3.3.2 Alkanes

Question 1

What is an alkane?

Answer 1

A saturated hydrocarbon.

Question 2

What does saturated mean?

Answer 2

Molecule containing no double bonds.

Question 3

What does petroleum consist mainly of, how is this separated?

Answer 3

Alkane hydrocarbons, fractional distillation.

Question 4

Define fraction.

Answer 4

Contains compounds that have similar b.ps.

Question 5

Different fractions have different what? What does this depend on?

Answer 5

B.ps, depends on the size or Mr of the hydrocarbons.

Question 6

What is the temperature gradient of a fractioning column?

Answer 6

Cold at the top (25), hot at the bottom (360).

Question 7

What does cracking involve?

Answer 7

Breaking C-C bonds in alkanes.

Question 8

Why do we do crack?

Answer 8

Longer chain fractions aren’t as useful and have a lower value economically.

Question 9

What does cracking produce?

Answer 9

Shorter, more useful chains. Alkenes.

Question 10

What are the conditions for thermal cracking? What does it produce?

Answer 10

400-900 C, 7000 KPa. Produces high proportion of alkenes to make plastics.

Question 11

What are the conditions for catalytic cracking? What does it produce?

Answer 11

450 C, slightly above atmospheric pressure, zeolite catalyst. Produces high proportion of branched alkanes, cycloalkanes and aromatic hydrocarbons.

Question 12

What is the general equation for complete combustion of alkanes?

Answer 12

CnH2n+2 + XO2 -> XCO2 + XH2O

Question 13

What is the general equation for incomplete combustion?

Answer 13

CnH2n+2 + XO2 -> XCO + XH2O
or
CnH2n+2 + XO2 -> XC + XH2O

Question 14

What are the problems with complete combustion of alkanes?

Answer 14

Produces CO2 + H2O, both greenhouse gases.

Question 15

What are the problems with incomplete combustion of alkanes?

Answer 15

Produces CO (toxic) and C (carbon particulates exacerbate asthma and causes cancer).

Question 16

What do exhaust fumes contain that’s harmful?

Answer 16

Contain unburnt alkanes, greenhouse gases and contribute to photochemical smog.

Question 17

What is harmful about burning petrol?

Answer 17

Burns at 1000 C, enough activation energy for N2 + O2 in the air-petrol vapour mix to react. Produces NOx gases.

Question 18

What are the equations for the reaction between N2 and O2? What do the gases produce?

Answer 18

N2 + O2 -> 2NO (Colourless)
2NO + O2 -> 2NO2 (Brown gas)
Builds up to give a brown haze and also reacts in water to give HNO3 (acid rain).

Question 19

What is harmful about using impure fossil fuels. What are the equations?

Answer 19

S impurities, major contributor to acid rain.
S + O2 -> SO2
SO2 + H2O -> H2SO3
2H2SO3 + O2 -> 2H2SO4

Question 20

What do catalytic converters do? How?

Answer 20

Remove carbon monoxide, nitrogen oxides and hydrocarbons from our car exhausts. Contain a honeycomb of ceramic material onto which metals such as platinum, palladium and rhodium are spread in a thin layer.

Question 21

Give the equations for reactions with catalytic converters. What do we still produce?

Answer 21

2CO + 2NO -> 2CO2 + N2
C8H18 + 25NO -> 8CO2 + 12N2 + 9 H2O
We still produce greenhouse gases.

Question 22

What happens to a mixture of methane and chlorine under UV light at room temp? What is the mechanism for this reaction called?

Answer 22

Reacts, very high temperature breaks Cl-Cl bond. Free radical substitution.

Question 23

What is a free radical?

Answer 23

A species that contains an odd number of electrons, with one e- not paired up with anyother.

Question 24

How are radicals formed?

Answer 24

When a covalent bond breaks with an equal splitting of the bond pair of electrons (homolytic fission).

Question 25

What is the first step in free radical substitution?

Answer 25

The initiation step. The UV provides enough energy needed to start the reaction by splitting some molecules into atoms (radicals).
Cl2 —-UV light—-> 2Cl•
Cl-Cl is weaker than the C-H bond in methane.

Question 26

What is the second step in free radical substitution?

Answer 26

Propagation step. In each step, a radical is used and a new radical is formed. Each step is exothermic, so the chain reaction might produce an explosion.
•Cl + CH4 -> •CH3 + HCl
•CH3 + Cl2 -> CH3Cl + Cl•
Overall reaction of the two propagation steps are
CH4 + Cl2 -> CH3Cl + HCl (radicals cancel)

Question 27

What is the equation for catalytic converter reaction?

Answer 27

2CO + 2NO -> 2CO2 + N2

Question 28

Why do we use a thin layer of metal in a catalytic converter?

Answer 28

Less material needed, more SA.

Question 29

Bromine water reaction?

Answer 29

Orange/ yellow to colourless.

Question 30

Saturated meaning?

Answer 30

Only single bonds, no double bonds.

Question 31

Why are sulphur impurities bad?

Answer 31

Produce SO2 (not sulphur oxides!!), contributes to acid rain.

Question 32

Explain how oxides of N are formed?

Answer 32

React with oxygen when burnt, at high temps.

Question 33

Catalyst for catalytic cracking?

Answer 33

Zeolite.

Question 34

Why is the m.p of dodecane higher than m.p of straight chain alkane when cracked?

Answer 34

More bonds, longer chain for dodecane, stronger VDW forces between molecules.

Question 35

What compound reacts with SO2 before it enters the atmosphere?

Answer 35

Calcium oxide/carbonate because it neutralises SO2.

Question 36

Why ethanol higher b.p than ethaniol?

Answer 36

Ethanol has H bonding, stronger VDW.

Question 37

Functional group isomer of enes?

Answer 37

Cyclic alkane.

Question 38

Ethene?

Answer 38

Antifreeze.

Question 39

What are the things you forget about the fractional distillation process?

Answer 39

Crude oil heated to vapourise it, fractions have different b.p, b.p depends on size.

Question 40

Characteristics of homologous series?

Answer 40

Same general formula, gradual change in physical properties, similar chemical properties, differs by CH2.

Question 41

Catalyst in converter?

Answer 41

Platinum / palladium / rhodium

Question 42

Why is cracking commercially good?

Answer 42

More high value products.