ALKANES Flashcards
Sources of alkanes
Natural gas- methane,ethane
Petrolium- Complex mixture, alkanes, cycloalkanes
Physical properties of alkanes
-Low melting and boiling point gases/liquids/solids because they aresimple molecular covalent non polar therefore have only weak van der waal attractions between neighbouring molecules
-Do not conduct in any state
Why do the boiling points of alkanes rise as the chain length gets longer?
Increasing chain length results in increasing number of electrons therefore stronger van der waal forces of attraction therefore it requires more energy to break these attractions therefore higher boiling points
Straight chain vs branched chain isomers
Branched alkanes have lower bpts than their straight chain isomers. This is because there are fewer points of contact between adjacent molecules; they do not pack together as efficiently :. havefewer van der waal forces of attraction :. lower bpt
Combustion of alkanes in a plentiful supply of oxygen
alkane + O2 -> CO2 + H20
Incomplete combustion of alkanes
alkane + O2 -> C + CO + H2O
Why is carbon monoxide known as the silent killer
Colourless odourless gas which binds irreversibly to haemoglobin, preventing oxygen being carried to cells for respiration
Why is using fossil fuels for energy undesirable
-Nonrenewable
-Contrain sulfur which produces a white choking gas sulfur dioxide when burnt
Acid rain
-Corrodes limestone, defoliates trees and pollutes lakes killing fish
-Can be minimised by removing sulfur from fuels before burning, fitting catalytic converters to cars and removing SO2 from power station emmissions by fitting scrubbers in the chimneys
Green house effect
-CO2 is a greenhouse gas that absorbs and traps infrared radiation causing an increase in temperatures on the earth’s surface
-CO2 levels have risen from 0.03% to 0.04%
How catalytic converters work
Molecules of polluting gases are pumped from the engine past the honeycomb catalyst. The catalyst splits up the molecules into their atoms. The atoms then recombine into molecules of relatively harmless substances such as carbon dioxide, nitrogen and water which blow out safely through the exhaust
Why is a catalytic converter described as being a heterogenous converter
A catalyst which is a different state from the reactants. Car catalytic converters are solids and the reactants are gases
What is chemisorption
Step 1- diffusion of reactant gases onto the surface of the catalyst
Step 2- molecules adsorb onto the surface
Step 3- desorption of products
Chemisorption weakens the bonds of the reactant gases and brings them in the correct orientation making it easier for the reaction
HALOGENATION
Reagent- halogen
Conditions- heat/sunlight UV
Product- halogenoalkane
Reaction type- free radical substitution
FRS reaction initiation reaction
Cl2 -> 2Cl .
