Alkanes

Question 1

Give the conditions for thermal cracking and catalytic cracking.

Answer 1

Thermal cracking has
-high temperature
-high pressure
Catalytic cracking has
-high temperature
-slight pressure
-zeolite catalyst

Question 2

Give the difference in product for thermal cracking and catalytic cracking. What are the applications?

Answer 2

Thermal cracking produces a high proportion of alkenes. Feedstock for polymers or alcohols.
Catalytic cracking produces
-branched hydrocarbons and cycloalkanes, used in motor fuels
-aromatic hydrocarbons for other uses

Question 3

Define cracking.

Answer 3

• the conversion of long-chain hydrocarbons into smaller-chain hydrocarbons by splitting of C-C bonds.

Question 4

Give the 2 economic reasons that cracking is carried out in industry.

Answer 4

1. to meet demand for short-chain hydrocarbons as fuels
2. the products of cracking are more valuable than the starting materials, so economic incentive.

Question 5

Sulfur-containing impurities, found in the heavy fuel oil fraction that is used in central heating, can cause issues to the environment. Why?

Answer 5

-on combustion/burning in air, sulfur forms SO2, which then causes acid rain.

Question 6

How can SO2 be removed from the waste gases of power stations? Give the equation.

Answer 6

• gases pass through a scrubber containing basic calcium oxide
• that neutralises the acidic SO2

SO2 + CaO -> CaSO3

Question 7

In a petrol car, NO2 is produced from the exhaust. Why does NO2 form in a car engine but not when petrol burns in open air?

Answer 7

• Nitrogen oxides form from reaction of N2 and O2 from the air
• High temperature AND sparks in the engine
• sufficient heat and energy MUST be provided to overcome activation energy and break the N to N triple bond.

Question 8

The pollutant gas CO is also found in the exhuast gases from petrol engines. Explain, using equations, how CO and NO are removed from the exhaust gases in a catalytic converter, and why the removal of each is desirable.

Why is the catalytic converter necessary for these reactions?

Answer 8

Reduction: 2NO -> N2 + O2
Oxidation: 2CO + O2 -> 2CO2
and 2NO + 2CO -> N2 + 2CO2
It is desirable to remove both as:
- CO is toxic
- NO is toxic and causes acid rain

The catalytic converter is necessary since it provides an alternative reaction route with a lower Ea.

Question 9

Describe the process of industrial crude oil fractional distillation.

Answer 9

-crude oil is heated in a furnace and vapourised
-vapours fed into a fractionating column
-which is cooler at the top and hotter at the bottom (negative temperature gradient)
-longer-chain hydrocarbons have stronger van der waals forces between them and so have a higher b.p
- and so condense lower down in the column, and viceversa

Question 10

Give 4 characteristics of a homologous series.

Answer 10

• differ by CH2
• have the same general formula
• have the same functional group
• similar chemical properties

Question 11

Outline the essential features of fractional distillation of crude oil that enable the crude oil to be seperated into fractions.

Answer 11

• different fractions have different b.p
• because larger-chain hydrocarbons have stronger VDW forces between molecules so have a higher b.p
• fractionating column is cooler at top and hotter at bottom (negative temp gradient)
• so higher boiling point fractions condense lower down in the column

Question 12

In industrial thermal cracking, a temperature of about 500 degrees and a pressure of 70 atmospheres is used.

State why high temperature are necessary for cracking reactions to occur.

Give one reason why thermal cracking reactions are carried out in industry.

Answer 12

TEMP
- high temperatures are required to break the single covalent C-C bonds
REASON
- to produce a large proportion of alkenes which are are in demand as feedstock for other industrial pipelines

Question 13

In the incomplete combustion of pentane, a solid pollutant is formed.

State the identity of this solid pollutant.

Suggest why this solid pollutant is an environmental problem.

Answer 13

• carbon/soot
• causes global dimming, and worsens asthma for people

Question 14

Sulfur dioxide can be removed from flue gases in power stations in a process called flue gas desulfurisation.

Identify a compound that is used to react with the sulfur dioxide in the products of combustion before they enter the atmosphere.

State a reason why this compound reacts with sulfur dioxide.

Answer 14

• calcium oxide
• it is basic and neutralises the acidic SO2