4.1.2 Alkanes Flashcards

1
Q

What’s an example of a mixture of hydrocarbons, that the majority of are alkanes

A

Crude oil

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2
Q

What are alkanes

A

Saturated hydrocarbons with no free spaces

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3
Q

What’s the general formula of alkanes

A

CnH2n+2

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4
Q

How can crude oil be separated into different fractions

A

Fractional distillation

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5
Q

How reactive are alkanes

A

Very unreactive due to very strong bonding between carbons (sigma bond)

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6
Q

3 reasons alkanes make good fuels

A
  • release large amount of energy when burnt (exothermic)
  • readily available at reasonable cost
  • convenient to transport + store
  • generally no toxic products
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7
Q

2 reasons alkanes are problematic as fuels

A
  • fossil fuels are being consumed faster than replaced (non-renewable)
  • can produce CO2, CO + C pollutants
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8
Q

A large quantity of what is produced when an alkane is completely combusted
Why is this bad

A

CO2

Is a greenhouse gas that contributes to global warming

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9
Q

What’s completely combusted

A

When plenty of oxygen is present to convert all hydrogen to H2O and all carbon to CO2

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10
Q

What situation may an alkane be incompletely combusted

A

If an engine is old/poorly maintained so oxygen is limited and less able to reach each alkane molecule

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11
Q

What’s produced in incomplete combustion

A

Carbon monoxide and carbon

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12
Q

Problem with carbon monoxide

A

Toxic - silent killer

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13
Q

Problem with carbon particles (soot)

A

Harmful by inhalation and can cause cancer, global dimming/asthma

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14
Q

What molecules are more susceptible to incomplete combustion

What flame do they burn with

A

Larger

Sooty flame

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15
Q

What happens to boiling point of alkanes as chain length increases
Why

A

Increases

Longer the chain more electrons + SA, more strength/number of IMFs, more IDDI, more surface contact

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16
Q

What happens to boiling point if there are branches

A

More branches
Less surface contact
Fewer IDDIs
Lower BP

17
Q

Whats formed when an alkane reacts with a halogen

A

A halogenoalkane (haloalkane)

18
Q

What does a chemical mechanism show

A

The movement of electrons during a reaction

19
Q

2 ways electrons move in a mechanism

What’s the way we’re looking at

A

Singly/in pairs

We’re focusing on electrons moving singly

20
Q

What mechanism is happening if electrons are moving singly

A

Free radical mechanism

21
Q

What’s a free radical

A

An atom/molecule/ion with unpaired valence electrons

22
Q

When do free radicals form

How

A

In homolytic fission

2 electrons in a covalent bond are returned to the atoms from which they originated during covalent bond formation

23
Q

What’s formed if a Cl2 molecule is split homylitically

What’s their reactivity like and why

A

2 chlorine radicals

Very reactive as no longer have full outer shells and will react with many species to regain a full outer shell

24
Q

What’s homolytic fission the reverse of

A

Covalent bond formation

25
Q

What does a fishhook arrow represent

A

Movement of only 1 electron (singly)

26
Q

What’s the mechanism for the reaction of an alkane + halogen

A

The free radical substitution mechanism

27
Q

What does the free radical substitution mechanism involve

A

Free radicals

Swapping of a H atom in Alkane by a halogen atom

28
Q

What are the 3 steps to this mechanism

A

Initiation
Propagation
Termination

29
Q

What is used in the initiation to split the molecule to make free radicals
What does this provide

A

UV light

Provides energy to break bonds

30
Q

What happens during propagation

A

Free radicals used up
Products are formed
More free radicals are regenerated

31
Q

What’s dangerous about the propagation steps

A

Explosive

32
Q

What steps of this mechanism are repeated explosively

A

2x Propagation steps

33
Q

What happens in termination

A

2 free radicals colliding will cancel each other out and form a stable molecule

34
Q

What is the result of adding up the 2 propagation Equations

A

The overall equation

35
Q

Why can side products be made from this mechanism (further substitution)

A

As free radicals are so reactive

36
Q

Why is free radical substitution not a reliable way of making haloalkanes
What’s the general way of making haloalkanes

A

Due to the many side products

By alkenes