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chemistry ocr a a level > Chapter 12 -Alkanes > Flashcards

Chapter 12 -Alkanes Flashcards

Properties of alkanes, Chemical reactions of alkanes.

1
Q

What is the main use of alkanes?

A

Fuel

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2
Q

What is the general formula of alkanes?

A

CnH2n+2

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3
Q

What kind of covalent bonds are formed in alkanes?

A

Sigma bonds

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4
Q

What is a sigma bond?

A

The overlap of two orbitals

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5
Q

How are alkanes separated?

A

Fractional distillation

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6
Q

How does chain length effect boiling point?

A

Longer chain length raises boiling point

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7
Q

Why does chain length have that effect on boiling point?

A

Increased chain length —>
Greater surface area of contact —>
Stronger London forces

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8
Q

What is the effect of branching on boiling point?

A

Lowers boiling point

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9
Q

Why does branching have that effect on boiling point?

A

More branching —>
Less surface area of contact —>
Weaker London forces

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10
Q

What are the reasons for alkanes being fairly unreactive?

A
  • C-C and C-H sigma bonds are strong
  • C-C bonds are non polar
  • similar electronegativity of C and H
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11
Q

What is the combustion reaction of an alkane in words?

A

Alkane + oxygen —> water + carbon dioxide

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12
Q

What is complete combustion?

A

when alkanes burn completely to produce carbon dioxide and water

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13
Q

What are the two potential products from incomplete combustion of alkanes?

A

carbon monoxide

carbon

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14
Q

What is the name of the mechanism for bromination of alkanes?

A

free radical substitution

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15
Q

What are the three stages of radical substitution?

A

initiation

propagation

termination

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16
Q

Describe the initiation stage of radical substitution

A

covalent bond in a bromine molecule is broken by homolytic fission

energy is provided by UV radiation (sunlight)

Br-Br –> Br* + Br*

17
Q

Describe the propagation stage of radical substitution

A

Two propagation steps :

Step 1 - CH4 + Br* -> *CH3 + HBr

Step 2 - CH3 + Br2 -> CH3Br + Br

18
Q

Describe the termination stage of radical substitution

A

Two radicals collide forming a molecule with all electrons paired.
Br* + Br* -> Br2

  • CH3 + *CH3 -> C2H6
  • CH3 + Br* -> CH3Br
19
Q

What can occur after a radical substitution reaction?

A

Further substitution

chemistry ocr a a level (35 decks)

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