Scottish Higher Higher Chemistry Flashcards

24 sample cards from the Higher Chemistry deck

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

1. Why are essential oils (fragrance compounds) often unsaturated, and what can happen on exposure to air?

   They contain carbon–carbon double bonds; these can be oxidised by oxygen in the air, which can alter or degrade the fragrance.

2. What do UVA/UVB sunscreens (e.g. with conjugated systems) do, and what does SPF measure?

   They absorb harmful UV light to protect skin. SPF (Sun Protection Factor) indicates how much longer protected skin can be exposed to UV before burning compared with unprotected skin.

3. What are free radicals and why are they harmful to skin?

   Free radicals are highly reactive species with unpaired electrons; in skin they damage cells and accelerate ageing. Free-radical scavengers (antioxidants) in skincare neutralise them.

4. State the relationship between number of moles, mass and molar mass.

   $$n = \frac{m}{M}$$ where $n$ = moles, $m$ = mass (g) and $M$ = molar mass ($\text{g mol}^{-1}$).

5. How is the number of moles of a solute calculated from concentration and volume?

   $$n = c \times V$$ where $c$ is concentration ($\text{mol l}^{-1}$) and $V$ is volume (litres).

6. What is the value of the molar volume assumption for gases, and how do you find moles of a gas from its volume?

   $$n = \frac{V}{V_m}$$ where $V_m$ is the molar volume (the same for all gases at a given temperature and pressure, e.g. about $24\ \text{l mol}^{-1}$).

7. Define the enthalpy of combustion of a substance.

   The enthalpy change ($\Delta H$) when one mole of a substance is burned completely in oxygen; it is always negative (exothermic).

8. Write the formula used to calculate the energy released when a substance heats water in a calorimetry experiment.

   $$E_h = c\, m\, \Delta T$$ where $c$ is the specific heat capacity of water ($4.18\ \text{kJ kg}^{-1}{}^{\circ}\text{C}^{-1}$), $m$ is the mass of water (kg) and $\Delta T$ the temperature change.

9. What is Hess's law?

   The enthalpy change for converting reactants into products is independent of the route taken, depending only on the initial and final states; so $\Delta H$ for an overall reaction equals the sum of the $\Delta H$ values of the steps.

10. In a reaction at dynamic equilibrium, what is true about the forward and reverse reactions?

    They occur at the same rate, so the concentrations of reactants and products remain constant (a closed system); the reaction has not stopped.

11. Using Le Chatelier's principle, what happens to the equilibrium $\ce{N2 + 3H2 <=> 2NH3}$ when pressure is increased?

    The equilibrium shifts to the side with fewer moles of gas (the product side, 2 moles vs 4 moles), increasing the yield of ammonia.

12. What effect does a catalyst have on the position of an equilibrium?

    None — a catalyst speeds up the forward and reverse reactions equally, so equilibrium is reached faster but the position (and yield) is unchanged.

13. How does increasing temperature shift an equilibrium for an exothermic forward reaction?

    It shifts the equilibrium in the endothermic (reverse) direction, decreasing the yield of products, because the system opposes the rise in temperature.

14. In chemical analysis, what colour change indicates the end-point of an acid–base titration using phenolphthalein (acid in flask, alkali in burette)?

    Phenolphthalein turns from colourless to pink (permanent pale pink) at the end-point when the solution becomes just alkaline.

15. What makes a titration result 'concordant', and why are concordant titres used?

    Concordant titres agree within $0.2\ \text{cm}^3$ of each other; they are averaged (and the rough titre discarded) to give an accurate, reliable result.

16. Name the standard laboratory apparatus used to (a) measure an accurate fixed volume and (b) deliver a variable, accurate volume in titration.

    (a) A pipette delivers a fixed accurate volume; (b) a burette delivers a variable, accurately measured volume.

17. What is filtration used to separate, and what are the residue and filtrate?

    Filtration separates an insoluble solid from a liquid; the solid trapped on the filter paper is the residue, and the liquid that passes through is the filtrate.

18. Distinguish between the accuracy and precision of a measurement.

    Accuracy is how close a measurement is to the true value; precision is how close repeated measurements are to one another (reproducibility).

19. What is the difference between a random error and a systematic error?

    Random errors scatter readings unpredictably around the true value and can be reduced by repeating and averaging; systematic errors shift all readings consistently in one direction (e.g. faulty calibration) and are not reduced by repetition.

20. Why are experiments repeated and average values calculated?

    To improve reliability by reducing the effect of random errors and to identify and discard anomalous (outlier) results.

21. What should a good experimental aim/hypothesis and a fair test include?

    A clear aim, one independent variable changed, the dependent variable measured, and all other variables controlled (kept constant) so the test is fair.

22. When evaluating an experiment, what is meant by an 'anomalous result' and how is it treated?

    A result that does not fit the pattern of the others (an outlier); it is identified, ideally investigated, and excluded from the average so it does not distort the conclusion.

23. What three pieces of information must a hazard/risk assessment identify before practical work?

    The hazards of the chemicals/procedures, the risks (likelihood and severity of harm), and the control measures needed to reduce the risk (e.g. goggles, fume cupboard, dilution).

24. Distinguish between a 'hazard' and a 'risk' in laboratory safety.

    A hazard is the potential of a substance or activity to cause harm; a risk is the chance (likelihood) that harm will actually occur under the conditions of use.

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