Alkanes Key Definitions

Question 1

Sigma bond

Answer 1

a type of covalent bond which forms when atomic orbitals overlap head-on. Sigma
(σ) bonds can rotate freely.

Question 2

Alkane

Answer 2

a homologous series with the general formula CnH2n+2.

Question 3

Bond Enthalpy

Answer 3

the energy required to break one mole of gaseous bonds.
Actual bond
enthalpies may differ from the average as the average bond enthalpy considers a particular
bond in a range of molecules

Question 4

Complete combustion

Answer 4

when a compound is burnt in a plentiful supply of oxygen.

When alkanes are completely combusted, the only products are water and carbon dioxide.

Question 5

covalent bond

Answer 5

a strong bond formed between 2 atoms due to the electrostatic attraction
between a shared pair of electrons and the atomic nuclei.

Question 6

Electron pair repulsion

Answer 6

pairs of electrons around a nucleus repel each other so the shape that a molecule adopts has these pairs of electrons positioned as far apart as possible.
As a result, carbon atoms in alkanes have a tetrahedral shape and a bond angle of 109.5°.

Question 7

homolytic fission

Answer 7

when a covalent bond breaks, each bonding atom receives one electron
from the bonding pair, forming 2 radicals.

Question 8

hydrocarbon

Answer 8

a compound which contains hydrogen and carbon atoms only.

Question 9

incomplete combustion

Answer 9

when a compound is burnt in a limited supply of oxygen.

When alkanes are incompletely combusted, water, carbon monoxide, particulates and some carbon dioxide may be produce

Question 10

Initiation

Answer 10

the first step in a radical substitution mechanism, involving the formation of the radical

Question 11

London forces

Answer 11

: induced dipole-dipole interactions caused when the random movement of
electrons creates a temporary dipole in one molecule which then induces a dipole in a neighbouring molecule

Question 12

polar bond

Answer 12

a covalent bond that has a permanent dipole due to the different
electronegativities of the atoms that make up the bond.

Question 13

Propagation

Answer 13

the intermediate steps in a radical substitution mechanism where a radical reacts with another species.

Question 14

radical

Answer 14

a species with an unpaired electron, E.g. Cl•.

Question 15

radical substitution

Answer 15

: a type of substitution reaction in which a radical replaces another
atom/ group of atoms in a compound.
Alkanes can undergo radical substitution reactions with chlorine or bromine in the presence of UV light, forming a mixture of organic products (further
substitution or reactions at different positions may occur)

Question 16

saturated

Answer 16

an organic compound which only contains single C-C bonds.

Question 17

termination

Answer 17

the final steps in a radical substitution mechanism in which 2 radicals react
together to form a species which only contains paired electrons.

Question 18

Tetrahedral

Answer 18

the shape of a molecule in which the central atom has 4 bonding pairs of electrons