Flashcards · YCH11 · AS Topic 1

Unit 1 – Structure, Bonding and Introduction to Organic Chemistry (AS Level) flashcards

38 cards on AS Topic 1, subtopics 1.1 to 1.5. Pick a subtopic or study them all.

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1.1 Formulae, Equations and Amount of Substance

Empirical formula vs molecular formula: Empirical: the lowest whole-number ratio of the atoms present. Molecular: the real number of atoms of each element in one molecule (C₂H₄ has empirical formula CH₂).
Relative molecular mass vs relative formula mass: Both are sums of Ar values and have no units. Relative molecular mass is for simple molecular substances; relative formula mass is for compounds with giant structures, such as NaCl and SiO₂.
The ideal gas equation, with the units it needs: pV = nRT, with p in Pa, V in m³, T in K and R = 8.31 J mol⁻¹ K⁻¹. Convert kPa × 1000, cm³ × 10⁻⁶, °C + 273.
1.06 g of sodium carbonate (M = 106.0 g mol⁻¹) is dissolved to make 100 cm³. Concentration?: n = 1.06 ÷ 106.0 = 0.0100 mol. c = 0.0100 ÷ 0.100 = 0.100 mol dm⁻³ (10.6 g dm⁻³).
Atom economy (as defined in this specification): Molar mass of the desired product ÷ sum of the molar masses of all products × 100%. It depends only on the equation; an addition reaction with one product has 100%.
Ionic equation for an acid reacting with a carbonate, and what you see: CO₃²⁻(aq) + 2H⁺(aq) → CO₂(g) + H₂O(l). Fizzing; the gas turns limewater cloudy.
Core Practical 1 (molar volume of a gas): two reasons the result is often low: Any two: gas escapes before the bung is fitted; gas dissolves in water if collected over water; an oxide layer on the metal; leaks at joints.

1.2 Atomic Structure and the Periodic Table

Electrons in a positive ion and in a negative ion: A positive ion has fewer electrons than protons; a negative ion has more electrons than protons. Neutrons = mass number − atomic number.
Why do isotopes of an element have the same chemical properties?: They have the same number of protons and so the same electron arrangement, which governs chemistry. They differ only in neutrons, so physical properties such as mass differ.
Relative atomic mass from a mass spectrum: chlorine is 75% ³⁵Cl and 25% ³⁷Cl: Weighted mean: (35 × 75 + 37 × 25) ÷ 100 = (2625 + 925) ÷ 100 = 35.5
How many molecular ion peaks does Cl₂ give, and at which m/z?: Three: ³⁵Cl–³⁵Cl at 70, ³⁵Cl–³⁷Cl at 72, ³⁷Cl–³⁷Cl at 74. Their heights follow the probability of each combination, so they are not equal.
First ionisation energy: The energy needed to remove one electron from each atom in one mole of gaseous atoms, forming one mole of gaseous 1+ ions: X(g) → X⁺(g) + e⁻.
What do successive ionisation energies show? Use magnesium as the example.: A large jump shows an electron being removed from a shell closer to the nucleus. For Mg the jump comes at the third electron: two electrons in the outer shell (shells 2, 8, 2).
Orbitals per sub-shell and electrons each holds: s: 1 orbital, 2 electrons. p: 3 orbitals, 6 electrons. d: 5 orbitals, 10 electrons. Each orbital holds two electrons of opposite spin; orbitals in a sub-shell fill singly before pairing.

1.3 Bonding and Structure

Ionic bonding: Strong net electrostatic attraction between oppositely charged ions in a giant lattice.
Covalent bond and dative covalent bond: Covalent: strong electrostatic attraction between two nuclei and the shared pair of electrons between them. Dative: a covalent bond where one atom supplies both electrons of the shared pair (e.g. N to H⁺ in NH₄⁺).
What gives an ionic compound covalent character?: Polarisation: a small, highly charged cation (high polarising power) distorts the electron cloud of a large, highly charged anion (high polarisability). Covalent character: AlF₃ < AlCl₃ < AlI₃.
Why do the radii of N³⁻, O²⁻, F⁻, Na⁺, Mg²⁺, Al³⁺ decrease in that order?: All have 10 electrons in the same arrangement, but the number of protons rises from 7 to 13, so the electrons are pulled in more strongly.
Shapes and bond angles of CH₄, NH₃ and H₂O: CH₄: 4 bonding pairs, tetrahedral, 109.5°. NH₃: 3 bonding pairs + 1 lone pair, pyramidal, about 107°. H₂O: 2 bonding pairs + 2 lone pairs, bent, about 104.5°. Lone pairs repel more than bonding pairs.
Shapes of gaseous PCl₅ and SF₆: PCl₅(g): 5 bonding pairs, trigonal bipyramidal, 120° and 90°. SF₆: 6 bonding pairs, octahedral, 90°.
Why is CO₂ non-polar but H₂O polar?: Both have polar bonds. CO₂ is linear and symmetrical, so the bond dipoles cancel. H₂O is bent, so they do not cancel and there is a net dipole.
Why do metals conduct electricity?: When a potential difference is applied, the mobile delocalised electrons drift through the lattice, carrying charge.
Why are metals malleable?: Layers of ions slide while the electron sea keeps holding them, since the bonds are not between fixed pairs of atoms.

1.4 Introductory Organic Chemistry and Alkanes

Hazard vs risk: The property versus the likelihood of harm. A hazard is something with the potential to cause harm; risk is the likelihood that it causes harm under the conditions of use.
Homolytic vs heterolytic fission: Radicals versus ions; half-arrows versus full arrows.
Homologous series: Members share a functional group and a general formula; neighbouring members differ by CH₂.
Structural isomers of C₅H₁₂: Three: pentane, 2-methylbutane and 2,2-dimethylpropane.
Free radical substitution of an alkane with chlorine: conditions and the three stages: UV light. Initiation: Cl₂ → 2Cl•. Propagation: Cl• + RH → R• + HCl; R• + Cl₂ → RCl + Cl•. Termination: two radicals join (e.g. Cl• + R• → RCl).
Why is free radical substitution of limited use in synthesis?: The product can be substituted further, and termination gives side products, so a mixture forms with a low yield of any one compound.
Cracking vs reforming: Cracking shortens chains and makes alkenes; reforming keeps the carbon count and changes shape (to branched or cyclic molecules).
Why is carbon monoxide toxic, and why is bioethanol not fully carbon-neutral?: CO binds to haemoglobin more strongly than O₂, reducing oxygen transport. Bioethanol: the CO₂ released was taken in by photosynthesis, but farming, processing and transport add CO₂.

1.5 Alkenes

σ bond vs π bond: σ: head-on overlap of orbitals on the axis between the two nuclei. π: sideways overlap of p orbitals, with electron density above and below the plane. The π bond is weaker.
Two conditions for E/Z (geometric) isomerism: A C=C, which has restricted rotation, and two different atoms or groups on each carbon of the C=C.
How are E and Z assigned?: On each C=C carbon, the attached atom with the higher atomic number has priority. Higher-priority groups on the same side = Z; on opposite sides = E.
Conditions for hydrogenation and hydration of an alkene: Hydrogenation: H₂, nickel catalyst, about 150 °C → alkane. Hydration: steam, phosphoric acid catalyst, high temperature and pressure → alcohol.
Propene + HBr: major product and why: 2-bromopropane. It forms via the secondary carbocation CH₃CH⁺CH₃, which is more stable than the primary one because more alkyl groups release electron density onto C⁺.
What evidence supports a two-step (carbocation) mechanism for Br₂ adding to ethene?: In sodium chloride solution, BrCH₂CH₂Cl forms as well as BrCH₂CH₂Br: a carbocation forms first and is attacked by any available nucleophile. No ClCH₂CH₂Cl forms.
Repeat unit of poly(chloroethene), PVC, and a problem with incinerating it: –[CH₂–CHCl]ₙ– from CH₂=CHCl. Incineration releases HCl; it is removed by scrubbers containing a base such as calcium hydroxide.