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Alcohols – Revision Pack (C6)
The formula of ethanol:
The molecular formula for ethanol is C2H5OH – ethanol is made using the process of
fermentation:
Glucose  Ethanol + Carbon Dioxide
C6H12O6  2C2H5OH + 2CO2
Ethanol is made by the fermentation of a glucose solution:
-
The reaction is catalysed (sped up) by the enzymes in yeast
This ONLY happens in the absence of oxygen
The dilute liquid made then undergoes fractional distillation to produce
ethanol
There’s more than one alcohol?
The alcohols are a group of compounds which have the general formula CnH2n+1OH
Alcohol
Methanol
Molecular Formula
CH3OH
Ethanol
C2H5OH
Propanol
C3H7OH
Butanol
C4H9OH
Pentanol
C5H11OH
Displayed Formula
The table above shows the molecular and displayed formula of alcohols with one to
five hydrocarbons (there are MANY more) – REMEMBER these!
Only ethanol is made using fermentation, using very specific conditions:
1) If the temperature is too low, the enzymes in the yeast are inactive
2) If the temperature is too high, the enzymes in the yeast are denatured
3) If air somehow gets in, ethanol is not made – ethanoic acid is by a different
reaction
Ethanol from Ethene:
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Ethanol made using fermentation can also be used as a fuel. This is a renewable fuel
because the plants that make the sugar (glucose) for the process can be grown
VERY quickly.
Ethanol that is used in industry can also be made from ethene. Ethanol made in this
way is NON-renewable. This is because the ethene it is made from comes from fossil
fuels. These fossil fuels CANNOT be replaced and are a finite (limited) resource.
The reaction used to make ethanol from ethene is called hydration because water is
added to the ethene molecule:
Ethene + Water  Ethanol
C2H4 + H2O  C2H5OH
A hot phosphoric acid catalyst is used in this reaction; ethene and steam are passed
over it.
Advantages and Disadvantages of Ethanol Production:
Processing
By Fermentation
35oC – LESS energy
needed
Batch (disadvantage)
By Hydration
Hot catalyst – MORE
energy needed
Continuous (advantage)
Sustainability
Sustainable (advantage)
Purification
Fractional Distillation
needed (disadvantage)
Low (disadvantage)
From a finite resource
(disadvantage)
Fairly pure on production
(advantage)
High (disadvantage)
Conditions
Percentage Yield / Atom
Economy
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The problem with the Ozone Layer:
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The ozone layer is located in the stratosphere.
While there are only very small amounts of
ozone in this layer, it still absorbs most of the
ultraviolet (UV) radiation from the Sun.
The more depleted the ozone layer becomes,
the more UV light can get through to the
earth’s surface.
When chlorofluorocarbons (CFCs) were first discovered, it was thought that they
were safe to use. However, since then scientists have discovered that CFC
molecules slowly move upwards into the stratosphere where they attack the ozone.
Society agreed with the view of the scientists’ that CFCs had been depleting the
ozone. This is why the use of CFCs has been banned in the UK, however it is a global
issue, and one country alone cannot solve it.
CFCs can be replaced with alkane or hydrofluorocarbons (HFCs) which DO NOT
damage the ozone layer when used.
How does the Ozone work?
UV radiation is part of the electromagnetic spectrum. Visible light is NOT absorbed
by the ozone layer and passes through it very easily. However, UV radiation is
absorbed by it:
-
The UV part of the electromagnetic spectrum has exactly the right frequency
to make ozone molecules vibrate
The energy of the UV radiation is converted into movement energy within
each ozone molecule
The thicker the ozone layer, the more UV radiation is absorbed
When CFCs were discovered in the 1930s, scientists were VERY excited because
these substances were inert (unreactive).
However, in the 1970s, scientists began to link the ozone depletion with CFCs.
CFCs, Ozone and Radicals:
In the stratosphere, the UV radiation from the sun breaks down the CFC molecules.
This makes highly reactive chlorine atoms. One of these reactive chlorine atoms is
known as a chlorine radical.
1) These chlorine radicals react with the ozone molecules, turning the ozone
back into oxygen gas and depleting the ozone layer
2) The highly reactive chlorine atoms are regenerated (made again) so can
react with more ozone molecules
3) UV light break down the CFCs very slowly, so they last for a very long time
CF2Cl2  CF2Cl + Cl
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NOTE – the dot means that there is an unpaired electron, making the atom very
reactive.
The main alternatives to CFCs are alkanes and hydrofluorocarbons. HFCs do NOT
contain chlorine, so cannot make chlorine radicals, and so are safer.
When a covalent bond is broke, it can split into equal halves to make radicals.
UV radiation causes radicals. Each radical sets off a chain reaction. One single
chlorine radical can cause the breakdown of 100,000 ozone molecules.
The chain reaction happens in three steps:
STEP 1 – UV light breaks a bond in the CFC molecules to form chlorine radicals (see
above)
STEP 2 – Chlorine radicals react with ozone molecules, creating more chlorine
radicals, for example:
Cl + O3  OCl
+ O2
OCl + O3  Cl
+ 2O2
If you combine these two equations, you get 2O3 + 3O2.
STEP 3 – A possible termination reaction (that ends the depletion) is:
Cl + Cl  Cl2
It is very common for CFCs to last for between 20 and 50 years before they are
completely broken down by UV radiation. For this reason, CFCs will continue to
deplete the ozone long after they have been banned.
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Low Temperature Washing:
It is good for the environment to wash clothes at 40oC instead of at higher
temperatures.
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Washing machines have to heat up a lot of water – this requires ENERGY, so the
lower the temperature of the water, the less energy is used and smaller volumes of
greenhouse gases are put into the atmosphere.
Washing clothes at low temperatures is also good for coloured clothes – this is
because many dyes are easily damaged by higher temperatures.
What are Detergents?
A detergent lifts grease stains off into the
water.
A detergent molecule has two parts. It has
a hydrophobic (oil-loving) tail and a
hydrophilic (water-loving) head.
How does a detergent work?
When we wash clothes using a detergent, the hydrophobic tail
of the detergent molecules forms strong intermolecular forces
with molecules of grease.
This leaves the hydrophilic head on the outside. The head
forms strong intermolecular forces with the water molecules.
Eventually, SO many detergent molecules have formed
intermolecular forces with the grease that the outside of the
grease has become covered with water.
The grease becomes surrounded with hydrophilic ends that
form strong intermolecular forces with the water molecules.
ts in the oil being lifted off the clothes into the water.
is made from oil or grease it WILL NOT dissolve in the water.
Dry Cleaning and how it works:
Some fabrics will be damaged if they are washed in water – they MUST be dry
cleaned.
A dry-cleaning machine washes the clothes in an organic solvent – the ‘dry’ doesn’t
mean no liquids are used, just that the liquid is NOT water.
Grease-bases stains do not dissolve in water but they do dissolve easily in a drycleaning solvent.
The solvent molecules surround the molecules of grease and pull them from the
fabric.
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STEP 1 – molecules of grease form intermolecular forces with other molecules of
grease
STEP 2 – molecules of solvent form intermolecular forces with other molecules of the
solvent
STEP 3 – molecules of the solvent form intermolecular forces with the molecules of
grease and are able to surround, and pull them off clothing
Water cannot be used to wash it off because the intermolecular forces between
water molecules are too strong, so they cannot form intermolecular forces with the
grease.
NOTE – THERE ARE NO PAST PAPER QUESTIONS ON THIS TOPIC
Electrolysis - Recap:
Electrolysis is the breaking down or
decomposition of a liquid by passing a
(direct) current power supply. For electrolysis
to happen, you need an electrolyte which is
the liquid that the current moves through.
You also need a positive electrode (anode)
and a negative electrode (cathode).
During electrolysis, ions in the electrolyte
move to electrodes and are discharged as
atoms or molecules there. REMEMBER –
opposites attract, so negative ions go to the
anode (these are called anions) and
positive ions (called cations) go to the
cathode.
You can tell whether your ion is an anion or a cation by looking at its chemical
formula. For example H+, Na+ and NH4+ all need to lose one electron to become
stable so are POSITIVELY charged and cations. Ions like OH-, Cl- and O2- all need to
gain electrons to become stable so are NEGATIVELY charged and anions.
Liquid Electrolyte:
Electrolytes are liquid. The charge moves through the molten liquid by the
movement of ions. If the electrolyte was to solidify, the ions would be in a fixed
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position and unable to move SO the current could not flow. Electrolysis is the flow of
charge by moving ions.
Electrolyte Decomposition:
The reactions that take place at the electrodes can be written as half equations.
For example, the electrolysis of sodium chloride:
At the Cathode
Na+ + e-  Na
At the Anode
2Cl- - 2e-  Cl2
The NaCl splits up into its ions, so they
are free to move.
The positive Na+ ions go to the negative
cathode.
Every Na+ ion gains an electron from the
cathode to become Na again –
REMEMEBER – in the ionic bond with
chlorine; sodium lost an electron to form
the Na+ ion.
Products of electrolysis:
The NaCl splits up into its ions so they
are free to move.
The negative Cl- ions go to the positive
anode.
The Cl- ions leave the anode as chlorine
gas (or Cl2).
Every Cl- ion loses its electron, and
combines with another Cl- in a covalent
bond – Cl2.
Electrolyte Solution
Sodium Hydroxide (NaOH)
Formed at the anode:
Oxygen
Formed at the cathode:
Hydrogen
Sulphuric Acid (H2SO4)
Oxygen
Hydrogen
Copper (III) Sulphate
Oxygen
Copper
(CuSO4)
Copper (II) Sulphate needs to be electrolysed with carbon electrodes.
The amount of substance formed during electrolysis varies with time and current.
More substance is formed when time is extended or more current is used. The
substances transferred at the electrodes are discharged (allowed out) ions.
The only thing that affects the amount of ions discharged is the amount of charge
transferred.
Electrode Reactions:
In both the electrolysis of NaOH and H2SO4 the electrode reactions are the same:
At the cathode – 2H+ +2e-  H2
At the anode – 4OH- - 4e-  2H2O + O2
You would expect the sodium (in NaOH) to form at the cathode but sodium is more
reactive that hydrogen so hydrogen is given off instead.
The electrode reaction in the electrolysis of CuSO4 with carbon electrodes is
interesting:
At the cathode – Cu2+ + 2e-  Cu
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At the anode – 4OH- - 4e-  2H2O + O2
Additional Notes:
You may be asked to calculate the amount of substance formed – this can be
calculated using simple ratio.
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Fuel Cells and Spacecraft:
Fuel cells are used in spacecraft for several reasons:
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The water produced is not wasted, the astronauts drink it
They are lightweight; normal batteries are much heavier
They are compact
There are NO moving parts
Fuel Cells and Cars:
The car industry is developing fuel cells for cars for a variety of reasons:
-
There are NO CO2 emissions when fuel cells are used in cars
They offer an alternative to the non-renewable fossil fuels currently being used
The main product of a hydrogen-powered fuel cell is water (this is NOT a
pollutant)
There is a plethora of hydrogen available through decomposing water
Advantages and Disadvantages of Fuel Cells:
Advantages of fuel cells
- There is direct energy transfer; the
energy goes straight from being
chemical energy to being electrical
energy
- There are few production stages, so
little energy is lost – this means it is an
efficient process
- They are less polluting; they do not
make nitric oxide
- Fuel cells last longer than conventional
batteries
Problems with fuel cells
- Poisonous catalysts are
used in fuel cells that need
to be disposed of when
the cell dies
- Fossil fuels are burnt in the
production of hydrogen
and oxygen that are
needed in fuel cells
Fuel cells are especially useful in mobile energy sources.
Getting energy from fuel:
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The reaction between oxygen and hydrogen by combustion is exothermic – it
releases energy to the surroundings. When hydrogen and oxygen (in the air) react
via burning, the chemical energy is released as heat energy.
Fuel cells use a reaction between the fuel and oxygen to produce electrical energy.
When hydrogen is used as this fuel, the reaction is:
2H2 + O2  2H2O
Fuel cells convert chemical energy DIRECTLY into electrical energy – there is NO
heat.
In an oxygen-hydrogen fuel cell, electrons are transferred from the cathode to the
anode.
The diagram shows how a fuel
cell generates electricity.
Energy-Level Diagrams:
When hydrogen burns in oxygen, water is
made.
The energy within the hydrogen and oxygen is
higher than the energy within the water made;
this means that energy is given out to the
surroundings – therefore, the reaction is
exothermic.
In fuel cells, it is electrical energy that is given
out, rather than heat energy.
We can look at the reactions at the anode and
cathode to see how the water is actually
made.
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At the cathode (negative electrode), the hydrogen loses its electrons:
2H2  4H+ + 4eThis is oxidation (Oxidation Is Loss) – the hydrogen ions then travel through the
electrolyte to the anode (positive electrode).
At the anode (positive electrode), the oxygen molecule combines with the four
hydrogen ions, and gain electrons from the circuit to form water.
O2 + 4H+ + 4e-  2H2O
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is therefore reduction (Reduction Is Gain). Overall, the reaction is:
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2H2 + O2  2H2O
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How is water hard?
Hard water is formed when rainwater dissolves some of the rock that it flows over.
Rainwater sometimes contains dissolved carbon dioxide. This makes it slightly acidic.
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Rocks such as chalk, limestone or marble are forms of calcium carbonate. These
react with water and carbon dioxide to form calcium hydrogencarbonate:
Calcium Carbonate + Water + Carbon Dioxide  Calcium Hydrogencarbonate
The calcium hydrogencarbonate dissolves to form temporary hard water.
Water hardness can be measured by how much soap is needed to form lather.
When a soap flake is shaken in a water sample, calcium ions in the water (from the
calcium hydrogencarbonate) react with the soap to form a nasty scum.
As you add more flakes, over time and after you’ve continually shaken the mixture,
the soap reacts with ALL of the calcium ions. After this point, any more soap
produces a stable lather for the first time. The number of soap flakes needed is a
measure of the hardness of the water. Less soap flakes means there were less
calcium ions present in the water, so it wasn’t hard.
Softening Water:
Temporary hardness can be removed by boiling. The calcium hydrogencarbonate
decomposes easily in hot water to form calcium carbonate (limescale), water and
carbon dioxide.
Heating removes the soluble calcium ions from the water and changes them into
insoluble calcium carbonate, which you often find at the bottom of your kettle.
Permanent hardness in water is NOT affected by heating – calcium sulphate (which
is present in PERMANENT hard water) is too stable.
For example:
An investigation is conducted to see the effect of boiling on how much soap is
needed to form lather in the water.
Sample
A
B
C
3
Volume of soap needed before boiling (cm )
20
20
20
Volume of soap needed after boiling (cm3)
2
20
14
Sample A contained temporary hard water because most of it was removed after
boiling, so less soap was needed to make lather.
Sample B contained only permanent hard water because none of it was removed
after boiling, so the same amount of soap was needed to make lather.
Sample C contained a mix of both permanent and temporary hard water because
some (NOT all) of it was removed, so less soap was needed to produce lather.
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Ion-Exchange Resins are used to remove
permanent hardness from water. The hard water
flows over beads of solid resin.
The calcium ions get trapped on the surface of
these beads and the ions exchange; the calcium
ions in the water displace the sodium ions on the
beads.
Ion-Exchange Resins removed BOTH temporary
and permanent hardness.
Thermal decomposition, which occurs when
water is boiled ONLY softens temporary hardness:
Ca(HCO3)2  CaCO3 + CO2 + H2O
Washing soda can soften both permanent and
temporary hardness in water. It is sodium
carbonate or Na2CO3
When washing soda dissolves, it reacts with calcium sulphate in the water and forms
insoluble calcium carbonate (limescale), thus ‘locking up’ the calcium ions:
CaSO4 + Na2CO3  Na2SO4 + CaCO3
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What are fats and oils?
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Fats and oils are all part of a common group of compounds called esters. They all
have chains of carbon atoms.
If the carbon chains contain only single carbon-carbon bonds, the fat is saturated.
If the carbon chains contain some double carbon-carbon (C=C) bonds, then the oil
is unsaturated.
To test for unsaturation, the oil is shaken with bromine water, which is orange:
-
Unsaturated compounds lose their colour (decolourise)
The colour remains orange with saturated compounds
An industrial use of vegetable oils is to make margarine. Vegetable oils are
unsaturated (so has C=C bonds):
STEP 1 – The first step taken is to ‘harden’ the oils and turn them into saturated
compounds; this is done by bubbling hydrogen through the oil at between 150oC
and 200oC using a nickel catalyst
STEP 2 – The hydrogen reacts with the carbon- carbon double bonds and turns them
into single bonds (see below)
Fats, Oils and Health:
Saturated fats and oils (no C=C bonds) normally come from animals, while
unsaturated fats and oils (has C=C bonds) normally come from plants.
‘Polyunsaturated’ means that the compound contains more than one C=C double
bond (think ‘poly’ means many)!
Bromine is an orange liquid. Bromine
reacts, in an addition reaction, with
the C=C double bonds in the chain.
The reaction uses the bromine
molecules, making a dibromo
compound (see left). This is colourless.
Saturated compounds CANNOT react with bromine since they have NO C=C bonds.
People who have a diet rich in unsaturated fats and oils have lower levels of the
type of cholesterol that causes heart disease.
Getting fats into water:
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Oil and water are immiscible liquids – they cannot be mixed together.
They do not dissolve in each other, but disperse into tiny droplets and form an
emulsion.
1) Milk is an oil-in water emulsion
2) Margarine and cold cream is a water-in-oil emulsion
Water
droplets
spread
through oil
Oil droplets
spread
though
water
Saponification:
Fats and oils can be split up using sodium hydroxide (an alkali) to make soap and
glycerol:
Fat + Sodium Hydroxide  Soap + Glycerol
The reaction of splitting up natural oils with alkalis is called saponification.
When an ester reacts with sodium hydroxide, saponification occurs:
-
The ester forms one glycerol molecule, and three soap molecules
This reaction is the same as the reaction of alkaline water so is called a
hydrolysis reaction:
Fat + Sodium Hydroxide  Soap + Glycerol
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Redox and Rusting:
‘Redox reaction’ references two processes that work together:
-
Reduction (Is Gain)
Oxidation (Is Loss)
Rusting is a redox reaction:
Iron + Oxygen + Water  Hydrated Iron (III) Oxide
Galvanizing iron protects it from rusting by covering it with a
protective layer of zinc. This zinc layer stops oxygen and water
from reaching the surface of the iron.
Zinc also acts a sacrificial metal, this is because it oxidises in
preference to iron (it loses or sacrifices electrons) – this is because
zinc is more reactive than iron.
Redox Extension:
Redox reactions can be thought of in terms of electrons:
A substance is said to have been oxidised if it loses electrons. An oxidising agent
takes electrons away from another object.
A substance is said to have been reduced if it gains electrons. A reducing agent
transfers electrons to another object.
An example of an oxidising agent is oxygen in rusting. The oxygen takes electrons
away from the iron – the oxygen gains these electrons, so in the rusting reaction:
-
Iron loses electrons – so is oxidised
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Oxygen gains electrons – so is reduced
Redox reactions can be thought of as reversible reactions; they can be forced in
either direction. In the redox reactions, one direction is oxidation and the other
reduction. For example:
1) The oxidation of iron = (Fe  Fe2+ + e-)
But this can be reversed to the reduction of Fe2+ = (Fe2+ + e-  Fe).
2) The oxidation of Fe2+ = (Fe2+  Fe3+ + e-)
But this can be reversed to the reduction of Fe3+ = (Fe3+ + e-  Fe2+)
3) The reduction of Cl2 = (Cl2 + 2e-  2Cl-)
But this can be reversed to the oxidation of 2Cl- = (2Cl-  Cl2 + 2e-)
Displacement Reactions:
When you write the word equation for a displacement reaction, the
more reactive metal swaps place with the less reactive one; for
example, sodium is more reactive than zinc, so zinc is displaced:
Sodium + Zinc Sulphate  Sodium Sulphate + Zinc
The order of reactivity (to the left) is needed to work out which metals
will displace other metals.
You will NEED to know the order of magnesium, zinc, iron and tin.
Magnesium is the most reactive, so will displace the other three.
Zinc is the second most reactive, so will displace iron and tin.
Iron is third most reactive, so will only displace tin.
Tin is the least reactive out of the four, so will not displace any.
Displacement – Explained:
Displacement reactions will happen between a reactive metal and compounds of a
less reactive metal. The general formula is:
Reactive
Metal
Element
+
Less
Reactive
Metal
Compound
Reactive
Metal
Compound
+
Less
Reactive
Metal
Element
All metals react by pushing electrons out and turning into ions – this is oxidation.
These electrons are forced onto the ions of other atoms that are not as reactive.
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The metal atoms that gain the electrons are reduced; for example:
The SO4 cancels out!
The Mg is oxidised.
Mg + ZnSO4  MgSO4 + Zn
Mg + Zn2+  Mg2+ + Zn
The Zn is reduced because the
electrons from the Mg are
forced onto the Zn2+.
If you coat iron in tin, rusting will happen at a faster rate because the tin is less
reactive. A layer of zinc or magnesium is often used because it is more reactive and
will lose electrons in preference to the iron.
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