JAMB UTME - Chemistry - 2025

Detalhes da Resposta

The compound [Cu(NH₃)₄]Cl₂ consists of two distinct parts:

• The complex cation: [Cu(NH₃)₄]²⁺ (copper(II) tetraamine ion)
• The anions: 2 Cl^- (chloride ions)

The question asks about the bond between the complex cation and the chloride ions. The complex cation carries a 2+ charge, and each chloride ion carries a 1- charge. The attraction between these oppositely charged ions is an ionic bond (electrostatic attraction).

It is important to distinguish this from the bonding within the complex ion. Inside [Cu(NH₃)₄]²⁺, each NH₃ molecule donates a lone pair of electrons from nitrogen to the Cu²⁺ ion, forming dative (coordinate) bonds. However, the question specifically asks about the bond between the complex and chlorine, which is ionic.

Detalhes da Resposta

A lone pair is a pair of valence electrons on an atom that is not shared in a bond. To find which molecule has the highest number of lone pairs, draw the Lewis structure of each molecule and count all lone pairs on every atom.

CH₄: Carbon has four bonding pairs (one to each hydrogen) and no lone pairs. Each hydrogen also has no lone pairs. Total lone pairs: 0.

NH₃: Nitrogen has three bonding pairs (one to each hydrogen) and one lone pair. Total lone pairs: 1.

H₂O: Oxygen has two bonding pairs (one to each hydrogen) and two lone pairs. Total lone pairs: 2.

CO₂: Carbon forms two double bonds (one to each oxygen) and has no lone pairs. Each oxygen in a double bond with carbon retains two lone pairs. Total lone pairs: 2 + 2 = 4.

CO₂ has the highest total number of lone pairs (four), making it the correct answer.

Exam tip: When counting lone pairs, remember to include those on every atom in the molecule, not just the central atom.

Detalhes da Resposta

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.

Detalhes da Resposta

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.

Detalhes da Resposta

To name this compound using IUPAC nomenclature, first expand the condensed structural formula (CH\(_3\))\(_2\)CHCH(OH)CH\(_2\)C(CH\(_3\))\(_3\):

\[\text{CH}_3-\underset{|}{\overset{\text{CH}_3}{\text{CH}}}-\underset{|}{\overset{\text{OH}}{\text{CH}}}-\text{CH}_2-\underset{|}{\overset{\text{CH}_3}{\underset{|}{\overset{}{\text{C}}}}}(\text{CH}_3)_2\]

Step 1: Find the longest carbon chain containing the OH group.

Tracing through the backbone: CH\(_3\)-CH-CH(OH)-CH\(_2\)-C-CH\(_3\) gives 6 carbons, so the parent chain is hexane.

Step 2: Number to give the OH group the lowest locant.

Numbering from the end nearest the OH group:

• C1: CH\(_3\) (end near the isopropyl group)
• C2: CH (bearing one CH\(_3\) branch)
• C3: CH-OH
• C4: CH\(_2\)
• C5: C (bearing two CH\(_3\) branches)
• C6: CH\(_3\)

OH is on carbon 3. Numbering from the other end would place OH on carbon 4, which is higher, so this direction is correct.

Step 3: Identify substituents.

• Carbon 2: one methyl group
• Carbon 5: two methyl groups

There are three methyl substituents at positions 2, 5, and 5.

Step 4: Construct the name.

The IUPAC name is 2,5,5-trimethylhexan-3-ol.

Detalhes da Resposta

In qualitative analysis, ions are classified as either acid radicals (anions) or basic radicals (cations).

• Acid radicals are the negative ions (anions) derived from acids. Examples include CO₃^2- (from H₂CO₃), SO₄^2- (from H₂SO₄), NO₃^- (from HNO₃), PO₄^3- (from H₃PO₄), and Cl^- (from HCl).
• Basic radicals are the positive ions (cations) derived from bases. Examples include Na⁺, K⁺, Mg²⁺, Ca²⁺, and Fe²⁺.

The question asks which group contains only acid radicals. Examining each option:

• CO₃^2-, PO₄^3-, Mg²⁺: contains Mg²⁺, which is a basic radical (cation).
• CO₃^2-, SO₄^2-, Na⁺: contains Na⁺, which is a basic radical (cation).
• CO₃^2-, SO₄^2-, NO₃^-: all three are anions derived from acids. This group contains only acid radicals.
• CO₃^2-, SO₄^2-, K⁺: contains K⁺, which is a basic radical (cation).

The correct answer is the group containing CO₃^2-, SO₄^2-, and NO₃^-, as all three are acid radicals.

Detalhes da Resposta

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.

Detalhes da Resposta

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.

Detalhes da Resposta

Esters (alkanoates) are the class of organic compounds responsible for the pleasant, fruity scents found in many fruits and perfumes. Esters are formed by the condensation reaction between an alkanol (alcohol) and an alkanoic acid (carboxylic acid) in the presence of a concentrated acid catalyst:

\[\text{Alkanoic acid} + \text{Alkanol} \xrightarrow{\text{H}^+} \text{Alkanoate (ester)} + \text{H}_2\text{O}\]

For example, ethyl ethanoate (CH\(_3\)COOC\(_2\)H\(_5\)) has a fruity smell resembling nail polish or pear drops. Different combinations of acids and alcohols produce esters with distinct aromas - banana, pineapple, apple, and many others.

The other options are different functional group classes with different characteristic properties:

• Alkanamides (amides, R-CO-NH\(_2\)) are generally odourless solids.
• Alkanones (ketones, R-CO-R') have sharp, often solvent-like odours - not fruity.
• Alkanals (aldehydes, R-CHO) tend to have pungent, irritating smells at low molecular weight.

Detalhes da Resposta

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.

Detalhes da Resposta

Aluminium (Al) is the metal widely used as a packaging material. It is used to make drink cans, food containers, and aluminium foil for wrapping food.

Aluminium is ideal for packaging because of several key properties:

• Low density - it is lightweight, making it easy to transport and handle.
• Malleability - it can be rolled into very thin sheets (foil) or pressed into various container shapes.
• Corrosion resistance - it forms a tough, protective oxide layer (\(\text{Al}_2\text{O}_3\)) on its surface that prevents further reaction with air or moisture, keeping the contents safe.
• Non-toxicity - it does not contaminate food or beverages.
• Recyclability - aluminium can be recycled repeatedly without losing its properties.

The other metals are unsuitable for packaging:

• Copper (Cu) is too heavy and expensive for everyday packaging, and it can impart a metallic taste to food.
• Iron (Fe) rusts readily when exposed to moisture and air, which would contaminate and spoil packaged goods.
• Sodium (Na) is an extremely reactive alkali metal that reacts violently with water and catches fire in air, making it entirely impractical for packaging.

Detalhes da Resposta

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.

Detalhes da Resposta

Sublimation is the phase transition in which a solid changes directly into a gas without passing through the liquid state.

This process requires energy to overcome the intermolecular forces holding the particles in the solid lattice so that they can escape into the gas phase. Since energy must be supplied to the substance, sublimation is an endothermic process, meaning it involves energy absorption.

The other options are incorrect:

• Vapourization refers specifically to the change from liquid to gas (boiling or evaporation), not from solid to gas.
• Melting is the change from solid to liquid, not solid to gas.
• Osmotic diffusion refers to the movement of solvent molecules through a semipermeable membrane, which is unrelated to phase changes.

Exam tip: Common examples of sublimation include solid carbon dioxide (dry ice) turning directly into CO₂ gas, iodine crystals forming purple vapour when heated, and solid naphthalene (mothballs) gradually disappearing into vapour.

Detalhes da Resposta

Magnesium tetraoxosulphate(VI) is the systematic name for magnesium sulphate (MgSO\(_4\)). In its hydrated form, MgSO\(_4\)\(\cdot\)7H\(_2\)O, it is commonly known as Epsom salt.

Epsom salt is widely used in medicine as a laxative. When taken orally, magnesium sulphate draws water into the intestines by osmosis (it is poorly absorbed), which softens the stool and stimulates bowel movement. This makes it an effective saline laxative.

The other options do not match:

• A disinfectant kills microorganisms - examples include sodium hypochlorite or phenol, not MgSO\(_4\).
• A plasticizer is added to polymers to increase flexibility - these are typically organic esters such as dioctyl phthalate.
• While magnesium and sulphur are plant nutrients, MgSO\(_4\) is not the standard compound referred to as a fertilizer at examination level. Its primary and best-known use is as a laxative.

Detalhes da Resposta

Copper is deposited from CuSO₄ solution by the reduction of Cu²⁺ ions:

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

This means each mole of copper requires 2 moles of electrons (2 faradays) to be deposited.

Step 1: Calculate the moles of copper to be deposited.

\[n_{\text{Cu}} = \frac{\text{mass}}{\text{molar mass}} = \frac{4.5}{64} = 0.0703125 \text{ mol}\]

Step 2: Calculate the total charge required.

Since 1 mole of Cu requires 2 faradays:

\[Q = n_{\text{Cu}} \times 2 \times F = 0.0703125 \times 2 \times 96500\]

\[Q = 0.140625 \times 96500 = 13570.3 \text{ C}\]

Step 3: Calculate the time using \(Q = It\).

\[t = \frac{Q}{I} = \frac{13570.3}{2.5} = 5428 \text{ sec}\]

The time required is 5428 seconds.

Detalhes da Resposta

When solid particles such as dust and sand are dispersed in a gas (air), the resulting system is classified based on particle size and behaviour.

A suspension is a heterogeneous mixture in which relatively large, visible particles are dispersed in a medium. The particles in a suspension are large enough to eventually settle out under gravity and can often be seen with the naked eye. Dust and sand particles in air fit this description: they are large, they scatter light visibly, and they settle when the air is still.

The other options do not fit:

• An emulsion is a mixture of two immiscible liquids (such as oil in water). Dust in air does not involve two liquids.
• A saturated solution is a homogeneous mixture where no more solute can dissolve at a given temperature. Dust particles are not dissolved in air.
• A dispersion is a broad term for any system in which one substance is spread throughout another, but it does not specifically describe the large-particle, settling behaviour of dust and sand in air.

Because dust and sand particles are large, visible, and settle out over time, the system is best classified as a suspension.

Detalhes da Resposta

This is a Faraday's law of electrolysis problem. The relationship between mass deposited, current, and time is:

\[m = \frac{M \times I \times t}{n \times F}\]

where \(m\) = mass deposited (g), \(M\) = molar mass, \(I\) = current (A), \(t\) = time (s), \(n\) = number of electrons transferred per ion, and \(F\) = Faraday constant (96500 C/mol).

For aluminium: \(\text{Al}^{3+} + 3e^- \rightarrow \text{Al}\), so \(n = 3\), \(M = 27\), \(m = 9\) g, \(I = 18\) A.

Rearranging for time:

\[t = \frac{m \times n \times F}{M \times I}\]

\[t = \frac{9 \times 3 \times 96500}{27 \times 18}\]

\[t = \frac{2\,605\,500}{486}\]

\[t = 5360.49 \text{ seconds}\]

Converting to minutes:

\[t = \frac{5360.49}{60} = 89.34 \text{ minutes}\]

The time required is 89.34 minutes.

Detalhes da Resposta

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.

Detalhes da Resposta

Reforming is a petroleum refinery process that rearranges the molecular structure of hydrocarbons to produce higher-octane fuels and aromatic compounds. The most common type is catalytic reforming, which converts naphthenes (cycloalkanes) and straight-chain alkanes into aromatic hydrocarbons such as benzene, toluene, and xylene, typically using a platinum-based catalyst at high temperature.

The conversion of cyclohexane to benzene is a classic example of reforming. In this reaction, cyclohexane (C₆H₁₂) undergoes catalytic dehydrogenation, losing three molecules of hydrogen to form benzene (C₆H₆):

\[ \text{C}_6\text{H}_{12} \xrightarrow{\text{Pt catalyst, heat}} \text{C}_6\text{H}_6 + 3\text{H}_2 \]

This aromatization reaction increases the octane rating of the fuel fraction and produces valuable aromatic feedstocks for the chemical industry.

The other options describe different processes:

• Converting hexene to hexane is hydrogenation (addition of hydrogen across a double bond).
• Converting hexane to carbon(IV) oxide and water vapour is combustion (burning in oxygen).
• Converting hexane to butane and ethane is cracking (breaking larger molecules into smaller ones).

Detalhes da Resposta

The fountain experiment demonstrates the very high solubility of certain gases in water. A round-bottom flask is filled with the gas and inverted over a trough of water (often containing an indicator). When a small amount of water enters the flask and dissolves the gas, the pressure inside drops dramatically. Atmospheric pressure then forces water up into the flask in a spectacular fountain.

For this experiment to work, the gas must be extremely soluble in water so that it dissolves almost instantly on contact, creating a near-vacuum inside the flask.

Hydrogen chloride (HCl) is the classic gas used. It is one of the most soluble gases in water: about 450 volumes of HCl dissolve in one volume of water at room temperature, forming hydrochloric acid. Ammonia (NH₃) is also commonly used for the same experiment, but it is not among the given options.

Hydrogen sulphide (H₂S) is only moderately soluble and is extremely toxic, making it unsuitable. Dinitrogen(I) oxide (N₂O, nitrous oxide) and nitrogen(II) oxide (NO, nitric oxide) are both poorly soluble in water and would not produce the dramatic pressure drop needed for the fountain effect.

Detalhes da Resposta

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.

Detalhes da Resposta

Solubility is defined as the maximum amount of a solute that can dissolve in a given quantity of solvent at a particular temperature to form a saturated solution. Its importance lies directly in the fact that it determines the amount of solute that can dissolve in a given amount of solvent.

This knowledge is practically essential in:

• Preparing solutions of known concentration in the laboratory
• Predicting whether a precipitate will form when solutions are mixed
• Understanding purification techniques such as recrystallisation
• Pharmaceutical formulation (ensuring a drug dissolves sufficiently for absorption)

The other options describe colligative properties - effects that arise after a solute has been dissolved:

• Lowering the vapour pressure and raising the boiling point are consequences of having dissolved particles in solution, not descriptions of what solubility itself tells us.
• Knowing chemical reactions of a gas with a liquid relates to chemical reactivity, not solubility as a concept.

Solubility is fundamentally about how much dissolves, and that is its primary importance.

Detalhes da Resposta

Litmus is an acid-base indicator. Red litmus paper turns blue only in the presence of a base (alkaline solution), which has a pH greater than 7.

Examining the given pH values:

• pH 2.5 - strongly acidic (no colour change; red litmus stays red)
• pH 3.5 - acidic (no colour change)
• pH 7.0 - neutral (no colour change; litmus remains the same colour)
• pH 8.0 - basic/alkaline (red litmus turns blue)

Only the solution with pH 8.0 is alkaline, so it is the only one that will turn red moist litmus paper blue. Acidic and neutral solutions cannot cause this change.

Detalhes da Resposta

Tetraoxosulphate(VI) acid is the IUPAC name for sulphuric acid, \(\text{H}_2\text{SO}_4\). Its large-scale industrial manufacture uses the Contact process.

The Contact process involves three main stages:

1. Sulphur is burned in air to produce sulphur(IV) oxide: \(\text{S} + \text{O}_2 \rightarrow \text{SO}_2\)
2. Sulphur(IV) oxide is oxidised to sulphur(VI) oxide using a vanadium(V) oxide (\(\text{V}_2\text{O}_5\)) catalyst at about 450 \(^\circ\text{C}\): \(2\text{SO}_2 + \text{O}_2 \rightleftharpoons 2\text{SO}_3\)
3. The \(\text{SO}_3\) is absorbed in concentrated \(\text{H}_2\text{SO}_4\) to form oleum (\(\text{H}_2\text{S}_2\text{O}_7\)), which is then carefully diluted with water to give concentrated sulphuric acid.

The other named processes serve different purposes. The Haber process manufactures ammonia from nitrogen and hydrogen. The Frasch process is used for mining sulphur deposits underground using superheated water. The Bosch process (or water-gas shift reaction) produces hydrogen from carbon monoxide and steam. None of these produces sulphuric acid.

Detalhes da Resposta

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.

Detalhes da Resposta

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.

Detalhes da Resposta

Ethyne (commonly known as acetylene, C₂H₂) is the organic gas used in the oxy-acetylene torch for welding and cutting metals. When ethyne burns in pure oxygen, it produces an extremely hot flame reaching temperatures above 3,000 °C - hot enough to melt steel and other metals.

The combustion reaction is:

\[2\text{C}_2\text{H}_2(g) + 5\text{O}_2(g) \rightarrow 4\text{CO}_2(g) + 2\text{H}_2\text{O}(g)\]

Ethyne produces such a high temperature because it is an unsaturated hydrocarbon with a carbon-carbon triple bond, which stores a large amount of energy. This makes it far superior to other hydrocarbons for metalwork.

Methane, propene, and butane can all burn, but their flames do not reach the extreme temperatures required to cut through metals. Ethyne's unique suitability for this industrial application is a frequently tested fact in organic chemistry.

Detalhes da Resposta

The structure shown is R-C(=O)-NH-H, which contains a carbonyl group (C=O) directly bonded to a nitrogen atom bearing hydrogen atoms. This is the defining arrangement of the amide functional group (-CONH₂).

An alkanamide (also called an amide) has the general formula R-CONH₂, where R is an alkyl group. The key feature distinguishing it from the other options is the simultaneous presence of both the C=O and the N-H bonds on the same carbon.

An alkylamine (R-NH₂) has nitrogen bonded to an alkyl group but no carbonyl. An alkanone (R-CO-R') has a carbonyl flanked by two carbon groups with no nitrogen. An amino acid would require both an amine group (-NH₂) and a carboxyl group (-COOH) on the same molecule, which is not the case here.

Detalhes da Resposta

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.

Detalhes da Resposta

According to Graham's law of diffusion, the rate of diffusion of a gas is inversely proportional to the square root of its molar mass:

\[\text{Rate} \propto \frac{1}{\sqrt{M}}\]

This means lighter gases diffuse faster and heavier gases diffuse slower. To arrange gases in order of increasing rate of diffusion, we arrange them from heaviest (slowest) to lightest (fastest).

Calculate the molar masses of the gases that appear in the options:

Arranging from heaviest to lightest (i.e., increasing rate of diffusion):

SO₂ (64) → O₂ (32) → NH₃ (17) → H₂ (2)

This matches the sequence SO₂, O₂, NH₃, H₂. The heaviest gas (SO₂) diffuses most slowly, and the lightest gas (H₂) diffuses most rapidly.

Detalhes da Resposta

Permalloy is an alloy composed of iron (Fe) and nickel (Ni), typically in a ratio of approximately 20% iron and 80% nickel, though the exact composition can vary.

Permalloy is valued for its exceptionally high magnetic permeability, meaning it is very easily magnetised even by weak magnetic fields. This property makes it useful in:

• Transformer cores
• Magnetic shielding
• Recording heads in data storage devices
• Sensitive magnetic instruments

The other metals listed form different alloys with iron:

• Iron + tin = tin plate (tin-coated iron, used for food cans)
• Iron + chromium (plus nickel and carbon) = stainless steel
• Iron + copper is not a standard named alloy

The defining feature of permalloy is that it is a nickel-iron alloy.

Detalhes da Resposta

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.

Detalhes da Resposta

To find the oxidation state of nitrogen in each compound, use the rule that oxygen has an oxidation state of \(-2\) and the sum of oxidation states in a neutral compound is zero.

For a compound of the form N\(_2\)O\(_x\):

\[2(\text{oxidation state of N}) + x(-2) = 0\]

\[\text{oxidation state of N} = \frac{2x}{2} = x\]

Wait - more precisely: \(\text{oxidation state of N} = \frac{2x}{2} = +x\). Let me calculate each:

• N\(_2\)O\(_5\): \(2n + 5(-2) = 0 \Rightarrow n = +5\)
• N\(_2\)O\(_3\): \(2n + 3(-2) = 0 \Rightarrow n = +3\) ✔
• N\(_2\)O\(_4\): \(2n + 4(-2) = 0 \Rightarrow n = +4\)
• N\(_2\)O: \(2n + 1(-2) = 0 \Rightarrow n = +1\)

The compound in which nitrogen has an oxidation state of +3 is N\(_2\)O\(_3\) (dinitrogen trioxide).

Detalhes da Resposta

Tristearin is a fat (triglyceride) formed from glycerol and three molecules of stearic acid. Alkaline hydrolysis of a triglyceride is the reaction of the fat with a strong alkali such as sodium hydroxide. This reaction is known as saponification and produces soap (the sodium salt of the fatty acid) and glycerol as a by-product:

\[\text{(C}_{17}\text{H}_{35}\text{COO)}_3\text{C}_3\text{H}_5 + 3\text{NaOH} \rightarrow 3\text{C}_{17}\text{H}_{35}\text{COONa} + \text{C}_3\text{H}_5\text{(OH)}_3\]

The by-product is glycerol, also known as propane-1,2,3-triol. Its structural formula shows three hydroxyl (-OH) groups, one on each of the three carbon atoms. An alcohol with three -OH groups is classified as a trihydric alkanol.

A dihydric alkanol has two -OH groups (e.g. ethane-1,2-diol). A secondary alkanol has the -OH group on a carbon bonded to two other carbon atoms. A tertiary alkanol has the -OH on a carbon bonded to three other carbons. Glycerol has two primary -OH groups and one secondary -OH group, but its defining classification is that it is trihydric, since it carries three hydroxyl groups in total.

Detalhes da Resposta

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.

Detalhes da Resposta

A metal is said to occur in the free state (or native state) in nature when it is found as the pure, uncombined element rather than in a compound (such as an ore). This happens only for metals that are very low in the reactivity series, meaning they are resistant to reaction with oxygen, water, and acids.

Silver (Ag) is one such metal. It is found as native silver in the earth's crust because of its very low chemical reactivity. Gold and platinum are other classic examples of metals found in the free state.

Zinc and iron are too reactive to exist as free metals in nature. They readily react with oxygen and moisture to form oxides and other compounds, so they are always found as ores (e.g., zinc as zinc blende ZnS, iron as haematite Fe₂O₃). While copper can occasionally be found native, silver is the stronger answer here because it is less reactive than copper and more commonly occurs in the uncombined state.

Detalhes da Resposta

The equation is:

\[2\text{X} + 2\text{HCl} \rightarrow 2\text{XCl} + \text{H}_2\]

The product formed is XCl, which tells us that element X combines with chlorine in a 1:1 ratio. This means X has a valency of +1 and forms a monovalent chloride.

Examining the options:

• K (potassium): Group 1 element with a valency of +1. It forms KCl. The equation becomes \(2\text{K} + 2\text{HCl} \rightarrow 2\text{KCl} + \text{H}_2\), which balances correctly.
• Mg (magnesium): Group 2 element with a valency of +2. It forms MgCl₂, not MgCl. Does not match.
• Ca (calcium): Group 2 element with a valency of +2. It forms CaCl₂, not CaCl. Does not match.
• Ba (barium): Group 2 element with a valency of +2. It forms BaCl₂, not BaCl. Does not match.

Only potassium (K) has a valency of +1 and forms a chloride with the formula XCl, making it the correct identity of X.

Detalhes da Resposta

The spontaneity of a reaction is determined by the Gibbs free energy change, given by:

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

A reaction is spontaneous when \(\Delta G\) is negative.

In this question:

• \(\Delta H\) is positive and small (slightly endothermic).
• \(\Delta S\) is positive and large (significant increase in disorder).

Substituting into the equation:

\[\Delta G = (\text{small positive}) - T \times (\text{large positive})\]

Since \(T\) (absolute temperature in Kelvin) is always positive, the term \(T\Delta S\) will be a large positive number. Subtracting this large positive value from a small positive \(\Delta H\) gives:

\[\Delta G = \text{small positive} - \text{large positive} = \text{negative}\]

A negative \(\Delta G\) means the reaction is spontaneous.

Exam tip: When \(\Delta H\) is positive but \(\Delta S\) is also positive and large, the entropy term dominates, and the reaction is spontaneous, especially at higher temperatures. This is called an entropy-driven reaction.

Detalhes da Resposta

Fermentation is the biochemical process in which enzymes (particularly zymase, found in yeast) convert glucose into ethanol and carbon dioxide. The overall equation is:

\[\text{C}_6\text{H}_{12}\text{O}_6 \xrightarrow{\text{zymase}} 2\text{C}_2\text{H}_5\text{OH} + 2\text{CO}_2\]

This is an anaerobic process, meaning it occurs without oxygen. The key word in the question is enzymatic, which points directly to fermentation, since it is the only process among the given options that is enzyme-catalysed.

Polymerization is the joining of small monomer molecules into a large polymer chain. Hydrogenation is the addition of hydrogen gas across unsaturated bonds, typically using a metal catalyst such as nickel. Saponification is the alkaline hydrolysis of fats or oils to produce soap and glycerol. None of these processes involves the enzymatic breakdown of glucose to ethanol.

Whenever a question mentions the biological or enzymatic conversion of sugars to alcohol, the answer is fermentation.

Detalhes da Resposta

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 \(\Delta S > 0\).