Chemistry - Fuels and Heats of Reaction

Organic Chemistry

the study of the compounds of carbon

Hydrocarbon

a compound that contains carbon and hydrogen only

Sources of Hydrocarbons

• coal

• natural gas (methane)

• petroleum (crude oil)

Fossil Fuels

fuels that were formed from the remains of plants and animals that lived millions of years ago

Alkanes

a family of hydrocarbons in which all the atoms are linked by single bonds

Saturated Compound

a compound in which there are only single bonds between the atoms in the molecules

Methane

CH4

Ethane

C2H6

Propane

C3H8

Butane

C4H10

Pentane

C5H12

Hexane

C6H14

Heptane

C7H16

Octane

C8H18

Nonane

C9H20

Decane

C10H22

Properties of Alkanes

• soluble in non-polar solvents, e.g. cyclohexane

• insoluble in water - alkanes only have Van der Waals forces in between molecules

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

  (properties of alkenes are the same)

Homologous Series

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

Structural Isomers

compounds with the same molecular formula but different structural formulas

Methyl Group

CH3

Ethyl Group

C2H5

Propyl Group

C3H7

Alkenes

contain one carbon-carbon double bond between two of the carbons in the molecule

Unsaturated Compound

a compound which contains one or more double or triple bonds between the atoms in the molecule

Alkynes

contain a carbon-carbon triple bond

Aliphatic Compound

an organic compound that consists of open chains of carbon atoms and closed chain compounds (rings) that resemble them in chemical properties

Aromatic Compounds

compounds that contain a benzene ring structure in their molecules

Benzene Ring

C6H6

Fractional Distillation

a process used to separate crude oil into a number of useful- crude oil is heated and separated on the bases of their boiling points

Refinery Gas

bottled gas

Gasoline

used as fuel for cars

Naphtha

used to make chemicals

Kerosene

used as fuel for aircrafts

Diesel

used as fuel for cars, lorries and buses

Fuel Oil

used as fuel for ships and power stations

Residue

used as bitumen for roads and roofs

Liquid Petroleum Gas

(LPG)

consists of butane and propane and is used in cooking

Mercaptans

sulphur compounds that are added to LPG in order to give it a smell so that leaks can be detected

Auto Ignition

premature ignition (explosion) of the petrol-air mixture before normal ignition of the mixture by a spark takes place

Octane Number

a measure of the tendency of a fuel to resist knocking (autoignition)

2,2,4-trimethylpentane

is assigned an octane number of 100, has a low tendency to auto-ignite due to branching

Heptane

is assigned an octane number of 0, very inefficient as it auto-ignites readily

Factors Affecting Octane Number

1. length of chain

2. degree of branching

3. straight-chain or cyclic structure

Length of Chain

the shorter the alkene chain, the higher the octane number

Degree of Branching

the more branched the chain, the higher the octane number

Straight Chain or Cyclic Structure

cyclic compounds have a higher octane number than straight chain compounds

Increasing the Octane Number of Petrol

1. isomerisation

2. catalytic cracking

3. dehydrocyclisation

4. adding oxygenates

Isomerisation

involves changing straight-chain alkanes into their isomers

Catalytic Cracking

the breaking down of long-chain hydrocarbon molecules by the action of heat and catalysts into short-chain molecules for which there is greater demand

Dehydrocyclisation

the use of catalysts to form ring compounds

Adding Oxygenates

three oxygen-containing compounds, methanol, ethanol and MTBE, are commonly added to petrol

Methyl Tertiary-Butyl Ether

MTBE

Benefits of Adding Oxygenates

• increases octane number of petrol

• give rise to very little pollution when they burn

Production of Hydrogen

can be produced by electrolysis of water, or steam reforming of natural gas

Exothermic Reaction

a chemical reaction that produces heat

Endothermic Reaction

a reaction that takes in heat

Exothermic Reaction

is indicated by a negative value of ΔH

Endothermic Reaction

is indicated by a positive value of ΔH

Heat of Reaction

the heat change when the number of moles of reactants indicated in the balanced equation for the reaction react completely

Heat of Combustion

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

Bomb Calorimeter

an instrument used for accurately measuring heats of combustion

Kilogram Calorific Value

the heat energy produced when 1kg of the fuel is completely burned in oxygen

Bond Energy

the average energy required to break one mole of a particular covalent bond and to separate the neutral atoms completely from each other

Heat of Neutralisation

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

Heat Given Out

mass × specific heat capacity × rise in temperature

Heat of Formation

the heat change that takes place when one mole of a compound in its standard state is formed from its elements in their standard state

Hess’s Law

states that if a chemical reaction takes place in a number of stages, the sum of the heat of changes in the separate stages is equal to the heat change if the reaction is carried out in one stage

Law of Conservation of Energy

states that energy cannot be created or destroyed but can only be converted from one form to another