12.1 Properties of the alkanes

Question 1

What is the general formula of alkanes?

Answer 1

CnH2n+2

Question 2

What are alkanes?

Answer 2

Saturated hydrocarbons containing only carbon and hydrogen single bonds only joined by a single covalent bond called a sigma bond (σ)

Question 3

What is meant by an alkane being saturated?

Answer 3

Each carbon is bonded 4 time with the maximum number of hydrogens possible

Question 4

What is the general formula of cycloalkanes?

Answer 4

CnH2n

Question 5

Are cycloalkanes saturated or unsaturated?

Answer 5

Saturated

Question 6

Definition of covalent bond

Answer 6

Shared pair of electrons

Question 7

Explain a σ bond

Answer 7

A bond formed by the overlap of two orbitals, one orbital from each bonding atom, consisting of two electrons with the electron density centred around the line directly between the nuclei of the two atoms

Question 8

Definition of a σ bond

Answer 8

The overlap of two orbitals, one from each bonding atom, directly between bonding atoms

Question 9

Where is a σ bond positioned?

Answer 9

Directly between the bonding atoms

Question 10

Draw an imagine of a sigma (σ) bond is between two carbon atoms

Answer 10

Question 11

What is the symbol for a sigma bond?

Answer 11

σ

Question 12

What is the shape of alkane molecules?

Answer 12

Tetrahedral

Question 13

What is the bond angle of alkane molecules?

Answer 13

109.5° - tetrahedral

Question 14

Explain the shape of an alkane molecule?

Answer 14

Each carbon atom is surrounded by 4 electron pairs in four σ bonds which all repel each other equally, as far apart as possible to produce tetrahedral arrangement with bond angle of 109.5°

Question 15

How do the σ bonds act in the shape of an alkane?

Answer 15

Act as an axis for the atoms to rotate round freely, so the shapes are not rigid but the distance between the bonds remain (mostly) the same

Question 16

How are the different alkanes in crude oil separated?

Answer 16

Oil refiners which separate crude oil into the different fractions using fractional distillation

Question 17

How is fractional distillation of alkanes possible?

Answer 17

All alkanes have different boiling points

Question 18

Definition of fractional distillation

Answer 18

The separation of components in a liquid mixture by their different boiling points into fractions with different compositions

Question 19

Draw a 3D shape of the bonding in a ethane molecule

Answer 19

Question 20

Explain the effect of chain length on boiling points in alkanes

Answer 20

As chain length increases, the greater the surface area so more surface contact is possible between molecules. The London forces are stronger, so it will take more energy to overcome these forces, therefore boiling point increases with chain length

Question 21

Explain the effect of branding on boiling points in alkanes

Answer 21

As branching increases, there are fewer surface contact points between molecules. The branches get in the way and prevent branched molecules form packing close together, so less surface contact. There are fewer London forces, so less energy is required to break the bonds

Question 22

What is the difference in boiling points into fractions between branched and straight chain alkanes?

Answer 22

Branched molecules have lower boiling points than straight chain alkanes