9. The Principles & Practice of Chemical
Production - Synthesising Molecules
(Suitable for AQA, Edexcel and OCR GCSE
See also 7.
Chemical & Pharmaceutical Industry Economics
Products of the
Chemical & Pharmaceutical Industries their Impact on Us
7. to 9.
are all connected as a survey of the chemical and pharmaceutical
industries, lots of overlap
INDEX of selected industrial chemistry sections
ALL my GCSE Chemistry Notes
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Principles & Practice of Chemical
Production - Synthesising Molecules
Chemical synthesis is the term
applied to describe the process of making a chemical compound which maybe a
single stage or multi-stage series of chemical reactions.
You are essentially taking some raw
material to produce, usually, relatively simple starter molecules and using them
to synthesise a complex compound.
A synthesis involves many factors
and stages e.g. methods and apparatus, calculation of amounts required, control
of process, maximise yield, all in all to achieve by the most efficiently
economic and profitable process - See 7.
How do we synthesise compounds?
does synthesis involve?
What are the starting point raw materials?
You need to know and find the
raw materials that you can convert into a chemical feedstock that
becomes the starting point for the synthesis of the chemical compound - the
e.g. you need sources of nitrogen and
hydrogen to synthesise ammonia using the
or sulfur, oxygen and water to
manufacture sulfuric acid by the
The phrase chemical
feedstock means the actual reactant molecules that are fed into the
reactor chamber e.g. hydrocarbons like octane or ethene, the alcohol ethanol, nitrogen
from air, hydrogen from methane etc.
The raw material usually
requires processing e.g. purifying or chemically modifying it to produce
a purer starting feedstock material.
Raw materials can be naturally
occurring substances like crude oil, natural gas, coal, deliberately 'grown'
biomass, water, oxygen and nitrogen from air, mineral ores.
There are several factors to be taken
into consideration before choosing the starting raw materials - that is
assuming there is a choice e.g.
The cost of extracting, separating
and purifying the starting chemicals from which to make the
e.g. is the raw material too
costly to process and make a profitable product.
Some metal ores have a very low
concentration of the desired metal and not economic to exploit -
though new techniques like bioleaching and phytomining are
extracting metals like copper from low grade ore.
Is the process economically viable
and the product ultimately profitable.
Can any of the raw materials or
energy be obtained from renewable sources e.g. the air (nitrogen),
What are the energy costs like - can
you operate the chemical process efficiently using the minimum energy -
. Lower temperatures and pressure conditions use less
energy and engineering costs are lowered too.
If it is an equilibrium reaction, how
far does it go to the desired product side.
The reaction conditions chosen must be
carefully controlled to allow an efficient rate of product production
(economically acceptable rate) and maximise the yield of product e.g. by
control of reactant concentrations, reactor pressure, reactor temperature
and appropriate catalyst.
right type of reaction
There are lots to choose from!
Types of Chemical Reactions or Chemical Processes (index at top of this page) e.g.
Addition * Contact Process * Cracking * Decomposition * Dehydration *
Displacement * Double decomposition * Electrolysis * Esterification *
Exothermic reaction * Fermentation * Galvanising * Haber Process * Hydration
* Neutralisation * Polymerisation * Precipitation * Substitution *
assessment of the reaction
This involves trying to
anticipate any dangers in the processes involved in the synthesis of the
compound e.g. identifying ANY potential hazards - hazardous chemicals and
their manipulation, risk of injury to personnel involved in the processes -
chemical plant workers, how can risks be reduced (risk is unavoidable in the
Hazard warning symbols
the quantities of the reactant chemicals required
You need to calculate how much
of the reactants and in what ratio to give the desired quantity of the
product. The starting point is balanced symbol equation from which you can
theoretically calculate the quantities of reactants needed. You can work in
reacting masses or reacting moles and convert to masses. The links below
offer pages on how to calculate the mass of products formed from a given
mass of reactants. You do not want waste any of the raw materials and
chemical feedstocks that go into the process of making the compound. Waste
makes the process less economic, waste is money down the drain!, though I
hope that's not where the waste goes!
Reacting mass ratio calculations of reactants and products
Calculation of how
much of a reactant is needed?
Connecting moles, mass and formula mass - the basis of reacting mole ratio calculations
the apparatus, reaction conditions and yield
The type of chemical plant required and
controlling the process e.g. in terms of temperature, pressure,
concentrations and catalysts.
The reaction must be carried out in appropriately designed apparatus
e.g. reactor chamber, distillation unit etc., depending on the type of
synthesis reaction being carried out. The chemical plant equipment must be
of the right size to accommodate the reactants and products needed for
sustainable commercial production.
If the reaction is very exothermic, heat
exchangers may be needed, but this 'waste heat' can be used to preheat
reactants or make hot water for heating offices, or steam to drive an
electrical generator. If its an endothermic reaction that only occurs at
elevated temperatures, a heating system is needed.
Decisions must be made on the
reaction conditions such as
temperature, pressure, concentration and a catalyst may be needed to give an
efficient and safe speed of reaction.
See 'industrial paragraphs' in
Rates of Reaction. Quite
often a compromise has to be made which I've discussed in detail for the
Haber Synthesis of ammonia.
The diagram is on the
above-right is a
generalised design for a chemical plant making ammonia. The separation of
the ammonia product is done at the bottom of the reactor and unreacted gases
diagram on the left shows the apparatus of the first stage of
preparing an ester in a
school or college laboratory are illustrated above.
I'm afraid you will have
to appreciate (without photographs) that in industry the equipment will be
rather larger, but in principle what you see in the above diagram would be
reproduced in a chemical plant by chemical engineers.
Electric heaters would
be used instead of bunsen burners!
is how much of the product you make from the reactants - usually expressed
as percentage yield.
% reaction yield and theoretical yield calculations,
why you can't get 100% yield
atom economy calculations
The reaction conditions often determine
the yield, especially if its an equilibrium reaction.
You want the equilibrium reaction to be
as much to the right as possible i.e. maximum possible yield, but sometimes,
as in the
Haber synthesis of ammonia,
its much more economic to get a low yield fast.
Be aware that conditions that give the fastest reaction
rate, may not give the highest yield of desired product.
rate always increases with temperature, but often a compromise is made to
get the highest yield as fast and as economic as possible.
If its a biochemical reaction, like fermentation, the catalytic
enzymes work best under optimum conditions e.g pH and temperature of
and isolating the product - dealing with by-products and waste too
The product needs to be
extracted or isolated from the reaction mixture which may involve
distillation, filtration, centrifuging, solvent extraction. Basically, you
want to separate out the crude product (since will still be impure)
and leave behind as much of the waste material as possible before doing the
final purification. The waste products, if of no use, must be disposed of
safely with no harm to workers, public and the environment - all of which is
governed by strictly enforced legal regulations.
It is possible sometimes
possible to recycle unused reactants - especially if it is a continuous
process like ammonia production (and not a batch process). But, the 'waste'
may include a useful by-product that can be sold, so further separate
but parallel processing may go on alongside the main product production. All
in all, all the products other than the main desired product to totally
waste material of new use at all, must all be dealt with.
The 2nd stage, distilling the ester from the reaction mixture, the flask is
simulating the 'reactor vessel' of a chemical plant.
Purifying requires the last of
any impurities to be removed from the isolated or extracted 'crude'
product. This may involve drying with a desiccating agent if water is
the only impurity, if its a solid it may be recrystallised from a suitable
solvent, it may involve an extra or multiple fractional distillation.
Further purification of the ester using a separating funnel.
The complete preparation of an
insoluble compound, mixing (preparation), filtration (separation), washing
and drying (purification).
yield and purity - quality control
You never get 100% of the
theoretical yield, but you do need to know how much product you have
actually got from the mass of reactants used. The success of the process can
be quantified as the actual percentage yield (100 x mass of actual
yield / theoretical maximum mass of product).
You also need to check on the
purity of the product for quality control and on every batch, or regular
monitoring if the process is continuous. The % purity analysis tells you
exactly how much of the product is actually the real product itself! Drugs
must be especially pure to avoid chemical contamination and side-effects in
the patient, which can be very serious (look up the infamous Thalidomide
Different levels of purity are
allowed if no hazard is involved (health, environment etc.), so you
may get cheaper impure grades or more expensive grades depending on the use
of the chemical. Why waste money on 'over-purification' if a very pure
product isn't required!
Explaining and calculating % purity of a product
Explaining and calculating % reaction yield,
reasons why never 100%
Explaining and calculating
and for more example
Calculating % yield and theoretical yield,
AND this page has a section on
to check on the purity of a compound' scroll down to 1.1h 'PURE
selected pages describing industrial processes:
- uses, thermal decomposition of carbonates, hydroxides and nitrates
Contact Process, the importance of sulfuric acid
metals be made more useful? (alloys of Al, Fe, steel etc.)
Instrumental Methods of Chemical Analysis
Chemical & Pharmaceutical Industry Economics & Sustainability
and Life Cycle Assessment
Products of the
Chemical & Pharmaceutical Industries & impact on us
The Principles & Practice of Chemical
Production - Synthesising Molecules
Extraction of Metals
- electrolysis and cells
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