fuels and heats of reaction

def. organic chemistry

the study of compounds of carbons

def. fossils duels

fuels that were formed from the remains of dead plants and animals that live

def. homologous series

series of compounds with similar chemical properties a general chemical formula and each successive member differing CH₂

The Alkanes

• single bonds between carbon atoms

• saturated compound

def. saturated compound

a compound where there are only single bonds between its atoms.

alkanes

general formula: C_nH_{2n +2}

properties of alkane

• soluble in non-polar solvents e.g cyclohexane

• insoluble in water because alkanes are non-polar - Van der Waals forces

• Larger molecules have stronger Van der Waals forces, increasing their boiling and melting points

def. structural isomer

compounds that have the same molecular formulae but different structural formulae

alkenes

the homologous series has a double bond between two carbon atom

unsaturated compounds

def. unsaturated compounds

a compound which contains one or more double or triple bonds between its atoms

general formula: C_nH_2n

properties of alkenes

soluble in non polar solvents e.g cyclohexane

insoluble in water because the alkenes are non-polar → Van der Waals forces between molecules

alkynes

• series has a triple bond between two carbon atoms

• they are unsaturated hydrocarbons

• ethyne

general formula of alkyne

C_nH_2n-2

properties of alkynes

• soluble in non-polar solvents e.g cyclohexane

• insoluble in water because alkynes are non-polar Van der Waals forces between the molecules

def. aromatic compounds

compounds that contain a benzene ring

e.g Benzene, methylbenzene and ethylbenzene

oil refining

crude oil is made up of a mixture of different hydrocarbons

fractional distillation

used to separate these different hydrocarbons into groups called fractions, based on their boiling points

frational distillation in order (top to bottom)

refiner gases (bottled gas)

gasoline (petrol)

naptha (making chemicals)

kerosene (aircraft fuel)

diesel oil (fuel for cars,lorries,buses)

fuel oil (fuel for ships, power stations)

residue (bitumen for roads and roofs)

def. octane number

a measure of its tendency to resist autoignition

def.autoignition

premature ignition of a fuel-air mixture before a spark is produced

reference hydrocarbons: heptane

octane number: 0

long chain

no branching

reference hydrcarbons: 2,2,4 - trimethylpentane

octane number:100

short chain

branching

factors that affect octane number

length of chain: shorter carbon chains have higher octane number

degree of branching: the more branching, higher octane number

presence of rings: ring structures give a fuel higher octane number

process that raise octane number of a fuel

isomerisation

catakyic cracking

dehydrocylisation

adding oxygenates

isomerisation

involves changing straight chain alkanes into their branched isomers to raise octane number

process of isomerisation

• heat in presence of a catalyst

• chain breaks

• bits rejoin to form a branched compound

catalytic cracking

involves using heat and a catalyst to break down long chain hydrocarbons into short chain molcules

high demand for short chain molecules with high octane numbers

one of the products will always be an alkenedeh

dehydrocyclisation

involves using catalysts to change linear molecules into cyclic molecules and cyclic molecules into aromatic compounds

adding oxygenates

addition of oxygen-containing compunds to raise its octane number - making it burn cleaner - less carbon monoide is formed when its burned

e.g oxygenates: methanol, ethanol, MTBE

hydrogen as a fuel

steam reforming a natural gas: steam is reacted with natural gas using a catalyst

CH₄ + H₂O → 3H₂ + CO

electrolysis of water: an electric current is passed through water

H₂O → H₂ + 1/2O₂

uses of hydrogen

burned as a fuel because it is environmentally friendly - its only waste product is water

note

Hydrogen forms a very explosive mixture with air and is therefore difficult to story and transport without risk of explosion

physical properties of hydrocarbons

boiling points: VAn der Waals occure between hydrocarbons resulting in low boiling points

solubility: only soluble in organic solvents because CH is non polar

e.g cyclohexane

thermochemistry

exothermic reaction

one that produces heat

endothermic

one that absorbs heat

if the heat change is negative

if the heat change is positive

negative - reaction is exothermic - gets hotter

positive - reaction is endothermic - gets cooler

def.heat of reaction

heat change when one mole of a substance is burned completely in excess oxygen

def.heat of combustion

heat of cumbustion is the heat change when one mole of a substance is bruned completely in excess oxygen

combustion equation

fuel + O₂ → CO₂ + H₂O

def. bond energy

the average energy required to break one mole of H+ ions from an acid reacts with one mole of OH^- ions from a base

def. heat of neutralisation

heat change when one mole of H⁺ ions from an acid reacts with one mole of OH^- ions from a base

def. Heat of formation

the heat change that takes place in one mole of a compound formed from its element in their standard stae

hess’s law

the sum of the heat changes in the separate stages is equal to the heat change if the reaction is carried out in one stage.