Ch. 4 - Sodium Hydroxide Flashcards
Mercury process advantages
- Produces very pure sodium hydroxide from pure water and mercury amalgam as the products of electrolysis are kept separate, hence there is no contamination from chloride ions - Avoids using asbestos, hence reducing health and environmental impacts - Mercury is mainly recycled, so there is an efficient use of resources
Explain the difference between galvanic cells and electrolytic cells in extremism of energy requirements.
In galvanic cell: - A spontaneous chemical reaction is used to generate electricity - Converts chemical energy into electrical energy without extra energy input requirements - Eo is positive In electrolytic cell: - A chemical reaction that doesn’t occur spontaneously is brought about - Converts electrical energy into chemical energy with extra energy input requirements - Eo is negative
Mercury difficulties (disadvantages)
TECHNICAL - Though the mercury is recycled, it is still costly to replace mercury lost in the process - Mercury cells require relatively high working voltages, so the cost of electricity is significant ENVIRONMENTAL - Mercury cells lose mercury to the environment, mainly from the mechanical transfer of mercury to the brine solution which is discharged to the environment - Metallic mercury is extremely insoluble and can be converted into other compounds by bacteria - Compounds of mercury (eg Hg(CH3)2 ) can be consumed by aquatic organisms and passed along the food chain with biomagnification to humans - Mercury can affect the nervous system of humans and cause brain damage, and accumulate in the body for long periods
Diaphragm process advantages
- Produces large quantities of sodium hydroxide and also chlorine (both high demand industrial chemicals) at reasonable costs - quite acceptable levels of purity for most purposes
Diaphragm process difficulties
TECHNICAL - Always a small amount of chloride in the NaOH produced - Chlorine combines with sodium hydroxide in the compartments to form sodium hypochlorite NaClO , hence reducing yield of both desirable products ENVIRONMENTAL - Small losses of asbestos to the environment can cause serious illnesses such as lung cancer - ClO- is a strong oxidant and must be removed before its discharge into the environment with brine
Membrane process advantages
- sodium hydroxide is virtually pure, with only 0.02% contamination by chloride - overcomes the disadvantages of mercury and diaphragm cells - produces large quantities of sodium hydroxide and also chlorine at quite reasonable costs and acceptable levels of purity for most uses - energy consumption is lowest for the membrane cell and it operates at the lowest voltage -> cost less - lower establishment and operating costs
Membrane process difficulties
TECHNICAL - high cost of polymer membranes, which are derived from the petrochemical industry - NaOH product still contains a tiny amount of chloride, which is absent if the mercury process is used ENVIRONMENTAL - contributes to depletion of fossil oil resources - the energy needs are a modern concern since the production of electricity requires burning vast amounts of fossil fuels, which releases CO2 to atmosphere
Outline steps in the industrial production of sodium hydroxide from sodium chloride solution
Concentrated sodium chloride solution (> 2 mol L-1), obtained from sea water or underground salt mines, is electrolysed
- Sodium ions migrate to the cathode, but they are too stable to be reduced, and instead water is reduced to hydrogen gas (and hydroxide ions)
Cathode: 2H2O (l) + 2e- → H2 (g) + 2OH- (aq)
- Chloride ions migrate to the anode and are oxidised to chlorine gas
Anode: 2Cl- (aq) → Cl2 (g) + 2e-
Net ionic: 2Cl- (aq) + 2H2O (l) → Cl2 (g) + H2 (g) + 2OH- (aq)
Full formulae equation: 2NaCl (aq) + 2H2O (l) → Cl2 (g) + H2 (g) + 2NaOH (aq)
Describe the mercury process
In a mercury cell, liquid mercury flows across the sloping floor of a large tank
At the titanium plate anodes, chloride ions are oxidised to form chlorine gas, which is removed
Anode: 2Cl- (aq) → Cl2 (g) + 2e-
At the mercury cathode, the mercury has the negative terminal of the voltage source immersed in it.
The sodium ions (rather than water) are preferentially reduced to sodium metal, which simultaneously dissolves in the mercury to form a sodium-mercury amalgam
Cathode: Na+ (aq) + e- → Na (Hg)
· _ Net ionic_ (distinct net ionic equation):
2Na+ (aq) + 2Cl- (aq) → Cl2 (g) + 2Na(Hg)
- The sodium amalgam flows from the electrolysis cell into the ‘decomposer’ which contains pure water. Sodium reacts with water in the presence of a graphite catalyst to form sodium hydroxide solution and hydrogen gas. The hydrogen gas is removed and the mercury is recycled.
2Na (s) (in Hg) + 2H2O (l) –graphite→ 2NaOH (aq) + H2 (g)
Overall reaction (same for all other methods):
2NaCl (aq) + 2H2O (l) → H2 (g) + Cl2 (g) + 2NaOH (aq)
Overall formulae equation for all three processes (diaphragm, mercury, membrane)
2NaCl (aq) + 2H2O (l) → H2 (g) + Cl2 (g) + 2NaOH (aq)
Visualise mercury process… now!
Describe the diaphragm process
- At the inert titanium plate anode, chloride ions are oxidised to form chlorine gas, which is pumped out as it is formed
Anode: 2Cl- (aq) → Cl2 (g) + 2e-
- At the iron mesh cathode, water is reduced to form hydrogen gas and sodium hydroxide, and the hydrogen has is also pumped out as it forms
Cathode: 2H2O (l) + 2e- → H2 (g) + 2OH- (aq)
Net ionic: 2Cl- (aq) + 2H2O (l) → Cl2 (g) + H2 (g) + 2OH- (aq)
Overall reaction: 2NaCl (aq) + 2H2O (l) → H2 (g) + Cl2 (g) + 2NaOH (aq)
Visualise diaphragm process… now!
Visualise membrane cell… now!
How is membrane cell different from diaphragm cell?
- The material and function of the membrane
- The electrolyte on the cathode side is pure water or dilute sodium hydroxide, instead of brine
- Purity of the sodium hydroxide product
First-hand investigation to identify the products of the electrolysis of sodium chloride - Setup
