Chapter 12: Alkanes (12.1-12.2)

Question 1

general formula of alkanes

Answer 1

CnH2n+2

Question 2

alkanes are saturated hydrocarbons, what does this mean

Answer 2

alkanes have single covalent bonds and contain single carbon-hydrogen bonds only (sigma bonds)

Question 3

properties of sigma bonds

Answer 3

formed from the overlap of orbitals directly between the bonding atoms
can rotate freely

Question 4

shape and bond angle of alkane

Answer 4

tetrahedral

109.5

Question 5

boiling point trends in straight-chain alkanes

Answer 5

as the carbon chain length increases, the boiling point increases
more electrons and more points of contact between molecules means strength of London forces between molecules increases. More energy is needed to break IMFs

Question 6

boiling point trends in branched-chain alkanes

Answer 6

as branching in isomers increases, the boiling point decreases
fewer points of contact between molecules means strength of London forces decreases
less energy needed to break IMFs

Question 7

solubility of alkanes

Answer 7

alkanes are non-polar molecules

not soluble in polar solvents (e.g. water). soluble in non-polar solvents (e.g. oil)

Question 8

reactivity of alkanes

Answer 8

low reactivity
sigma bonds (C-C and C-H) are have high enthalpy (strong, hard to break)
C-C and H-H bonds are non-polar (making them less reactive)

Question 9

what is a fuel

Answer 9

fuels are substances that can be burnt in oxygen to release heat energy

Question 10

how does an alkane act as a fuel

Answer 10

although unreactive, many alkanes react exothermically with oxygen to make good fuels

Question 11

alkane + oxygen –>

excess oxygen, complete combustion

Answer 11

CO2 (g) + H2O (l)

Question 12

alkane + oxygen –>

excess alkane, incomplete combustion

Answer 12

CO (g) + H2O (l)

carbon only undergoes partial oxidation, unburnt carbon atoms can be released as carbon particles (soot)

Question 13

dangers of carbon monoxide

Answer 13

toxic gas that binds with haemoglobin in red blood cells, preventing haemoglobin from carrying oxygen
difficult to detect: colourless and odourless

Question 14

reactions of alkanes with halogens

Answer 14

alkane + halogen –> haloalkane + hydrogen-hailide

radical substitution, needs UV light to start

Question 15

steps of radical substitution (alkane + halogen)

Answer 15

1) initiation. UV radiation provides energy for homolytic fission of halogen molecule to form 2 radicals
2) propagation. 2 reactions of radicals reacting with molecules to form a molecule and a new radical (radical will be on carbon a molecule or a lone halogen)
3) termination. 3 reactions of 2 radicals reacting to form a molecule

Question 16

limitations of radical substitution for organic synthesis (production of organic molecules on an industrial level)

Answer 16

a large mixture of products can be formed:
Further substitution: the main product will still contain hydrogen atoms (easily replaced)
Structural isomers: for alkanes with a chain of 3 or more C atoms, substitution can take place at different points along the chain giving different structural isomers