Alkanes

Question 1

Bond angles in alkanes

Answer 1

109.5°

Question 2

Forces that hold Alkanes together

Answer 2

Van der Waals forces

Question 3

How do the Van der Waals forces arise?

Answer 3

Electron movement in the first molecule creates a temporary dipole, this induces a dipole in the neighbouring molecule, the δ+ side of one molecule is attracted to the δ- side of another molecule

Question 4

How are bonds in Alkanes formed?

Answer 4

Covalent bonds occur from the sharing of electrons

Question 5

Explain why alkanes are saturated hydrocarbons and explain what saturated means.

Answer 5

They contain only C-C single bonds, saturated means there are no C=C double bonds

Question 6

Polarity of Alkanes

Answer 6

Non-polar, electronegativities of carbon and hydrogen are very similar.
Only contain Van der Waals forces

Question 7

Boiling points of Alkanes

Answer 7

The boiling points of Alkanes increase as the chain length increases, Alkanes with branched chains have lower boiling points compared to straight chains due to packing closely which makes the Van der Waals forces ineffective

Question 8

Solubility of Alkanes

Answer 8

Insoluble in water- due to strong hydrogen bonds in water

Question 9

Reactivity of Alkanes

Answer 9

Unreactive- strong C-C and C-H bonds
Burn with Alkenes under suitable conditions

Question 10

Essential features of Fractional Distillation of Crude Oil

Answer 10

Temperature gradient up the column from hotter to cooler

Hydrocarbons rise up and condense when they reach their boiling points

Hydrocarbons with longer chains have higher boiling points condense further down the column whilst shorter chain hydrocarbons condense further up the column

Question 11

Cracking

Answer 11

Breaking of C-C bonds in Alkanes
Larger chains are broken into smaller chains making efficient fuels
Alkenes are also formed which are used in the polymer industry

Question 12

Thermal Cracking

Answer 12

High temperature
High pressure

Produces alkenes, for polymer industry

Question 13

Catalytic Cracking

Answer 13

Zeolite catalyst
Moderately high temperature
Slight pressure

Produces branched alkanes. for motor fuels and aromatic hydrocarbons

Question 14

Main use of Alkanes

Answer 14

Fuels

Question 15

Two types of combustion

Answer 15

Complete and Incomplete

Question 16

CO pollutant

Answer 16

Formed by incomplete combustion, poisonous gas

Question 17

NOx pollutants

Answer 17

Found in combustion engines
N2(g) + O2 (g) —> 2NO(g)
Respiratory issues

Question 18

SO2 pollutant

Answer 18

Formed when sulfur-containing impurities in crude oil are burned
Forms Acid Rain

Question 19

C pollutant

Answer 19

Formed from incomplete combustion
Causes global dimming

Question 20

How are catalytic converters used to remove harmful gases?

Answer 20

Polluting gas pass over the platinum catalyst, they react with each other to form less harmful products

2CO (g) + 2NO (g) —> N2 (g) + 2CO2 (g)

Question 21

Flue gas desulfurisation

Answer 21

CaCO3 (s) + 1.5O2 (g) + SO2 (g) —> CaSO4 (s) + CO2 (g)

Question 22

Why are Alkanes unreactive?

Answer 22

Atoms in Alkanes are held together by non-polar sigma bonds, electronegativities of carbon and hydrogen is very similar, sigma bonds are very strong.

Question 23

Free radical substitution of methane

Answer 23

1. Initiation

Cl-Cl —> (UV) 2CL*

2. Propagation

CH4 + Cl* —> HCL + CH3*
CH3* + Cl2 —> CH3Cl + Cl*

3. Termination

Cl* + Cl* —> Cl2
Cl* + CH3* —> CH3Cl
CH3* + CH3* —> C2H6

Question 24

Free radical

Answer 24

Species with an unpaired electron represented by a single dot

Question 25

Formation of Ozone

Answer 25

O2 (g) —> (UV) 2O (g)
O2 (g) + O (g) —> O3(g)

Question 26

Propagation- Ozone Depletion

Answer 26

• Cl + O3 —> * ClO + O2
• ClO + O3 —> 2O2 + * Cl

Question 27

Overall equation of Ozone depletion

Answer 27

2O3 —> 3O2
<—