Scottish Higher Higher Chemistry Syllabus

How are oxidation and reduction half-equations combined to give a redox equation?

Balance the electrons lost and gained (multiply equations as needed) then add the half-equations together, cancelling the electrons and any species common to both sides.

What is the functional group of an ester and how is it formed?

The ester linkage $\ce{-COO-}$ (carboxyl–oxygen), formed by a condensation reaction between a carboxylic acid and an alcohol with the elimination of water.

Name the type of reaction that breaks an ester back into a carboxylic acid and an alcohol.

Hydrolysis — the ester reacts with water (the reverse of the condensation/esterification reaction).

What is the structural difference between a fat and an oil?

Oils contain more carbon–carbon double bonds (more unsaturated), so their molecules pack less closely, giving lower melting points (liquid at room temperature); fats are more saturated and solid at room temperature.

What are fats and oils chemically (in terms of building blocks)?

They are esters (triglycerides) formed from the condensation of glycerol (propane-1,2,3-triol) with three fatty acid (long-chain carboxylic acid) molecules.

Which functional groups join to form the amide (peptide) link in proteins?

An amino group ($\ce{-NH2}$) of one amino acid condenses with the carboxyl group ($\ce{-COOH}$) of another, eliminating water and forming the amide/peptide link $\ce{-CONH-}$.

What is an essential amino acid?

An amino acid that cannot be synthesised by the body and so must be obtained from the diet (from protein in food).

What is the difference between a fibrous and a globular protein in function?

Fibrous proteins are long and insoluble, giving structural/support roles (e.g. collagen); globular proteins are folded into compact shapes with regulatory or functional roles (e.g. enzymes, haemoglobin).

What happens to a protein when it is denatured?

Its structure (shape) is changed by breaking the bonds that maintain the folded shape (e.g. by heat or pH change), so it can no longer perform its function — for enzymes this stops them working.

Why are unsaturated fats and oils susceptible to oxidation, and what is the consequence?

The carbon–carbon double bonds react with oxygen, causing the fats/oils to go rancid (develop off flavours and odours).

How do antioxidants prevent oxidation in food?

Antioxidants are reducing agents (electron donors) that are oxidised in preference to the food molecules, so they protect the food. Example ion-electron: $$\ce{2I- -> I2 + 2e-}$$

Why are vitamin C and other antioxidants described as reducing agents?

They readily donate electrons (are oxidised), preventing oxygen from oxidising food molecules; donating electrons is the defining behaviour of a reducing agent.

How is soap manufactured from fats and oils?

By alkaline hydrolysis (saponification) of fats/oils with sodium or potassium hydroxide, producing glycerol and the sodium/potassium salts of fatty acids (soap).

Describe the two ends of a soap (or detergent) molecule and their behaviour.

A hydrophilic (water-loving) ionic head that dissolves in water, and a hydrophobic (water-hating) non-polar hydrocarbon tail that dissolves in oil/grease.

What is a micelle in the context of soap cleaning action?

A ball-shaped cluster where the hydrophobic tails embed in a droplet of oil/grease and the hydrophilic heads point outwards into the water, keeping the grease suspended so it can be rinsed away.

Why do soaps form scum in hard water whereas soapless detergents do not?

Soaps react with the $\ce{Ca^2+}$ and $\ce{Mg^2+}$ ions in hard water to form an insoluble precipitate (scum); detergents form soluble products and so do not.

What is an emulsion and what is the role of an emulsifier?

An emulsion is a mixture of small droplets of one liquid suspended in another immiscible liquid (e.g. oil in water). An emulsifier has hydrophilic and hydrophobic ends that stabilise the emulsion by preventing the droplets from coalescing.

What are terpenes and from what unit are they built?

Terpenes are unsaturated compounds found in essential oils (fragrances), built up from joining together isoprene ($\text{C}_5$) units.

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.

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.

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.

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 ($\text{g mol}^{-1}$).

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).

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 $24\ \text{l mol}^{-1}$).

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).

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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).

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.

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.

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.

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.

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).

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.