Unit 1-Structure And Bonding

Question 1

What name is given to bonding:

a) inside molecules
b) between molecules

Answer 1

a) inside molecules (intramolecular bonds)
b) between molecules (intermolecular bonds).

Question 2

Describe how a covalent bond is held together.

Answer 2

Two positive nuclei being held together by their attraction for the shared pair of electrons.

Question 3

What are the two types of covalent bonding? (Hint-Electronegativity)

Answer 3

Non-polar (pure) covalent and polar covalent.

Question 4

What causes a polar covalent bond to be formed?

Answer 4

Attraction of the atoms for the pair of bonding electrons is different (different electronegativity).

Question 5

How is a polar covalent bond shown in a diagram?

Answer 5

Delta positive (δ+) and delta negative (δ-) notation can be used to indicate the partial charges on atoms, which give rise to a dipole (eg H^δ+ ̶ Cl^δ-).

Question 6

What causes a non-polar (pure) covalent bond to be formed?

Answer 6

The attraction of the atoms for the pair of bonding electrons are similar (same electronegativity).

Question 7

What causes an ionic bond to form?

Answer 7

Large difference in electronegativity, causes one atom to lose electrons and the other to gain electrons. Oppositely charged ions are

Question 8

What is meant by the bonding continuum?

Answer 8

Pure covalent bonding and ionic bonding can be considered as being at opposite ends of a continuous range with polar covalent bonding lying between these two extremes.

Question 9

What are the weakest intermolecular forces and how do they arise?

Answer 9

LDFs occur when there is an uneven distribution of electrons within an atom or molecule. This causes temporary dipoles resulting in weak attractions between molecules.

Question 10

What affects the strength of London dispersion forces?

Answer 10

Number of electrons.
Larger atoms and molecules have more electrons. This leads to larger dipoles being established.

Question 11

Explain why water is polar but carbon tetrachloride is non-polar despite both containing polar bonds.

Answer 11

A substance that contains polar covalent bonds may not be overall polar due to symmetry.

Water is angular, the molecule is polar.

Carbon tetrachloride is a tetrahedral molecule, so the charges are symmetrical and the molecule is non-polar.

Question 12

Describe what is meant by permanent dipole to permanent dipole interactions.

Answer 12

The δ+ and δ- ends of polar molecules attract each other.

Question 13

What name is given to the strongest type of intermolecular bond?

Answer 13

Hydrogen (bonding).

Question 14

What is needed for hydrogen bonding to occur?

Answer 14

Hydrogen bonding occurs when a hydrogen atom is covalently bonded to a highly electronegative element such as nitrogen, oxygen or fluorine.

As with permanent dipole to permanent dipole attractions, the oppositely charged ends of molecules attract.

Question 15

How do the melting and boiling points of a polar substance compare to a non-polar substance with similar numbers of electrons?

Answer 15

The melting and boiling points of polar substances are higher than the melting and boiling points of non-polar substances with similar numbers of electrons.

Question 16

Describe the properties (melting and boiling points, conductivity) of ionic compounds.

Answer 16

All ionic compounds have a high melting point and boiling point. They conduct when molten or in solution as the ions are free to move.

Question 17

Describe the properties (melting and boiling points, conductivity) of covalent network compounds.

Answer 17

All covalent network structures have very high melting points and boiling points. They are all hard and do not conduct electricity.

Question 18

Describe the properties (melting and boiling points, conductivity) of covalent molecular compounds.

Answer 18

They have low melting points and boiling points. They do not conduct electricity. Some covalent molecular compounds have higher melting points than expected.

Question 19

Which of the following molecules would have the higher melting and boiling points and why?

Answer 19

Ethanol would have higher MP and BP.

Both ethanol and ether have the same molecular mass, but ethanol contains hydrogen bonding.

Ether is a symmetrical molecule, so even though it contains polar bonds, the molecule itself is non-polar and the only intermolecular force present will be weak London dispersion forces.

Question 20

Describe the solubility of ionic compounds, polar molecules and non-polar molecules.

Answer 20

Ionic compounds and polar molecular compounds tend to be soluble in polar solvents such as water and insoluble in non-polar solvents. Non-polar molecular substances tend to be soluble in non-polar solvents and insoluble in polar solvents.

As a general rule “like dissolves like” so, polar molecules are soluble in polar solvents (such as water) and vice-versa.