Chemistry: Alkanes
This flashcard set covers the general characteristics of alkanes, including their formula, bonding, molecular shape, and bond angles. It also introduces structural isomers—compounds with the same molecular formula but different structural arrangements.
General formula alkanes
CnH2n+2
Key Terms
General formula alkanes
CnH2n+2
Alkanes
Contain single carbon-carbon bonds only
How many bonding pairs of electrons is each carbon atoms in an alkane surrounded by
4
What is the shape and bond angle around a carbon atom in an alkane
Tetrahedral
109.5
Structural isomers
Compounds with the same molecular formula but a different structural formula
Cycloalkanes have…
Two less hydrogen atoms than the open chain alkane and are names according to the largest ring
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| Term | Definition |
|---|---|
| CnH2n+2 |
Alkanes | Contain single carbon-carbon bonds only |
How many bonding pairs of electrons is each carbon atoms in an alkane surrounded by | 4 |
What is the shape and bond angle around a carbon atom in an alkane | Tetrahedral 109.5 |
Structural isomers | Compounds with the same molecular formula but a different structural formula |
Cycloalkanes have… | Two less hydrogen atoms than the open chain alkane and are names according to the largest ring |
Why are alkanes not soluble in water | Water molecules held together by hydrogen bonds which are stronger than VDWs in alkanes |
What is the correlation between length of carbon chain and boiling point of the alkabe | As length or carbon chain increases, boiling point of alkane increases |
Why does boiling point increase with length of carbon chain | More electrons- VDWs stronger- require more energy to break |
What is the correlation between number of branches and boiling point | As number of branches increase boiling point decreases |
Why does boiling point decreases as more branches are added | Fewer points of contact between molecules- VDWs weaker- require less energy to break |
Crude oil | A mixture consisting mainly of alkane hydrocarbons- main source of fuels and petrochemicals |
Fractional distillation | The separation of the components of a liquid into fractions which differ in boiling point |
Each fraction is a group of… | Compounds that have similar boiling points and are removed at the same level |
What happens to boiling points as you go up the column | They decrease |
Process of fractional distillation |
|
what is the issue with crude oil | demand for shorter chain hydrocarbons doesn’t match abundancies in crude oil |
how are the issues with crude oil solved | longer, less useful alkanes converted into more useful molecules through cracking |
two types of cracking | thermal and catalytic |
what conditions is thermal cracking carried out under | very high temp and pressure |
what does thermal cracking produce | alkanes and a high percentage of alkenes |
why can thermal cracking create a mixture of products | c-c bonds break at different positions in the chain |
what can the products of thermal cracking be used for | to make polymers |
under what conditions is catalytic cracking carried out | high temp, slight pressure and in presence of a zeolite |
what does catalytic cracking produce | cycloalkanes, branched alkanes and aromatic hydrocarbons |
what are the products of catalytic cracking used for | motor fuels |
what is obtained from alkane fuels when they're combusted | energy |
what do the products formed in the combustion of an alkane depend on | the supply of oxygen |
what products are formed in complete combustion | carbon dioxide and water |
what products form during incomplete combustion | carbon monoxide and water |
when does incomplete combustion occur | in a limited supply of oxygen |
what products form during further incomplete combustion | solid carbon (soot) and water |
when does further incomplete combustion occur | in an even more limited supply of oxygen |
what does the combustion of hydrocarbon fuels produce | pollutants |
what is the effect of unburned hydrocarbons | low level ozone (respiratory problems) |
how do unburned hydrocarbons produce low level ozone | reactions with NO gases |
what is the effect of carbon dioxide | global warming |
what is carbon dioxide produced from | combustion of fuels |
what effect does carbon monoxide have | toxic gas |
how is carbon monoxide produced | incomplete combustion of fuels in limited supply of oxygen |
effect of carbon | particles exacerbate asthma |
how is carbon produced | incomplete combustion of fuels in a very limited supply of oxygen |
effect of nitrogen oxides | acid rain and photochemical smog |
how are nitrogen oxides produced | nitrogen and oxygen from the air react at high temperatures in engines |
effect of sulphur dioxide | acid rain |
how is sulphur dioxide produced | sulphur from fuel impurities reacts with oxygen in the air |
catalytic converters | fitted to cars to remove CO, NO and unburned hydrocarbons contain honeycomb structure coated with Pt/Pd/Rh metals honeycomb structure- large surface area |
how are NO and CO removed | react to produce less polluting products eg carbon dioxide and nitrogen |
flue gas desulfarisation | chimneys/flues coated with CaO or CaCO3 which absorb and react with SO2 produced |
what is halogenation | reacting an alkane with a halogen |
why are alkanes unreactive generally | non-polar bonds lots of strong covalent bonds need to be broken |
what conditions are needed for halogenation | uv light |
what type of reaction is halogenation | substitution- one atom replaced by other |
what is formed when alkanes with 3 or more carbons react with a halogen | a mixture of position isomers |
what happens if an alkane is reacted with an excess halogen | each hydrogen atom can be replaced by halogen atom |
What is the mechanism for halogenation of alkanes | Free radical substitution |
What is a radical | A species with an unpaired electron |
What are the 3 stages of free radical substitution | Initiation Propagation Termination |
What is initiation | Formation of radicals |
What is propagation | Formation of products |
what is termination | removal of radicals |
how is the unpaired electron of a radical shown | by a dot |