Alkanes and Nomenclature and the properties of Alkanes Flashcards
what are the alkanes?
the alkanes are saturated hydrocarbons that only contain single c-c ,c-h bonds.
(C2H2n+2)
describe the shape and give the bond angle for an alkane molecule.
the molecule will form a tetrahedral shape around each carbon as each carbon atom has four pairs of bonding electrons around it which repel each other equally.
the bond angles around each carbon will be 109.5 degrees.
describe the bonding in alkanes and how the bonds are formed
In alkanes, atomic orbitals on carbon and hydrogen atoms are positioned so that they overlap in a straight line to form a new, shared orbital lying directly between the bonded atoms. This type of covalent bond is called a sigma bond.
-the high electron density between the nuclei means there is a strong electrostatic attraction between the nuclei and the shared pair of electrons.
-this means that the sigma bond has a high bond elthalpy.
-the bonded atoms are free to rotate around the sigma bond as whichever way the atoms point, the atomic orbitals between them will always overlap.
describe the reactivity of the alkanes.
-pretty unreactive
-can be explained by the bonding in alkane molecules.
-the c-c,c-h sigma bonds have a large bond enthalpy, making them very strong and therefore difficult to break.
-the bonds are also non polar, meaning that they won’t attract any positive or negatively charged particles to react with them.
describe the variety in boiling points for the alkanes.
-boiling point depends on size and shape
-due to differences in their molecular forces
-between the molecules are induced dipole-dipole interactions which hold them together.
-the longer the carbon chain, the more induced dipole-dipole interactions there are.
-due to having larger molecular surface area, and so there is more surface contact between the molecules and there are more electrons to interact.
-so as the molecule gets longer, it takes more energy to overcome the London forces and separate them, and so boiling point rises.
-a branched-chain alkane has a lower boiling point than its straight- chain isomer.
-branched can’t pack closely together and they have smaller molecular surface areas, and so interactions are reduced.
describe briefly what a combustion reaction of the alkanes involves
-if you burn alkanes with enough oxygen, you get carbon dioxide and water (complete combustion.)
-complete combustion reactions happen between gases, so liquid alkanes have to be vaporised first.
explain why alkanes make excellect fuels and give some examples.
-when alkanes burn, some energy is used to break the covalent bonds but more energy is released by forming the new product bonds in Co2 and H2O.
-this means that larger alkanes release more energy per mole than smaller alkanes, because more CO2 and H2O are formed.
overall, alkanes make excellent fuels because they release so much energy when they burn.
examples:
-propane is used as a central heating and cooking fuel
-butane is bottled and sold as camping gas
-petrol and diesel are both made up of a mixture of alkanes too.
why does an incomplete combustion reaction occur and how can this lead to problems?
-if there isn’t much oxygen available, an alkane will still burn, but it will produce carbon monoxide and water.
-incomplete combustion is a problem as CO is poisonous.
-CO is better at binding to haemoglobin than oxygen is and so if you breath in air with a high conc of CO it will bind to the haemoglobin in bloodstream before the oxygen can, meaning less oxygen will reach your cells. you will suffer from oxygen deprivation.
-all appliances that use an alkane-based fuel need to be properly ventilated, advised to get a carbon monoxide detector. they should also be checked and maintained regularly.
