Production of materials
Ethylene (ethene)
Although ethylene is a widely used raw material
very little of it is found in either natural gas or
crude oil. Instead it has to be produced from
other hydrocarbons by a process called cracking.
Cracking: process in which large hydrocarbons
are broken down into smaller ones with the help
of heat and/or a catalyst.
Cracking
During the cracking process bonds within the
hydrocarbon molecule are broken. Ethene is
produced in one of two ways:
From crude oil by catalytic cracking of fractions from the distillation column
From natural gas by thermal cracking
One possible reaction involving the cracking of hydrocarbon C15H32 might be
http://www.chemguide.co.uk/organicprops/alkanes/cracking.html
Reactions of ethane and ethene
What do ethane and ethene have in common?
2 carbon atom chain, non-polar (insoluble in water, low melting and boiling points, both undergo combustion with oxygen
How do they differ?
ethene has a double bond and therefore is much more reactive
Reactions of ethene
Like all alkenes, ethene undergoes addition
reactions. Why?
Answer: When the double bond is broken
additional atoms or groups of atoms can be
added – one to each C atom previously linked
by the double bond.
Addition of hydrogen
Addition of hydrogen to ethene results in the formation of ethane.
Ethylene + hydrogen ethane
CH2= CH2 (g) + H2 (g) CH3-CH3 (g)
Alkene + hydrogen alkane
Addition of halogens
Addition of a halogen (eg: Cl, Br) to ethene results in the formation of a haloalkane
ethene + bromine 1,2 dibromoethane
CH2= CH2 (g) + Br2 (l) CH2Br-CH2Br
(g)
Alkene + halogen di-halo-alkane
Addition of hydrogen halides
Addition of a hydrogen halides (eg: HCl) to ethylene also produces haloalkanes.
Ethylene + hydrogen chloroethane
chloride
CH2= CH2 (g) + HCl (g) CH3-CH2Cl(l)Alkene + hydrogen haloalkane
halide
Addition of water
Addition of water (in the presence of an acidic catalyst) to ethylene produces an ethanol
Ethylene + water ethanol
CH2= CH2 (g) + H2O (l) CH3-CH2OH
(l)
Alkene + water alkanol
Reactions of alkanes
1. Combustion reaction: Alkanes burn in air to produce CO2 and H2O
C3H8 (g) + 5O2 (g) 3CO2 (g) + 4H2O (g)
propane + oxygen carbon + water
dioxideAlkane + oxygen carbon + water
dioxide
Reactions of alkanes
1. Substitution reaction: Alkanes react with Cl2, Br2, I2 (halogens) when exposed to ultraviolet light
C6H14 (l) + Br2 (l) C6H13Br(l) + HBr(aq)
hexane + bromine bromohexane
Alkane + halogen haloalkane
Polymerisation
Reaction in which many small molecules
(monomers) combine to form one large
molecule (polymer). There are two main types
of polymerisation reactions:
Addition polymerisation Condensation polymerisation
Addition polymerisation
In the process of addition polymerisation
monomers simply add together without the loss
of any atoms. Basically the double bond opens
out to form single bonds with neighbouring
molecules.
Addition reactions involve unsaturated
hydrocarbons.
Addition polymerisation
Condensation polymerisation
Condensation polymerisation involves a
reaction between two monomers which have
different functional groups. Small molecules
such as water are eliminated during this
reaction.
Carboxylic acid functional group
COOH
Condensation polymerisation
Amine functional group –NH 2
Alcohol functional group -OH
Synthetic polmers
Ethene is the simplest monomer capable of
undergoing addition polymerisation. Some
important synthetic polymers formed from
ethene include: Poly(ethene) (polyethelyne) Poly (vinyl chloride) PVC Poly (styrene) Poly (acylonitrile) PAN Poly (propene) (polypropylene)
Biopolymers
Polymers produced by living organisms are
called biopolymers. Examples include:
Cellulose Starch Proteins Nucleic acids
Alcohols
Alcohols are a family of carbon compounds that contain the hydroxy group (-OH).
Alkanols are a specific group of alcohols where one or more hydrogen atoms in an alkane are replaced by an –OH functional group.
Alkanols are represented by the general formula ROH where R = alkyl group
Ethanol
Alcohols : Nomenclature
Add the suffix ‘ol’ in place of the ‘e’ on the name of the hydrocarbon to which the –OH group is attached.
A number indicates the position of the carbon atom containing the -OH group.
If there are more than one –OH group add the suffixes ‘-diol’, ‘-triol’ and so on.
Ethanol 1,2 ethanediol
Primary alcohol
The carbon atom attached to the _OH group
has two carbon atoms bonded to it.
Secondary alcohol
The carbon atom attached to the _OH group
has two carbon atoms bonded to it.
Tertiary alcohol
The carbon atom attached to the _OH group
has two carbon atoms bonded to it.
Ethanol as a solvent
Ethanol is a good solvent because it is a very
polar molecule. When ethanol and water are mixed
they readily dissolve in each other. This is due to
the polar nature of the O-H bond.
The polar end of the ethanol molecule interacts with other
polar molecules to form dipole-dipole forces or
hydrogen bonds eg: with water
C δ+ O δ-
Hδ+
Ethanol as a solvent
Ethanol and hexane (a non-polar molecule)
readily dissolve in each other.
The non - polar end of the ethanol molecule (the alkyl
chain) forms dispersion forces with other non-polar
molecules. This enables ethanol to act as a solvent for
some non-polar molecules.
Production of Ethanol
1. Hydration of ethanol: industrial ethanol is produced by the acid catalysed addition of water to ethene, represented by the equation:
CH2 = CH2 (g) + H20 (g) CH3 - CH2OH(g)
Production of ethanol
2. Fermentation: process in which glucose is broken down to ethanol and carbon dioxide by the action of enzymes in yeast (these act as catalyst).
C6H12O6 (aq) 2CH3 - CH2OH (aq) +2CO2 (g)
This process is exothermic.
Ethanol as a fuels
The combustion of ethanol is an exothermic
reaction.
C2H5OH(g) + 3O2 2CO2(g) + 3H2O(g)
The amount of heat released can be expressed
as the molar heat of combustion:
‘Heat liberated on complete combustion of one
mole of a substance’
Calorimetry
Calorimetry is a method used to determine heat of combustion. Essentially we measure the change in temperature of measured mass of water heated by the combustion of a measured amount of fuel. This is then used to calculate heat energy release per mole of substance burned.
Molar heat of combustion
1. Find the mass of the fuel burned ??? by weighing the fuel and container before
and after heating
2. Calculate the moles of fuel burned ??? n= m/M
Molar heat of combustion
3. From the rise in water temperature calculate heat produced by combustion of that many moles of fuel???
ΔH = m C ΔT
4. Calculate how much heat could have been produced by one mole of the substance
Oxidation-reduction
Reactions which involve the transfer of electrons are called oxidation-reduction reactions.
OXIDATION = LOSS OF ELECTRONSREDUCTION = GAIN OF ELECTRONS
Zn (s) + 2HCl (aq) ZnCl2 (aq) + H2 (g)
Oxidation: Zn (s) Zn 2+ (aq) + 2e-
Reduction: 2H+(aq) + 2e- H2 (g)
Displacement reactions
Displacement reactions are oxidation-reduction
reactions in which a metal converts the ion of
another metal to the neutral atom.
In these reactions the metal dissolves and the
ions of the other metal are reduced to elemental
metal and deposit out of solution.
Example:
Cu 2+ ions have a greater tendency to gain electrons than Zn 2+ ions.
As a result there is a transfer of electrons from the Zn metal to the Cu (II) ions.
As the reaction proceeds Zn metal dissolves and goes into solution as Zn ions and Cu metal is formed
The activity series
The activity series can be used to predict whether a metal will displace the ions of another metal.
K>Na>Mg>Al>Zn>Fe>Sn>Pb>Cu>Ag>Hg>Pt>Au
The more reactive metal will displace another metal from a solution of its ions.
Oxidation states
In many oxidation-reduction reactions it is not obvious which species has been reduced and which has been oxidised.
To overcome this problem we use a system of assigning oxidation states to atoms to keep track of the number of electrons transferred or shared in oxidation-reduction reactions.
Oxidation state is an arbitrary number assigned according to a set of rules.
Rules for determining oxidation state
1. Uncombined elements have an oxidation state of 0
2. Ions have an oxidation state equal to their charge (eg: Na+ = +1)
3. Oxygen in compounds has a charge of -2 in oxides and -1 in peroxides
4. Hydrogen in compounds has a charge of +1 when combined with non-metals and -1 when combined with metals
5. The oxidation state of a compound or polyatomic ion is the sum of the oxidation states of all its atoms.
Oxidation state
Oxidation involves an increase in
oxidation state.
Half equation Zn (s) Zn 2+ (aq) + 2e-
Oxidation state 0 2+
Reduction involves a decrease in
oxidation state.
Half equation: 2H+(aq) + 2e- H2 (g)
Oxidation state: 1+ 0
Note that the number of electrons lost or gained = change in oxidation state
Electrochemical cells
Redox reactions can be used to generate electricity in a galvanic cell
Example: When zinc metal is placed in CuSO4 solution,
following reaction take place: Zn(s) + CuSO4(aq) ZnSO4(aq) + Cu(s)
Oxidation: Zn(s) → Zn+2 + 2e- Reduction: Cu+2 + 2e- → Cu Overall: Zn(s) + Cu+2 → Zn+2 + Cu(s)
Galvanic cell
Each of the two parts of the cell is called a half cell.Each half cell is connected by a salt bridge which completes the circuit and allowsions to travel between each half cell.
How does the galvanic cell work?
Zn loses electrons to form Zn ions in solution. The Zn strip dissolves Zn ions travel through the external circuit to
the Cu strip where they are accepted by the Cu ions
The Cu ions are reduced to Cu atoms which deposit on the strip
How does a galvanic cell work?
As reaction continues excess Zn2+ ions build up in the ZnSO4
2- solution
excess negative SO42- ions build up in the
CuSO42- solution
To maintain electrical neutrality in the half cell solutions positive Na+ ions move into the copper half cell from the salt bridge at the same time NO3
- ions move into the zinc half cell
Standard Reduction potentials
Standard reduction potential (EO) is a measure of the relative tendency of a substance to gain one or more electrons compared to the standard hydrogen half cell. The larger the EO value the greater the oxidising power of a substance
e.m.f or voltage of a galvanic cell is the difference in the reduction potentials of the two couples making up the cell.