Alkanes Flashcards
what are alkanes?
alkanes are hydrocarbons that fit the general formula CnH2n+2
what are alkanes made up of?
they are made of simple molecules containing C and H atoms held together by covalent bonds
what does the name of the alkane show?
- the start of the name shows how many carbon atoms it has
- the end of the name is ‘-ane’ to show its an alkane
what is the formula of methane?
CH4
how many carbon atoms does methane have?
1
what is the formula of ethane?
C2H6
how many carbon atoms does ethane have?
2
what is the formula of propane?
C3H8
how many carbon atoms has propane got?
3
what is the formula of butane?
C4H10
how many carbon atoms does butane have?
4
what is the formula of pentane?
C5H12
how many carbon atoms does pentane have?
5
what are the names of alkanes after the first five?
the names follow the 2D shapes: hexane, heptane, octane etc.
what are the four different formulas of alkanes?
- displayed formula
- structural formula
- molecular formula
- empirical formula
what is the displayed formula of an alkane?
a diagram showing the positions of every atom and every bond (C and H’s)
what is the structural formula of an alkane?
this shows the structure of the molecule in one line, read from left to right like a word
eg. CH3CH2CH2CH3
what is the molecular formula of an alkane?
this only shows how many carbon and hydrogen atoms there are
e.g. C4H10
what is the empirical formula of an alkane?
this only shows the simplest whole-number ratio of carbon to hydrogen
e.g. C2H5
what is a homologous series?
alkanes are a homologous series of organic molecules - this means they are a family of molecules
what do alkanes all have?
- similar chemical reactions
- trends in physical properties (e.g. boiling points, viscosity or colour)
- the same general formula
how many series of homologous are there in the IGSCE course?
five:
- alkenes
- alcohols
- carboxylic acids
- esters
- alkanes
what are isomers?
isomers are molecules with the same molecular formula but different structural formulae
how many isomers does C4H10 have?
two
how many isomers does C5H12?
three
what makes isomers different?
the branching in the carbon chain - you can’t make a different isomer just by bending the molecule
why are most alkanes used as fuels?
they release a lot of energy when combusted
what are the two types of combustion that alkanes can undergo, depending on how much oxygen is available?
- complete combustion
- incomplete combustion
what is complete combustion of alkanes?
- this occurs when there is excess oxygen
- it produces only CO2 and H2O:
alkane + oxygen → carbon dioxide + water
what is the incomplete combustion of alkanes?
- this occurs when there isn’t enough oxygen
- instead of CO2 this produces a nasty mixture of:
- carbon monoxide (CO) - a toxic gas which lowers oxygen-carrying capacity of blood
- soot (C) - which causes lung diseases
- it also releases less energy so it makes the alkane less useful as a fuel
what are the three significant pollutants of the combustion of alkanes into the atmosphere?
- carbon dioxide (CO2)
- nitrogen oxides (NOx)
- sulphur dioxide (SO2)
how does alkane release carbon dioxide (CO2) into the atmosphere?
- this forms directly from the combustion of the alkane fuel
- it is a greenhouse gas that traps heat in the atmosphere and causes climate change
how does alkane release nitrogen oxides (NOx) into the atmosphere?
- this family of compounds form when nitrogen from the air combusts
- usually nitrogen doesn’t combust but car engines are really, really hot
- they dissolve in rainwater to form acid rain, which corrodes structures and is harmful to plant and aquatic life
how does alkane release sulphur dioxide (SO2) into the atmosphere?
- this forms when sulphur impurities in the fuel combust
- like nitrous oxides, it also dissolves in rainwater to form acid rain
how do alkanes react with halogens?
alkanes react with halogens by swapping one hydrogen atom for one halogen atom
e.g. for example, when ethane (C2H6) reacts with bromine (Br2):
C2H6 + Br2 → C2H5Br + HBr
does alkane reactions with halogens work all the time?
- these reactions would work with any alkane (e.g. pentane) any halogen (e.g. chlorine)
- all that happens is a hydrogen swaps with a halogen
what does a halogen and alkane reaction require?
this reaction requires UV light - because the alkane is pretty unreactive
what type of reaction is a halogen and alkane reaction?
its a substitution reaction - because the H swaps with the Br
