Scottish Advanced Higher Chemistry Flashcards

24 sample cards from the Chemistry deck

Sampled from the end of the deck, so these are different cards from the ones shown on the syllabus page.

1. Distinguish between a sigma ($\sigma$) bond and a pi ($\pi$) bond.

   A $\sigma$ bond forms by head-on overlap of orbitals along the internuclear axis; a $\pi$ bond forms by side-on overlap of parallel p orbitals above and below the axis.

2. Use VSEPR theory to predict the shapes of molecules with 2, 3, 4, 5 and 6 electron domains.

   2: linear; 3: trigonal planar; 4: tetrahedral; 5: trigonal bipyramidal; 6: octahedral — domains arrange to minimise repulsion.

3. Explain why $\ce{NH3}$ is pyramidal with a bond angle of about $107^\circ$ rather than $109.5^\circ$.

   The lone pair on nitrogen repels the bonding pairs more strongly than bonding pairs repel each other, compressing the $\ce{H-N-H}$ angle below the ideal tetrahedral value.

4. What is the difference between homolytic and heterolytic bond fission?

   Homolytic fission splits a bond so each atom keeps one electron, forming two radicals; heterolytic fission gives both electrons to one atom, forming a cation and an anion.

5. Define an electrophile and a nucleophile.

   An electrophile is an electron-pair acceptor (electron-deficient, attracted to negative centres); a nucleophile is an electron-pair donor (electron-rich, attracted to positive centres).

6. Compare the $S_N1$ and $S_N2$ nucleophilic substitution mechanisms.

   $S_N1$ is two-step via a carbocation intermediate, first order ($\text{rate} = k[\text{halide}]$), favoured by tertiary halides; $S_N2$ is one-step concerted, second order ($\text{rate} = k[\text{halide}][\text{Nu}]$), favoured by primary halides and gives inversion of configuration.

7. Describe the mechanism type and conditions for halogenation of an alkane.

   Free-radical substitution requiring UV light: initiation (homolysis of $\ce{X2}$), propagation (radical chain) and termination (radical combination).

8. What is electrophilic addition, and to which class of compounds does it typically apply?

   Addition of an electrophile across a multiple bond; it applies to alkenes (and alkynes), e.g. $\ce{HBr}$ adding across a $\ce{C=C}$ double bond.

9. State Markovnikov's rule for electrophilic addition of $\ce{HX}$ to an unsymmetrical alkene.

   The hydrogen of $\ce{HX}$ adds to the carbon already bearing the most hydrogens, so the halogen attaches to the more substituted carbon (forming the more stable carbocation).

10. In IR spectroscopy, what molecular property is probed and where does an $\ce{O-H}$ or $\ce{N-H}$ stretch appear?

    IR probes bond vibrations; a broad $\ce{O-H}$ (or $\ce{N-H}$) stretch appears around $3200$–$3550\ \text{cm}^{-1}$.

11. What IR absorption is characteristic of a carbonyl ($\ce{C=O}$) group?

    A strong, sharp absorption around $1680$–$1750\ \text{cm}^{-1}$.

12. In $^{1}\text{H}$ NMR, what do the chemical shift and the integration (peak area) tell you?

    Chemical shift ($\delta$, in ppm) indicates the chemical environment of the hydrogens; the integration (relative peak area) is proportional to the number of hydrogens in each environment.

13. State the $n+1$ rule for spin-spin coupling in $^{1}\text{H}$ NMR.

    A hydrogen with $n$ equivalent neighbouring hydrogens is split into $n+1$ peaks; e.g. a $\ce{CH3}$ next to a $\ce{CH2}$ appears as a triplet.

14. What information does mass spectrometry give, and what is the molecular ion peak?

    It gives relative molecular mass and structural fragments; the molecular ion peak ($M^{+}$) is at the highest $m/z$ and equals the relative molecular mass of the intact molecule.

15. Explain the underlying principle of chromatographic separation.

    Components partition between a stationary phase and a mobile phase; those with greater affinity for the stationary phase (or less for the mobile phase) move more slowly, achieving separation.

16. Define the retention factor $R_f$ in thin-layer or paper chromatography.

    $R_f = \dfrac{\text{distance moved by component}}{\text{distance moved by solvent front}}$, a value between 0 and 1 characteristic of a substance in a given system.

17. In gas chromatography, what is retention time and what does peak area indicate?

    Retention time is the time taken for a component to pass from injection to the detector; the peak area is proportional to the quantity of that component present.

18. Define a pharmacophore in the context of drug action.

    The specific arrangement of atoms or functional groups in a drug molecule responsible for its pharmacological activity and binding to the biological target.

19. Distinguish between an agonist and an antagonist drug.

    An agonist binds to a receptor and produces a response by mimicking the natural active compound; an antagonist binds to the receptor (often blocking it) without producing a response, preventing the natural compound from acting.

20. What does a structure-activity relationship (SAR) describe, and why is it useful?

    SAR describes how changes to a molecule's structure (functional groups, shape) alter its biological activity; it guides the design of more effective and selective drugs.

21. Outline how to carry out an accurate acid-base titration and how the end-point is identified.

    Rinse and fill the burette with titrant, pipette a known volume of analyte into a conical flask, add a suitable indicator, run titrant in dropwise near the end-point with swirling, and record the volume at the colour change, repeating for concordant titres within $0.1\ \text{cm}^3$.

22. Write the relationship used to calculate the number of moles of a solute from a solution's concentration and volume.

    $n = c \times V$, where $n$ is moles, $c$ is concentration in $\text{mol}\,\text{dm}^{-3}$ and $V$ is volume in $\text{dm}^{3}$.

23. Define percentage yield and write its formula.

    The proportion of product actually obtained relative to the theoretical maximum: $\%\ \text{yield} = \dfrac{\text{actual yield}}{\text{theoretical yield}} \times 100$.

24. Distinguish between a natural product and a synthetic compound in organic chemistry.

    A natural product is a chemical compound produced by a living organism (e.g. terpenes, alkaloids); a synthetic compound is one made artificially in the laboratory by chemical reactions, which may copy or modify natural structures.

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