Ch10 - Alkanes

State the type(s) of intermolecular interactions between alkane molecules, and why.

Intermolecular forces of attraction between alkane molecules are weak instantaneous dipole-induced dipole interactions.

Alkanes are non-polar.

State the 2 factors that affect the boiling and melting points of alkanes.

• Number of carbon atoms

• Degree of branching

Explain how number of carbon atoms affect boiling and melting points of alkanes.

• When number of carbon atoms increases,

• number of electron increases,

• hence electron cloud size increases.

• The ease of polarisation of electron cloud increases,

• hence strength of instantaneous dipole-induced dipole attractive forces between alkane molecules increases,

• requiring increasingly large amounts of energy to overcome.

• Hence melting and boiling points increases.

Explain how degree of branching affects boiling and melting points of alkanes.

• When degree of branching increases,

• alkane molcules become more compact,

• decreasing surface area available for intermolecular forces of attraction,

• decreasing the extent of interaction with neighbouring molecules,

• hence weaker instantaneous dipole-induced dipole interactions between alkane molecules,

• requiring less energy to overome,

• hence melting and boiling points decreases.

Explain the solubility of alkanes in non-polar and polar solvents.

• soluble in non-polar solvents (e.g. benzene)

• insoluble in polar solvents (e.g. water)

Explain alkanes as solvents.

Alkanes can be used as non-polar solvents (e.g. hexane), they dissolve compounds of low polarity only.

State the density of alkanes relative to water.

Alkanes are less dense than water. (they tend to level off ~0.8g/cm³)

State factor(s) affecting density of alkanes.

Molecular size

State the type(s) of reaction(s) for preparation of alkanes, as well as reagents and conditions.

Reaction: Reduction of Alkenes

Reagents: H₂(g)

Conditions: Ni catalyst

State the type(s) of reaction(s) that alkanes undergo.

• Reaction: Free Radical Substitution
  Reagents: Cl₂ / Br₂
  Conditions: ultraviolet light at room temperature

• Reaction: Combustion
  Reagents: excess O₂(g)

Define homolytic fission.

Homolytic fission is when a covalent bond breaks such that each of the atom involved retains one of the two shared electrons.

Explain the general lack of reactivity of alkanes.

1. Alkanes are non-polar.
   They do not contain regions of high electron density to attract electrophilic reagents.
   They do not contain electron-deficient sites to attract nucleophilic reagents.

2. Alkanes have relatively strong C-C and C-H bonds which do not break under normal conditions.

State the 3 steps in the FRS mechanism.

1. Initiation (homolytic fission)

2. Propagation (a. radical attack & formation of HX, b. regeneration of radical)

3. Termination

Explain the stability of the secondary radical, relative to its primary radical.

The secondary radical is more stable than the primary radical due to the presence of one more electron-donating group attached to the electron-deficient C atom. Therefore, it gives rise to a greater proportion of the product formed from the secondary radical than expected.

Generally explain the environmental consequences formed from hydrocarbons as fuels.

• Formation of CO(g) due to incomplete combustion and NO_x(g) formed by oxidation of atmospheric nitrogen due to high temperatures in the engine.

• Incomplete combustion of hydrocarbons may have occurred due to the reaction time in the engine being too short (despite excess O₂(g) provided)

Explain the adverse effects of CO(g).

• CO(g) is a greenhouse gas, contributing to the enhanced greenhouse effect.

• CO(g) binds irreversibly with haemoglobin in the bloodstream, rendering it ineffective to oxygen.

• Affects mental alertness. 10% in air in confined spaces, fatal in 2 minutes.

Explain how to reduce CO(g), unburnt hydrocarbons and carbon particulates in the atmosphere.

1. Careful adjustment of fuel/air proportions and design of engines.

2. Inclusion of catalytic converters in exhaust assembly to finish combustion reaction.

Explain the adverse effects of unburnt hydrocarbons and carbon particulates.

Formation of photochemical smog (in the presence of sunlight)

• irritates the respiratory tract and results in chest pains and breathing difficulties

• harmful effects on plants

Explain the adverse effects of nitrogen oxides, NO_x(g).

1. Acidic gas, contributing to acid rain.

• renders farmlands unsuitable for cultivation

• corrosion of architecture

• harmful to marine life (increase in acidity of water)

2. Contributor to photochemical smog.

Explain how to reduce NO_x(g) in the atmosphere.

The use of catalytic converters to reduce the oxides to nitrogen.

State what catalytic converters do.

Catalytic converters remove main pollutants i.e. CO, NO_x and unburnt hydrocarbons, by converting them into CO₂, N₂ and water vapour.

Explain the enhanced greenhouse effect, and its adverse effects.

• The enhanced greenhouse effect is caused by the increase in concentration of greenhouse gases in the atmosphere traps more heat in the atmosphere, as less infrared radiation radiates into space.

Effect(s):

• more frequent and intense hot and cold extreme temperatures, precipitation events, droughts and hurricanes.

• rising global sea levels due to thermal expansion of warming oceans and water added to oceans by melting glaciers and ice sheets.