H10
Alkalis and Detergents
Industrial processes are continually being improved to make them more efficient or
environmentally safe. This chapter investigates the industrial production of sodium
hydroxide and sodium carbonate.
Properties of sodium
hydroxide
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Uses for sodium
hydroxide
Sodium hydroxide is used in the manufacture of:
 Soap and other detergents
 Rayon and other plastics
 Paper
 Oven and drain cleaners
Very soluble in water
Very strong base, completely ionising into Na+ and OHions in aqueous solution
Sodium hydroxide is also used in the manufacture of:
 Extraction of alumina from bauxite
 Removal of H2S and SO2 in the processing of natural gas
and oil.
Galvanic cell
Device in which a chemical reaction generates electricity by
converting chemical energy to electrical energy.
Electrolysis
Process in which electrical energy is converted to chemical
energy and therefore to bring about chemical change.
Electrolytic cell
A cell in which electrolysis is carried out
Industrial preparation
of sodium hydroxide
Sodium hydroxide is made industrially by electrolysing a
concentrated sodium chloride solution (brine).
2NaCl(aq) + 2H2O(l)
2NaOH(aq) + Cl2(g) + H2(g)
Three different electrolytic cells have been used over the last
100 years.
 Diaphragm cell
 Mercury cell
 Membrane cell
Diaphragm cell
Salt bridge:
Anode:
Cathode:
Asbestos diaphragm
Titanium
Iron mesh
Oxidation reaction
2Cl-
Cl2 + 2e-
Reduction reaction
2H2O + 2e-
H2 + 2OH-
Advantages
 Produces large quantities of sodium hydroxide and
chlorine
 Reasonable cost
 Acceptable levels of purity
Disadvantages
 Small amount of chloride in the sodium hydroxide
 Health and environmental problems associated with
making and using the asbestos diaphragms
 There may be hypochlorite, ClO-, (a strong oxidant) in
the waste brine solution
Mercury cell
Salt bridge:
None, sodium metal is drained off as a
mercury amalgam.
Anode:
Cathode:
Titanium
Liquid mercury
Oxidation reaction
2Cl-
Reduction reaction
Na+ + e-
Cl2 + 2eNa (dissolved in Hg)
Advantages
 Produces very pure sodium hydroxide
 Avoids using mercury
Disadvantage
 Loss of mercury to the environment
Membrane cell
Salt bridge:
Anode:
Cathode:
Polytetrafluoroethylene (PTFE)
diaphragm
Titanium
Iron mesh
Oxidation reaction
2Cl-
Cl2 + 2e-
Reduction reaction
2H2O + 2e-
H2 + 2OH-
Advantages
 Virtually pure sodium hydroxide
 No asbestos or mercury
 No hypochlorite, ClO-, (a strong oxidant) in the waste
brine solution
Disadvantages
 Not known as yet
Electrolysis of
sodium chloride
under different
conditions
Faraday constant
(Insert Conquering Chemistry HSC page 346 Table 10.1)


Quantity of electricity carried by one mole of electrons.
96 500coulombs per mole (C mol-1)
Coulomb

Amount of charge involved when a current of one ampere
flows for one second.
Q = It

Hydrolysis
Reaction between a compound and water.
Saponification


Fats and oils




Soap


Reaction between an ester and hydroxide ion to form an
alcohol and a carboxylate anion.
Saponification is the hydrolysis of an ester under alkaline
conditions.
Esters of glycerol (1, 2, 3-propanetriol)
Each glycerol reacts with three fatty acid molecules to
form a triglyceride.
Saturated fatty acids lead to solid fats
Unsaturated fatty acids lead to liquid oils
Soluble salt of a fatty acid, most commonly sodium
tristearate or sodium palmitate.
Made by the saponification of fats and oils.
Surfactant




Surface acting substance
Substance that decreases the surface tension of water or,
alternatively, “solubilizes” dirt and grease.
Consists of a hydrophilic carboxylate ion at one end of
the molecule attached to a hydrophobic hydrocarbon
chain.
Synthetic surfactants do not form precipitates with
magnesium and calcium ions and consequently can be
used with hard water.
Emulsions
Dispersion of small droplets of one liquid throughout another
liquid.
Different classes of
surfactants
(Insert Conquering Chemistry HSC page 356 Table 10.3)
Environmental
impact of soaps and
detergents




Solvay process
Soaps are biodegradable
Earliest synthetic surfactants (anionic branched-chain
alkylbenzene sulfonates) were not biodegradable.
Later linear-chain alkylbenzene sulfonates have a similar
biodegradability to that of soaps.
The phosphate content of washing powders and liquids is
the major environmental concern with detergents.
The key steps in the Solvay process used to manufacture
sodium carbonate are
 Saturate a concentrated brine solution with ammonia.
 Bubble carbon dioxide through this to form a precipitate
of sodium hydrogen carbonate
 Filter off the sodium hydrogen carbonate
 Heat the sodium hydrogen carbonate to form sodium
carbonate
 Regenerate the ammonia from the filtrate in the third
step.
Environmental issues associated with the Solvay process
include:
 Disposal of calcium chloride
 Loss of ammonia
 Waste heat
 Mining