2.5 Hydrocarbons

Question 1

What are the only products when a
hydrocarbon fuel undergoes complete
combustion?

Answer 1

Water (H2O) and carbon dioxide (CO2)

Question 2

Write a balanced chemical equation for the complete combustion of ethane

Answer 2

2C2H6 + 7O2 → 4CO2 + 6H2O

Question 3

Write a balanced chemical equation for the complete combustion of ethane

Answer 3

2C2H6 + 7O2 → 4CO2 + 6H2O

Question 4

When does incomplete combustion occur? What is required to ensure complete combustion occurs?

Answer 4

Incomplete combustion occurs when there is an insufficient supply of oxygen.
To ensure complete combustion occurs, the
reaction should be carried out with excess oxygen.

Question 5

What are the products of incomplete
combustion?

Answer 5

Carbon particulates - soot (C)
Carbon dioxide (CO2)
Carbon monoxide (CO)
Water (H2O)

Question 6

Write a balanced symbol equation for the
incomplete combustion of methane to
form carbon monoxide and water

Answer 6

2CH4 + 3O2→ 2CO + 4H2O

Question 7

Write a balanced symbol equation for the
incomplete combustion of methane to
form carbon monoxide and water

Answer 7

2CH4 + 3O2→ 2CO + 4H2O

Question 8

advantages of fossil fuels

Answer 8

produces large amounts of energy
currently relatively easily available

Question 9

disadvantages of fossil fuels

Answer 9

uses infinite resources
releases CO2 which contributes to global warming
releases carbon monoxide which is toxic
produces acidic gases which causes acid rain

Question 10

incomplete combustion eg propane

Answer 10

propane + oxygen—>carbon dioxide + carbon monoxide

Question 11

What type of bonds are the C-C and C-H
bonds in alkanes?

Answer 11

σ-bond
(Sigma bond)

Question 12

Give an example of a type of reaction
that uses a radical substitution
mechanism?

Answer 12

photochlorination of alkanes

Question 13

What is a photochlorination reaction?

Answer 13

A photochlorination reaction occurs when
one of the hydrogen atoms on an alkane
is replaced by a chlorine atom. The
reaction is initiated by light.

Question 14

Compare the difference in reactivity of
alkanes and alkenes

Answer 14

Alkenes have a high electron density in the
carbon-carbon double bond, C=C. This makes
them more susceptible to attacks from
electrophiles, compared to alkanes, making
alkenes more reactive.

Question 15

Describe the C=C bond in alkenes in
terms of the types of covalent bonds
present

Answer 15

The C=C has a normal σ-bond as well as a π-bond located above and below the σ-bond.
σ-bond: sigma bond
π-bond: pi bond

Question 16

What is stereoisomerism (in terms of E-Z
isomerism)?

Answer 16

Stereoisomerism occurs when isomers have the same structural formula but a different arrangements of atoms in space due to the limited rotation around the carbon-carbon double bond.
Isomers are normally referred to as E- or Z- isomers.

Question 17

How do you determine whether an isomer is E- or Z-?

Answer 17

● In E- isomers, high priority groups are on
opposite sides of the C=C bond.
● In Z- isomers, high priority groups are on the same side of the C=C bond.

Question 18

Why can alkenes undergo electrophilic
addition?

Answer 18

Alkenes are susceptible to attack from
electrophiles because of the high
electron density in the C=C double bond

Question 19

Which qualitative tests can be carried out
to test for alkenes?

Answer 19

• Add bromine water: The orange solution decolourises if the alkene C=C bond is present.
• Add acidified potassium dichromate: The purple solution decolorises if the alkene C=C bond is present.

Question 20

Explain why the electrophilic addition of
bromine to propene can produce a major
and a minor product

Answer 20

When propene undergoes electrophilic addition, two possible products can be formed: the major and the minor product. This is because two different carbocation intermediates can be formed - the primary and the secondary carbocation. The secondary carbocation is more stable so this produces the major product
2- bromopropene. The primary carbocation forms the minor product 1-bromopropane.

Question 21

What reaction does an alkene undergo to form an alkane? Name the reagents and conditions

Answer 21

● Reaction type: Hydrogenation
● Reagents: Hydrogen gas, H2
● Conditions: Nickel catalyst, 150℃

Question 22

What is addition polymerisation?

Answer 22

Addition polymerization occurs when lots of monomers with carbon-carbon double bonds join together to form a long-chain polymer.
The 𝝅 bond breaks and the electrons are used to form 𝛔 bonds with other monomers on either side of it.
Atom economy is 100%.

Question 23

what is a repeat unit?

Answer 23

A single unit of a polymer derived from its monomer, the repetition of which
would result in the polymer.