Transcript Slide 1

Fuels and Heats of
Reaction
Chapter 21 Pg 302
Fuels
Coal
• Plants and animals died
and were immediately
covered by sediment in
seas or swamps.
• This stopped them
decaying.
• Further layers of
sediment buried the
plant and animal remains
deeper and deeper.
• After millions of years
of pressure and heat
(900C to 1200C), these
remains turned into
COAL
Oil and Gas
• The same process
is used for oil and
gas
• Oil and gas occur
together and
were formed
from both plants
and animals being
buried.
Coal, oil and gas are all
hydrocarbons
• Hydrocarbons are compounds consisting of
carbon and hydrogen only, bonded together
covalently
Aromatic Hydrocarbons
• These are chains of
carbon and hydrogen
that contain a
benzene ring
• You may see benzene
drawn a number of
different ways
• (We take s closer look
at aromatics later)
Aliphatic Hydrocarbons
• These hydrocarbons consist of straight
and branch chains of carbon atoms.
They also contain non aromatic rings
Homologous Series
These are a family of compounds that
have:
1. The same general formula
2. Successive members that differ by a
single CH2
3. Similar chemical properties
Naming Organic Compounds
• Organic compounds are named according
to the IUPAC system
• This separates the naming of organic
compounds according to two parts
• The number of carbons in the longest
chain -the root
• The family (homologous series) - suffix
No. Of Carbon Atoms
Root
1
Meth-
2
Eth-
3
Prop-
4
But-
5
Pent-
6
Hex-
7
Hept-
8
Oct-
9
Non-
10
Dec-
Family
bonds
Suffix
Alkanes
Single
-ane
Alkenes
Double
-ene
Alkynes
Triple
-yne
Step 1: Parent Chain
• This is the longest unbroken chain of
H
H
H
carbons
H
H
H
C
H
C
C
C
H
H
C
H
C
H
H
H
H
• Once you have found it count the
number of carbons it contains and assign
H
H
H
the prefix
H
H
H
C
• 6 carbons = Hex
H
C
C
C
H
H
C
C
H
H
H
H
H
2. Bonding / Groups
H
H
H
H
C
H
C
H
C
C
H
H
H
C
C
H
H
H
H
H
1. What type of bonding is present?
Single bonds only
Now what is the suffix
-ane
2. Does the molecule contain any
additional groups (halogens) ?
No
Name the Compound
• 6 Carbons = Hex
• All single bonds = -ane
HEXANE
H
H
H
H
C
H
C
H
C
C
H
H
H
C
C
H
H
H
H
H
Branches
Carbon (alkyl) branches
The name of the branches
depends on the number of
carbons
1 Carbon = Methyl = CH3
2 carbons =Ethyl = CH3CH2
3 carbons =Propyl = CH3CH2CH2
Etc....
Halogen Branches
The name of the branch
depends on the type of
halogen attached
– Fluoro (F-)
– Chloro (Cl-)
– Bromo (Br-)
– Iodo (I-)
Designating where the branch goes
• Number the parent chain so that the
attached groups are on the lowest
numbers
Methyl is on carbon #2 of the parent chain
Methyl is on carbon #4 of the parent chain
1
5
2
4
3
3
4
2
5
1
GREEN is the right
way for this one
And Finally......
• Alphabetize the branches , combine like
groups, and assemble the name
• Prefixes are not considered when
alphabetizing (Example: dimethyl = m for
alphabetizing)
• Parent chain goes LAST
2-methyl
• 2-methylpentane
1
2
3
4
5
1. Identify the
longest unbroken
chain of carbons
5 Carbons = pent
2. What bonding is
present
Single = -ane
3. is there anything
else attached to
the main chain
Yes a CH3 branch
H
H
H
H
C
H
C
C
C
H
H
H
H
CH2
CH3
H
H
H
H
C
H
C
C
C
H
H
H
H
CH2
CH3
H
H
H
H
C
H
C
C
H
H
H
C
H
CH2
CH3
A CH3 branch = methyl and it is on the
third carbon (no matter which way we
count)
H
H
H
• So the name of our
H
compound is
3-methylpentane
1
5C
H
2
4
C
H
C3
H
H
C
3
4
2
CH2
5
1
CH3
H
Examples
H
H
H
C
H
C
Cl
C
H
H
H
1. longest unbroken chain of carbons :
3 Carbons = propH
2. Bonds?
All single = -ane
3. Branches
H
Yes, Chlorine = Chloro
4. Where
1
st
H
C
1 carbon
3
5. Name
H
1-Chloropropane
H
H
H
C
Cl
C
C
H
H
H
H
H
2
C
2
3
1C
H
H
Cl
CH3
H
1. Longest unbroken chain of carbons :
3 Carbons = prop-
C
H
C
C
H
H
CH3
H
H
C
C
H
Cl
C
H
H
4. Where
1st carbon (Cl) and 2nd Carbon (-CH3)
5. Name
1-Chloro,2-methylpropane
Cl
H
2. Bonds?
All single = -ane
3. Branches
Yes, Chlorine = Chloro
And a methyl or -CH3
H
H
H
CH3
H 2
1
3
C
H
C
2
3
Cl
1C
H
H
H
Learning Check.......
Name These Compounds
H
H
H
H
C
H
C
H
C
C
H
H
H
H
H
C
H
CH2
C
C
H
C
H
H
H
H
H
H
H
Cl
C
C
C
H
H
H
H
CH3
H
F
H
Br
H
C
C
H
C
H
C
H
C
H
H
H
H
C
H
H
H
C
H
H
H
H
C
Br
Br
H
C
C
H
C
CH2
H
H
CH3
H3C
H
Draw these Simple Alkanes
• 2-methylpentane
• 3-ethylhexane
• 2,2-dimethylbutane
• 2,3-dimethylbutane
Alkanes
• Single bonds
• Molecules that only contain single bonds
are saturated
• General Formula CnH2n+2
• Zig –Zag arrangement
• Straight chained or branched
• Named according to the longest chain
Properties of Alkanes
• Methane → Butane are gasses ( 1 to 4
carbons)
• 5 to 15 carbons are liquids
• 16 + are waxy solids
• The more carbons in the chain the
higher the boiling point
• They are non-polar and insoluble in
water
Alkenes
• Double bonds
• Molecules that contain double or triple
bonds are unsaturated
• The basic formula for alkenes is CnH2n
Physical Properties of Alkenes
1. All up to butene are gasses at room
temperature above this alkenes are liquids
and solids
2. They are non-polar or slightly polar which
means they have a VERY LOW solubility in
water. They are however soluble in nonpolar solvents such as cyclohexane
Naming Alkenes
• Alkenes as with all organic compounds
are named they same way as alkanes
The difference being.......
• The suffix is –ene
• You must state the carbon the double
bond is formed at and it must be given
the lowest number
Examples
1. Longest unbroken chain of
carbons :
H
5 Carbons = Pent
1
2. Bonds?
C
Double = -ene
5
3. Where
H
st
1 carbon
4. Branches
No
5. Name
1-pentene
H
H
H
2
C
4
H
4
3
C3
C
2
H
5
C1
H
H
H
Examples
1. Longest unbroken chain of
carbons :
5 Carbons = Pent
2. Bonds?
H
Double = -ene
3. Where
2nd carbon
4. Branches
No
5. Name
2-pentene
H
H
2
C
1
C
5
H
H
4
H
4
3
3C
H
H
C
2
5
C
1
H
H
Examples
1. Longest unbroken chain of
carbons :
5 Carbons = Pent
2. Bonds?
H
Double = -ene
3. Where
2nd carbon
4. Branches
Yes, Cl on 5th Carbon
5. Name
5-chloro-2-pentene
H
H
H
2
C
1
C
5
H
4
3
3C
H
Cl
4
C
2
H
5
C
1
H
H
Alkynes
• They contain at least one C≡C triple
bond
• They are highly unsaturated
• The basic formula for alkynes is CnH2n-2
• When naming alkynes the suffix is –yne
• As with alkenes the position of the
triple bond has to be identified in the
formula (and given the smallest number)
Uses of Ethyne
H
C
C
H
• Ethyne is more commonly known as acetylene.
• When burned on its own it produces a sooty
flame but burnt in the presence of oxygen it
burns a clean flames that can reach
temperatures of up to 3000oC making it
suitable for cutting and welding
Examples
1. Longest unbroken chain
of carbons :
4 Carbons = but
2. Bonds?
triple = -yne
3. Where
1st carbon
4. Branches
No
5. Name
1-butyne
H
H
2
C
1
C
4
H
3
4
3
C2
C
1
H
H
H
Examples
1. Longest unbroken chain
of carbons :
4 Carbons = but
2. Bonds?
triple = -yne
3. Where
1st carbon
4. Branches
Yes, Bromine (bromo)
on the 4th Carbon
5. Name
4-bromo-1-butyne
Br
H
2
C
1
C
4
H
3
4
3
C2
C
1
H
H
H
Specified Demonstration 1
• The solubility properties of methane,
ethane and ethyne.
Page 241 and 242 of book
Mandatory Experiment 13.1
Preparation and Properties of
Ethyne
Page 243
Aromatic Hydrocarbons
• Alkenes and alkynes where said to be
unsaturated
• Aromatics – another group of unsaturated
compounds
• Benzene is an important hydrocarbon as it
forms the basis for all aromatic compounds
Benzene (C6H6)
H
H
1. Six carbon atoms joined H
together by double and
single bonds to form a ring.
H
2. These double and single
bonds are constantly
switching.
3. The bonds are best
represented by a circle in
the centre of the ring
4. The bottom picture
represents how benzene is
drawn in diagrams
H
C
C
C
C
C
C
H
C
C
C
C
H
H
C
H
H
H
H
H
H
C
C
C
C
C
C
H
H
C
H
H
Aromatic Compounds
CH3
Methylbenzene
C7H8
CH2CH3
Ethylbenzene
C8H10
Physical Properties of
Aromatic Hydrocarbons
1. Similar to aliphatic hydrocarbons
2. Aromatics are non-polar they do not dissolve
to any great extent in water
3. They will dissolve in polar solvents
4. Aromatic hydrocarbons such as methylbenzene is better known in industry as
toluene and is an excellent non-polar solvent
5. Aromatic hydrocarbons with low molecular
mass are liquid at room temperature while
those of higher molecular mass are solids
Specified Demonstration 2
• Solubility properties of methylbenzene
Page 247 of book
Thermochemistry
The relationship between heat and
energy
Heat of Reaction (H)
• The mixing of any two substance in a
chemical reaction will result in a change
in heat or energy
• The heat of reaction(ΔH) of a
chemical reaction is the amount of
heat in kilojoules released or
absorbed when the number of moles
of reactant indicated in the balanced
equation describing the reaction,
react completely
Chemical Equations
H2(g) + 1/2O2(g)
2H2(g) +O2(g)
H2O(g) ΔH = -242 kJ/mol
2 H2O(g) ΔH = -484 kJ/mol
• You can see when you double the number of moles of
reactants you double the energy released
• Equations like these are called Thermochemical
equations , they must be balanced, ΔH must be given
and the physical states of the reactants must be given
• If heat (H) is released during a
reaction it is said to be exothermic
• If heat (H) is absorbed during a
reaction it is said to be endothermic
(H)
– ΔH
• Heat / energy is released during a reaction
• Symbolised by a – ΔH
• Mixing of sodium hydroxide and hydrochloric
acid releases heat therefore is exothermic
(H)
+ΔH
• Heat / energy is absorbed during a reaction
• Symbolised by a + ΔH
• the reaction between ethanoic acid and sodium
carbonate absorbs heat and therefore is
endothermic
Heat Released During a Reaction
Heat Change (ΔH) = mc ΔT
m = mass (kg)
c = specific heat capacity
ΔT = change in temperature (K)
K = Kelvin (oC + 273)
Specified Demonstration 3
• Demonstration of an exothermic and
endothermic reactions
So what is the source of
this change on heat/energy
in a reaction?
Consider this reaction....
CH4 + 2O2 → CO2 + 2H2O
• The burning of methane gives off heat and so is
exothermic but if you take a closer look at the
reaction and look at the bonds that are broken and
formed you will be able to find out where this heat
comes from
O
H
O
H
C
O
H
O
H
O
H
C
H
O
O
O
H
H
Bonds Broken
4 x C-H
2 x O-O
Bonds Formed
2 x C-O
2 x O-H
Which takes in energy
Which gives out energy
Heat of Reaction = Energy taken in – Energy given out
Bond Energy
The amount of Energy in kilo joules needed
to break one mole of covalent bonds and
to separate the neutral atoms completely
from each other,(all species being in a
gaseous state)
The bond energy for the breaking up of CH4 is found to be
CH4 → C + 4H
∆H = 1648 kJ mol -1
There are four C-H bonds so to obtain the bond energy for a single
C-H bond we divide 1648 kJ mol-1 by 4
E(C-H) = ¼ (1648) = 412 kJ mol -1
Heat of Combustion
• The heat of combustion of a substance is the heat
change in kilojoules when one mole of a substance is
completely burnt in excess oxygen
• An important phrase in the definition is the term “One
Mole”
• Consider this reaction
2C 4H10(g) + 13 O2(g) → 8CO2(g) + 10H2O ΔH= - 5720 kJ/mol-1
• The figure given for ΔH is per 2 moles of C 4H10 and so
must be divided by two to obtain the heat of combustion
C 4H10(g) + 6½ O2(g) → 4CO2(g) + 5H2O ΔH = -2860 kJ/mol-1
Measuring Heats of Combustion
 Heats of combustion are accurately measured
using a bomb calorimeter
 A calorimeter is any container used to measure
heat changes
 A bomb calorimeter consists of a small metal
container (known as “The Bomb”) with a screw on
cap
 The sample whose heat of combustion is to be
measured is placed in a crucible in the bomb
 The bomb is placed in a container of water (the
calorimeter)
 Oxygen is pumped into the bomb and it is ignited
with electric wires
 By measuring the rise in temperature of the
water and applying mathematic formulas based on
the ability of water to absorb heat it is possible
to calculate the heat of combustion of the fuel
 The kilogram calorific value is defined as the
heat energy produced per kilogram of fuel
 See pg327 of your book for values
Heat of Formation
• The heat of formation is the heat change in
kilojoules, when one mole of a substance is
formed from its elements in their standard
states
• Once again it is important to remember the
phrase 1 moles is very important
The heat of formation of an element is 0 (ZERO)
NB: an element in its common form i.e. C / H2 / O2 / N2
• The standard state of an element or
compound is its normal form at 25⁰C and
at 1 atmosphere pressure
• The heat of formation of water is -285.8
kj mol-1 and is represented as follows
H2(g) + ½O2(g)
H2O(l) ∆H = -285.8 kj mol-1
• Why does the following Equation NOT
represent the heat of formation of
water?
H2(g) + ½O2(g)
H2O(g) ∆H = -241.8 kj mol-1
• Does this equation represent the heat of
formation of water?
2H2(g) + O2(g)
2H2O(l) ∆H = -571.6 kj mol-1
• Study table 21.5 p333 to see the heats
of formation of some common
compounds
• Also note the heat of formation
equations shown on this page, note how
they are balanced using fractions
Hess’s Law
• Law of conservation of energy state that
energy cannot be made or destroyed it can
only be changed from one form to another
• Hess’s Law states if a chemical reaction takes
place in a number of stages, 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
• Hess’s law is very useful to find the
heat of reaction of a chemical reaction
that may be difficult to measure
directly
• We will read through the examples on
pages 333, 334 and 335 to learn how to
apply Hess’s law in this way
2004 Q 6 (HL)
• CH4(g) + 2O2(g) → CO2(g) + 2H2O(l)
ΔH = − 890.4 kJ mol-1
• The standard heats of formation of carbon dioxide and
water are −394 and −286 kJ mol-1 respectively.
• Calculate the heat of formation of methane.
For CO2
C + 2O → CO2 ΔH = -394 kJ mol-1
For H2O
H2 + ½ O2 → H2O ΔH = -286 kJ mol-1
Fuels
Crude oil
• Crude oil was formed from the bodies
of tiny sea creatures that died millions
of years ago
• Crude oil is pumped from underneath
the ground as is a thick black substance
with an unpleasant smell
• It does not burn easily and must be
undergo fractional distillation to
separate out all the useful components
Fractional Distillation
• The crude oil is heated in a furnace and starts
to evaporate, the longer carbon chains are
heavier and do not rise as easily as shorter
(lighter) carbon chains
• This means that the longer chains carbon
fractions are collected first while the shorter
chains get collected higher up the column
• Remember larger hydrocarbons have higher
boiling points while smaller hydrocarbons have
lower boiling points
You do not need to
memorise the
temperatures but you
do need to learn the
fractions and the uses
of the products of each
fraction
Fractionating Column at
Whitegate Oil Refinery
Refinery Gas
• Top of the column, methane,
ethane, propane + butane
are gases at 25⁰C
• Some used as fuel mostly
bottled for sale
• Since these are odourless
sulfur compounds called
Metacarptants are added for
safety
• Bottled gas is mainly a mix of
propane + butane which are
liquefied under high pressure
Petrol (Light Gasoline)
• Used as motor fuel
• A mixture of at least
100 compounds mostly
hydrocarbons of 5-10
carbons
Naphta
• Very useful to
petrochemical industry
• It is a source for a huge
number of useful
chemicals eg,
medicines, plastics,
synthetic fibres,
detergents, solvents
etc.
• 7-10 carbons in length
Kerosene/Parrafin
• Used as aviation fuel
and in certain lamps
• 10-14 carbon length
• NB as the number of
carbon atoms increases
so does the boiling
point as more heat is
required to break the
larger number of bonds
Diesel Oil/GasOil
• More difficult to
vaporise than petrol
• Therefore diesel engine
has a different design
• Trucks, buses, trains
and some cars use
diesel
• 14-19 carbon length
Lubricating Oil
• 19-35 carbons in length
• Cannot be vaporised
easily so cannot be
used as a fuel
• Used as a lubricant to
reduce wear and tear
Fuel Oil
• 30-40 carbons in length
• Used in ships, power
stations, heating plants,
oil heating in homes
Bitumen
• More than 35 carbon
atoms in length
• Very high boiling point
• Used in Tar to resurface
roads
Petrol and the internal
combustion engine
How a car engine works (VIDEO)
Knocking
• Smooth running of an engine depends on
this explosion occurring at exactly the
right time
• If the explosion occurs too early the
pistons vibrate and a metallic noise is
heard from the engine
• Early explosion can occur if the petrol +
air explode when they are compressed
instead of waiting for the spark
• This early explosion is referred to as
knocking or auto-ignition
• Straight chain alkanes such as nonane,
octane and heptane ignite very easily
and explode too soon
• Branched chain alkanes such as 2,2,4trimethylpentane (iso-octane) do not
tend to auto-ignite
• Petrol mixtures with large amounts of
branched chain alkanes are more
efficient than those which contain
straight chain molecules
• To indicate the efficiency of a
particular type of petrol a number
called the Octane Number is assigned
to it
Octane Numbers
• The octane number of a fuel is a measure of
the tendency of the fuel to resist knocking
• Since 2,2,4-trimethylpentane is a very efficient
fuel it is assigned an octane number of 100
• Heptane is not an efficient fuel and is assigned
an octane number of 0
• The octane number of a fuel is calculated by
comparing it’s efficiency in an engine to 2,2,4trimethylpentane and heptane
• Standard unleaded petrol usually has an
octane number of 95
• The shorter the alkane chain the higher
the octane number
• The more branched the chain the higher
the octane number
• Cyclic compounds have a higher octane
number than straight chain compounds
Which do you think would be
the best fuel
heptane,
3-methylhexane,
2,3,-dimethylpentane
Petrol
• In the 1920’s it was found that adding small
amounts of lead compounds to petrol helped
reduce the amount of knocking
• Due to its toxicity, lead has been phased out
of petroleum used in many countries. It is
illegal in the following countries to sell or
distribute leaded fuels for road vehicles
• Austria was the first to ban leaded petrol in
1989, in 2000 it was banned across the EU
• The alternative is unleaded petrol which are
used in conjunction with catalytic converters
to give cleaner emissions from motor cars
Increasing octane number of fuel
• This is now achieved in one of the following 4
ways;
1. Isomerisation
2. Catalytic Cracking
3. Reforming
4. Adding Oxygenates
Isomerisation
• This involves changing straight chain alkanes into
their isomers
• Alkanes are heated in the presence of a suitable
catalyst and the chains break
• The chains are allowed to reconnect but they are
more likely to reform in branched-chains than in
straight-chains
• This is commonly done on a large industrial scale
with pentane and hexane (NB fig 13.24)
Catalytic Cracking
• Catalytic cracking is the breaking down of long
chain hydrocarbon molecules into short chain
molecules for which there is greater demand
• In oil refineries the heavier fractions ( such as fuel
oil, diesel oil and kerosene) are heated in the
presence of a catalyst
• The short chain alkanes produced tend to be
highly branched and hence have a high octane
number
Reforming (Dehydrocyclisation)
• Reforming involves the use of catalysts to form ring
compounds
• Straight chain alkanes are converted to cycloalkanes
and these are converted to aromatic compounds
• Aromatic compounds have high octane numbers and
petrol contains 3-4% benzene, because benzene is a
carcinogen this is a cause for concern
• Hydrogen may also be produced in reforming reaction,
this is a useful substance and may be piped away for
various purposes
Adding Oxygenates
• An oxygenate is any fuel that contains oxygen in its
molecules
• 3 oxygen containing compounds, methanol, ethanol
and MTBE are commonly added to petrol to increase
its octane number
• Another advantage to adding oxygenates to fuel is that
they cause very little pollution when they burn and are
cleaner fuels
Hydrogen Fuel
• Hydrogen is a colourless odourless gas that is
insoluble in water
• It has received considerable attention as a
possible fuel for motor cars
• It burns more efficiently than petrol and is
non polluting with the only product of burning
being water
• Many companies are currently researching the
feasibility of building hydrogen cars, and most
of the automobile manufacturers had begun
developing hydrogen cars
Steam Reforming
• Natural Gas is reacted with steam
• CH4 + H2O → CO + 3H2
• The mixture is then reacted with more steam
carbon monoxide is oxidised to carbon dioxide
• CO + H2O → CO2 + H2
• This process converts 70% of methane to
hydrogen
Electrolysis of Water
• This is expensive and uses large amounts of
electricity
• Platinum or carbon electrodes with hydrogen
liberated at the negative electrode (the
cathode) while oxygen is liberated at the
positive electrode (anode)
• H2O → H2 + ½ O2
Electric Cars
An electric car is an alternative fuel car that utilizes electric
motors and motor controllers, instead of an internal combustion
engine
It is recharged by plugging it into a normal socket
Advantage: clean emissions???????
Disadvantage: Takes time to recharge
Hybrid Cars
• Is a vehicle that uses two or more distinct power sources to
move the vehicle. The term most commonly refers to
hybrid electric vehicles (HEVs), which combine an internal
combustion engine and one or more electric motors.
• Advantage: Uses less Petrol than a normal car and lower
CO2 emissions
• Disadvantage: Concerns about materials used to make
batteries
Hydrogen Cars
In hydrogen internal combustion engine vehicles, the hydrogen is
combusted in engines in fundamentally the same method as
traditional internal combustion engine vehicles.
In fuel-cell conversion, the hydrogen is reacted with oxygen to
produce water and electricity, the latter being used to power an
electric traction motor.
Disadvantages: Hydrogen is extremely expensive to produce at the
moment, lack of filling stations, CO2 emissions form steam
reformation