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OCR H032 AS Chemistry > Alkanes > Flashcards

Alkanes Flashcards

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1
Q

What is the general formula

A

CnH2n+2

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2
Q

At rtp which of these are gases, liquids or solids

(In the case of straight chains)

C1-C4 C5-C16 C17+

A

C1-C4 gas
C5-C16 liquid
C17+ solid

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3
Q

As the size of the chain increases so does the mp and bp

But why?

A

The forces of attraction between molecules is stronger

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4
Q

In alkanes what are the intermolecular forces that occur (also occur in other homologous series)

A

London forces

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5
Q

What is the effect of branching

A 
Branched isomers have:
Lower mp and bp
Increased volatility 
Decreased density
Lower viscosity
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6
Q

Why do branched isomers have a lower mp and bp

A

Branching causes the molecules to pack less closely together therefore there is a smaller surface area of contact between molecules
Therefore reduced induced dipole
Therefore weaker London forces
Therefore lower mp and bp

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7
Q

What colour are alkanes when pure

A

Colourless

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8
Q

Are they soluble or insoluble in water and why

A

Insoluble they are non polar, water is very polar.
They are less dense than water and form an immiscible upper layer
They dissolve very well in other non polar solvents like each other

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9
Q

All bonds in alkanes are…….
A) sigma bonds
B) pi bonds
C) delta bonds

A

A) sigma bonds

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10
Q

What is the characteristic bond angle in alkanes between hydrogen and hydrogen

A

109.5 degrees

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11
Q

What is the shape of an alkane

A

Tetrahedral

Bonding electrons repel each other and the tetrahedral shape allows for the greatest separation of them

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12
Q

Are alkanes:

a) very reactive
b) very unreactive

Towards ionic and polar substances

A

B) very unreactive

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13
Q

Why are alkanes so unreactive

A

C-C and C-H bonds very strong and non polar

Relatively high bond enthralpies

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14
Q

Why are alkanes used as fuel for combustion

A

All alkanes react with a plentiful supply of oxygen and don’t produce toxic products
Readily available
Easy to transport

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15
Q

Are combustion reactions of alkanes endothermic or exothermic

A

Very exothermic

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16
Q

Are combustion reactions of alkanes spontaneous or not and why

A

Not (activation energy is required) because combustion involves a free radical mechanism

17
Q

What is the general equation of complete combustion of alkanes

A

CxHy + (X+Y/4)O2 –> XCO2 +y/2 H2O

18
Q

What products for when alkanes incompletely combust

A

Carbon or carbon monoxide

Water

19
Q

Alkanes react with halogens in what conditions

A

Uv light (sunlight)

20
Q

What is the reaction called between alkanes and halogens

A

Substitution reaction

21
Q

What is the mechanism called for bromination of alkanes

A

Free radical substitution

22
Q

What are the three steps called for the bromination of alkanes

A

Initiation
Propagation
Termination

23
Q

What happens in the initiation step

A

Primary free radicals form

24
Q

What is the activation energy for the initiation step and why is it needed

A

Uv light to break bonds by homolytic fission

25
Q

Finish this initiation step

   Uv light Br2 ----------> ?
A

2Br^. ( free radical)

26
Q

What happens in the propagation step

A

Secondary free radicals form, this is described as a chain reaction

27
Q

Finish these equations
Br ^. + CH4 -> ? + HBr

? + Br2 -> CH3Br + ?

A 
?1 = ^.CH3
?2= ^.CH3
?3= Br^.
28
Q

What happens in the termination stage

A

Two radicals collide and react, both radicals are removed from the reaction mixture stopping the reaction

29
Q

Complete these termination reactions

Br^. + Br^. -> ?
^.CH3 + ^.CH3 ->?
^.CH3 +^.Br -> ?

A

Br2
C2H6
CH3Br

30
Q

If the molar ration of Cl2:CH4 (halogen:molar ratio) what is the major product

A

CH3Cl

31
Q

How would you purely obtain the major organic product from the free radical mechanism

A

Distillation

33
Q

What are the limitations of obtaining haloalkanes via free radical substitution

A

Many different products are made not just the desired haloalkane
Further substitutions may occur
In larger alkanes substitution may occur in different positions

34
Q

Free radicals occur in a ? Phase only, ? And solid alkanes Must be ? first

A

Gas
Liquid
Vapourised

OCR H032 AS Chemistry (12 decks)

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