Transcript Production of materials

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 +
CH2= CH2 (g) +
hydrogen
H2 (g)
Alkene + hydrogen
ethane
CH3-CH3 (g)
alkane
Addition of halogens
 Addition of a halogen (eg: Cl, Br) to ethene
results in the formation of a haloalkane
ethene + bromine
CH2= CH2 (g) +
Br2 (l)
Alkene + halogen
1,2 dibromoethane
CH2Br-CH2Br (g)
di-halo-alkane
Addition of hydrogen halides
 Addition of a hydrogen halides (eg: HCl) to
ethylene also produces haloalkanes.
Ethylene + hydrogen
chloride
chloroethane
CH2= CH2 (g) +
CH3-CH2Cl(l)
HCl (g)
Alkene + hydrogen
halide
haloalkane
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)
propane + oxygen
Alkane + oxygen
3CO2 (g) + 4H2O (g)
carbon + water
dioxide
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)
hexane
+ bromine
Alkane + halogen
C6H13Br(l) + HBr(aq)
bromohexane
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.
C δ+
O δHδ+
The polar end of the ethanol
molecule interacts with other
polar molecules to form
dipole-dipole forces or
hydrogen bonds eg: with water
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 ELECTRONS
REDUCTION = GAIN OF ELECTRONS
Zn (s) + 2HCl (aq)
Oxidation:
Zn (s)
Reduction: 2H+(aq) + 2e-
ZnCl2 (aq) + H2 (g)
Zn 2+ (aq) + 2eH2 (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.
2.
3.
4.
5.
Uncombined elements have an oxidation state of 0
Ions have an oxidation state equal to their charge (eg:
Na+ = +1)
Oxygen in compounds has a charge of -2 in oxides
and -1 in peroxides
Hydrogen in compounds has a charge of +1 when
combined with non-metals and -1 when combined
with metals
The oxidation state of a compound or polyatomic ion
is the sum of the oxidation states of all its atoms.
Oxidation state
Note that the number of
electrons lost or gained =
change in oxidation state
Oxidation involves an increase in
oxidation state.
Half equation
Zn (s)
Zn 2+ (aq) + 2eOxidation state 0
2+
Reduction involves a decrease in
oxidation state.
Half equation:
2H+(aq) + 2eOxidation state: 1+
H2 (g)
0
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 + 2eReduction: 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 allows
ions 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 ZnSO42solution
 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.