NCEA Level 2 Chemistry 91164 Bonding, structure, properties and energy changes: past papers, practice and marking
- NCEA Level 2
- Chemistry
- AS91164
- 5 credits
- External exam
About 91164
AS91164, Demonstrate understanding of bonding, structure, properties and energy changes. The four types of solid and why they melt, conduct, bend or shatter, what dissolves in what, Lewis structures, shapes and polarity, and the energy in reactions and changes of state: the 5-credit external, in the seven question types it keeps reusing.
What 91164 covers
- Types of solidMolecular, ionic, metallic and covalent network (3D and 2D) solids: the particles, the attractive forces between them, and the energy needed to overcome those forces to melt, boil or sublime
- Properties of solidsMobile charged particles for conduction (delocalised electrons or ions), ionic solids only when molten or dissolved, graphite against diamond, non-directional metallic bonds letting layers slide, strong directional covalent bonds making a network hard
- SolubilityPolar and non-polar solvents, solute–solvent attractions that must overcome solute–solute and solvent–solvent attractions, hydrated ions with the water dipoles the right way round, never just “like dissolves like”
- Lewis structuresLewis structures with every valence electron, multiple bonds as one region, VSEPR: regions of electron density repel to maximum separation, parent geometry and bond angle, lone pairs giving bent and trigonal pyramid shapes
- PolarityElectronegativity, polar bonds and bond dipoles, symmetry and dipole cancellation, unequal dipoles that do not cancel even in a symmetrical shape
- Enthalpy changesExothermic and endothermic reactions and changes of state, enthalpy diagrams, n = m/M then energy from ΔrH with the equation coefficients, kg to g, energy per kilogram, density steps
- Bond enthalpyΔrH = Σ(bonds broken) − Σ(bonds formed), counting bonds from structures, finding an unknown bond enthalpy, linking the sign to bond breaking and making
91164 past papers
Open a 91164 past paper beside your own Ako page, then get it marked, part by part.
Work through a past paper All NCEA Level 2 Chemistry past papers
Practice questions
Question 1: Types of solid
Mothballs are made of naphthalene, C₁₀H₈, a molecular solid that melts at 80 °C. Furnace bricks are made of magnesium oxide, MgO, an ionic solid that melts at 2852 °C.
- Identify the particles and the attractive forces between them in naphthalene and in magnesium oxide.(up to Achieved)
- Describe what happens to the particles in naphthalene when it melts.(up to Achieved)
Check your answer
Part a: what gets the marks
- Naphthalene is made of C₁₀H₈ molecules held to each other by weak intermolecular forces.
- Magnesium oxide is made of Mg²⁺ and O²⁻ ions held by ionic bonds in a 3D lattice.
A full answer: Naphthalene: C₁₀H₈ molecules; weak intermolecular forces.
Magnesium oxide: Mg²⁺ and O²⁻ ions; ionic bonds.
Part b: what gets the marks
- The naphthalene molecules gain heat energy until the weak intermolecular forces between them are overcome and they can move past each other, while the covalent bonds inside each molecule stay unbroken.
A full answer: The molecules gain heat energy and vibrate more until the weak intermolecular forces between them are overcome; the molecules can then move past one another as a liquid. The molecules stay whole: no covalent bonds break.
What gets each grade
Question 2: Types of solid
Calcium fluoride, CaF₂, melts at 1418 °C. White phosphorus, P₄, melts at 44 °C.
- Explain why white phosphorus melts at a much lower temperature than calcium fluoride.(up to Excellence)
- Calcium fluoride is brittle and shatters when it is hit with a hammer. Explain why.(up to Merit)
Check your answer
Part a: what gets the marks
- To melt a solid the forces between its particles must be overcome, so the stronger those forces, the higher the melting point.
- Phosphorus is made of P₄ molecules held to each other only by weak intermolecular forces, which need only a small amount of heat energy to overcome, so it melts at 44 °C.
- Calcium fluoride is a 3D lattice of Ca²⁺ and F⁻ ions held by strong ionic bonds, which need a large amount of heat energy to overcome, so it melts at 1418 °C.
- The ionic bonds overcome when calcium fluoride melts are far stronger than the intermolecular forces overcome when phosphorus melts, so much more energy is needed and its melting point is much higher.
A full answer: Melting means overcoming the forces between the particles; the stronger the forces, the more heat energy is needed. Phosphorus is a molecular solid: its P₄ molecules are held together by weak intermolecular forces (the P–P covalent bonds inside each molecule do not break), so only a small amount of energy is needed and it melts at 44 °C. Calcium fluoride is ionic: a 3D lattice of Ca²⁺ and F⁻ ions held by strong ionic bonds, so a large amount of energy is needed and it melts at 1418 °C. The ionic bonds in CaF₂ are much stronger than the intermolecular forces in P₄, so CaF₂ needs far more energy and melts at a far higher temperature.
Part b: what gets the marks
- When the crystal is struck, a layer of ions shifts along by one ion, so Ca²⁺ ions end up next to Ca²⁺ ions and F⁻ ions next to F⁻ ions.
- Ions with the same charge repel each other strongly, so the lattice is forced apart and the crystal breaks.
A full answer: Calcium fluoride is an ionic lattice of Ca²⁺ and F⁻ ions. A blow shifts one layer of ions along, so ions of the same charge come to lie next to each other. These repel strongly and force the layers apart, so the crystal shatters.
What gets each grade
Question 3: Types of solid
Silicon, Si, melts at 1414 °C. White phosphorus, P₄, melts at 44 °C. A student writes: "Phosphorus melts easily because its P–P covalent bonds are weak, and silicon melts at a high temperature because its Si–Si covalent bonds are strong."
- Discuss what is wrong with the student's reasoning, comparing what must be overcome when each solid melts.(up to Excellence)
- Explain why silicon is very hard.(up to Merit)
Check your answer
Part a: what gets the marks
- Phosphorus is a molecular solid: the P–P covalent bonds are inside each P₄ molecule, and the molecules are held to each other by weak intermolecular forces.
- When phosphorus melts the P–P covalent bonds are not broken; only the weak intermolecular forces between P₄ molecules are overcome, which needs little energy, so it melts at 44 °C.
- Silicon is a covalent network: every Si atom is joined to four others by strong covalent bonds throughout the solid, so melting means breaking covalent bonds, which needs a very large amount of energy, so it melts at 1414 °C.
- Both solids contain strong covalent bonds, but only in silicon do they have to be broken to melt, so the student is wrong: the difference is the weak forces between molecules in P₄ against the covalent bonds between atoms in Si.
A full answer: The student has the wrong forces for phosphorus. Phosphorus is a molecular solid: its P–P covalent bonds are inside each P₄ molecule and are not broken when it melts. Only the weak intermolecular forces between the molecules are overcome, which needs little energy, so it melts at 44 °C. Silicon is a covalent network: each Si atom is bonded to four others by strong covalent bonds right through the solid, so to melt it covalent bonds must be broken, which needs a very large amount of energy, so it melts at 1414 °C. Both contain strong covalent bonds; the difference is that in silicon they are what holds the particles together, while in phosphorus only weak intermolecular forces do.
Part b: what gets the marks
- Silicon is a 3D covalent network in which every Si atom is joined to four other Si atoms by strong covalent bonds.
- To scratch or dent silicon, covalent bonds have to be broken, which needs a very large force, so silicon is very hard.
A full answer: Silicon is a 3D covalent network: each Si atom is held to four others by strong covalent bonds right through the solid. To scratch or dent it, covalent bonds must be broken, which needs a very large force, so silicon is very hard.