-Hydrogen bond is a nonpolar-covalent bond.

In general, if the elctronegativity difference between participating atoms  <0.3 the bond is nonpolar-covalent.

Covalent bonds in which bonded electrons are shared unequally between the participating atoms are called polar covalent bonds .

Example: H-Cl bond is a polar-covalent bond with cholorine side being more negative and hydrogen side being more positive.

In general, if the electronegativity differeence between participating atoms is between 0.3 and 1.7 then the bond formed is polar-covalent.

Bonus question:

4c) What determines whether atoms will form chemical bonds?

The number of valence electrons  determines whether the element will form bond with other atoms. If the element already has its outer orbital shells fully occupied then those elements are are unlikely to form bond.

For example Group VIII elements (He, Ne, Ar,Kr, Xe, Rn).

On other hand elements like Cl, H, and others have orbitals in outer energy level that are empty or partially filled.  They are willing to give up or gain electrons to have their outermost energy level filled (in the case of Cl) or empty (in the case of H).  An atom with outermost energy level that is partially filled carries high potential energy naturally.

In general, atoms don’t exist normally by themselves. They have high potential energey and nature favors  them to find  a more stable energy state by forming bonds with other atoms.

Read: P175

 

5)

 

Electronegativity

difference

Bond type

Electronegative

atom in the bond

H and I

H=2.1, I=2.5
eneg diff = 0.4

polar Covalent

I

S and O

S=2.5, O=3.4
eneg diff=0.9

polar covalent

O

K and Br

K=.82, Br=2.96

eneg diff = 2.16

Ionic

Br

Si and Cl

Si=1.9, cl=3.16

eneg diff= 1.26

polar covalent

Cl

K and Cl

k=0.82, Cl=3.16

eneg diff= 2.34

Ionic

Cl

Se and S

Se=2.55, S=2.5

eneg diff=0.05

covalent

Se

C and H

C=2.55, H=2.20

eneg diff=0.35

polar Covalent

C

Read: P176 and P161.

For teachers : if you are creating test paper with questions that ask students to identify bond type then  it is advisable to use open book or use of  copies of periodic table with electronegativity noted for each element. It doesn’t serve any purpose to have them remember electronegativity of each atom via rote memorization process.  It would be more useful to focus on the process and logic than some hard numbers which could be fictional and not something they will understand anyway.

6) Rank the above list by Covalent bond character 1 being the least covalent and 7 being the most.

 

Electronegativity

Eneg Diff

Bond type

Electronegative

atom in the bond

Rank by Covalent Bond character

K and Cl

k=0.82, Cl=3.16

2.34

Ionic

Cl

1

K and Br

K=.82, Br=2.96

2.16

Ionic

Br

2

Si and Cl

Si=1.9, Cl=3.16

1.26

polar covalent

Cl

3

S and O

S=2.5, O=3.4

0.9

polar covalent

O

4

H and I

H=2.1, I=2.5

0.4

polar Covalent

I

5

C and H

C=2.55, H=2.20

0.35

polar Covalent

C

6

Se and S

Se=2.55, S=2.5

0.05

covalent

Se

7

Read: P176 and P161.

Note to teachers: see above.

7) Use orbital notation to illustrate the bonding in each of the following molecules:

  Answer
Chlore,Cl2 Cl: 1s22s2p63s2p5
Outermost sub shell p: [↑↓]  [↑↓] [↑ ]
Cl . – . Cl
The unpaired valence electron is involved in the covalent bond.
 Oxygen, O2 O: 1s22s2p4
Outermost sub shell 3p: [↑↓]  [↑] [↑ ]
O : =: O
Each unpaired valence electron is involved in forming a covalent bond. Since there are two unpaired electrons the bond formed is  thus a double bond.
Hydrogen Fluoride, HF H: 1s1
F: 1s22s2p5
Outermost sub-shell 2p:  [↑↓]  [↑] [↑ ]
H . -. F
Each of the unpaired valence electron from H and F are involved in the covalent bond.

Read: P184, Lewis dot notation

8) The lattice energy of sodium chloride, NaCl, is -787.5 kJ/mol. The lattice energy of potassium chloride, KCl, is -715 kJ/mol. In which compound is the bonding between ions stronger? Why?

NaCl bong is stronger because the amount of energy released is is higher by 72KJ/mo compared to that released in the formation of KCl. 

Covalent Bonding and Molecular Compounds

9) What is a molecule?

10.a) What determines bond length?

10.b) How are bond energies and bond lengths related?

11) Describe the general location of the electrons in a covalent bond.

12) As applied to covalent bonding, what is meant by an unshared or line pair of electrons?

 

13) Describe the octet rule in terms of noble-gas configurations and potential energy?

14) Determine the number of  valence electrons in an atom of each of the following elements

 

27.a) Lattice Energy

is defined as the energy released when one mole of an ionic crystalline compound is formed from gaseous ions. 

27.b) Relation between Lattice Energy and Strength of Ionic bond

Bonds formed with release of higher lattice energy are stronger than ones formed with release of smaller lattice energy.  Ionic bond strength is therefore measured by the amount of lattice energy.

28.a) How do ionic and molecular compounds compare in terms of melting points, boiling points, and ease of vaporization

Melting point, boiling point and hardness of a compound depend on how strongly its basic units are attracted to each other.

28.b) What accounts for the observed difference in the properties of ionic and molecular compounds

  • Ionic compounds:  because they have strong attractive forces beween molecules, they tend to have higher melting and boiling points.
  • Covalent compounds:  as forces of attraction between molecules are weaker they have lower melting /boiling points.  Many of the compounds are already gaseous at room temperature.

28.c)  Cite three or more physical properties of ionic compounds.

 Ionic compounds are hard, malleable and  brittle

Bonus:  include general characteristic of  ionic and covalent compounds side by side.

Ionic compounds Covalent compounds
Strong attraction between positive and negative charge Attraction between molecules are weaker compared to ionic compounds
Hard and brittle  
Melting, Boiling points are higher because the attraction forces between units of the compound.   Have lower melting and boiling point because of weaker attraction between units of the compound
In solid state they are not electrical conductors  
In molten state they are good conductors because the ions can move freely to carry electrical current.  
Many can dissolve in water to form solution that can conduct electricity.  

44. For each of the following polar molecules, indicate the direction of the resulting dipole:

  Answer
H-F -+—->

 H  –   F

H-Cl  -+—->

 H  –   Cl

H-Br -+—->

 H  –   Br

H-I -+—->

 H  –   I

46. For each of the following polar molecules, indicate the direction of the resulting dipole

  Answer
H-H Non polar
H-O -+—->

 H  –   O

H-F -+—->

 H  –   f

Br-Br Non polar
H-Cl -+—->

 H  –   Cl

H-N -+—->

 H  –  Nf

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Written by ellanti

December 23, 2011 at 4:21 pm

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