Covalent bonding extended

Question 1

Describe a sigma bond.

Answer 1

Head to head overlap of orbitals between two nuclei. Orbital turns from an atomic orbital to a molecular orbital. the sigma bond lies on the internuclear axis, which makes it a stronger bond.

Question 2

Describe a Pi bond

Answer 2

Two atoms have formed a sigma bond between them, and both atoms happen to have a P orbital with an unpaired electron, they can align and overlap side to side. the Pi bond is parallel either side of the sigma bond, the Pi bond contains two electrons that are simultaneously in both sides of the Pi bond. The molecular orbital that forms in a Pi bond lies either side of the internuclear axis, which means it is a weaker bond.

Question 3

Explain the strength of a Pi bond relative to a sigma bond

Answer 3

A pi bond is weaker than a sigma bond as it is further from the internuclear axis which means it experiences less electrostatic attraction to the positive nuclei.

Question 4

Explain the strength of a sigma bond relative to a pi bond

Answer 4

Stronger than a pi bond as it lies on the internuclear axis which means it experiences the highest possible electrostatic attraction to the positive nuclei.

Question 5

Describe the location of a sigma bond

Answer 5

Lies on the internuclear axis

Question 6

Describe the location of a Pi bond

Answer 6

Lies either side of the internuclear axis, parallel to the sigma bond.

Question 7

Describe the addition reactions of alkenes

Answer 7

1. alkene + Br2 at RTP
2. Alkene + Conc HBr (aq) at RTP
3. a)
   Alkene + H2 (g) at high temp and high pressure with fine powder Ni (nickel) catalyst.
   b)
   Alkene + H2 (g) at RTP with Pt (platinum) catalyst
4. a)
   Alkene + H2SO4 heated under reflux followed by H2O

b)
Alkene + H2O (g) (water vapour) w/ Phosphoric acid catalyst at high temp and high pressure.

Question 8

Definition of an electrophile

Answer 8

A chemical species with a positive charge, that can accept a pair of e- from a region of high e- density (e.g C=C), forming a covalent bond.

Question 9

describe an endothermic reaction in terms of the bond interactions

Answer 9

in a reaction, bonds are broken which requires energy. Energy is then released to reform these bonds. Endothermic reactions break strong, stable bonds which requires more energy, and form weak, unstable bonds which releases less energy, but holds a higher chemical potential energy. This results in a positive enthalpy change .

Question 10

describe an exothermic reaction in terms of the bond interactions

Answer 10

In a reaction, bonds are broken which requires energy, and bonds are reformed which releases energy. In an exothermic reaction, weak unstable bonds are broken which requires relatively little energy. Strong, stable bonds are then formed which releases more energy but hold less chemical potential energy. this results in a negative enthalpy change

Question 11

define the standard enthalpy change of combustion

Answer 11

The enthalpy change when 1 mole of a fuel combusts fully at rtp