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Flashcards · J248 · Topic 2
Topic C2 – Elements, Compounds and Mixtures flashcards
25 cards on Topic 2, subtopics C2.1 to C2.3. Pick a subtopic or study them all.
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C2.1 Purity and Separating Mixtures
- Pure substance (chemical sense), and how melting point shows purity
- A single chemical substance. A pure substance melts and boils at a single, sharp temperature; impurities lower the melting point and widen its range. "Pure" fruit juice is a mixture.
- Filtration separates…
- An insoluble solid from a liquid.
- Crystallisation separates…
- A dissolved solid from its solvent: evaporate the solvent and the solid remains.
- Simple distillation vs fractional distillation
- Simple: a liquid from dissolved solids, or liquids with very different boiling points. Fractional: miscible liquids with similar boiling points; needs a fractionating column.
- Rf value, and the Rf of a spot that moves 3.2 cm when the solvent front moves 8.0 cm
- Rf = distance travelled by spot ÷ distance travelled by solvent. 3.2 ÷ 8.0 = 0.4
- How does a chromatogram show purity, and when can Rf values be compared?
- A pure substance gives a single spot; an impure sample gives several. Compare Rf values only under the same conditions (same solvent, paper, temperature), ideally on the same chromatogram.
- A compound contains 2.4 g of carbon and 0.6 g of hydrogen. Empirical formula?
- C: 2.4 ÷ 12 = 0.2. H: 0.6 ÷ 1 = 0.6. Divide by 0.2: 1 : 3, so CH₃.
- How do you recover dry sand and dry salt from a mixture of sand and salt water?
- Filter: the insoluble sand stays on the paper. Then crystallise the filtrate: heat to evaporate water until crystals form, cool and dry.
C2.2 Bonding
- Ion
- An atom or group of atoms with a charge, because electrons have been lost or gained. Protons never change.
- Ionic bond
- The electrostatic pull holding positive and negative ions together, acting in all directions.
- Covalent bond
- An electron pair held between two non-metal atoms, with both nuclei pulling on it.
- Metallic bond
- Positive metal ions in a regular lattice pulled towards the delocalised electrons that surround them.
- Lone pair
- A pair of outer electrons on one atom that is not shared.
- Ion charges from the group number
- Groups 1, 2, 3: +1, +2, +3. Groups 5, 6, 7: −3, −2, −1.
- Deduce the formula of calcium nitride
- Each Ca loses 2 electrons; each N gains 3. Lowest common number 6: 3 Ca²⁺ and 2 N³⁻, so Ca₃N₂.
- Metal oxides vs non-metal oxides
- Metal oxides are basic (they neutralise acids; soluble ones are alkaline), e.g. Li₂O. Non-metal oxides are acidic, e.g. CO₂.
- What a dot and cross diagram does not show
- The 3D shape, that all electrons are identical, and the relative sizes of the atoms or ions.
C2.3 Properties of Materials
- Why does carbon form so many compounds?
- It forms four covalent bonds, it bonds strongly to other carbon atoms, so it makes chains (straight or branched) and rings, and families of similar compounds differing by CH₂.
- Diamond: bonding and properties
- Each carbon bonded to 4 others in a giant 3D tetrahedral network. Very hard, very high melting point; does not conduct (no delocalised electrons).
- Graphite: bonding and properties
- Each carbon bonded to 3 others in stacked hexagonal layers. Soft and slippery (weak forces between layers); conducts along the layers (one delocalised electron per carbon).
- Graphene and fullerenes
- Graphene: a single graphite sheet; very strong and conducts. Fullerenes: molecules such as hollow C₆₀ spheres or nanotubes. C₆₀ is soft (weak forces between molecules); nanotubes are strong and conduct.
- When water boils, what is overcome?
- The weak intermolecular forces between water molecules. The covalent bonds inside the molecules do not break.
- Size of a nanoparticle, and 1 nm in metres
- Nanoparticles are 1 nm to 100 nm across. 1 nm = 1 × 10⁻⁹ m.
- Surface area to volume ratio of a cube of side 4 nm
- Area 6 × 4² = 96 nm²; volume 4³ = 64 nm³; ratio 96 ÷ 64 = 1.5 nm⁻¹. Smaller particles have a larger ratio (6 ÷ x).
- Two uses of nanoparticles, and one risk linked to a property
- Silver: antibacterial (wound dressings). Titanium dioxide or zinc oxide: block UV in sunscreens. Risk: they are so small they may pass through skin or be inhaled, and long-term effects are not fully known.