Industrial chemistry

Sodium Hydroxide(Lye, caustic soda)

NaOH

Instruction:

•Properties of sodium hydroxide1. Physical2. Chemical3. Structural

•Manufacture1. The mercury cell process (Castner-Kellner)2. Nelson diaphragm cell process3. Membrane cell process

•Industrial use

•Environmental and health issues

Physical Properties

•Appearance: A white crystalline substance (solid)

•Smell: Odourless

•Transparent: Only in liquid form

•Poisonous: Yes (causes burns when inhaled)

•Molecular Weight: 39.997g/mol

•Boiling point: 1390C (2534F)

•Melting point: 318C (604F)

•Density: 2.13g/cm^3

•Flammable: No

•Acidity: 13-14 pH (basic)

•Basic Type: Caustic Metallic Base

•Corrosive: High

•Reactivity: Medium

•Hygroscopic: Yes (absorbs CO2 and moisture from air)

Chemical Properties

•Solubility in Water: 1110g/L (20C)

•Soluble: in water, acids and alcohol

Neutralization

•The hydroxide ion makes sodium hydroxide a very strong base which react with any acid which will neutralise each other.

•In general, neutralisation reactions can be represented as follows in an ionic equation:OH−(aq) + H+(aq) → H2O(l)

•Sodium hydroxide reacting with an acid will generally result in a neutralisation process

•For instance, sodium hydroxide reacts with hydrochloride acid:NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l)

Structural properties

Manufacture

The major method for producing sodium hydroxide is electrolysis of concentrated brine (sodium chloride solution) produces chloride gas, hydrogen gas and sodium hydroxide.

2NaCl(aq) + 2H2O(l) -----> H2(g) + Cl2(g) + 2NaOH(aq)

At the anode (positive electrode), the chloride ions (Cl-) are oxidised to chloride gas (Cl2(g)). At the cathode (negative electrode), the sodium ions (Na+) and water (H2O(l)) are reduced to hydrogen gas (H2(g)) and sodium hydroxide (NaOH(aq)).

The mercury cell process (Castner-Kellner)

At the anode, titanium is the metal and chloride ions are oxidised to chloride gas:2Cl-

(aq) -----> Cl2(g) + 2e-

At the cathode, mercury is flowing along the bottom of the cell. Sodium ions are reduced to sodium metal:Na+

(aq) + e- -----> Na(s)

An amalgam is formed when sodium metal dissolves in the liquid mercury and it is removed to the decomposer. The amalgam then reacts with water to form sodium hydroxide, hydrogen gas and mercury:2Na/Hg(l) + 2H2O(l) -----> 2NaOH(aq) + H2(g) + 2Hg(l)

Nelson diaphragm cell process

The saturated brine flows through the diaphragm and into the cathode compartment.

At the anode, carbon or titanium coated with Ru-Ti, the chlorine ions are oxidized. At the cathode, steel mesh, hydrogen gas and hydroxide are produced: 2H2O(l) + 2e -----> H2(g) + 2OH-

(aq)

The diaphragm allows the sodium ions to migrate across it and to combine with hydroxide, forming sodium hydroxide: 2H2O(l) + 2Cl-

(aq) + 2Na+(aq) ----->

2NaOH(aq) + H2(g) + Cl2(g)

Solid sodium hydroxide can be crystallised out.

Membrane cell process At the anode, titanium is the metal.

At the cathode the metal is nickel and water is reduced:2H2O(l) + 2e -----> H2(g) + 2OH-

(aq)

Sodium ions migrate across the membrane and combine with hydroxide to form sodium hydroxide in the cathode compartment:

2H2O(l) + 2Cl-(aq) + 2Na+

(aq) -----> 2NaOH(aq) + H2(g) + Cl2(g)

Soap making:

The common fats, triglyceride esters react with sodium hydroxide to form glycerol and the sodium salt of the fatty acid which is soap.

The equation can be simplified to:C18H36O2 + NaOH -> alcohol + salt of the carboxylic acid (soap)

The reaction between a strong base and a fat is called ‘saponification’, can be presented by the equation

Cleaning agent:

•Sodium hydroxide is usually added to hot water and then can be used to dissolve fat, grease, oil and protein based deposits.

•Sodium hydroxide solution is widely used in cleaning the oil which is used in making metal products.

•Compared to CFC (chlorofluorocarbon, a powerful substance to clean the oil), it does not pollute the environment.

•The industrial oil does not react easily, so it needs energy to break it up to react.

•The hot water provides the energy required to make the sodium hydroxide work efficiently.

Aluminum production:

•Aluminium ore is usually contained in the mineral bauxite that is a mixture of hydrated aluminium oxides Al2O3

and hydrated iron oxides, Fe2O3.

•In aluminum refining, sodium hydroxide will only react with aluminium oxide and form sodium aluminates (red mud), NaAl(OH)4

•After that, aluminum oxide will precipitate then be washed and heated to form pure aluminum.

The ionic equation will be: Al2O3(s) + 2 OH−

(aq) + 3 H2O(l) → 2 [Al(OH)4] (aq)

−

The equation with NaOH:Al2O3(s) + 3H2O(l) + 2NaOH(aq) -----> 2NaAl(OH)4(aq)

Cryolite, Na3AlF6 Production:

•Cryolite is a useful material that is used in the production of potassium aluminium sulphate, sodium carbonate and aluminium sulphate.

•To produce this material, aluminium oxide, hydrogen fluoride and sodium hydroxide are required. With the similar equation as aluminium production:

Al2O3 + 2NaOH = 2NaAlO2 (Aluminate) + H2O

6HF + 3NaAlO2 = Na3AlF6 (Cryolite) + 2Al(OH)3

Paper pulp industry:

•In the process of paper making, there are two main parts: pulping and papermaking.

•There are always some excess chemicals used to pulp the paper, sodium hydroxide will help regenerate these chemicals to reduce the cost.

•Because of the corrosive property of sodium hydroxide, it will also be added to the process of bleaching to increasing the efficiency.

•It also has a function of PH control. PH stays at around 8 where the hypochlorite bleach works the most efficiently.

Environmental and health issues• Sodium hydroxide can cause

severe burns and damage to any tissues of the body, because of:

1. The reaction of sodium hydroxide with moisture generates heat and causes thermal burns (exothermic reaction).

2. The reaction of sodium hydroxide with organic molecules results in chemical burns.

•In the atmosphere, sodium hydroxide can dissolve in water vapour and form aerosols or mists which are very corrosive.

•Sodium hydroxide’s primary effect would cause the water’s pH level to raise due to the hydroxyl ion (OH-).

•Due to the neutralization of pH, sodium hydroxide allows specific plants to grow more efficiently by adjusting the balance of nutrients