- Brass is an alloy of copper and zinc. Which statement best explains why brass is harder and less malleable than pure copper?
Answer: The different sizes of copper and zinc atoms disrupt the regular lattice, preventing layers from sliding easily.
In pure metals, the atoms are of uniform size and arranged in regular layers that can slide over one another, making the metal malleable. In an alloy like brass, the differently sized zinc atoms disrupt this regular arrangement. This disruption makes it more difficult for the layers of atoms to slide, resulting in a material that is harder and less malleable than the pure constituent metal. The bonding remains metallic, not ionic or covalent.
- An unknown solid compound is found to be brittle, have a melting point of 980 °C, and conduct electricity only when in a molten state. Based on these properties, the bonding in the compound is best described as:
Answer: Ionic, with ions held in a fixed crystal lattice that are free to move when molten.
The combination of properties—high melting point, brittleness, and electrical conductivity only in the molten state—is characteristic of an ionic compound. In the solid state, ions are held in fixed positions in a crystal lattice and cannot move to carry charge. When melted, the ions become mobile and can conduct electricity. Metallic solids conduct electricity in the solid state. Covalent molecular solids have low melting points. Covalent network solids have very high melting points but do not conduct electricity in any state (with exceptions like graphite).
- A solid exhibits the following thermal expansion behavior: it expands significantly more in one direction than the other two when heated. Chemical analysis shows it contains only nonmetallic elements. X-ray crystallography reveals long chains of atoms connected by short bonds, with longer distances between chains. Which structural model best correlates with these observations?
Answer: One-dimensional polymer chains with strong intramolecular bonds and weak intermolecular interactions
When analyzing crystal structures, you need to connect macroscopic properties (like thermal expansion) with microscopic arrangements of atoms and bonding patterns. The key insight here is recognizing how different bonding strengths in different directions create anisotropic (directional) behavior.
The observations point to a structure with strong bonds in one direction and weak interactions in others. The significant expansion in one direction indicates weak intermolecular forces that easily stretch when heated, while minimal expansion in other directions suggests strong covalent bonds that resist thermal motion. Since the material contains only nonmetallic elements and shows long chains with short intramolecular bonds, this describes polymer chains held together by weak van der Waals forces or hydrogen bonds.
Option D correctly identifies this as one-dimensional polymer chains. The strong covalent bonds along the chain resist expansion, while weak intermolecular forces between chains allow significant thermal expansion perpendicular to the chain direction.
Option A is wrong because three-dimensional networks show isotropic (uniform) expansion in all directions, contradicting the observed anisotropy. Option B describes molecular crystals, but these typically don't form long chains and would show more uniform expansion. Option C suggests an ionic structure, but the material contains only nonmetallic elements, ruling out ionic bonding.
Remember: anisotropic thermal expansion is a telltale sign of structural anisotropy. Look for materials where bonding strength varies significantly with direction—polymers, layered materials, and chain structures are prime examples on IB Chemistry exams.
- Bronze is a substitutional alloy where some copper atoms in the lattice are replaced by larger tin atoms. How does this substitution primarily affect the physical properties of the metal?
Answer: It decreases malleability because the irregular atom sizes disrupt the slip planes in the metallic lattice.
The key feature of alloys that makes them harder and less malleable than pure metals is the disruption of the regular crystal lattice. In bronze, the larger tin atoms prevent the layers of copper atoms from sliding past one another easily. This resistance to slipping of atomic layers is observed as decreased malleability and increased hardness. The bonding remains non-directional and metallic. While alloying does affect conductivity (usually decreasing it) and melting point, the primary effect on malleability is due to lattice disruption.
- Stainless steel is an alloy of iron and chromium. Why is stainless steel significantly more resistant to corrosion than pure iron?
Answer: Chromium atoms on the surface rapidly form a very thin, continuous, and non-reactive layer of chromium(III) oxide.
The corrosion resistance of stainless steel is due to a phenomenon called passivation. The chromium in the alloy reacts with oxygen in the air to form a very thin, invisible, and highly stable layer of chromium(III) oxide (Cr₂O₃) on the surface. This oxide layer is non-porous and self-healing; it protects the underlying iron from reacting with oxygen and water, thus preventing rust (iron oxide) from forming. This is the specific chemical reason for its enhanced property.
- Which statement best explains the difference between addition and condensation polymerization at the molecular level? [AHL]
Answer: The empirical formula of an addition polymer is identical to its monomer, whereas for a condensation polymer it is different.
Addition polymerization involves monomers (typically alkenes) adding to one another in such a way that the polymer contains all the atoms of the monomer unit. Therefore, the empirical formula of the polymer is the same as that of the monomer. In condensation polymerization, two monomers combine with the elimination of a small molecule, usually water. Because atoms are lost from the monomers when the polymer forms, the empirical formula of the repeating unit is different from the sum of the monomers. Condensation has a lower atom economy.
- Nylon 6,6 is a condensation polymer. Why does this polymer have a relatively high melting point and tensile strength compared to an addition polymer of similar chain length like poly(hex-1-ene)? [AHL]
Answer: The presence of polar amide groups in nylon allows for strong hydrogen bonding between the polymer chains.
The physical properties like melting point and tensile strength of polymers are largely determined by the strength of the intermolecular forces between polymer chains. Poly(hex-1-ene) is a non-polar hydrocarbon and only has weak London dispersion forces between its chains. Nylon 6,6 contains amide functional groups (–CONH–). The N–H and C=O groups are polar, allowing for strong hydrogen bonds to form between adjacent chains. These hydrogen bonds are much stronger than LDFs, holding the chains together more tightly and resulting in a higher melting point and greater strength.
- The bonding in silicon carbide, SiC, can be characterized using electronegativity (EN) values: Si = 1.90, C = 2.55. Which statement most accurately describes the bonding and resulting properties?
Answer: With ΔEN = 0.65, it has polar covalent bonds within a covalent network structure, making it very hard.
The electronegativity difference is 2.55 - 1.90 = 0.65. This value indicates polar covalent bonding. Silicon carbide does not form discrete molecules; instead, it forms a giant covalent network structure similar to diamond, where each silicon is bonded to four carbons and vice versa. This network of strong covalent bonds makes the material extremely hard and gives it a very high melting point. The bonding is not ionic enough to be soluble, and it is a semiconductor, not a good conductor.
- An alloy is prepared by mixing 95% aluminum with 5% copper. Which description of the resulting material is most accurate?
Answer: It is a substitutional alloy where metallic bonding is maintained, but the lattice is distorted, making it harder than pure aluminum.
Mixing two metals forms an alloy, where bonding remains metallic. The atomic radii of aluminum (143 pm) and copper (128 pm) are similar, so copper atoms will replace aluminum atoms in the crystal lattice, forming a substitutional alloy. This substitution disrupts the regular layers of atoms, making it more difficult for them to slide past one another. Consequently, the alloy (e.g., duralumin) is harder and stronger than pure aluminum. It is not an ionic or covalent material. It is not an interstitial alloy because copper atoms are not small enough to fit into the interstices of the aluminum lattice.
- Poly(ethenol), also known as polyvinyl alcohol (PVA), has the repeating unit –[CH(OH)CH₂]–. It is soluble in water, unlike poly(ethene). Which statement best explains this difference in solubility?
Answer: The C–O and O–H bonds in poly(ethenol) make the polymer chain polar and capable of hydrogen bonding with water.
The principle of 'like dissolves like' governs solubility. Water is a highly polar solvent that can form hydrogen bonds. Poly(ethene) is a non-polar hydrocarbon and cannot interact favorably with water. Poly(ethenol) has hydroxyl (–OH) groups along its chain. These groups are polar and can form strong hydrogen bonds with water molecules. These favorable polymer-water interactions overcome the polymer-polymer interactions, allowing the polymer to dissolve. Both are molecular materials, not network solids.
- What is the systematic IUPAC name for the monomer that polymerizes to form the repeating unit -[O-(CH₂)₄-CO]-? [AHL]
Answer: 5-hydroxypentanoic acid
This polymer is a polyester. The repeating unit is -[O-(CH₂)₄-CO]-. This unit is formed via condensation polymerization of a single monomer containing both a hydroxyl (-OH) group and a carboxylic acid (-COOH) group. Let's reconstruct the monomer. The -CO- part comes from a -COOH group, and the -O- part comes from an -OH group. The monomer must be HO-(CH₂)₄-COOH. To name this, we number the carbon chain starting from the carboxylic acid carbon as C1. This makes the chain five carbons long (a pentanoic acid). The hydroxyl group is on the fifth carbon, so the name is 5-hydroxypentanoic acid.
- High-density poly(ethene) (HDPE) consists mainly of linear polymer chains, while low-density poly(ethene) (LDPE) has significant chain branching. Which statement correctly explains why HDPE has a higher melting point than LDPE?
Answer: The linear chains in HDPE can pack together more closely, leading to stronger London dispersion forces between them.
The melting of a polymer involves overcoming the intermolecular forces between polymer chains. HDPE has linear chains that can align and pack closely together in an ordered, crystalline fashion. This close packing maximizes the surface area contact between chains, resulting in stronger overall London dispersion forces (LDFs). In contrast, the branches on LDPE chains prevent them from packing efficiently, leading to weaker LDFs and a lower melting point. The monomer (ethene) and the covalent bond strengths are the same for both.
- A material exhibits excellent thermal and electrical conductivity, high tensile strength, and can be deformed without breaking. When analyzed by X-ray diffraction, it shows a face-centered cubic (fcc) structure. Which combination of bonding model and structural feature best explains these macroscopic properties?
Answer: Metallic bonding with delocalized electrons and close-packed arrangement allowing slip planes
The combination of excellent conductivity (thermal and electrical), high strength, malleability, and fcc structure is characteristic of metallic bonding. The delocalized electron sea explains conductivity, while the close-packed fcc structure with slip planes explains the ability to deform without breaking. Option B describes covalent networks (like diamond) which are hard but brittle. Option C describes ionic compounds which are generally insulators and brittle. Option D describes molecular solids which have poor conductivity and low strength.
- Analysis of a material's structure reveals that it has a coordination number of 12 for each atom, with electrons that can move freely throughout the structure. However, unlike typical materials with these characteristics, this substance is relatively soft and has a low melting point. Which factor most likely accounts for this unexpected combination of properties?
Answer: Large atomic size results in diffuse electron clouds and weaker electrostatic attractions
Coordination number 12 and freely moving electrons indicate metallic bonding in a close-packed structure. The unexpected softness and low melting point suggest weak metallic bonding, which occurs with large atoms (like alkali metals) where the valence electrons are far from the nucleus and the metallic bonds are weak due to large atomic radii. Option A doesn't typically apply to simple metals. Option C doesn't explain the structural features. Option D would affect mechanical properties but not necessarily the melting point.
- Consider the oxides MgO, Al₂O₃, and SiO₂. Using electronegativity values from the data booklet, which statement correctly describes the trend in bonding character across these period 3 oxides?
Answer: The bonding character transitions from predominantly ionic to polar covalent as the electronegativity difference decreases.
From the data booklet, EN values are: Mg=1.31, Al=1.61, Si=1.90, O=3.44. The electronegativity difference (ΔEN) for each oxide is: MgO: 3.44-1.31=2.13; Al₂O₃: 3.44-1.61=1.83; SiO₂: 3.44-1.90=1.54. As we move from Mg to Si, the electronegativity of the element increases, so its ΔEN with oxygen decreases. This corresponds to a trend from predominantly ionic bonding (MgO) towards polar covalent bonding (SiO₂), with Al₂O₃ being intermediate (amphoteric).
- Nitinol is a 'memory-metal' alloy of nickel and titanium. It can be deformed and will return to its original shape upon heating. This property depends on a reversible solid-state phase change. Which statement about the bonding in nitinol provides the best explanation for this ability?
Answer: The non-directional metallic bonding allows atoms to rearrange into different crystal structures without fracturing.
Alloys are held together by metallic bonding, which involves a lattice of positive ions and a 'sea' of delocalized electrons. This bonding is non-directional, meaning the attraction is maintained even if the atoms shift positions. This allows the atoms in nitinol to rearrange from one solid crystal structure to another (a phase change) without breaking the material, which is essential for the shape-memory effect. Covalent bonds are directional and would lead to a brittle material, while ionic and intermolecular force models are incorrect for a metallic alloy.
- Two materials, X and Y, have similar molecular formulas but vastly different melting points (X: 1050°C, Y: 78°C). X is insoluble in water but conducts electricity when molten. Y is highly soluble in polar solvents and does not conduct electricity in any state. What structural difference most likely accounts for these contrasting properties?
Answer: X has ionic bonding in a crystal lattice while Y has covalent bonding in discrete molecules
The high melting point and electrical conductivity when molten (but not when solid) indicates X is an ionic compound. The low melting point, solubility in polar solvents, and lack of conductivity indicates Y is a polar molecular compound. Option A doesn't explain the conductivity difference. Option B doesn't explain why X conducts when molten. Option D doesn't explain the solubility behavior of Y or why X only conducts when molten.
- On a bonding triangle diagram, a substance is located in the region corresponding to a moderate electronegativity difference and a high average electronegativity. Which properties are most likely for this substance?
Answer: A very high melting point solid that is an electrical insulator and insoluble in water.
A moderate electronegativity difference and high average electronegativity are characteristic of polar covalent bonds, often found in covalent network solids (e.g., SiO₂, SiC). These materials have atoms held together by a vast network of strong covalent bonds, resulting in very high melting points, hardness, and general insolubility. They are typically electrical insulators because the electrons are localized in bonds. Choice C matches this description. Choice A describes a molecular solid, and choice D describes a metal.
- A new material is synthesized. It is a very hard, brittle solid with a melting point above 2500 °C. It does not conduct electricity under any conditions and is insoluble in both water and hexane. The bonding in this material is best described as:
Answer: Covalent network
The combination of very high melting point, extreme hardness, brittleness, and being an electrical insulator in all states is characteristic of a covalent network solid (or giant covalent structure). In these materials, atoms are held together by a vast, three-dimensional lattice of strong covalent bonds (e.g., diamond, silicon dioxide). Ionic compounds would conduct when molten, metallic compounds would conduct as solids, and molecular solids would have much lower melting points.
- A material scientist observes that compound Z can be cleaved along specific crystallographic planes with relatively little force, yet individual layers are extremely difficult to compress. The material is a good conductor parallel to the layers but an insulator perpendicular to them. What structural model best explains this anisotropic behavior?
Answer: Layered structure with strong intralayer covalent bonds and weak interlayer van der Waals interactions
The combination of easy cleavage along planes, high resistance to compression within layers, and anisotropic conductivity strongly suggests a layered structure like graphite. Strong covalent bonds within layers explain the compression resistance and in-plane conductivity, while weak van der Waals forces between layers explain easy cleavage and poor perpendicular conductivity. Option A doesn't explain the easy cleavage. Option B doesn't typically show such pronounced anisotropy. Option D doesn't explain the conductivity anisotropy.
