Fractions
Properties of fractions
Each In fractional distillation, such as that of crude oil, the different parts of the original mixture are called fractions. The substances in each fraction have similar boiling points to each other. contains a Two or more substances that are not joined together. The substances can be elements, compounds, or both. of A compound that contains hydrogen and carbon only.. The hydrocarbons in a fraction are mostly hydrocarbons called Saturated hydrocarbon. A compound of hydrogen and carbon only, with no C=C bonds.. They have similar but not identical:
- numbers of hydrogen and carbon atoms in their A collection of two or more atoms held together by chemical bonds.
- The temperature at which a substance rapidly changes from a liquid to a gas.
- How easily something will burn or ignite.
- A measure of how difficult it is for a substance to flow - the higher the viscosity, the 'thicker' it is.
For example, the gases fraction contains hydrocarbons with between one and four carbon atoms. These have boiling points below room temperature. They are very flammable.
The hydrocarbons in different fractions differ in these properties. For example, the bitumen fraction contains hydrocarbons with more than 35 carbon atoms. These have boiling points well above room temperature. They are very difficult to ignite and have a high viscosity.
Alkanes as a homologous series
A A 'family' of organic compounds that have the same functional group and similar chemical properties. is series of A substance formed by the chemical union of two or more elements. which:
- have the same An algebraic formula that sets out a rule or trend which is followed by all members of a homologous series. For example, all non-cyclic alkanes have the general formula CnH2n+2.
- differ by CH2 in the Chemical formula showing the actual number of atoms of each element in a molecule. of neighbouring compounds
- show a gradual variation in physical The characteristics of something. In chemistry, chemical properties include the reactions a substance can take part in. Physical properties include colour and boiling point., such as their boiling points
- have similar chemical properties
The table shows four alkanes, their molecular formulae and their structures.
The general formula for the alkanes is CnH2n+2, with n standing for the number of carbon atoms.
As the number of carbon atoms increases:
- the Weak attractive forces between molecules. When a simple molecular substance melts or boils, it is the intermolecular forces that are broken (not the covalent bonds in each molecule). between alkane molecules become stronger
- more The capacity of a system to do work or the quantity required for mechanical work to take place. Measured in joules (J). For example, a man transfers 100 J of energy when moving a wheelbarrow. is needed to overcome them
- the boiling point increases
Question
Describe how you can tell from their molecular formulae that ethane, propane and butane are neighbouring compounds in the same homologous series.
Their molecular formulae differ by CH2 from one compound to the next. For example, ethane is C2H6 but propane is C3H8 and the next one, butane, is C4H10.