20.1 Materials | Materials | Chemistry 12

Introduction to Materials

Chemistry plays a central role in the development and production of modern materials. Two of the most important classes of synthetic materials are polymers, produced through industrial chemical processes. Understanding how these materials are made, their properties, and their uses is essential for FBISE Grade 12 Chemistry.

Polymerization

Polymerization is the chemical process by which small molecules called monomers join together to form large molecules called polymers.

There are two main types of polymerization:

Addition Polymerization
Condensation Polymerization

Addition Polymerization

In addition polymerization, monomers containing a carbon–carbon double bond (C=C) react together. The double bond opens and the monomers link in a chain with no atoms lost — the molecular formula of the repeat unit is the same as the monomer.

General process: $n CH_{2} = CH_{2} catalyst, pressure [- CH_{2} - CH_{2} -]_{n}$

Examples of Addition Polymers

Monomer	Polymer	Uses
Ethene ( $CH_{2} = CH_{2}$ )	Poly(ethene) / Polyethylene (PE)	Plastic bags, bottles
Propene ( $CH_{3} CH = CH_{2}$ )	Poly(propene) / Polypropylene (PP)	Ropes, containers
Chloroethene ( $CH_{2} = CHCl$ )	Poly(chloroethene) / PVC	Pipes, insulation
Tetrafluoroethene ( $CF_{2} = CF_{2}$ )	PTFE (Teflon)	Non-stick coatings
Styrene ( $C_{6} H_{5} CH = CH_{2}$ )	Polystyrene (PS)	Packaging foam

Properties of Addition Polymers

Generally thermoplastic (soften on heating)
Non-biodegradable — resistant to chemical attack
Low density (polyethene) to moderate density
Good electrical insulators

Condensation Polymerization

In condensation polymerization, monomers join together with the elimination of a small molecule (usually water, $H_{2} O$ , or hydrogen chloride, $HCl$ ) at each step.

Condensation polymers require monomers with two functional groups (bifunctional monomers).

Types of Condensation Polymers

1. Polyesters

Formed from a diol (two $- OH$ groups) and a dicarboxylic acid (two $- COOH$ groups). Water is eliminated.

Example: Terylene (PET — Polyethylene terephthalate) $HO-CH_{2} CH_{2} -OH + HOOC-C_{6} H_{4} -COOH \to [- O-CH_{2} CH_{2} -OOC-C_{6} H_{4} -CO-]_{n} + n H_{2} O$

Uses: Clothing fibres, plastic bottles (PET bottles), food packaging

2. Polyamides (Nylons)

Formed from a diamine (two $- NH_{2}$ groups) and a dicarboxylic acid (two $- COOH$ groups). Water is eliminated, forming amide linkages ( $- CO-NH-$ ).

Example: Nylon-6,6 $H_{2} N-(CH_{2})_{6} -NH_{2} + HOOC-(CH_{2})_{4} -COOH \to [- NH-(CH_{2})_{6} -NH-CO-(CH_{2})_{4} -CO-]_{n} + n H_{2} O$

Uses: Clothing, ropes, parachutes, toothbrush bristles, engineering components

3. Proteins (Natural Condensation Polymers)

Proteins are natural polyamides formed from amino acids joined by peptide bonds ( $- CO-NH-$ ).

Properties of Condensation Polymers

Contain ester ( $- COO-$ ) or amide ( $- CO-NH-$ ) linkages in the backbone
Generally stronger and more resistant to heat than addition polymers
Can be hydrolysed (broken down by water under acidic or alkaline conditions)
Polyamides can form hydrogen bonds between chains, giving high tensile strength

Comparison: Addition vs Condensation Polymerization

Feature	Addition	Condensation
Monomer requirement	One type with C=C	Two types with two functional groups each
By-product	None	Small molecule (e.g., $H_{2} O$ )
Repeat unit formula	Same as monomer	Different from monomer
Examples	PE, PVC, PTFE	Nylon, Terylene, proteins
Linkage	C–C bonds only	Ester or amide bonds

Cognitive Biases in Scientific Reasoning

Scientists must be aware of cognitive biases — systematic errors in thinking that can distort scientific reasoning and lead to flawed conclusions.

Common Cognitive Biases and Fallacies

Bias / Fallacy	Description	Example in Chemistry Context
Confirmation bias	Seeking or interpreting evidence that confirms existing beliefs; ignoring contradictory data	Only recording experimental results that match the expected outcome
Hasty generalization	Drawing broad conclusions from too few observations	Concluding a reaction mechanism applies to all compounds after testing only one
Post hoc ergo propter hoc	Assuming that because B followed A, A caused B	Assuming a new catalyst caused a yield increase when temperature also changed
Straw man fallacy	Misrepresenting an opponent's argument to make it easier to attack	Distorting a rival theory to dismiss it
Appeal to tradition	Assuming something is correct because it has always been done that way	Rejecting a new synthesis route because the old method is traditional
False authority	Accepting a claim because of who said it, not the evidence	Accepting an incorrect mechanism because a famous chemist proposed it
Occam's Razor (failing)	Choosing a needlessly complex explanation when a simpler one fits the data	Proposing a multi-step mechanism when a single-step one explains all observations
Argument from non-testable hypothesis	Using a hypothesis that cannot be tested or falsified	Claiming a reaction works due to an untestable mystical property
Begging the question	Using the conclusion as a premise in the argument	"This compound is acidic because it donates protons, and it donates protons because it is acidic"
Fallacy of exclusion	Ignoring relevant evidence that contradicts the conclusion	Reporting only trials where the catalyst worked, ignoring failed trials
Faulty analogy	Drawing conclusions based on a misleading comparison	Assuming two structurally similar compounds have identical reactivity without testing
Redefinition	Changing the meaning of a term mid-argument to avoid refutation	Redefining "yield" to make a poor synthesis appear successful

Why This Matters in Chemistry

Sound scientific reasoning requires:

Objectivity — evaluating all evidence, not just supporting evidence
Falsifiability — hypotheses must be testable
Reproducibility — results must be independently verifiable
Parsimony — preferring the simplest explanation consistent with the data (Occam's Razor)

Recognizing these biases helps chemists design better experiments, interpret data honestly, and communicate findings accurately.

Introduction to Materials

Polymerization

Polymerization is the chemical process by which small molecules called monomers join together to form large molecules called polymers.

There are two main types of polymerization:

Addition Polymerization
Condensation Polymerization

Addition Polymerization

General process: $n CH_{2} = CH_{2} catalyst, pressure [- CH_{2} - CH_{2} -]_{n}$

Examples of Addition Polymers

Monomer	Polymer	Uses
Ethene ( $CH_{2} = CH_{2}$ )	Poly(ethene) / Polyethylene (PE)	Plastic bags, bottles
Propene ( $CH_{3} CH = CH_{2}$ )	Poly(propene) / Polypropylene (PP)	Ropes, containers
Chloroethene ( $CH_{2} = CHCl$ )	Poly(chloroethene) / PVC	Pipes, insulation
Tetrafluoroethene ( $CF_{2} = CF_{2}$ )	PTFE (Teflon)	Non-stick coatings
Styrene ( $C_{6} H_{5} CH = CH_{2}$ )	Polystyrene (PS)	Packaging foam

Properties of Addition Polymers

Generally thermoplastic (soften on heating)
Non-biodegradable — resistant to chemical attack
Low density (polyethene) to moderate density
Good electrical insulators

Condensation Polymerization

In condensation polymerization, monomers join together with the elimination of a small molecule (usually water, $H_{2} O$ , or hydrogen chloride, $HCl$ ) at each step.

Condensation polymers require monomers with two functional groups (bifunctional monomers).

Types of Condensation Polymers

1. Polyesters

Formed from a diol (two $- OH$ groups) and a dicarboxylic acid (two $- COOH$ groups). Water is eliminated.

Example: Terylene (PET — Polyethylene terephthalate) $HO-CH_{2} CH_{2} -OH + HOOC-C_{6} H_{4} -COOH \to [- O-CH_{2} CH_{2} -OOC-C_{6} H_{4} -CO-]_{n} + n H_{2} O$

Uses: Clothing fibres, plastic bottles (PET bottles), food packaging

2. Polyamides (Nylons)

Formed from a diamine (two $- NH_{2}$ groups) and a dicarboxylic acid (two $- COOH$ groups). Water is eliminated, forming amide linkages ( $- CO-NH-$ ).

Example: Nylon-6,6 $H_{2} N-(CH_{2})_{6} -NH_{2} + HOOC-(CH_{2})_{4} -COOH \to [- NH-(CH_{2})_{6} -NH-CO-(CH_{2})_{4} -CO-]_{n} + n H_{2} O$

Uses: Clothing, ropes, parachutes, toothbrush bristles, engineering components

3. Proteins (Natural Condensation Polymers)

Proteins are natural polyamides formed from amino acids joined by peptide bonds ( $- CO-NH-$ ).

Properties of Condensation Polymers

Contain ester ( $- COO-$ ) or amide ( $- CO-NH-$ ) linkages in the backbone
Generally stronger and more resistant to heat than addition polymers
Can be hydrolysed (broken down by water under acidic or alkaline conditions)
Polyamides can form hydrogen bonds between chains, giving high tensile strength

Comparison: Addition vs Condensation Polymerization

Feature	Addition	Condensation
Monomer requirement	One type with C=C	Two types with two functional groups each
By-product	None	Small molecule (e.g., $H_{2} O$ )
Repeat unit formula	Same as monomer	Different from monomer
Examples	PE, PVC, PTFE	Nylon, Terylene, proteins
Linkage	C–C bonds only	Ester or amide bonds

Cognitive Biases in Scientific Reasoning

Scientists must be aware of cognitive biases — systematic errors in thinking that can distort scientific reasoning and lead to flawed conclusions.

Common Cognitive Biases and Fallacies

Bias / Fallacy	Description	Example in Chemistry Context
Confirmation bias	Seeking or interpreting evidence that confirms existing beliefs; ignoring contradictory data	Only recording experimental results that match the expected outcome
Hasty generalization	Drawing broad conclusions from too few observations	Concluding a reaction mechanism applies to all compounds after testing only one
Post hoc ergo propter hoc	Assuming that because B followed A, A caused B	Assuming a new catalyst caused a yield increase when temperature also changed
Straw man fallacy	Misrepresenting an opponent's argument to make it easier to attack	Distorting a rival theory to dismiss it
Appeal to tradition	Assuming something is correct because it has always been done that way	Rejecting a new synthesis route because the old method is traditional
False authority	Accepting a claim because of who said it, not the evidence	Accepting an incorrect mechanism because a famous chemist proposed it
Occam's Razor (failing)	Choosing a needlessly complex explanation when a simpler one fits the data	Proposing a multi-step mechanism when a single-step one explains all observations
Argument from non-testable hypothesis	Using a hypothesis that cannot be tested or falsified	Claiming a reaction works due to an untestable mystical property
Begging the question	Using the conclusion as a premise in the argument	"This compound is acidic because it donates protons, and it donates protons because it is acidic"
Fallacy of exclusion	Ignoring relevant evidence that contradicts the conclusion	Reporting only trials where the catalyst worked, ignoring failed trials
Faulty analogy	Drawing conclusions based on a misleading comparison	Assuming two structurally similar compounds have identical reactivity without testing
Redefinition	Changing the meaning of a term mid-argument to avoid refutation	Redefining "yield" to make a poor synthesis appear successful

Why This Matters in Chemistry

Sound scientific reasoning requires:

Objectivity — evaluating all evidence, not just supporting evidence
Falsifiability — hypotheses must be testable
Reproducibility — results must be independently verifiable
Parsimony — preferring the simplest explanation consistent with the data (Occam's Razor)

Recognizing these biases helps chemists design better experiments, interpret data honestly, and communicate findings accurately.