From models to materials: notes and practice questions
- This topic explores how bonding and structure influence material properties, using models to explain them.
- Bonding is best described as a continuum between ionic, covalent, and metallic models, represented by a bonding triangle.
- The position of a compound in the bonding triangle is determined by the relative contributions of the three bonding types.
- Properties of a compound can be predicted based on its position in the bonding triangle.
- Alloys are mixtures of metals or metals and non-metals, with enhanced properties due to non-directional bonding.
- Polymers are large molecules (macromolecules) made from repeating monomer subunits.
- Addition polymers form by breaking double bonds in monomers, while condensation polymers form by reactions between functional groups with the release of a small molecule.
How it is examined
Bonding triangle questions give the electronegativity data and ask for a position or a predicted property, 1 to 2 marks. Polymer questions run both ways: draw the repeating unit from monomers, or deduce the monomer from a drawn polymer. May 2025 HL Paper 2 TZ1 asked candidates to deduce the monomer that forms a given polymer [1] and then describe a chemical property that makes it useful [1]. The repeating-unit convention (brackets, continuation bonds through the brackets, subscript n) is marked, so a drawing without the trailing bonds can be refused.
The bonding triangle and electronegativity values. Everything else is recall or is supplied in the question stem, including monomer structures.
- 2.4.1 Bonding is best described as a continuum between the ionic, covalent and metallic models, and can be represented by a bonding triangle. Students use bonding models to explain the properties of a material.
- 2.4.2 The position of a compound in the bonding triangle is determined by the relative contributions of the three bonding types to the overall bond. Students determine the position of a compound in the bonding triangle from electronegativity data, and predict the properties of a compound based on its position.
- 2.4.3 Alloys are mixtures of a metal and other metals or non-metals. They have enhanced properties. Students explain the properties of alloys in terms of non-directional bonding.
- 2.4.4 Polymers are large molecules, or macromolecules, made from repeating subunits called monomers. Students describe the common properties of plastics in terms of their structure.
Calculations of percentage ionic character are not required. The bonding triangle is read off, not computed into a percentage.
2.4.6 Condensation polymers form by the reaction between functional groups in each monomer with the release of a small molecule. Students represent the repeating unit of polyamides and polyesters from given monomer structures.
Guiding questions
- What role do bonding and structure have in the design of materials?
Linking questions
- Structure 3.1 How do the trends in properties of period 3 oxides reflect the trend in their bonding?
- Nature of science, Structures 2.1, 2.2 What are the limitations of discrete bonding categories?
- Structure 2.1, 2.2, 2.3 Why do composites like reinforced concretes, which are made from ionic and covalently bonded components and steel bars, have unique properties?
- Structure 1.1 Why are alloys more correctly described as mixtures rather than as compounds?
- Structure 3.2 What are the structural features of some plastics that make them biodegradable? What functional groups in molecules can enable them to act as monomers for addition reactions? (HL) for condensation reactions?
- Reactivity 2.1 Why is the atom economy 100% for an addition polymerization reaction?
- Structure 3.2 (HL) What is the nature of the reaction that occurs when two amino acids form a dipeptide?
Practice questions
15 questions · 9 easy · 5 medium · 1 hardQuestion 1
EasyPaper 1A · calculator1 markWhich force is primarily responsible for the significantly higher boiling point of water, , compared to hydrogen sulfide, ?
A. Covalent bonds
B. London (dispersion) forces
C. Dipole-dipole forces
D. Hydrogen bonds
Consider the types of intermolecular forces present in each molecule. One type of force is present in water but is negligible in hydrogen sulfide, and it is much stronger than the other forces.
Question 2
MediumPaper 1A · calculator1 markThe properties of four allotropes of carbon are described below.
Description 1: A molecular solid composed of spherical molecules, soluble in some non-polar solvents.
Description 2: A 2D material with exceptionally high tensile strength and electrical conductivity.
Description 3: A hard, transparent crystal that is an electrical insulator, with each atom bonded to four others.
Description 4: A soft, black solid that conducts electricity and is used as a lubricant.
Which row correctly matches the allotropes to their descriptions?
| Description 1 | Description 2 | Description 3 | Description 4 | |
|---|---|---|---|---|
| A. | Buckminsterfullerene | Graphene | Diamond | Graphite |
| B. | Diamond | Graphite | Graphene | Buckminsterfullerene |
| C. | Graphene | Diamond | Graphite | Buckminsterfullerene |
| D. | Graphite | Buckminsterfullerene | Diamond | Graphene |
Consider the bonding and structure of each carbon allotrope. How does the arrangement of atoms (layers, 3D network, single sheet, discrete molecules) influence properties like hardness, conductivity, and solubility?
Question 3
HardPaper 1B · calculator16 marksNitrogen dioxide, a pollutant from car exhausts, reacts with water in the atmosphere to form nitric acid and nitrous acid. This is a disproportionation reaction.
(a) Deduce the oxidation states of nitrogen in the reactant and products.
Reactant:
Products: ,
(b) Explain, with reference to the equilibrium, why more nitrogen dioxide gas dissolves when the reaction occurs in alkaline rainwater.
(c) The solubility of nitrogen dioxide gas in water was measured by different scientific groups. A summary of their results is shown.
| Source | Temperature / °C | Solubility of gas in 0.100 dm³ of water |
|---|---|---|
| A | 0 | 0.380 dm³ |
| B | 10 | 0.28 dm³ |
| C | 20 | 200 cm³ |
| D | 25 | 0.15 L |
| E | 30 | 0.120 dm³ |
(i) Identify a problem in comparing the data from the different sources as presented in the table.
(ii) The units of solubility are converted to mol dm⁻³. Complete the table by calculating the value for source A. Assume the atmospheric pressure is 100 kPa and the density of the resulting solution is 1.00 g cm⁻³.
(iii) Suggest an explanation for the effect of temperature on the solubility of nitrogen dioxide gas.
(d) Suggest one reason why nitrogen dioxide is considered a major air pollutant.
(e) Nitrous acid, , is a weak acid. The graph shows the percentage of nitrous acid and its conjugate base, the nitrite ion (), present at different pH values.
(GRAPH IS A STANDARD SPECIATION PLOT FOR A WEAK ACID. X-AXIS: pH from 0 to 8. Y-AXIS: Percentage from 0 to 100. A curve for HA starts at 100% and goes down, a curve for A- starts at 0% and goes up. The two curves cross at pH = 3.3, where each is at 50%.)
(i) Deduce the pH range where nitrous acid, , is the dominant nitrogen-containing species in the solution.
(ii) Determine, with reference to the graph, the of nitrous acid.
(f) Nitrous acid can react with secondary amines to form N-nitrosamines, which are potent carcinogens. An example is the reaction with dimethylamine, .
(i) Deduce a balanced chemical equation for the formation of N-nitrosodimethylamine, , from dimethylamine and nitrous acid.
(ii) The rate of N-nitrosamine formation is highly dependent on pH. The reaction rate is highest under mildly acidic conditions where there is a sufficient concentration of both the unprotonated amine and nitrous acid. State two conditions that could be maintained in an industrial process to minimize the formation of N-nitrosamines.
(g) To combat the effects of acid rain, powdered limestone () is sometimes added to lakes. Suggest two distinct reasons why this 'liming' process is effective at restoring the aquatic ecosystem.
Remember the rules for assigning oxidation states. Oxygen is usually -2 and hydrogen is usually +1. The sum of oxidation states in a neutral molecule is zero.
Alkaline conditions imply the presence of a base, like hydroxide ions (). How would a base react with the acidic products of the forward reaction? Consider Le Châtelier's principle.
Look closely at the units used for solubility in the table. Are they all consistent? What other variable that affects gas solubility might be missing?
To find the concentration in mol dm⁻³, you first need to find the moles of gas. You are given the volume of the gas, the temperature, and the pressure. The ideal gas law might be useful here.
Observe the trend in the table: as temperature increases, what happens to the volume of gas that dissolves? The dissolution of a gas in a liquid is an equilibrium process. How does temperature affect exothermic and endothermic equilibria?
Consider the direct effect of on human health or its role in forming other harmful substances in the environment.
The 'dominant' species is the one present in a higher concentration (or percentage). Find the part of the graph where the curve for is above the curve for .
The has a special significance on a speciation graph. It is the pH at which the concentrations of the weak acid and its conjugate base are equal.
The reactants are and . The products are and one other small molecule. Identify the atoms that are not part of the nitrosamine product to deduce the other product.
To minimize the formation of a product, you can either remove one of the reactants or change the conditions (like pH) to make the reaction much slower. Consider what pH would minimize the concentration of one of the key reactants, .
Think about the direct chemical effect of adding a base () to an acidified lake. Then, consider how this action might affect the equilibrium involving the atmospheric pollutant that caused the acid rain in the first place.
Question 4
EasyPaper 1A · calculator1 markAn unknown silvery solid is analysed and found to have the following properties:
• High melting point ( °C)
• Excellent electrical conductor as a solid
• Excellent electrical conductor when molten
• Malleable and ductile
What is the most probable structure of this substance?
A. Ionic lattice
B. Metallic lattice
C. Simple molecular
D. Covalent network
Consider how the mobility of charged particles (electrons or ions) in the solid and molten states affects electrical conductivity for each bonding type. Also, think about how the layers of particles can move relative to each other in each structure.
Question 5
MediumPaper 2 · calculator3 marksHigh-voltage power lines are typically made of aluminium rather than copper, despite copper having a higher electrical conductivity. Explain, with reference to two distinct physical properties, why aluminium is often the preferred material for this application.
Consider the practical challenges of suspending long, heavy cables between pylons over large distances. What properties, besides conductivity, would be important for the material used?
Question 6
EasyPaper 1A · calculator1 markWhich substance has the lowest melting point?
A. Diamond
B. Graphite
C. Fullerene ()
D. Silicon dioxide ()
Compare the structures of the substances. Melting a substance with a giant covalent structure requires breaking strong covalent bonds, while melting a substance with a simple molecular structure only requires overcoming weak intermolecular forces.
Question 7
MediumPaper 1A · calculator1 markIn which set do all species contain delocalized electrons?
A. , , (diamond)
B. , ,
C. , ,
D. , ,
Consider which of these molecules or ions can be represented by two or more valid Lewis structures (resonance structures). Also, think about the bonding in different allotropes of carbon and in aromatic compounds.
Question 8
EasyPaper 2 · calculator2 marksPure gold, also known as 24-carat gold, is a relatively soft metal. For use in jewelry, it is often alloyed with other metals like copper to produce 18-carat gold.
(a) Explain, with reference to their structures, why 18-carat gold is harder than 24-carat gold.
Consider the arrangement of atoms in a pure metal compared to an alloy. How does the presence of atoms of different sizes affect the way layers of atoms can move past each other?
Question 9
MediumPaper 2 · calculator8 marksSilicon, a key component in electronics, can be purified to an exceptional degree, making it one of the purest elements commercially available. It reacts with oxygen to form silicon dioxide, a compound with a vast range of applications from glassmaking to fibre optics.
(a) Distinguish between the terms element and compound.
(b) Justify, using electronegativity values from the data booklet, why the Si-O bond is polar.
(c) Contrast the electrical conductivity of silicon and silicon dioxide. Relate the difference to their bonding and structure.
(d) The first ionization of a gaseous silicon atom can be initiated by electromagnetic radiation. Calculate the wavelength, in m, of the radiation corresponding to the first ionization energy of silicon. Use the data booklet.
Think about the fundamental particles that make up each type of substance. What is the key difference in their composition at the atomic level?
Find the electronegativity values for silicon and oxygen in the data booklet and calculate the difference. A significant difference indicates a polar bond.
Consider the type of bonding and structure for each substance and how this affects the mobility of electrons.
First, find the ionization energy for a single atom in Joules by converting from kJ mol⁻¹ using Avogadro's constant. Then, use the equations relating energy, Planck's constant, and wavelength.
Question 10
EasyPaper 2 · calculator2 marksPolytetrafluoroethene (PTFE), widely known by the brand name Teflon, is a synthetic fluoropolymer of tetrafluoroethene. It has numerous applications due to its non-reactive and non-stick properties. The monomer is tetrafluoroethene, .
(a) Describe the key changes in bonding that occur when tetrafluoroethene monomers polymerize to form PTFE.
Focus on the type of covalent bond present in the monomer that is not present in the polymer, and the type of covalent bond that is formed to link the monomer units together.
Question 11
MediumPaper 1A · calculator1 markIn which allotropes of carbon is each carbon atom covalently bonded to three other carbon atoms?
I. Diamond
II. Graphite
III. Buckminsterfullerene ()
A. I and II only
B. I and III only
C. II and III only
D. I, II and III
Consider the giant covalent network structure of diamond versus the layered structure of graphite and the cage-like structure of buckminsterfullerene. How many neighbours does a typical carbon atom have in each structure?
Question 12
EasyPaper 1A · calculator1 markA white crystalline solid is investigated. It is found to be brittle, has a high melting point, and does not conduct electricity in the solid state. However, it conducts electricity when dissolved in water. Which type of bonding best describes this solid?
A. Covalent molecular
B. Covalent network
C. Ionic
D. Metallic
Consider the state in which the substance conducts electricity. What does this tell you about the charge carriers (electrons or ions) and their mobility in different states?
Question 13
EasyPaper 1A · calculator1 markAn unknown substance is a lustrous solid at room temperature. It is found to have a high melting point and is an excellent electrical conductor in its solid form. Which structure best describes this substance?
A. Ionic lattice
B. Metallic lattice
C. Simple molecular
D. Covalent network
Consider the properties of each type of structure. Which one contains mobile charged particles in the solid state that would allow for electrical conductivity?
Question 14
EasyPaper 1A · calculator1 markWhat is the reason for the electrical conductivity of graphite?
A. The presence of mobile ions.
B. Strong covalent bonds within the layers.
C. Weak intermolecular forces between the layers.
D. The presence of delocalized electrons.
Consider what particles must be free to move for a substance to conduct electricity. Relate this to the bonding model of graphite.
Question 15
EasyPaper 1A · calculator1 markA white crystalline solid is found to have a high melting point. It does not conduct electricity in the solid state, but it does conduct electricity when molten. Which type of structure and bonding is present in the solid?
A. Metallic
B. Ionic
C. Covalent molecular
D. Covalent network
Consider what particles are responsible for electrical conductivity. In which types of substances are these particles mobile in the solid state, and in which are they only mobile when the substance is molten or dissolved?
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Where marks are lost
- Reaching for "human error" or "only one trial." A source of error has to be a specific step in the method, not a general apology for the result.
- Joining the dots instead of drawing a curve.
- Naming a chemical instead of the property that distinguishes it, or vice versa. Answering with the nearest fact that comes to mind rather than the fact the command term and stem jointly ask for is a recurring way to answer a question that was not, quite, the one asked.
