fuels

hydrocarbons

compounds that contain carbon and hydrogen only

crude oil

• mixture of hydrocarbons

• contains molecules where carbon atoms are in chains or rings

• important source of useful substances - fuel/feedstock for petrochemical industry

• finite resource

how to refine crude oil

• fractionating column very hot at bottom and cool at top

• crude oil enters column, heated so vapours rise

• vapours of hydrocarbons with different boiling points condense into liquid at different heights, ad are tapped off

• bigger hydrocarbons normally collected nearer bottom, smaller nearer top

why do bigger hydrocarbons have higher boiling points

stronger intermolecular forces of attraction, because more atoms in molecule and larger surface area - more energy needed to overcome

uses of different fractions

good people keep driving fast bikes

gas = domestic heating and cooking

petrol = fuel for cars

kerosene = fuel for aircraft

diesel oil = fuel for cars and trains

fuel oil = fuel for large ships and power stations

bitumen = surfacing roads and roofs

how do fractions differ from each other (going up fractionating column)

number of carbon and hydrogen atoms in molecules = decreases

boiling points = decreases

ease of ignition = increases

viscosity = decreases

and are mostly alkanes

homologous series

series of compounds which:

• have same general formula

• differ by CH2 group in molecular formulae from neighbouring compounds

• show a gradual variation in physical properties (e.g. boiling points)

• have similar chemical properties

complete combustion of hydrocarbons

reactants = hydrocarbon, oxygen

produced = carbon dioxide, water, energy given out

incomplete combustion

reactants = hydrocarbon, not enough oxygen

produced = carbon monoxide/carbon, water, energy given out

occurs in limited supply of oxygen - carbon dioxide needs more oxygen as it has CO2 than CO or C

why is carbon monoxide dangerous

is a toxic gas:

• binds to haemoglobin

• reduces how much oxygen can bind to haemoglobin

• reduces blood’s ability to carry oxygen around the body

• can lead to death and suffocation

problems of incomplete combustion

carbon monoxide = toxic gas, reduces blood’s ability to carry oxygen around the body

carbon/soot = can cause respirational problems, block pipes carrying away waste from appliance, blacken buildings or appliance

how is sulfur dioxide produced

• sulfur impurities in hydrocarbon fuel

• sulfur combusts alongside hydrocarbons under high temperatures

• reacts with oxygen to produce sulfur dioxide

acid rain causes and problems

• sulfur dioxide produced from combustion of sulfur impurities in hydrocarbon fuel

• sulfur dioxide dissolves in water in atmosphere to form sulfuric acid

• water with sulfuric acid falls as acid rain

• can kill trees and crops, go into streams and water bodies and kill fish, acidify soils, damage buildings

nitrogen oxides

• nitrogen and oxygen in air react at high temperatures in an engine when fuels are burned

• produce nitrogen oxides

• are pollutants - can cause acid rain and respiratory problems

• can be prevented from reacting by using a catalytic converter in engines

advantages of hydrogen fuel over petrol

• releases more energy per kilogram

• does not pollute as it only produces water on combustion

disadvantages of hydrogen fuel over petrol

• expensive to produce and requires energy for production process

• difficult and dangerous to store and transport (usually stores as liquid hydrogen in highly pressurised containers)

• production of hydrogen releases carbon dioxide

fossil fuel examples

from crude oil = petrol, kerosene, diesel

natural gas = methane

all non-renewable

cracking

• long-chained saturated hydrocarbons (alkanes) are broken down to form a mixture of shorter-chained alkanes and unsaturated hydrocarbons (alkenes)

• a single covalent bond between two carbon atoms is broken

• one product is always an alkane, another an alkene (or can break in multiple places, producing a mixture

• requires a temperature of 600-700 degrees C, and a catalyst (silica)

alkenes can be tested for by decolourisation of bromine water from brown

alkanes vs alkenes

alkanes = saturated - hydrocarbons with only single bonds between atoms

alkenes = unsaturated - hydrocarbons with one or more double covalent bonds between atoms

why is cracking necessary

greater demand for shorter-chained hydrocarbons:

shorter-chained alkanes = more volatile and flammable, make better fuels

shorter-chained alkenes = more reactive than alkanes and can be used to produce polymers, alcohols, and wide variety of organic molecules