Bishops Science Department
THE CHEMICAL INDUSTRY
This topic considers resources, chemistry and production with respect to the chloroalkali and
fertilizer industries, as well as the application of redox chemistry in batteries.
THE CHLOROALKALI INDUSTRY.
The electrolysis of saturated sodium chloride (brine) produces chlorine, sodium hydroxide and
hydrogen. The electrolytic process is very expensive (due to the vast amount of electricity used) but
the process is economic due to the large number of uses of the products.
Define electrolysis: ………………………………………………………………………
……………………………………………………………………………………………
Electrolysis takes place in an electrolytic or electrochemical cell. A cell has an anode and cathode at
which half-reactions occur.
The overall reaction is
2NaCl (aq) + H2O (l) → 2NaOH (aq) + Cl2 (g) + H2 (g)
The electrolysis of sodium chloride can take place in three types of cells, namely the membrane cell,
the diaphragm cell or the mercury cell. The latter two are being phased out due to environmental
risks associated with them.
Consider the membrane cell. The anode is made from titanium and the cathode of nickel or steel. An
ion-exchange membrane up the middle lets sodium ions (and water) through to the cathode but keeps
the gases apart. As a result, the NaOH only forms in the cathode compartment and does not mix with
the NaCl.
The membrane is a polymer impregnated with negative charges that help draw positive charges
through it but repel negative charges.
Label the following diagram of a membrane cell:
(State whether oxidation or reduction is occurring at
each electrode)
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Hydrogen is produced at the cathode: Half reaction - …………………………………
(The H+ ions are present from the water)
Chlorine is produced at the anode: Half reaction - ……………………………………
Where do the OH- ions come from? ………………………………
There are no environmental risks associated with the membrane cell. Care just needs to be taken when
handling the products. The mercury cell poses problems due to the highly poisonous nature of
mercury. The diaphragm cell uses asbestos as its diaphragm, and this also has documented risks.
Uses of chlorine, sodium hydroxide and hydrogen:
Chlorine: Properties…………………………………………………………………….
…………………………………………………………………………………..
Uses: In the preparation of PVC, solvents (eg trichloroethane for dry cleaning), paints and
dyes, hydrochloric acid, bleaches, weedkillers and pesticides, pharamaceuticals.
Also use in water purification – killing bacteria in tap water and swimming pools.
Sodium hydroxide: Properties ………………………………………………………….
……………………………………………………………………………………
Uses: In the preparation of soaps, detergents, paper, textiles, ceramics.
Hydrogen: Properties ……………………………………………………………………
…………………………………………………………………………………….
Uses:
In the preparation of nylon, hydrogen peroxide, margarine, ammonia and also used as a
rocket fuel.
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THE FERTILIZER INDUSTRY.
Plants need carbon dioxide and water to provide the essential nutrients (carbon, hydrogen and oxygen)
for photosynthesis.
Provide an equation for the photosynthesis reaction:
…………………………………………………………………………………………..
Plants also need three other primary nutrients – nitrogen, potassium and phosphorous. These are
obtained from the soil. Nitrogen is needed to make proteins for growth and root development.
Potassium is needed to promote growth. Phosphorous is needed to help leaves develop. Some
bacteria (called nitrifying bacteria and they live in the soil or root nodules of plants) also convert the
nitrogen in the air into nitrates – this process is call nitrogen fixing, fixation or nitration.
Other elements such as calcium, iron and sulphur are also needed to a lesser degree.
When the same crops are grown in the same soil year after year, the supply of nutrients get exhausted
and the crop growth declines. Fertilizers are added to the soil to replace elements used up by the
growing plants. Animal manure is a natural (organic) fertilizer. Artificial inorganic fertilizers include
ammonium nitrate (formula ……………..), ammonium sulphate ( ………………….), ammonium
phosphate (……………………) and potassium chloride (KCl).
Historically, nitrogen used to be sourced from bird guano, phosphorous from bone meal and potassium
from German mines. These sources cannot supply today’s demands and thus most fertilizers nowdays
are produced in factories. Fertilizers are often bought as NPK compound fertilizers, with different
mixtures suiting different needs. A 20:8:14 NPK fertilizer would consist of 20 % nitrogen, 8 %
phosphorous and 14 % potassium, by mass.
Humans also require elements to survive. The four primary nutrients required are carbon, hydrogen,
oxygen and nitrogen. These are obtained from food, water and the atmosphere. The following table
lists the nature of nutrients in plants and humans, as well their use to humans:
Element
Nutrient in plants
C, H, O
N
P
K
Ca
Mg
S
Cl
Fe
Major element
Primary nutrient
Primary nutrient
Primary nutrient
Secondary nutrient
Secondary nutrient
Secondary nutrient
Secondary nutrient
Micronutrient
Nutrient in
humans
Primary nutrient
Primary nutrient
Macromineral
Macromineral
Macromineral
Macromineral
Macromineral
Macromineral
Micromineral
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Uses in human
Organic compounds
Proteins, enzymes
Bones, teeth, DNA
Salt in body fluids
Bones, teeth, blood clotting
Enzymes, energy metabolism
Proteins
Salt in body fluids
Haemoglobin (carry O2 in blood)
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Industrial manufacture of fertilizers.
Three processes are significant in this industry. The Haber process makes ammonia
(formula…………) which becomes the basis of ammonium (formula……….) fertilizers. The Ostwald
process makes nitric acid (formula………….) which is converted to nitrates (formula…………). The
Contact process makes sulphuric acid (formula……………) which is converted to sulphates
(formula………….). Potassium is obtained from potash (mined potassium salts such as potassium
nitrate and potassium sulphate).
Most fertilizers are made in a neutralization reaction. Consider, for example, ammonium nitrate:
NH3 (aq) + HNO3 (aq) → NH4NO3 (aq)
The Haber process – the production of ammonia
Nitrogen and hydrogen are needed as reactants. Nitrogen is obtained from the fractional distillation
of air. Describe this process by using the boiling points of nitrogen and oxygen:
…………………………………………………………………………………………….
…………………………………………………………………………………………….
…………………………………………………………………………………………….
……………………………………………………………………………………………..
……………………………………………………………………………………………..
……………………………………………………………………………………………..
Hydrogen is produced at SASOL by reacting methane and steam, which forms hydrogen and carbon
dioxide. Write an equation for this reaction:
……………………………………………………………………………………………...
Ammonia is produced according to the following reversible reaction:
N2 (g) + 3 H2
(g)
2 NH3 (g)
∆H = -92 kJ.mol-1
To understand the operating conditions, you need to know Le Chateliers principle. State Le
Chateliers principle:
………………………………………………………………………………………………
………………………………………………………………………………………………
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According to Le Chateliers principle, what would the ideal temperature and pressure conditions be to
maximize the yield of ammonia?
Temperature: …………………………..
Pressure: ……………………………….
In order to maximize the yield of ammonia, a compromise of approximately 400 oC and 200
atmospheres is use. The following graph illustrates the effect of temperature and pressure on the yield
of ammonia:
What yield of ammonia would be obtained at 350 oC and 100 atmospheres? ………..
What yield of ammonia is obtained at 450 oC and 200 atmospheres? ……………
Why does a higher ammonia yield result from a higher temperature?
…………………………………………………………………………………………..
Why is a higher operating pressure not used? ………………………………………….
…………………………………………………………………………………………..
An iron catalyst is also used to further increase the yield.
140 Million tons of ammonia is produced annually worldwide, of which 80 % is used in the
manufacture of fertilizers.
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The Ostwald process – production of nitric acid
Ammonia, air and water are needed as reactants. The air provides O2.
In general terms, the stages of conversion are as follows:
NH3
NO
NO2
HNO3
The reactions for these stages are:
→ 4 NO (g) + 6 H2O (g)
4 NH3 (g) + 5 O2
(g)
2 NO (g) + O2
→ 2 NO2 (g)
(g)
4 NO2 (g) + O2 (g) + 2 H2O (l) → 4 HNO3 (aq)
A platinum catalyst is used, in conjunction with operating conditions of 900 oC and
4 – 10 atmospheres, in the first stage.
60 Million tons of nitric acid is produced annually worldwide, of which 85 % is used in the
manufacture of fertilizers.
The Contact process – production of sulphuric acid
In general terms, the stages of conversion are as follows:
S
SO2
SO3
H2S2O7
H2SO4
The reactions for these stages are:
S (s) + O2 (g) → SO2 (g)
SO2 (g) + O2 (g) → SO3 (g)
SO3 (g) + H2SO4 (aq) → H2S2O7 (aq)
H2S2O7 (aq) + H2O (l) → 2 H2SO4 (aq)
The second step is known as the contact step. Operating conditions are approximately 430 oC and 2
atmospheres, and a vanadium pentoxide (V2O5) catalyst is used.
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Problems associated with fertilizers
Nitrates are very soluble. Thus they can end up in rivers by either being washed by rainwater or by
draining through groundwater.
In the rivers, the nitrates promote growth of tiny water plants called algae. When this algae dies,
bacteria feeds off this but also uses up oxygen dissolved in the water. This results in oxygen
starvation which results in fish and other river life dieing. This is called eutrophication.
Some algae are poisonous to fish and humans which can result in eye infections, rashes, vomiting and
diarrhea.
If excess nitrates get into drinking water, it increases the risk of blue-baby syndrome. The nitrates
get converted in nitrites which gets taken up by babies haemoglobin, instead of oxygen. This can
result in death.
Phosphates are less soluble than nitrates and are not as significant a hazard since so little leaching
occurs.
Farmers thus need to use fertilizers sparingly and appropriately – for example, not when it is about to
rain.
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Bishops Science Department
BATTERIES
A battery is a collection of galvanic or electrochemical cells. They provide a supply of direct current
as a consequence of a redox reaction between two chemical. The two half reactions occur at two
different electrodes. Electrons pass from the reducing agent to the oxidizing agent via the external
electric circuit.
Batteries can be classified as primary or secondary. Primary batteries or cells are discarded when they
go flat, while secondary batteries can be recharged.
The cell used in a school laboratory separates the two electrodes, as illustrated below.
Using the following Standard Electrode potentials, answer the questions below:
Half reaction
2+
Zn + 2eZn
Cu2+ + 2eCu
Eθ (volts)
-0,76
+0,34
a)
Which substance is the oxidizing agent? …………………………….
b)
Write down the oxidation half reaction: ……………………………..
c)
Give the international notation of the cell: …………………………..
d)
What is the overall equation: …………………………………………
e)
Determine the cell potential: …………………………………………
………………………………………………………………………..
………………………………………………………………………...
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Bishops Science Department
The diagram below illustrates a primary cell in its commonly found form. A primary cell (eg, a torch
‘battery’) has a central graphite rod (B) acting as an anode and a zinc outer casing (C) which acts as a
cathode. A sodium hydroxide/zinc chloride paste acts as the electrolyte/salt bridge(D). A metal cap
(A) covers the graphite rod.
The above cell is also known as the Leclanche cell.
The energy supplied to a circuit by a cell or battery is related to the charge and potential difference:
W = Vq
(W – Joules, V – Volts, q- Coulombs)
The charge that flows depends on the current and time:
Q = It
(I – Amperes, t – seconds)
The capacity or rating of a battery indicates how long a battery will last while delivering a set current.
The unit of capacity or rating is the ampere-hour (Ah) and use the formula q = It with t measured in
hours. A car battery of 40 Ah can delivery 1 ampere for 40 hours, or 5 amperes for 8 hours. Smaller
batteries can have their rating expressed as milliampere-hours.
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