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A-level Chemistry > 4.2 - Hydrocarbons > Flashcards

4.2 - Hydrocarbons Flashcards

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1
Q

What is an alkane ?

A

A saturated hydrocarbon with the general formula CnH2n+2

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2
Q

What is the general formula of alkanes ?

A

CnH2n+2

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3
Q

What are the physical properties of alkanes ?

A
  • Tetrahedral shape around each carbon atom
  • Bond angle = 109.5o
  • Non-polar molecules and therefore only London forces between molecules
  • Boiling point increases when more electrons are present and there is less branching
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4
Q

Explain why the boiling point of alkanes increases due to an increased number of electrons ?

A
  • If there are more electrons present this means that London forces are stronger and therefore take more energy to overcome
  • This causes the boiling point to increase
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5
Q

Explain why the boiling point of alkanes increases due to less branching ?

A
  • If there is less branching, then there are more points of surface contact between molecules
  • This means there are more London forces which take more energy to overcome
  • This results in a higher boiling point
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6
Q

What are the chemical properties of alkanes increases due?

A
  • Alkanes are saturated hydrocarbons with the general formula CnH2n+2
  • They contain sigma bonds
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7
Q

Define the term ‘sigma bond’ ( σ bond ) ?

A

Sigma bond - The shared pair of electrons between the nuclei of two bonded atoms

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8
Q

Describe how a sigma bond forms ?

A

The overlap of orbitals directly between the nuclei of two atoms

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9
Q

What are the two types of combustion reactions for alkanes ?

A
  • Complete combustion
  • Incomplete combustion
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10
Q

What are the conditions and products for the complete combustion of an alkane ?

A
  • Complete combustion requires plenty/ excess oxygen
  • It produces COv2 and Hv20
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11
Q

What are the conditions and products of the incomplete combustion of alkanes ?

A
  • Incomplete combustion occurs in limited supply of oxygen
  • It products C or CO and Hv20
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12
Q

Do you know how to balance combustion equation of alkanes ?

A

Yes

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13
Q

What are the reagents for forming haloalkanes ?

A
  • Alkane
  • Halogen ( Fluorine, Chlorine, Bromine, etc.. )
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14
Q

What are the conditions for the formation of haloalkanes ?

A

UV light ( provides energy to break the bond in the halogen )

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15
Q

What type of reaction is the formation of haloalkanes ?

A

A substitution reaction

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16
Q

Define the term ‘substitution reaction’ ?

A

Substitution reaction - A reaction where an atom or group of atoms is replaced by another atom or group of atoms

17
Q

Define the term ‘addition reaction’ ?

A

Addition reaction - A reaction in which a reactant is
added to an unsaturated molecule to make a one
saturated molecule

18
Q

Define the term ‘elimination reaction’ ?

A

Elimination reaction - The removal of a molecule
from a saturated molecule to make an unsaturated molecule

19
Q

What are the mechanisms of the formation of haloalkanes ?

A

1 ) Initiation
2 ) Propagation
3 ) Termination

20
Q

What happens during ‘Initiation’ mechanism when forming haloalkanes ?

A
  • Involves the homolytic fission of the covalent bond in the halogen
  • This forms two highly reactive radicals
  • The energy to break the bonds is provided by UV light
21
Q

Explain how the formation of two chlorine radicals from a chlorine molecule during initiation is an example of homolytic fission ?

A

One electron from the bond pair goes to each chlorine atom to form two chlorine radicals

22
Q

What happens during ‘Propagation’ mechanism when forming haloalkanes ?

A
  • A chain reaction is propagated via two propagations steps
  • The halogen radical first reacts with a C-H bond in the alkane forming an alkyl radical and a molecule of hydrogen halide
  • The alkyl radical then reacts with another halogen molecule to form a haloalkane, while reforming a halogen radical
23
Q

Explain how a chain reaction is is propagated by the ‘propagation’ mechanism ?

A

The new halogen radical produce is then able to attack another alkane molecule, so the china reaction can continue

24
Q

What happens during ‘termination’ mechanism when forming haloalkanes ?

A
  • Two radicals collide ( by chance ) forming a product that has no unpaired electrons
  • By removing radicals from the reaction mixture in this way, the reaction is stopped
25
Q

What are the limitations of forming haloalkanes ?

A
  • You can get further substitution and the desired product made could react again
  • Substitution can occur at different points on the carbon chain producing a mixture of chain isomers in your product
26
Q

Why may radical substitution not be the best way to form a product ?

A
  • This is because desired products may undergo further substitution
  • Substitution may occur at different points in the carbon chain resulting in a mixture of position isomers that are costly/ time consuming to seperate
27
Q

Define the term ‘homolytic fission’ ?

A

Homolytic fission - Each bonding atom receiving one electron from the bonded pair

28
Q

Define the term ‘heterolytic fission’ ?

A

Heterolytic fission - One bonding atom receiving both electrons from bonded pair

29
Q

Define the term ‘radical’ ?

A

Radical - A species with an unpaired electron

30
Q

Write the initiation mechanism for the following reaction : CHv4 + CLv2 —> CHv3Cl + HCl ?

A
31
Q

Write the propagation mechanism for the following reaction : CHv4 + CLv2 —> CHv3Cl + HCl ?

A
32
Q

Write the propagation mechanism for the following reaction : CHv4 + CLv2 —> CHv3Cl + HCl ?

A
33
Q

What are ‘curly arrows’ ?

A

Arrows that represent the movement of an electron pair

34
Q

Draw curly arrows to represent heterolytic fission ?

A
35
Q

Draw curly arrows to represent homolytic fission ?

A
36
Q

Do you know how to draw ‘curly arrows’ to describe reaction mechanisms with relevant dipoles ?

A

yes

A-level Chemistry (29 decks)

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