Applications of Acids and Bases (Grade 12 NSC Matric Physical Sciences): Revision Notes

Applications of Acids and Bases

Industrial production of chlorine and sodium hydroxide

The chlorine-alkali industry represents one of the most important industrial applications of acids and bases in modern chemistry. This industry produces chlorine gas and sodium hydroxide through a process that demonstrates the practical significance of acid-base chemistry in manufacturing.

The electrolysis process

The most common industrial method involves the electrolysis of concentrated brine (sodium chloride solution). This electrochemical process separates the components of salt water using electrical energy to drive chemical reactions that would not occur naturally.

During electrolysis, the following chemical reactions occur at the electrodes:

At the anode (positive electrode): $2Cl^-(aq) \rightarrow Cl_2(g) + 2e^-$

At the cathode (negative electrode): $2H_2O(l) + 2e^- \rightarrow H_2(g) + 2OH^-(aq)$

The sodium ions (Na⁺) remain in solution with the hydroxide ions (OH⁻), forming the overall reaction:

$2NaCl(aq) + 2H_2O(l) \rightarrow Cl_2(g) + H_2(g) + 2NaOH(aq)$

Temperature effects on chlorine reactions

When chlorine and hydroxide ions interact, the temperature significantly affects the products formed. Understanding these temperature-dependent reactions is crucial for industrial control.

At low temperatures (below 60°C): $Cl_2(g) + 2OH^-(aq) \rightarrow Cl^-(aq) + ClO^-(aq) + H_2O(l)$

This reaction produces sodium hypochlorite (NaClO), commonly found in household bleach.

At high temperatures (above 60°C): $3Cl_2(g) + 6OH^-(aq) \rightarrow 5Cl^-(aq) + ClO_3^-(aq) + 3H_2O(l)$

This reaction produces sodium chlorate (NaClO₃), which has different industrial applications.

Industrial applications and uses

Uses of chlorine include:

• Water purification - chlorine kills harmful bacteria and viruses in drinking water
• Disinfectant - used in swimming pools and cleaning products
• Chemical production for manufacturing:
  • Hypochlorous acid (kills bacteria in drinking water)
  • Chloroform and carbon tetrachloride
  • Paper and food processing chemicals
  • Antiseptics, insecticides, and medicines
  • Textiles and fabric treatments
  • Paints, petroleum products, solvents, and plastics (like PVC)

Uses of sodium hydroxide include:

• Soap and cleaning agent production - reacts with fats to make soap
• Aluminium purification - removes impurities from bauxite ore
• Paper manufacturing - breaks down wood fibres
• Rayon production - creates artificial silk fibres

The versatility of these products demonstrates how acid-base chemistry supports numerous industries and everyday applications.

Hair chemistry and structure

Hair provides an excellent example of how acid-base chemistry affects biological materials. Understanding hair chemistry helps explain how various hair treatments work and why pH control is essential in cosmetic applications.

Keratin protein structure

Hair consists primarily of a protein called keratin. This protein forms the structural foundation of hair and contains numerous amino acids linked together in long chains. The strength and flexibility of hair depend on the chemical bonds within and between these protein chains.

Amino acids are molecules that contain both an amino group (NH₂) - which acts as a base - and a carboxylic acid group (COOH) - which acts as an acid.

Cysteine and disulfide bonds

The amino acid cysteine plays a particularly important role in hair structure. Cysteine contains a sulfur-containing side chain with a thiol group (-SH). When two cysteine molecules come close together, they can form a covalent bond called a disulfide bond (S-S linkage).

The formation of cystine (two cysteine residues connected by a disulfide bond) creates cross-links between protein chains. These cross-links give hair its natural strength, elasticity, and shape. The more disulfide bonds present, the curlier and stronger the hair appears.

Natural pH of hair

Proteins naturally give hair a slightly acidic pH between 4 and 5. This acidic environment helps maintain the hair's structure and protects it from damage. When hair treatments alter this pH, they can break existing chemical bonds and allow new ones to form, changing the hair's properties.

Hair treatment applications

Hair treatments demonstrate practical applications of acid-base chemistry in everyday life. These treatments manipulate the pH of hair to break and reform chemical bonds, achieving desired cosmetic effects.

Permanent waving (perms)

Permanent waving or perming changes straight hair into curly hair through controlled chemical reactions. The process involves breaking disulfide bonds in the hair and reforming them in new positions.

The chemical used in perms is ammonium thioglycolate, which acts as a reducing agent. The equilibrium reaction that occurs is:

$HSCH_2COO^-(aq) + NH_4^+(aq) \rightleftharpoons HSCH_2COOH(aq) + NH_3(aq)$

Hair relaxers

Hair relaxers work by doing controlled damage to hair structure, similar to perms but with the goal of straightening rather than curling hair. There are two main types of hair relaxers with different pH levels and safety profiles.

Lye hair relaxers:

• Contain sodium hydroxide (NaOH)
• Have very high pH (12-14) making them strongly alkaline
• Provide quick application time
• Cause significant damage to scalp and hair due to extreme alkalinity
• Often require additional heating, which increases damage risk

No-lye hair relaxers:

• Have lower pH (9-11) making them less alkaline
• Are gentler on the scalp
• Take longer to work but cause less immediate damage
• Can be left on too long, leading to hair dryness
• May cause calcium buildup over time

Hair colouring chemistry

Hair colouring involves different chemical processes depending on the desired permanence and colour change. The chemistry varies significantly between different types of hair dyes.

Permanent hair dyes:

• Contain a diamino compound (molecule with two amino groups)
• Include a coupling agent and an oxidising agent
• Use hydrogen peroxide (H₂O₂) to oxidise the diamino compound, enabling it to bond with the coupling agent
• Have basic pH (9-11, usually from ammonia NH₃) to open hair cuticles and allow dye penetration
• Create larger, more complex molecules that cannot be easily washed out

Demi-permanent hair dyes:

• Use ethanolamine or sodium carbonate as gentler bases (pH 8-9)
• Contain hydrogen peroxide but in lower concentrations
• Cannot lighten dark hair to lighter colours
• Last approximately 12 washes but gradually fade
• Cause less hair damage than permanent dyes

Semi-permanent hair dyes:

• Contain small dye molecules that partially penetrate hair without requiring strong bases
• Use minimal hydrogen peroxide or ammonia
• Last about 4-5 washes
• Depend heavily on original hair colour and porosity
• Do not lighten hair

Temporary hair dyes:

• Use large dye molecules that cannot penetrate hair structure
• Stick to the hair surface only
• Can usually be removed with single washing
• Work better on damaged hair where cuticles are more open

Safety considerations and health effects

Understanding the pH levels and chemical nature of hair treatments helps explain their potential health effects and the importance of proper safety precautions.

pH-related damage

Different pH levels cause different types of damage to hair and scalp:

Potential health risks

Safety precautions

Professional hair treatments require careful handling of chemicals:

• Use protective equipment (gloves, safety glasses)
• Handle all chemicals with care
• Follow timing instructions precisely
• Perform patch tests before full application
• Ensure proper ventilation
• Have neutralising agents available

The chemistry of hair treatments demonstrates how understanding acid-base principles enables both beneficial cosmetic effects and helps prevent serious chemical injuries.