JAMB UTME - Chemistry - 2025

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Oxidation and reduction are defined in terms of electron transfer:

• Oxidation is the loss (removal) of electrons from a species.
• Reduction is the gain (addition) of electrons to a species.

A useful mnemonic is OIL RIG: Oxidation Is Loss, Reduction Is Gain.

For example, when iron is oxidised:

\[\text{Fe} \rightarrow \text{Fe}^{2+} + 2e^-\]

Iron loses two electrons, so its oxidation state increases from 0 to +2. The electrons are removed from the iron atom.

The other options are incorrect: "added" describes reduction (the opposite process), while "hydrolysed" (broken down by water) and "hydrated" (combined with water molecules) are unrelated to the electron-transfer definition of oxidation.

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Iron is extracted from its ore in a blast furnace. The iron that comes directly out of the blast furnace is called pig iron. It contains about 3-4% carbon along with smaller amounts of impurities such as silicon, manganese, phosphorus, and sulphur.

Pig iron is brittle due to its high carbon content and is not suitable for most engineering applications in its raw form. It must be further processed to produce more useful forms of iron and steel:

• Cast iron is produced by remelting pig iron and allowing it to cool in moulds. It still has a relatively high carbon content (2-4%) but has improved properties for casting.
• Wrought iron is nearly pure iron (less than 0.1% carbon) produced by removing most of the carbon and impurities from pig iron. It is soft, malleable, and resistant to corrosion.
• Carbon steel is produced by carefully controlling the carbon content of iron (0.1-1.5%) to achieve specific mechanical properties.

The iron that comes directly from the blast furnace, before any further refining, is pig iron.

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The empirical formula is the simplest whole-number ratio of atoms in a compound. The empirical formula mass (sometimes called empirical mass) is the molar mass corresponding to that simplest formula.

The molecular formula given is C₆H₁₂O₆. To find the empirical formula, divide all subscripts by their greatest common factor:

\[\text{GCF of } 6, 12, 6 = 6\]

\[\text{Empirical formula} = \text{C}_1\text{H}_2\text{O}_1 = \text{CH}_2\text{O}\]

Now calculate the empirical formula mass using the given atomic masses (H = 1, C = 12, O = 16):

\[\text{Empirical mass} = 12 + 2(1) + 16 = 30\]

As a check, the molecular mass of C₆H₁₂O₆ is 6(12) + 12(1) + 6(16) = 72 + 12 + 96 = 180. Dividing by the empirical mass: 180 / 30 = 6, confirming that the molecular formula is exactly 6 times the empirical formula.

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To determine the group and period of an element from its electron configuration, two pieces of information are needed:

• The period is the highest principal quantum number (the largest shell number) occupied by electrons.
• The group (for s-block and p-block elements) is determined by the number of electrons in the outermost shell.

The electron configuration given is \(1s^2\,2s^2\,2p^6\,3s^2\). The total number of electrons is \(2 + 2 + 6 + 2 = 12\), which identifies the element as magnesium (Mg).

The highest principal quantum number is 3 (from the \(3s^2\) subshell), so the element is in Period 3.

The outermost shell (\(n = 3\)) contains only 2 electrons (both in the \(3s\) subshell). Since these are s-block electrons, the element is in Group 2.

Therefore, the element belongs to Group 2 and Period 3.

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Sulphuric acid (H₂SO₄) is a diprotic acid, meaning each molecule donates two hydrogen ions when it dissociates completely in water:

\[\text{H}_2\text{SO}_4(aq) \rightarrow 2\text{H}^+(aq) + \text{SO}_4^{2-}(aq)\]

Step 1: Find [H⁺]

Since each mole of H₂SO₄ produces 2 moles of H⁺:

\[[\text{H}^+] = 2 \times 0.06 = 0.12 \text{ mol dm}^{-3} = 1.2 \times 10^{-1} \text{ mol dm}^{-3}\]

Step 2: Find [OH^-]

Using the ionic product of water at 25 °C:

\[K_w = [\text{H}^+][\text{OH}^-] = 1.0 \times 10^{-14}\]

\[[\text{OH}^-] = \frac{K_w}{[\text{H}^+]} = \frac{1.0 \times 10^{-14}}{1.2 \times 10^{-1}}\]

\[[\text{OH}^-] = \frac{1.0}{1.2} \times 10^{-14+1} = 0.833 \times 10^{-13} = 8.3 \times 10^{-14} \text{ mol dm}^{-3}\]

Therefore [H⁺] = \(1.2 \times 10^{-1}\) mol dm^-3 and [OH^-] = \(8.3 \times 10^{-14}\) mol dm^-3.

A common mistake is forgetting that sulphuric acid is diprotic and using [H⁺] = 0.06 instead of 0.12, which would give \(1.2 \times 10^{-2}\) instead of \(1.2 \times 10^{-1}\) and an incorrect OH^- concentration of \(8.3 \times 10^{-13}\).

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The compound CH₃-CH₂-COOCH₂-CH₃ contains the ester functional group (-COO-). To identify the reactants that formed this ester, split the structure at the ester linkage (between the carbonyl carbon and the oxygen bonded to the alkyl group).

The ester bond in -COO- comes from two parts:

• The acid part (to the left of -COO-): CH₃-CH₂-COO-. This corresponds to the carboxylate portion of propanoic acid (CH₃CH₂COOH), a three-carbon carboxylic acid.
• The alcohol part (to the right of -COO-): -CH₂-CH₃. This corresponds to the alkyl group from ethanol (CH₃CH₂OH), a two-carbon alcohol.

The ester is therefore ethyl propanoate, formed from propanoic acid and ethanol:

\[\text{CH}_3\text{CH}_2\text{COOH} + \text{CH}_3\text{CH}_2\text{OH} \rightleftharpoons \text{CH}_3\text{CH}_2\text{COOCH}_2\text{CH}_3 + \text{H}_2\text{O}\]

The correct reactants are propanoic acid and ethanol.

Exam tip: To identify the parent acid and alcohol of an ester, break the molecule at the single-bond oxygen in the -COO- group. The fragment bonded to the carbonyl (C=O) gives the acid; the fragment bonded through the oxygen gives the alcohol.

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In the nitrogen cycle, when organisms die, their proteins and other nitrogenous compounds are broken down by decomposing bacteria in a process called ammonification (or decay). The first product of this decomposition is ammonia (NH₃).

The process occurs in stages:

1. Proteins in dead material are broken down into amino acids by proteolytic enzymes from decomposing bacteria and fungi.
2. The amino acids are further deaminated (the amino group -NH₂ is removed), releasing ammonia (NH₃).

After ammonia is produced, nitrifying bacteria can convert it further: first to nitrites (dioxonitrate(III), NO₂^-) by Nitrosomonas, then to nitrates (trioxonitrate(V), NO₃^-) by Nitrobacter. However, the question asks specifically about the first conversion product of nitrogenous compounds in dead materials, which is ammonia.

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A blowing agent is a substance used to produce a cellular structure (foam) in materials such as plastics, rubber, and insulation. The term "blowing" refers specifically to the process of expanding a material by generating gas bubbles within it during manufacture.

Chlorofluorocarbons (CFCs) were widely used as blowing agents in the foam industry because they vaporise at low temperatures, creating uniform gas pockets that give foam products their lightweight, insulating structure. Expanded polystyrene, polyurethane foam, and similar products were traditionally manufactured using CFCs as the blowing agent.

While CFCs also have applications as refrigerants and aerosol propellants, those uses are distinct from the role of a blowing agent. A refrigerant absorbs and releases heat during phase changes in a cooling cycle, and a propellant provides pressure to expel contents from a container. Neither of these functions involves creating foam. The tyre industry does not use CFCs as blowing agents.

The key word in the question is "blowing agent," which points specifically to foam production.

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The diagram shows the equation 2NaCl(l) → 2Na(l) + Cl₂(g) with electricity as the energy source. This is the electrolysis of molten sodium chloride to produce metallic sodium and chlorine gas.

This industrial process is known as the Downs process, named after J.C. Downs who patented the Downs cell in 1924. In the Downs cell, molten NaCl (often mixed with CaCl₂ to lower the melting point from 801°C to about 600°C) is electrolysed. At the cathode, Na⁺ ions are reduced to liquid sodium metal, while at the anode, Cl⁻ ions are oxidised to produce chlorine gas.

The Bosch process produces hydrogen gas from water gas. The Chlor-alkali process electrolyses aqueous (not molten) NaCl to give NaOH, Cl₂, and H₂. The Browning process is not a standard industrial chemistry term in this context.

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The structural formula shows H₃C-CH₂-C(=O)-O-CH₂-CH₃, which is an ester. To name an ester using IUPAC nomenclature, identify two parts:

The acid component (to the left of the ester linkage -C(=O)-O-): There are three carbon atoms (CH₃-CH₂-C=O), which corresponds to propanoic acid. In the ester name, this becomes propanoate.

The alkyl component (to the right of the ester oxygen): There are two carbon atoms (-O-CH₂-CH₃), which is an ethyl group.

Combining both parts, the ester is named ethyl propanoate. The alkyl group name comes first, followed by the name derived from the parent carboxylic acid with the -ic acid suffix replaced by -ate.

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Petroleum is a naturally occurring substance found in underground rock formations. Its composition varies from one source to another because petroleum is a mixture of many different hydrocarbons and other organic compounds, not a single pure substance.

A pure substance (element or compound) has a fixed, definite composition regardless of its source. A mixture, however, consists of two or more substances combined in no fixed ratio, so its composition can differ from sample to sample.

Petroleum contains alkanes, cycloalkanes, aromatic hydrocarbons, and other compounds in varying proportions depending on the geological conditions under which it formed. This variable composition is precisely what defines it as a mixture and is why it must be separated into useful fractions by fractional distillation.

While petroleum is indeed a hydrocarbon-containing substance, a liquid, and a natural resource, none of those properties explain why its composition varies. Only the fact that it is a mixture accounts for this variability.

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Colloids are classified based on the physical states of the dispersed phase (the substance spread throughout) and the dispersion medium (the substance in which it is spread). The main types include:

A commercial deodorant spray works by dispersing tiny liquid droplets (the fragrance and active ingredients) into the air (a gas). This makes it an aerosol - a colloid in which a liquid is dispersed in a gas.

It is not a foam (gas in liquid/solid), not an emulsion (liquid in liquid), and not a sol (solid in liquid).

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The expression shown is V \(\propto\) nT/P. This can be derived from the ideal gas equation PV = nRT, which rearranges to V = nRT/P. Since R is a constant, V is directly proportional to nT/P.

This expression combines three individual gas laws into one:

• Boyle's law: V \(\propto\) 1/P (at constant n and T)
• Charles' law: V \(\propto\) T (at constant n and P)
• Avogadro's law: V \(\propto\) n (at constant T and P)

Because the expression accounts for changes in all three variables (amount of substance n, temperature T, and pressure P) simultaneously, it represents the general gas law, not any single individual law.

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The homologous series of alkenes are unsaturated hydrocarbons that contain exactly one carbon-carbon double bond (C=C). Their general molecular formula is \(\text{C}_n\text{H}_{2n}\), where n is the number of carbon atoms (n >= 2).

To verify, consider a few members:

• Ethene (n = 2): C₂H₄ = C₂H₂₍₂₎
• Propene (n = 3): C₃H₆ = C₃H₂₍₃₎
• Butene (n = 4): C₄H₈ = C₄H₂₍₄₎

Each formula fits \(\text{C}_n\text{H}_{2n}\).

The other general formulae belong to different homologous series: \(\text{C}_n\text{H}_{2n+2}\) represents alkanes (saturated hydrocarbons), \(\text{C}_n\text{H}_{2n-2}\) represents alkynes (with a triple bond), and \(\text{C}_n\text{H}_{2n+1}\text{OH}\) represents alkanols (alcohols).

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A reaction is feasible (spontaneous) when the Gibbs free energy change is negative: \(\Delta G < 0\). The relationship between Gibbs free energy, enthalpy, and entropy is:

\[\Delta G = \Delta H - T\Delta S\]

The question states that \(\Delta G^\circ\) is negative. To determine which combination of \(\Delta H\) and \(\Delta S\) guarantees this, consider each option:

• If \(\Delta H\) is positive and \(\Delta S\) is positive: \(\Delta G = (+) - T(+)\). This is negative only at sufficiently high temperatures, so feasibility is not guaranteed at all temperatures.
• If \(\Delta H = 0\) and \(\Delta S\) is positive: \(\Delta G = 0 - T(+) = -T\Delta S\), which is always negative at any temperature above 0 K. This guarantees a negative \(\Delta G\).
• If \(\Delta H\) is positive and \(\Delta S\) is negative: \(\Delta G = (+) - T(-) = (+) + T|\Delta S|\), which is always positive. The reaction would never be feasible.
• If \(\Delta S = 0\): \(\Delta G = \Delta H\), and feasibility depends entirely on the sign of \(\Delta H\), which is not specified.

The only option that ensures \(\Delta G\) is negative under all conditions is \(\Delta H = 0\) and \(\Delta S\) is positive, because the \(-T\Delta S\) term is always negative when \(\Delta S > 0\).

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For a reaction that follows first-order kinetics, the half-life is related to the rate constant by the formula:

\[t_{1/2} = \frac{\ln 2}{k} = \frac{0.693}{k}\]

Given \(k_1 = 3.85 \times 10^{-3}\, \text{s}^{-1}\):

\[t_{1/2} = \frac{0.693}{3.85 \times 10^{-3}}\]

\[t_{1/2} = \frac{0.693}{0.00385}\]

\[t_{1/2} = 180\, \text{s}\]

The half-life of radioactive phosphorus is 180 s.

An important feature of first-order kinetics is that the half-life is independent of the initial concentration - it depends only on the rate constant. This is why radioactive decay (which always follows first-order kinetics) has a constant half-life regardless of how much of the substance remains.

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Hard water contains dissolved calcium and magnesium ions (Ca²⁺ and Mg²⁺). Several properties of hard water are well established:

• The calcium ions help build strong teeth and bones in animals - this is a genuine advantage of hard water.
• Hard water tastes better than soft water because of its mineral content - this is widely accepted.
• Hard water can actually be safely supplied through lead pipes. The dissolved calcium and magnesium salts react with lead to form insoluble compounds (such as lead carbonate) that coat the inside of the pipe, creating a protective lining. This prevents further lead from dissolving into the water. In contrast, soft water in lead pipes is dangerous because it can dissolve lead, causing lead poisoning.

The statement that hard water cannot be supplied in pipes made of lead is false. The opposite is true: hard water is safer in lead pipes than soft water, precisely because the mineral deposits form a barrier that prevents lead contamination.

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Transition metals are widely used as catalysts in both industrial and biological processes. The key electronic property that makes them effective catalysts is the presence of partially filled d-orbitals.

Partially filled d-orbitals allow transition metals to:

• Exhibit variable oxidation states by losing different numbers of d-electrons, enabling them to participate in electron-transfer steps during a reaction.
• Form temporary bonds with reactant molecules by accepting or donating electron density through their available d-orbitals, which lowers the activation energy of the reaction.
• Provide a suitable surface or coordination environment for reactants to adsorb, orient, and react more readily.

A filled d-orbital would mean the metal has no vacant d-orbitals to accept electron density from reactants, greatly reducing its catalytic ability. Similarly, f-orbitals (whether filled or partially filled) are associated with the inner transition metals (lanthanides and actinides) and are not the primary reason for catalytic activity in the common transition metal catalysts such as iron, nickel, platinum, and palladium.

Exam tip: The two key properties of transition metals to remember are variable oxidation states and catalytic activity, both arising from partially filled d-orbitals.

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The pH of a salt solution depends on the strength of the acid and base from which the salt was formed. A salt formed from a strong acid and a strong base produces a neutral solution with a pH of 7, because neither ion undergoes hydrolysis in water.

Sodium chloride (NaCl) is formed from hydrochloric acid (HCl, a strong acid) and sodium hydroxide (NaOH, a strong base). When dissolved in water, the Na⁺ and Cl^- ions do not react with water, so the solution remains neutral at pH 7.

The other salts behave differently:

• Ammonium chloride (NH₄Cl) is formed from a weak base (NH₃) and a strong acid (HCl). The ammonium ion hydrolyses in water to release H⁺ ions, making the solution acidic (pH below 7).
• Ammonium tetraoxosulphate(VI) ((NH₄)₂SO₄) is also formed from a weak base and a strong acid, producing an acidic solution.
• Sodium trioxocarbonate(IV) (Na₂CO₃) is formed from a strong base (NaOH) and a weak acid (H₂CO₃). The carbonate ion hydrolyses in water to produce OH^- ions, making the solution basic (pH above 7).

Exam tip: To predict the pH of a salt solution, identify the parent acid and base. Strong acid + strong base gives pH 7; strong acid + weak base gives pH below 7; weak acid + strong base gives pH above 7.

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Concentrated sulphuric acid (H\(_2\)SO\(_4\)) has a very strong affinity for water. It is one of the most powerful dehydrating agents in chemistry, meaning it removes water (or the elements of water, H and O in a 2:1 ratio) from other substances.

This dehydrating ability is demonstrated in several ways:

• It chars sugar (sucrose, C\(_{12}\)H\(_{22}\)O\(_{11}\)) by removing water, leaving behind carbon.
• It is used in the laboratory to dry gases that do not react with it (e.g., drying chlorine, oxygen, or carbon dioxide).
• It dehydrates ethanol to produce ethene: \(\text{C}_2\text{H}_5\text{OH} \xrightarrow{\text{conc. H}_2\text{SO}_4} \text{C}_2\text{H}_4 + \text{H}_2\text{O}\)

The other options describe properties that are not characteristic of concentrated H\(_2\)SO\(_4\). Reacting with metals to produce hydrogen and neutralising alkalis are properties of dilute acids, not concentrated sulphuric acid specifically. Producing a cryolite precipitate is not a recognised reaction of sulphuric acid.

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Le Chatelier's principle states that if a system at equilibrium is subjected to a change in conditions, the equilibrium shifts in the direction that tends to counteract that change. For pressure changes, the key factor is the difference in the total number of moles of gas on each side of the equation.

If there is no change in volume during a gaseous reaction, this means the total number of moles of gaseous products equals the total number of moles of gaseous reactants. In such a case, increasing or decreasing the pressure gives the system no direction in which to shift, because neither the forward nor the backward reaction would reduce the total number of gas molecules.

Therefore, a change in pressure will have no effect on the equilibrium position when the reaction involves equal moles of gas on both sides.

For example, in the reaction:

\[ \text{H}_2(g) + \text{I}_2(g) \rightleftharpoons 2\text{HI}(g) \]

there are 2 moles of gas on each side, so pressure changes do not shift the equilibrium.

Pressure only affects equilibrium when there is an unequal number of moles of gas on either side. In those cases, increasing pressure favours the side with fewer moles of gas, and decreasing pressure favours the side with more moles.

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The cell notation (also called line notation) for an electrochemical cell follows the convention:

Anode | Anode ion || Cathode ion | Cathode

where the single vertical line (|) represents a phase boundary, and the double vertical line (||) represents the salt bridge separating the two half-cells.

For the reaction Mg + Pb²⁺ → Mg²⁺ + Pb:

• Anode (oxidation): Mg → Mg²⁺ + 2e^-. Magnesium is oxidized, so it is the anode.
• Cathode (reduction): Pb²⁺ + 2e^- → Pb. Lead ions are reduced, so lead is the cathode.

Applying the convention:

Mg | Mg²⁺ || Pb²⁺ | Pb

Using the notation in the options (where I = | and II = ||), this is written as Mg|Mg²⁺||Pb²⁺|Pb.

The anode always appears on the left and the cathode on the right. Within each half-cell, the metal (solid phase) is written adjacent to the outer edge, and the ion (aqueous phase) is written adjacent to the salt bridge.

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Water has an unusually high boiling point (100 °C) for a molecule of its small size (relative molecular mass = 18). To understand why, compare water (H₂O) with hydrogen sulphide (H₂S), which has a larger relative molecular mass of 34 but is a gas at room temperature (boiling point -60 °C). The difference lies in the type of intermolecular forces present.

Each water molecule can form up to four hydrogen bonds with neighbouring molecules. Oxygen is highly electronegative, creating a large partial positive charge on the hydrogen atoms. These hydrogen atoms are attracted to the lone pairs on the oxygen of adjacent water molecules, forming strong intermolecular hydrogen bonds. This extensive hydrogen-bonding network requires a large amount of energy to break, which raises the boiling point far above what the molecular mass alone would predict.

The covalent bonds within each water molecule (O-H bonds) hold the atoms together inside one molecule, but they do not determine the physical state. Van der Waals forces are present in all molecules but are too weak on their own to keep such a light molecule in the liquid state at room temperature. Electrovalent (ionic) bonds do not exist in water, which is a covalent molecular substance.

It is therefore the strong hydrogen bonding between water molecules that keeps water liquid at room temperature.

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An alkaline gas is a gas that dissolves in water to produce a solution with a pH greater than 7 (a basic solution).

NH₃ (ammonia) is the classic example. When ammonia dissolves in water, it reacts to form ammonium hydroxide:

\[\text{NH}_3(g) + \text{H}_2\text{O}(l) \rightleftharpoons \text{NH}_4^+(aq) + \text{OH}^-(aq)\]

The production of hydroxide ions (OH^-) makes the solution alkaline.

The other gases are not alkaline:

• HCl (hydrogen chloride) dissolves in water to form hydrochloric acid, which is strongly acidic.
• N₂ (nitrogen) is an inert, unreactive gas that does not dissolve appreciably in water and produces neither acidic nor basic solutions.
• NO₂ (nitrogen dioxide) dissolves in water to form a mixture of nitric acid and nitrous acid, both of which are acidic.

Exam tip: Ammonia is the only common alkaline gas encountered at this level. Its characteristic pungent smell and ability to turn moist red litmus paper blue are standard identification tests.

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A reaction is spontaneous when the Gibbs free energy change is negative, that is, \(\Delta G < 0\). The Gibbs equation relates enthalpy, entropy, and temperature:

\[\Delta G = \Delta H - T\Delta S\]

For the decomposition of phosphorus pentachloride:

\[\text{PCl}_5(g) \rightarrow \text{PCl}_3(s) + \text{Cl}_2(g)\]

Consider the entropy change. On the reactant side there is 1 mole of gas, and on the product side there is 1 mole of solid and 1 mole of gas. Since a solid has much lower entropy than a gas, the total entropy of the products is lower than that of the reactant. Therefore \(\Delta S\) is negative.

With \(\Delta S < 0\), the term \(-T\Delta S\) becomes positive, which adds to \(\Delta G\). For \(\Delta G\) to still be negative (spontaneous), \(\Delta H\) must be sufficiently negative to overcome the positive \(-T\Delta S\) contribution:

\[\Delta G = \Delta H - T\Delta S < 0\]

\[\Delta H < T\Delta S \quad (\text{where } \Delta S < 0, \text{ so } T\Delta S < 0)\]

This means \(\Delta H\) must be negative. An exothermic reaction (\(\Delta H\) is negative) releases enough energy to drive the process forward despite the unfavourable entropy change.

Exam tip: When \(\Delta S\) is negative, only a sufficiently negative \(\Delta H\) can make \(\Delta G\) negative, and such reactions tend to be spontaneous only at low temperatures.

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KOH is a strong base that dissociates completely in water:

\[\text{KOH} \rightarrow \text{K}^+ + \text{OH}^-\]

For a 0.001 M KOH solution, the concentration of hydroxide ions is:

\[[\text{OH}^-] = 0.001\;\text{M} = 10^{-3}\;\text{M}\]

First, calculate the pOH:

\[\text{pOH} = -\log[\text{OH}^-] = -\log(10^{-3}) = 3\]

Then, use the relationship between pH and pOH at 25 \(^\circ\)C:

\[\text{pH} + \text{pOH} = 14\]

\[\text{pH} = 14 - 3 = 11\]

The pH of 0.001 M KOH solution is 11.

Exam tip: For strong bases, first find [OH^-] from the molarity, calculate pOH, then subtract from 14 to get pH. A pH of 11 confirms a basic solution, which is consistent with KOH being a strong alkali.

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Baking powder is a mixture of sodium hydrogen carbonate (baking soda) and a solid acid. The acid is needed because sodium hydrogen carbonate only releases carbon dioxide gas, the substance that makes dough rise, when it reacts with an acid:

\[ \text{NaHCO}_3 + \text{acid} \rightarrow \text{CO}_2 + \text{H}_2\text{O} + \text{salt} \]

Tartaric acid, \( \text{C}_4\text{H}_6\text{O}_6 \), is a naturally occurring organic acid obtained mainly from grapes, and it is one of the classic solid acids used to formulate baking powder because it is stable when dry but dissolves and reacts quickly once water is added to the dough or batter. The same acid, because it has a pleasant sharp taste and is safe to consume in small amounts, is also added to soft drinks to give them their tart, refreshing flavour and to help balance the sweetness of the sugar in the drink.

The other acids listed do not fit both uses. Fatty acids are found in oils and fats and are not used as leavening or flavouring agents in drinks. Boric acid is toxic in the concentrations relevant to food and is not permitted as a food additive. Citric acid is common in fruit-flavoured drinks but is not the acid traditionally paired with sodium hydrogen carbonate in the classic baking-soda and soft-drink formulation being tested here.

When a question links a food acid to two different uses at once, check that the acid is both chemically suited to the reaction involved (reacting with a base to release gas) and safe and pleasant enough to be consumed directly, since not every food-grade acid satisfies both conditions.

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Freons are a group of chlorofluorocarbons (CFCs) - synthetic compounds containing chlorine, fluorine, and carbon. They were widely used as propellants in aerosol cans, as refrigerants in air conditioners and refrigerators, and as solvents in industrial cleaning.

When released into the atmosphere from these sources, freons rise to the stratosphere where ultraviolet radiation breaks them down, releasing chlorine atoms. These chlorine atoms catalytically destroy ozone molecules, contributing to the depletion of the ozone layer.

Fossil fuel combustion releases carbon dioxide, sulphur dioxide, and nitrogen oxides, but not freons. Photosynthesis is a biological process that produces oxygen and consumes carbon dioxide. Organic decay releases methane and carbon dioxide. None of these processes involve freons.

The Montreal Protocol (1987) restricted the production and use of CFCs, leading to a gradual recovery of the ozone layer.

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The electron configuration 1s² 2s² 2p⁶ 3s² 3p⁵ has a total of 2 + 2 + 6 + 2 + 5 = 17 electrons, which identifies the element as chlorine (Cl, atomic number 17).

The group number of an element in the periodic table is determined by the number of electrons in its outermost (valence) shell. For chlorine, the outermost shell is the third shell (n = 3), which contains:

\[3s^2\,3p^5 = 2 + 5 = 7 \text{ electrons}\]

Therefore, chlorine belongs to Group 7 (also called Group VII or Group 17 in modern IUPAC numbering). Group 7 elements are the halogens: fluorine, chlorine, bromine, iodine, and astatine. They all have seven electrons in their outermost shell, giving them the general outer-shell configuration ns² np⁵.

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The reaction is:

\[\text{NH}_3(g) + \text{HCl}(g) \rightarrow \text{NH}_4\text{Cl}(s)\]

On the reactant side, there are 2 moles of gas (1 mole of NH₃ + 1 mole of HCl). On the product side, NH₄Cl is a solid, so there are effectively 0 moles of gas.

According to Le Chatelier's principle, when the pressure of a gaseous system at equilibrium is increased, the equilibrium shifts towards the side with fewer moles of gas to reduce the pressure.

Since the product side has fewer gaseous moles than the reactant side, increasing the pressure will shift the equilibrium to the right, favouring the product (NH₄Cl).

Note that changing pressure shifts the position of equilibrium but does not change the equilibrium constant (K). The equilibrium constant is only affected by changes in temperature, not pressure or concentration.

Exam tip: When applying Le Chatelier's principle to pressure changes, count only the moles of gaseous species on each side. Solids and liquids are not affected by pressure changes.

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Crude oil (petroleum) is a complex mixture of hydrocarbons with different boiling points. To separate it into useful fractions (such as petrol/gasoline, kerosene, diesel, lubricating oil, and bitumen), fractional distillation is used.

In fractional distillation, crude oil is heated in a furnace until most of it vaporises. The vapour enters a tall fractionating column that is hot at the bottom and cool at the top. As the vapour rises through the column:

• Hydrocarbons with higher boiling points condense lower in the column (e.g., bitumen, heavy fuel oil).
• Hydrocarbons with lower boiling points rise higher before condensing (e.g., petrol, refinery gas).

The column contains trays at different heights where each fraction is collected.

The other separation methods are not suitable:

• Steam distillation is used for heat-sensitive compounds (like essential oils) that would decompose at their normal boiling points.
• Fractional crystallisation separates solids from solution based on differences in solubility at different temperatures.
• Precipitation is used to separate insoluble solids from a solution by adding a reagent.

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Brine is a concentrated solution of sodium chloride (NaCl) in water. When brine is electrolysed using inert (neutral) electrodes such as carbon or platinum, the ions present in solution are:

• Cations: Na⁺ and H⁺ (from water)
• Anions: Cl^- and OH^- (from water)

At the anode (positive electrode), anions migrate and are discharged. Both Cl^- and OH^- are present, but Cl^- is preferentially discharged because it is present in much higher concentration in the brine solution. Despite OH^- having a lower discharge potential, the high concentration of Cl^- gives it priority at the anode.

The half-equation at the anode is:

\[2\text{Cl}^- \rightarrow \text{Cl}_2 + 2e^-\]

Chlorine gas (Cl₂) is released at the anode.

Na⁺ and H⁺ are cations and migrate to the cathode, not the anode. At the cathode, H⁺ is discharged (since Na⁺ has a very high discharge potential), producing hydrogen gas.

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Surface tension is the property of a liquid that causes its surface to behave like a stretched elastic membrane. It arises because molecules at the surface of a liquid experience a net inward pull from neighbouring molecules below and beside them, but not from above. This inward force causes the surface to contract to the smallest possible area.

For a given volume of liquid, the shape with the smallest surface area is a sphere. Therefore, when water forms small droplets (such as raindrops or drops on a waxy surface), surface tension pulls the water into a spherical shape.

The other properties do not explain the spherical shape:

• Polarity explains why water molecules attract each other (through hydrogen bonding), which contributes to surface tension, but polarity itself does not directly cause the spherical shape.
• Viscosity is the resistance of a liquid to flow. It affects how quickly water moves, not the shape of its droplets.
• Density is mass per unit volume and determines whether a substance sinks or floats, not the shape of droplets.

Exam tip: Surface tension explains several everyday observations: water forming spherical drops, insects walking on water surfaces, and a needle floating when placed gently on water.

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A water pollutant is any substance or condition that degrades the quality of a water body and harms aquatic life or makes the water unsuitable for its intended use.

Oxygen gas is not a water pollutant. In fact, dissolved oxygen is essential for aquatic life. Fish and other aquatic organisms depend on dissolved oxygen for respiration. A water body with adequate dissolved oxygen levels is considered healthy.

The other options are all recognised water pollutants:

• Warm water effluent (thermal pollution): Discharge of heated water from power plants or factories raises the temperature of natural water bodies, which reduces the solubility of dissolved oxygen and disrupts aquatic ecosystems.
• Inorganic fertilizers: Nitrates and phosphates from agricultural runoff cause eutrophication, leading to excessive algal growth, oxygen depletion, and death of aquatic organisms.
• Biodegradable waste: Organic matter such as sewage increases the biological oxygen demand (BOD) of the water, depleting dissolved oxygen as microorganisms decompose the waste.

Since oxygen gas is a natural and beneficial component of water, it is not classified as a pollutant.

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The equation shown is:

2XY₃(g) ⇌ X₂(g) + 3Y₂(g)

The equilibrium constant \(K_c\) is defined as the ratio of the product concentrations raised to their stoichiometric coefficients divided by the reactant concentrations raised to their stoichiometric coefficients.

From the balanced equation, the products are X₂ (coefficient 1) and Y₂ (coefficient 3), while the reactant is XY₃ (coefficient 2). Therefore:

\[K_c = \frac{[X_2][Y_2]^3}{[XY_3]^2}\]

The coefficients become exponents in the equilibrium expression, not multipliers placed in front of the concentration brackets. This is a fundamental distinction: writing \([2XY_3]\) or \([3Y_2]\) treats the coefficient as part of the concentration term, which is incorrect.

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The energy profile diagram shows the energy changes during a chemical reaction. The reactants (A+B) start at an energy level of approximately 30 units, while the products (C+D) end at approximately 50 units. The activation energy peak reaches about 80 units.

Since the products have a higher energy level than the reactants, the reaction has absorbed energy from the surroundings. This net gain in energy by the reacting system is the defining characteristic of an endothermic reaction. The energy difference between products and reactants (\ (\Delta H\)) is positive, confirming that heat was taken in rather than released.

An exothermic reaction would show products at a lower energy level than reactants, indicating a release of energy. Here, the upward shift from reactants to products clearly indicates energy absorption.

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The basicity of an acid is the number of replaceable hydrogen ions (\(\text{H}^+\)) that one molecule of the acid can donate in a reaction with a base.

The compound \(\text{C}_2\text{H}_2\text{O}_4\) is oxalic acid (also called ethanedioic acid). Its structural formula is:

\(\text{HOOC-COOH}\)

Oxalic acid contains two carboxyl groups (\(-\text{COOH}\)). Each carboxyl group carries one hydrogen atom that can be released as \(\text{H}^+\) during a neutralisation reaction. The remaining hydrogen atoms in the molecule are bonded to carbon and are not ionisable.

Since there are two replaceable hydrogen atoms, the basicity of oxalic acid is 2. This means it is a dibasic acid (also called a diprotic acid).

The neutralisation reaction with sodium hydroxide confirms this:

Two moles of NaOH are required to completely neutralise one mole of oxalic acid, confirming a basicity of 2.

When determining basicity, count only the hydrogen atoms bonded to oxygen in carboxyl or hydroxyl groups, not those bonded directly to carbon.

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When air is passed through a series of reagents, each one absorbs a specific component:

1. Alkaline pyrogalol (alkaline solution of pyrogallol) absorbs oxygen (O₂). Pyrogallol in alkaline solution reacts readily with oxygen and turns dark brown.
2. Potash (concentrated potassium hydroxide solution, KOH) absorbs carbon dioxide (CO₂). The CO₂ reacts with KOH to form potassium carbonate: CO₂ + 2KOH → K₂CO₃ + H₂O.
3. Fused calcium chloride (CaCl₂) in a U-tube absorbs water vapour (H₂O). Anhydrous CaCl₂ is a powerful desiccant (drying agent).

The main components of air are nitrogen (~78%), oxygen (~21%), argon and other noble gases (~0.9%), carbon dioxide (~0.04%), and water vapour (variable). After removing oxygen, carbon dioxide, and water vapour, the components that remain unabsorbed are noble gases and nitrogen. These are chemically inert (noble gases) or unreactive with the reagents used (nitrogen), so none of the three reagents can remove them.

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The boiling point of a substance depends on the strength of its intermolecular forces and, to a lesser extent, its molecular mass. The key intermolecular forces in order of strength are: hydrogen bonding > dipole-dipole > van der Waals (London dispersion).

Consider the four compounds:

• CH₃CH₂OH (ethanol, M_r = 46): contains an -OH group, so it forms hydrogen bonds. Boiling point approximately 78 °C.
• CH₃CH₂CH₂Cl (1-chloropropane, M_r = 78.5): polar C-Cl bond gives dipole-dipole interactions but no hydrogen bonding. Boiling point approximately 47 °C.
• CH₃CH₂CH₂CH₂CH₂CH₃ (hexane, M_r = 86): non-polar, only van der Waals forces. Boiling point approximately 69 °C.
• CH₃CH₂CH₂OH (propan-1-ol, M_r = 60): contains an -OH group, so it forms hydrogen bonds, and has a higher molecular mass than ethanol. Boiling point approximately 97 °C.

Propan-1-ol (CH₃CH₂CH₂OH) has the highest boiling point. It combines hydrogen bonding (the strongest intermolecular force among these molecules) with a greater molecular mass than ethanol, giving it stronger overall intermolecular attractions.

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Amines are classified based on the number of carbon-containing groups (alkyl or aryl groups) directly bonded to the nitrogen atom:

• Primary amine: nitrogen is bonded to one carbon group and two hydrogen atoms (R-NH₂).
• Secondary amine: nitrogen is bonded to two carbon groups and one hydrogen atom (R₂-NH).
• Tertiary amine: nitrogen is bonded to three carbon groups and no hydrogen atoms (R₃-N).

In CH₃CH(NH₂)CH₂CH₂CH₃, the nitrogen atom in the -NH₂ group is bonded to one carbon atom (the CH group in the chain) and two hydrogen atoms. This fits the definition of a primary amine.

The fact that the nitrogen is attached to a secondary carbon (a carbon bonded to two other carbons) does not change the amine classification. The classification depends only on how many carbons are bonded directly to nitrogen, not on the type of carbon.

Exam tip: Do not confuse amine classification (based on bonds to nitrogen) with alcohol classification (based on the type of carbon bearing the -OH group). A primary amine simply means nitrogen has one C-N bond.