Module 4 - Alkanes Flashcards
what are alkanes
-saturated hydrocarbons with the general formula of CnH2n+2
how many hydrogens per carbon in cycloalkanes
-2
describe the shape of alkanes
-tetrahedral
-109.5
-as bonds repel each other equally
-to keep as far apart as possible
what are the different shapes/lengths of alkanes
-unbranched that leads to more surface contact
-branched
pattern of increasing boiling points in alkanes
-increasing number of electrons in bigger molecules
-which causes an increase in the size of the london forces between molecules
explain how long, straight chain hydrocarbons have higher boiling points
-more london forces
-more energy needed to overcome these forces
-so boiling point increases
explain how branched hydrocarbons have lower boiling points
-branches mean less points of contact
-so weaker london forces
-less energy needed to overcome these forces
-so boiling point decreases
how is a sigma bond formed in alkanes
-one sp3 orbital from each carbon overlap to form a single C-C bond called a σ bond
why are alkanes quite unreactive
-C-C and C-H bonds are strong
-C-C bonds are non polar
-C-H bonds are virtually non polar
-molecules are symmetrical so are non polar
what does fuel do
-releases heat energy when burnt
describe how alkanes burn in oxygen
-burn completely and readily
-highly exothermic
-form CO2 and H2O
-so used as fuels
what is the combustion of alkanes used for
-powering vehicles
-most electricity
describe what happens when complete combustion occurs
-in a plentiful supply of oxygen
-complete combustion of an alkane occurs
-producing CO2 and H2O
equation for complete combustion
fuel + oxygen -> carbon dioxide + water
how to balance complete combustion equations
-half the number of H in the fuel and put this number in front of H2O
-put number of carbons from the fuel in front of the CO2
-work out missing oxygen from reactants side
describe what happens when incomplete combustion occurs
-in a limited supply of oxygen
-incomplete combustion of an alkane occurs
-producing CO, C and H2O
equation for incomplete combustion
fuel + oxygen -> carbon + carbon monoxide + water
difference in energy produced in complete and incomplete combustion
-incomplete combustion produces less energy per mole than complete combustion
features of carbon monoxide
-highly toxic
-odourless
-can cause death if builds up in an enclosed space due to a faulty heating appliance
-can form a strong bond with haemoglobin in red blood cells that is stronger than that made with oxygen
-so oxygen is prevented from attaching to haemoglobin
-can be removed using a catalytic converter
features of carbon (soot)
-can cause global dimming
-reflection of the suns light
what is bond fission
-the breaking of a covalent bond
when do alkanes react with halogens
-in the presence of UV light
what is the mechanism for the reaction between an alkane and halogen
-free radical substitution
what are the steps of a free radical substitution
-initiation
-propagation
-termination
describe the initiation step of free radical substitution
-the UV light supplies the energy to break the covalent bond in a diatomic halogen molecule (homolytic fission)
-it is broken in preference to the others as it is the weakest
-the broken bond leads to formation of free radicals
what is the initiation step in free radical substitution also called
-homolytic fission
define a free radical
-a highly reactive species due to having an unpaired electron
features of free radicals
-do not have a charge
-represented by a *
describe the propagation step in free radical substitution
-the free radicals are very reactive and remove a H from the alkane
-the alkane free radical reacts with the halogen molecule to produce the main product and another halogen free radical
-producing a new molecule and new free radical
features of propagation
-always two steps
-always have a free radical in the reactants and in the products
-as the halogen free radical is regenerated, it can react with several more alkane molecules in a chain reaction
describe the termination step in free radical substitution
-two free radicals react to form a new non radical molecule
-this ends the chain reaction
where does the dot go on a free radical
-needs to be on the correct carbon in the propagation stages
products of free radical reactions
-further substitution can occur with more Hs being replaced by halogens
-substitution can also occur at different points in a carbon chain (isomers formed)
-the reaction produces a mixture of products
disadvantage of free radical substitution
-not efficient
-as limited use in synthesis as making a desired product takes multiple steps
