alkanes Flashcards
What are alkanes ?
They are saturated hydrocarbons
What is the definition of a hydrocarbon?
They are composed of hydrogen and carbon atoms only
What is the definition of saturated?
They only contain single c-c bonds
Alkanes homologous series stats
- general formula : CnH2n+2
- -ane
- have a gradation i physical and chemical properties
- ## can be both straight chained, cyclic and branched
What is unique about carbon bonding in alkanes?
Each carbon atom can form 4 covalent bonds, the bonds formed are sigma bonds and directly overlap orbitals
What is the shape that a methane takes?
Tetrahedral with a bond angle of 109 degrees
What is a similarity between hydrogen and carbon?
They have similar values of electronegativity
What are alkanes in terms of electronegativity
- the alkane molecules are not polar
- weak intermolecular forces of attraction occur because the constant movement of electrons allows for temporary dipoles to occur , these are induced in neighbouring molecules
- London forces of attraction
What is the trend in boiling point in alkanes
As the alkane chain length increases, their relative molecular mass increases. The molecules become larger.
The larger molecules have a greater surface area
>there is a greater degree of surface area contact between adjacent molecules.
What is the trend in London forces in alkanes?
It increases
> more energy is required to overcome these increasingly strong intermolecular forces of attraction
> the more branching a molecule has, the less surface area over which intermolecular interactions are able to occur .
> more branched molecules have a lower degree of london forces
> the more branching in the molecule the lower boiling point
What is the trend in 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
What is the trend of combustion of alkanes ?
Alkanes can be completely and incompletely combusted . What is the stats in both completely and incompletely combustion?
Complete:
In excess air, alkanes will undergo complete combustion. Maximum amount of energy is transferred . Burns with a clean blue flame . Forms water and carbon dioxide
Incomplete:
In a limited air supply of air, alkanes will undergo incomplete combustion . Burns with a dirty yellow flame. Forms carbon and carbon monoxide
How can impurities in hydrocarbons be removed?
Atoms of sulfur can be contained in hydrocarbon molecules.> processing such as fractional distillation and cracking may not have removed these impurities.
They contribute to acid rain .
