Alkanes | A-level Chemistry | OCR, AQA, Edexcel

Alkanes in a Snap!

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The key points covered in this video include:
1. Introduction to Alkanes
2. Bonding in Alkanes
3. Intermolecular Forces of Attraction
4. Trends: Boiling Points
5. The Reactivity of Alkanes
6. Reactions
a) Combustion
b) Products of Combustion
c) Radical Substitution
7. Catalytic Converters

What are Alkanes?

Alkanes are: Hydrocarbons. They are composed of only carbon and hydrogen atoms. Saturated. They contain only single C-C bonds. A homologous series. Have a general formula, Have a gradation in physical and chemical properties. Alkanes can be: Straight Chain, Branched, Cyclic.

Alkanes: Bonding

Carbon atoms have an electronic arrangement of 2,4. Each carbon atom can form 4 covalent bonds. The bonds formed are sigma bonds. Direct overlapping of orbitals.

Alkanes: Intermolecular Forces

Carbon and hydrogen have very similar values of electronegativity. Molecules are not polar. Weak intermolecular forces of attraction occur. Constant movement of electrons allows for temporary dipoles to occur. These are induced in neighbouring molecules. London Forces of attraction.

Alkanes: Boiling Points

We see a trend in the boiling point of alkanes. Length of Carbon Chain: As alkanes increase in length, their relative molecular mass increase. The molecules become larger. The larger molecules have a greater surface area. There is a greater degree of surface area contact between adjacent molecules. The London Forces increases. More energy is required to overcome these increasingly strong intermolecular forces of attraction. As the chain length increases, the boiling point does too. Branching of the Carbon Chain: The more branching a molecule has, the less surface area over which intermolecular interactions are able to occur. Molecules are not able to fit together as well. E.g. Pentane, Boiling point: 36.1°C, Methylbutane, Boiling Point: 27.8°C. More branched molecules have a lower degree of London Forces, The more branching in the molecule the lower the boiling point.

The Reactivity of Alkanes

Alkanes have a relatively low reactivity. The C-H bond has a very low polarity. The sigma bonds have a low polarity, The electronegativity of carbon and hydrogen are very similar. The Bonds have large bond enthalpies. Lots of energy is required to break these bonds.

Combustion of Alkanes

During the process of combustion oxygen from the air is combined with an alkane. This process is rapid. Alkanes are frequently used as, or as a component of, a fuel source. Through combustion their chemical energy is transferred to thermal energy. Alkanes can be completely and incomplete combusted. Complete Combustion: In excess air, alkanes will undergo complete combustion, Maximum amount of energy is transferred, Burns with a clean blue flame. Incomplete Combustion. In a limited supply of air, alkanes will undergo incomplete combustion, Burns with a dirty yellow flame.

The Products of Combustion

Complete Combustion
Water: The formation of this product is not an issue as it simply adds to the worlds existing water supplies. Carbon Dioxide: Greenhouse Gas. Increasing levels of CO2 are thought to be responsible for increasing global temperatures. Incomplete Combustion. Carbon: Solid carbon can be formed through incomplete combustion. Soot. Carbon Monoxide: CO is a colourless, odourless gas. It is the Silent Killer. Prevents the effective transport of oxygen around the body, Binds to haemoglobin in place of the oxygen. Unburned Hydrocarbons: Some unburned hydrocarbons exist. Impurities. Oxides of Sulfur: Atoms of sulfur can be contained in hyrdocarbon molecules. Processing, such as fractional distillation and carcking, may not have removed these impurities. Sulfur, sulfur dioxide and sulfur trioxide may be formed in combustion. These gases are acidic oxides, When they dissolve in water, they form sulfuric and sulfurous acid. These contribute to acid rain. Oxides of Nitrogen: Combustion of Nitrogen containing alkanes will lead to the formation of nitrogen oxides. Nitrogen monoxide, Nitrogen dioxide. This can dissolve in water to form acid rain.