3.2 Alkanes Flashcards
What is the c-c-c angle of an unbranched chain?
109.5 degrees
Give an example of an unbranched chain
Pentane
Give an example of a branched chain
Methylbutane which is an isomer of pentane
What is the general molecular formula of ring alkanes and explain why
CnH2n because the end hydrogens are not required
Name the first six alkanes
Methane Ethane Propane Butane Pentane Hexane
How do you name straight chain alkanes?
Alkanes are named from the root, which tells us the number of carbon atoms, and the suffix -ane, denoting an alkane.
How do you name branched chain alkanes
You must first find the longest unbranched chain which gives the root name. Then name the branches as prefixes and add numbers to say which carbon atoms the side chains are attached to.
What happens to the number of isomers when the chain length increases
The number of isomers rise rapidly with chain length
Panay is the general formula for all chain alkanes
CnH2n+2
What is the polarity of alkanes and why
They are almost non-polar because the electronegativities of carbon and hydrogen are so similar. So the only intermolecular forces are between the molecules are weak van der waals and the larger the molecule the stronger the van der waals.
Explain the boiling points of alkanes
The shorter chains are gases at room temperature. At a chain length of about 18 carbons, the alkanes become solids at room temperature. Alkanes with branched chains have a lower melting point than straight chain alkanes with the same number of carbon atoms. This is because they cannot be packet together as closely as unbranched chains so the van der waals forces are not as effective.
Explain the solubility of alkanes
They are insoluble in water because the water molecules are held together by hydrogen bonds which are much stronger than the van der waals forces that act between the alkane molecules. However alkanes do mix with other relatively non-polar liquids.
How do alkanes react
They are relatively unreactive. They have strong carbon-carbon and carbon-hydrogen bonds. They do not react with acids,bases,oxidising agents and reducing agents. However they burn and react with halogens under suitable conditions. They burn in a plentiful supply oxygen to form carbon dioxide and water (in a restricted supply of oxygen they form carbon monoxide or carbon)
What are alkanes
Alkanes are saturated hydrocarbons which contain only carbon-carbon and carbon-hydrogen single bonds. They are among the least reactive organic compounds.
What is the main source of alkanes
Crude oil
Why are alkanes very important to industry
They are used as fuels and lubricants and as starting materials for a range of other compounds.
Why are long chain hydrocarbons shortened?
Shorter hydrocarbons are more useful so sell for more. Therefore long chain hydrocarbons are often ‘cracked’ into smaller hydrocarbons to meet demand and sell for more money?
How does thermal cracking work?
Between 700K and 1200K and 7000kPa. C-C bonds get broken to form 2 free radicals. One forms an alkene and loses a hydrogen. The other takes a hydrogen and becomes a smaller alkane.
How does catalytic cracking work?
720K, lower pressure than thermal cracking but more than atmospheric. Zeolite catalyst (honeycomb structure of silicon and aluminium dioxides). Produces motor fuels, branched alkanes and some cycloalkanes.
Why do sulfur impurities in hydrocarbon fuels make it particularly bad for the environment?
The sulfur reacts with water vapour and oxygen to form sulfuric acid, which makes acid rain.
What is flue gas desulfurisation?
The process of removing sulfur from the gases given out by power stations.
What are the 2 ways in which flue gas desulfurisation is done?
1) with CaO, water and oxygen to make gypsum which can be sold. CaO+2H2O+1/2O2+SO2–> CaSO4-2H2O 2)With calcium carbonate CaCO3+1/2O2+SO2–> CaSO4+CO2
What is the essential condition needed for free radical substitution?
It needs to be under UV light.
What is the initiation step for C2H6 +F2?
F2–>2F. with UV light as a catalyst.
What are tthe 2 propogation steps for C2H6 +F2?
C2H6+F.–> C2H5.+HF C2H5. + F2 C2H5F +F.
What are two possible termination steps for C2H6 +F2?
F. + C2H5. –> C2H5F C2H5. + C2H5. –> C4H10
What is the general formula for an alkane?
C2H2n+2
Are alkanes saturated or unsaturated?
Saturated because no more bonds can be made
What are the first six alkanes (length of carbon chains)?
Methane, ethane, propane, butane, pentane, hexane
Alkanes are useful as fuels. Write a balanced symbol equation for the complete combustion of propane
C3H8+5O2 -> 3CO2+4H2O
Write a balanced symbol equation for the incomplete combustion of methane.
CH4+1.5O2 -> CO+H2O
What are the three main greenhouse gases?
Carbon dioxide, water vapour, methane
What are the two types of cracking called?
Catalytic cracking and thermal cracking
Give three features of thermal cracking
-high temperature -high pressure -produces lots of alkenes
Give three features of catalytic cracking
-uses a zeolite catalyst -therefore there is a low temperature and pressure -makes motor fuels and aromatic hydrocarbons
Write a molecular formula for hexane
C6H14
