WAEC SSCE - Chemistry - 2014

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The correct relationship that expresses Boyle's law is "P \(\alpha\) \(\frac{1}{V}\) at constant T." This means that at a constant temperature, the pressure of a gas is inversely proportional to its volume. In other words, as the volume of a gas decreases, its pressure will increase, and vice versa, as long as the temperature remains constant. This relationship is often written as PV = constant, where P is the pressure, V is the volume, and the product of the two is constant as long as the temperature remains constant.

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The two elements ₁₁X and ₁₉Y are in the same group because they have the same number of valence electrons. The elements in the same group of the periodic table have similar chemical and physical properties, and this is largely due to the fact that they have the same number of valence electrons in their outermost energy level. Valence electrons are the electrons in the outermost energy level of an atom that are involved in chemical bonding with other atoms. Elements in the same group have the same number of valence electrons, which gives them similar reactivity and bonding patterns. In this case, both ₁₁X and ₁₉Y have the same number of valence electrons, which means that they belong to the same group of the periodic table.

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The salt that would give an alkaline solution upon hydrolysis is CH₃COONa. When CH₃COONa is hydrolyzed, it reacts with water to form acetic acid (CH₃COOH) and sodium hydroxide (NaOH). Sodium hydroxide is a strong base and will dissociate in water to produce hydroxide ions (OH^-), which will increase the pH of the solution and make it alkaline. Acetic acid is a weak acid and will only slightly lower the pH of the solution. On the other hand, NH₄CI, AICI₃, and NH₄NO₃ are all salts that would give an acidic solution upon hydrolysis. When NH₄CI is hydrolyzed, it forms ammonium hydroxide (NH₄OH) and hydrochloric acid (HCI), which is an acidic solution. AICI₃ and NH₄NO₃ would also give acidic solutions upon hydrolysis.

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The production of biogas from the fermentation of waste materials is an example of recycling. This is because biogas is produced by the breakdown of organic waste materials, such as food scraps and animal manure, in the absence of oxygen. The waste materials are recycled and converted into a useful energy source, biogas, which can be used for heating, electricity generation, or as a fuel for vehicles. Recycling is the process of converting waste materials into new products or energy sources, rather than disposing of them as garbage. In the case of biogas production, organic waste materials that would otherwise be discarded are recycled into a valuable resource.

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The most common process in the extraction of metals from their ore is **reduction**. In this process, the metal oxide in the ore is reduced by carbon or another reducing agent to obtain the metal. Reduction involves the removal of oxygen or addition of electrons to the metal ion, resulting in the formation of the metal. This process is used for extracting metals such as iron, copper, and zinc from their ores. Electrolysis is also used in the extraction of certain metals, but it is not as common as reduction.

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The process used in food preservation in an industry is the irradiation of gamma rays. Gamma irradiation is a process that exposes food to a controlled amount of ionizing radiation to eliminate bacteria, fungi, and other microorganisms that cause food spoilage and foodborne illness. The gamma rays penetrate the food, disrupting the genetic material of microorganisms and rendering them unable to reproduce. This process does not make the food radioactive and does not significantly change the taste, texture, or nutritional value of the food. Therefore, the correct option is "Irradiation of gamma rays".

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The compound that determines the octane rating of petrol is 2,2,4-trimethylpentane, also known as iso-octane. Octane rating is a measure of a fuel's ability to resist "knocking" or detonation that can occur inside an engine, causing damage and reducing efficiency. Iso-octane has a high resistance to knocking, which makes it an ideal reference standard for determining the octane rating of petrol. A fuel with a higher percentage of iso-octane will have a higher octane rating and will be less likely to cause knocking in an engine.

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The equilibrium constant (K) is a value that indicates the extent of a chemical reaction at equilibrium. It is determined by the concentrations of the reactants and products at equilibrium, and is unaffected by any external factors such as the initial concentrations or the rate of the reaction. However, the equilibrium constant is affected by changes in temperature. When the temperature of a system at equilibrium is changed, the equilibrium constant will also change. This is because the forward and reverse reactions of a chemical reaction are affected differently by changes in temperature, which alters the equilibrium position. Similarly, changes in pressure or volume of a system at equilibrium can also affect the equilibrium constant. For example, an increase in pressure will cause the equilibrium to shift towards the side with fewer moles of gas, while a decrease in volume will have the same effect. These changes in the equilibrium position will alter the concentrations of the reactants and products, and hence change the equilibrium constant. Overall, concentration alone cannot affect the equilibrium constant as it is determined by the concentrations of the reactants and products at equilibrium, and is not dependent on the initial concentrations. However, changes in temperature, pressure, or volume can affect the equilibrium constant by altering the equilibrium position of the chemical reaction.

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When a strong acid and a strong base are mixed, they undergo a chemical reaction called neutralization. During this reaction, the hydrogen ions (H+) from the acid combine with the hydroxide ions (OH-) from the base to form water (H2O). The mixing of a strong acid to a strong base results in an exothermic reaction, which means that heat is released. This is because the breaking of the strong acid and base bonds releases energy, and the formation of the new water bonds also releases energy. The heat released can be felt if you touch the container in which the acid and base are mixed. There is no electrical, light, or sound energy released during the mixing of a strong acid to a strong base. Therefore, the correct answer is heat.

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The shape of CH₄ is tetrahedral. This is because the carbon atom is surrounded by four hydrogen atoms, and the molecule has a symmetrical arrangement, with the hydrogen atoms forming the corners of a tetrahedron around the carbon atom. The bond angles between the hydrogen atoms in the CH₄ molecule are 109.5 degrees, which is characteristic of a tetrahedral shape.

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The vapour pressure of a given volume of liquid increases when the temperature of the liquid rises. Vapor pressure is the pressure exerted by the gas molecules that are in equilibrium with the liquid phase of a substance. In simple terms, it is the pressure created by the gas molecules that escape from the surface of a liquid. The amount of vapor pressure that a liquid can exert is directly related to its temperature. As the temperature of a liquid increases, the average kinetic energy of the molecules in the liquid also increases. This increase in kinetic energy allows more molecules to escape from the surface of the liquid, leading to an increase in the number of gas molecules in the vapor phase. Consequently, the vapor pressure of the liquid increases. On the other hand, when the temperature of the liquid falls, the average kinetic energy of the molecules in the liquid decreases. As a result, fewer molecules escape from the surface of the liquid, leading to a decrease in the number of gas molecules in the vapor phase. This results in a decrease in the vapor pressure of the liquid. Therefore, the correct option is "temperature of the liquid rises" since it results in an increase in the vapor pressure of the liquid. The other options are incorrect because the volume of the liquid and its saturation with a salt have no significant effect on the vapor pressure of the liquid, while a decrease in temperature would lead to a decrease in the vapor pressure of the liquid.

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The type of reaction illustrated by the given equation is "cracking". In cracking, a large hydrocarbon molecule is broken down into smaller molecules by heating it at high temperatures or using a catalyst. In this reaction, C₁₂H₂₆ (a large hydrocarbon molecule) is broken down into smaller molecules of C ₂H₄ and C ₂H₆ by heating. Therefore, the correct answer is "Cracking".

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The liquid hydrocarbon likely to be found in the fraction of crude oil used for domestic cooking is C₅H₁₂, which is also known as Pentane. Crude oil is a complex mixture of hydrocarbons of varying lengths and structures. These hydrocarbons are separated into different fractions based on their boiling points, using a process called fractional distillation. The fraction of crude oil used for domestic cooking is commonly known as cooking gas or liquefied petroleum gas (LPG). Pentane is a colorless and odorless liquid hydrocarbon that is commonly found in the LPG fraction of crude oil. It has a boiling point of approximately 36°C and is highly volatile, which makes it suitable for use as a fuel for domestic cooking. Pentane is often mixed with other hydrocarbons such as butane and propane to create a more stable and efficient fuel for cooking. In summary, the liquid hydrocarbon likely to be found in the fraction of crude oil used for domestic cooking is C₅H₁₂, which is pentane.

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The atomic number of an element is the number of protons in the nucleus of an atom of that element. Since the atom W has 17 electrons, it must also have 17 protons to balance the electrical charge. Therefore, its atomic number is 17. The atomic mass number of an atom is the sum of the number of protons and neutrons in its nucleus. We are told that atom W has 18 neutrons, so its atomic mass number is 17 + 18 = 35. Therefore, the correct representation of the atom W is ³⁵₁₇W, where the superscript indicates the atomic mass number and the subscript indicates the atomic number.

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Ethene is a molecule that consists of two carbon atoms and four hydrogen atoms. The carbon atoms in ethene each have one unpaired electron, which they can use to form a covalent bond with another carbon atom. When many ethene molecules are added together, the carbon atoms can bond with each other to form a long chain of carbon atoms, with hydrogen atoms attached to each carbon atom. This chain of repeating units is called a polymer. The process of adding ethene molecules to each other to form a polymer is called polymerization. In this process, the double bond between the carbon atoms in ethene is broken, and the carbon atoms can bond with each other and with other molecules to form a long chain. Therefore, the correct answer is polymer, as ethene molecules can be added to each other to form a long chain polymer.

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The balanced chemical equation for the reaction between Na₂CO_3(aq) and HCl_(aq) is: Na₂CO_3(aq) + 2HCl_(aq) → 2NaCl_(aq) + H₂O_(l) + CO_2(g) From the equation, we can see that 1 mole of Na₂CO_3(aq) reacts with 2 moles of HCl_(aq) to produce 2 moles of NaCl_(aq), 1 mole of H₂O_(l), and 1 mole of CO_2(g). Therefore, the number of moles of HCl_(aq) that reacted can be calculated as: number of moles = concentration x volume number of moles = 0.10 mol dm^-3 x 0.040 dm³ number of moles = 0.004 mol From the balanced equation, we know that 1 mole of Na₂CO_3(aq) reacts with 2 moles of HCl_(aq). Therefore, the number of moles of Na₂CO_3(aq) required to completely neutralize the HCl_(aq) can be calculated as: number of moles = 0.5 x number of moles of HCl_(aq) number of moles = 0.5 x 0.004 number of moles = 0.002 Finally, we can use the concentration and number of moles to calculate the required volume of Na₂CO_3(aq) as: volume = number of moles / concentration volume = 0.002 mol / 0.02 mol dm^-3 volume = 0.1 dm³ = 100 cm³ Therefore, the correct answer is 100 cm³.

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Zinc oxide (ZnO) is called amphoteric because it can react as both an acid and a base. When it reacts with a strong acid, it behaves like a base and neutralizes the acid to form a salt and water. On the other hand, when it reacts with a strong base, it behaves like an acid and neutralizes the base to form a salt and water. This property of zinc oxide makes it useful in a variety of industrial and chemical processes.

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The enzyme that catalyzes the conversion of glucose to ethanol and carbon dioxide is called zymase. Zymase is a complex of enzymes that are found in yeast cells and are involved in the fermentation of glucose into ethanol and carbon dioxide. The process of fermentation involves the breakdown of glucose into pyruvate, which is then converted into ethanol and carbon dioxide by the action of zymase. During the fermentation process, zymase catalyzes two important reactions: the conversion of glucose to pyruvate by the action of the enzyme hexokinase, and the conversion of pyruvate to ethanol and carbon dioxide by the action of the enzymes pyruvate decarboxylase and alcohol dehydrogenase. Therefore, the correct option is "zymase" because it is the enzyme complex that is responsible for the conversion of glucose to ethanol and carbon dioxide. The other options are incorrect because diastase and maltase are enzymes involved in the breakdown of carbohydrates into simpler sugars, while ptyalin is an enzyme found in saliva that helps to break down starch into maltose.

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The correct statement about a chemical system in equilibrium is that "equilibrium is re-established when a stress is applied." Chemical equilibrium is a state where the forward and backward reactions occur at equal rates, resulting in no net change in the concentration of reactants or products. It is a dynamic state where the system appears to be static because the rates of forward and backward reactions are equal. However, a system in equilibrium is not unchangeable and can be disturbed by changes in concentration, pressure, or temperature. Such changes are referred to as "stresses." When a stress is applied to an equilibrium system, the system responds by shifting the equilibrium position in a direction that reduces the stress. This means that the concentration of the reactants and products will change until a new equilibrium is established. Therefore, the only correct option is that "equilibrium is re-established when a stress is applied." The other options are incorrect because a system in equilibrium is reversible, and the reaction may not go to completion. Additionally, the equilibrium position is not affected by a catalyst because it only speeds up the rate of the reaction without affecting the position of equilibrium.

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In Fe₂(SO₄)₃, there are two iron (Fe) atoms and three sulfate (SO₄) ions. Each sulfate ion has a charge of -2, and the total negative charge of the three sulfate ions is -6. Since the compound as a whole is neutral, the total positive charge must be +6. To find the oxidation number of sulfur (S) in sulfate (SO₄) ion, we need to apply the following rule: the sum of the oxidation numbers of all atoms in a compound must be equal to the overall charge of the compound. In sulfate ion, oxygen (O) always has an oxidation number of -2, so the total oxidation number contributed by four oxygen atoms in SO₄ is -8. Therefore, the oxidation number of S in SO₄ can be calculated as follows: (-2 × 4) + (oxidation number of S) = -2 Simplifying the above equation, we get: -8 + (oxidation number of S) = -2 (oxidation number of S) = +6 So, the oxidation number of sulfur in Fe₂(SO₄)₃ is +6. Therefore, the correct option is +6.

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An aqueous solution of Na2CO3 (sodium carbonate) is alkaline. When sodium carbonate is dissolved in water, it dissociates into sodium ions (Na+) and carbonate ions (CO32-). The carbonate ions react with water molecules to form bicarbonate ions (HCO3-) and hydroxide ions (OH-). The presence of hydroxide ions makes the solution alkaline, with a pH greater than 7. Therefore, an aqueous solution of Na2CO3 is an alkaline solution.

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Hydrogen bonds are a type of intermolecular force that forms between molecules containing a hydrogen atom bonded to a strongly electronegative atom such as oxygen, nitrogen or fluorine. These atoms have a partial negative charge due to their high electronegativity, which attracts the partially positive hydrogen atom of another molecule, forming a weak bond between them. This type of bonding is important in many biological processes, such as the structure of DNA and the folding of proteins, and also plays a role in the physical properties of substances such as water. Hydrogen bonds do not form with non-polar species, diatomic elements, or complex ions because they lack the necessary partial charges for the interaction to occur.

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The purity of a solid sample can best be determined by its melting point. Melting point is defined as the temperature at which a solid substance melts to become a liquid. Pure substances have a fixed melting point, which means that they melt at a specific temperature under a given pressure. Impurities in a solid sample can lower the melting point and cause it to melt over a range of temperatures instead of at a fixed temperature. Therefore, by measuring the melting point of a solid sample, we can determine its purity. If the sample has a narrow melting point range, it is likely to be pure. If the sample has a wide melting point range or melts over a range of temperatures, it is likely to be impure. In summary, the purity of a solid sample can be best determined by its melting point because pure substances have a fixed melting point, whereas impurities lower the melting point and cause the sample to melt over a range of temperatures.

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The crystal layers in graphite are held together by van der Waal's forces. These forces are relatively weak and are caused by the attraction between temporary dipoles in adjacent atoms or molecules. In graphite, each layer is made up of carbon atoms arranged in a hexagonal lattice, with each carbon atom bonded to three neighboring carbon atoms through strong covalent bonds. However, the layers themselves are held together by van der Waal's forces, which allows them to slide past each other easily. This is why graphite is a soft, slippery material, and is commonly used as a lubricant.

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Milk of magnesia with a pH value of 10.5 is an alkali. Alkalis are substances that have pH values greater than 7.0. The higher the pH value, the more alkaline the substance is. In this case, the substance with the highest pH value among the given options is Milk of magnesia, with a pH of 10.5. Therefore, it is the alkali among the given substances.

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Volatile liquids are substances that have a tendency to evaporate easily at room temperature. This is because the molecules of volatile liquids have weak intermolecular forces, which allow them to escape easily from the surface of the liquid and form a gas. The vapour pressure of a liquid is a measure of the tendency of its molecules to escape into the gas phase. Volatile liquids have a high vapour pressure because a large number of their molecules can escape from the surface of the liquid and enter the gas phase, even at relatively low temperatures. The boiling point of a liquid is the temperature at which its vapour pressure becomes equal to the external pressure on the liquid. At this temperature, the molecules of the liquid have enough energy to overcome the intermolecular forces holding them together and escape into the gas phase. Therefore, the correct statement about volatile liquids is that they have a high vapour pressure and a low boiling point. This is because they can easily form a gas at room temperature due to their weak intermolecular forces, and their vapour pressure is high because a large number of their molecules can escape from the surface of the liquid. In addition, their low boiling point indicates that they can boil at a relatively low temperature, as their vapour pressure is already high at lower temperatures. The other options are not correct because they describe properties that are not characteristic of volatile liquids. Volatile liquids do not have a low vapour pressure or a high boiling point, as they can easily form a gas at low temperatures and boil at relatively low temperatures.

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Argon is used to retard the vaporization of the filament in a light bulb. When a light bulb is turned on, the filament heats up and vaporizes, leading to a loss of material and eventual failure of the bulb. However, if the bulb is filled with an inert gas like argon, the gas helps to slow down the rate of vaporization by reducing the rate of evaporation of the filament. This prolongs the life of the filament and the bulb. Helium and hydrogen are not commonly used because they are too light and can escape through small leaks in the bulb. Oxygen is not used because it can react with the hot filament, leading to oxidation and failure of the bulb.

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To calculate the volume of NaOH (aq) used, we need to use the formula: moles of solute = mass of solute / molar mass of solute First, we need to find the number of moles of NaOH used to produce the 5.0 g of solid NaOH. Molar mass of NaOH = 23.0 + 16.0 + 1.0 = 40.0 g/mol Number of moles of NaOH = mass of NaOH / molar mass of NaOH = 5.0 g / 40.0 g/mol = 0.125 mol Now, we can use the formula for concentration to find the volume of NaOH (aq) used. Concentration of NaOH (aq) = 0.20 mol/dm³ Number of moles of NaOH (aq) = concentration × volume 0.125 mol = 0.20 mol/dm³ × volume Rearranging the equation to isolate the volume: volume = number of moles / concentration volume = 0.125 mol / 0.20 mol/dm³ volume = 0.625 dm³ Converting dm³ to cm³ by multiplying by 1000: volume = 0.625 dm³ × 1000 cm³/dm³ volume = 625 cm³ Therefore, the volume of NaOH (aq) used is 625 cm³. The correct option is (B).

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(a)(i) Purification of water for town supply

1. Screening to remove large floating debris.
2. Sedimentation/coagulation: a coagulant such as alum is added to make fine suspended particles clump and settle.
3. Filtration through beds of sand and gravel to remove remaining suspended matter.
4. Chlorination (sterilization) to kill harmful micro-organisms.
5. Storage in covered reservoirs and distribution.

(ii) Cations causing hardness: Ca²⁺ and Mg²⁺.

(iii) Disadvantages of hard water: it wastes soap (forms scum); it deposits scale/fur in kettles, boilers and pipes.

(iv) Sources of water pollution: industrial effluents; sewage/domestic waste (also agricultural run-off, oil spillage).

(b)(i) Water is a polar solvent. Sodium chloride is ionic, so its Na⁺ and Cl^- ions are strongly attracted to and become surrounded (solvated) by the polar water molecules, and it dissolves. Oil is non-polar (covalent) and has no charged centres for water to attract, so water does not dissolve it (like dissolves like).

(ii) Function of H₂SO₄

• C₂H₅OH → C₂H₄: dehydrating agent (removes water).
• MgO + H₂SO₄ → MgSO₄ + H₂O: acid (undergoes neutralization).
• C + conc. H₂SO₄ → CO + SO₂ + H₂O: oxidizing agent.

(c)(i) When chlorine water stands in sunlight the hypochlorous acid decomposes and the two products are hydrochloric acid (HCl) and oxygen (O₂):

2Cl₂ + 2H₂O → 4HCl + O₂

(ii) Neither of these two products (an acid and oxygen) gives an alkaline solution, so neither is suitable for preparing a standard alkaline solution; both leave an acidic/neutral solution. A primary standard alkali (e.g. anhydrous Na₂CO₃) would instead be needed, which is not among these products.

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(a)(i) An ionic (electrovalent) bond is the electrostatic force of attraction between oppositely charged ions formed by the complete transfer of one or more electrons from a metal atom to a non-metal atom.

(ii) I. Magnesium oxide (MgO): ionic (electrovalent) bond. II. Ammonium ion (NH₄⁺): covalent bonds, one of which is a coordinate (dative) covalent bond from N to the extra H⁺.

(b) Oxidation number of sulphur in sodium thiosulphate, Na₂S₂O₃

(Average oxidation number of S = +2. If the formula is read strictly as Na₂S₂O₂, the same method gives S = +1.)

(c) Faraday's first law: The mass of a substance liberated or deposited at an electrode during electrolysis is directly proportional to the quantity of electricity (charge) passed through the electrolyte.

(d)(i) Acid salt: sodium hydrogentrioxocarbonate(IV), NaHCO₃ (or NaHSO₄). (ii) Basic salt: magnesium hydroxychloride, Mg(OH)Cl (or basic lead carbonate).

(e)(i) I_2(s) → I_2(g): sublimation, energy is absorbed (endothermic, enthalpy of sublimation). (ii) Cl_(g) + e^- → Cl^-_(g): energy is released (exothermic, electron affinity/electron gain enthalpy).

(f)(i) Na₂SO_4(aq): neutral salt, no effect on litmus. (ii) AlCl_3(aq): acidic (salt hydrolyses), turns blue litmus red.

(g) Efflorescence is the process by which a hydrated crystalline substance loses its water of crystallization to the atmosphere on exposure to air, becoming powdery.

(h) Two uses of activated charcoal: as an adsorbent to decolourize/purify (e.g. sugar solutions, water); in gas masks to adsorb poisonous gases.

(i) (i) Hydrogenation of vegetable oil: manufacture of margarine (solid cooking fats). (ii) Cracking: production of petrol and of alkenes for making plastics. (iii) Esterification: manufacture of esters used as flavourings, perfumes and solvents.

(j) Silver deposited (Ag⁺ + e^- → Ag, so 1 mol needs 96500 C):

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(a)(i) A functional group is an atom, a bond, or a group of atoms in an organic compound which determines its characteristic chemical properties.

(a)(ii) The functional groups present are:

• hydroxyl (alkanol) group, −OH;
• alkanoate (ester) group, −COO−.

(b)(i) Compound Q is cyclopropane.

(b)(ii) The balanced equation for its complete combustion is:

\[2C_3H_6 + 9O_2 \rightarrow 6CO_2 + 6H_2O\]

(b)(iii) Cyclopropane undergoes a substitution reaction with chlorine.

(b)(iv) The alkene isomer of cyclopropane is propene, with structural formula:

(c)(i)

Sample	Family of organic compound
A	Alkene
B	Alkyne
C	Alkanol
D	Alkanoic acid

(c)(ii) Samples A and B decolourize bromine in tetrachloromethane because they are unsaturated compounds. They contain carbon-to-carbon multiple bonds which add bromine.

(c)(iii) Sample B is a terminal alkyne. Its terminal hydrogen is replaced by silver ions in ammoniacal silver trioxonitrate(V) solution to form an insoluble white silver alkynide precipitate.

(c)(iv) The reaction between C and D is esterification.

(c)(v) Concentrated tetraoxosulphate(VI) acid acts as:

1. a catalyst; and
2. a dehydrating agent, removing water formed and favouring ester formation.

(d)(i) Organic waste such as animal dung and plant materials is mixed with water and fed into an airtight biodigester. In the absence of air, anaerobic bacteria decompose the waste and produce biogas, mainly methane with carbon(IV) oxide. The gas collects in the dome of the digester and is taken out through a gas outlet. The residual slurry can be used as manure.

(d)(ii) Uses of biogas include:

1. as a fuel for cooking or heating;
2. for lighting or generating electricity.

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(a)(i) An ion is an atom or group of atoms that carries a net electrical charge because it has lost or gained one or more electrons. (ii) Isotopes are atoms of the same element having the same atomic number (number of protons) but different mass numbers (different numbers of neutrons).

(b)(i) I. Electron configurations

• ₁₂Mg: 1s² 2s² 2p⁶ 3s² (2, 8, 2)
• ₁₃Al: 1s² 2s² 2p⁶ 3s² 3p¹ (2, 8, 3)

II. The first ionization energy of Mg is greater than that of Al because in Mg the outermost electron is removed from a stable, completely filled 3s² sub-shell, whereas in Al the outermost electron is removed from the higher-energy 3p sub-shell, which is partly shielded by the 3s electrons and so is more easily removed. Hence less energy is needed for Al.

(ii) Three oxides of period 3 elements that react with water: Na₂O, SO₃, P₄O₁₀.

(c)(i) Allotropes are different structural forms of the same element existing in the same physical state. (ii) Diamond and graphite. (iii) Diamond: used in cutting/drilling tools and as an abrasive; graphite: used as a lubricant and as electrodes.

(d) Warming NaCl with concentrated H₂SO₄ evolves hydrogen chloride gas.

(i) Two physical properties of HCl: colourless gas; pungent (choking) smell; very soluble in water; fumes in moist air. (ii)

NaCl_(s) + H₂SO_4(l) → NaHSO_4(s) + HCl_(g)

(e) Amount of W: (5.68 mg = 0.00568 g)

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(a)(i) I. Anode: the electrode at which oxidation (loss of electrons) occurs; in electrolysis it is the positive electrode, where anions are discharged. II. Cathode: the electrode at which reduction (gain of electrons) occurs; in electrolysis it is the negative electrode, where cations are discharged.

(ii) Extraction equations

I. Sodium (molten NaCl):
Cathode: Na⁺ + e^- → Na
Anode: 2Cl^- → Cl₂ + 2e^-

II. Aluminium (molten Al₂O₃):
Cathode: Al³⁺ + 3e^- → Al
Anode: 2O^2- → O₂ + 4e^-

(iii) In the extraction of aluminium the gas discharged at the anode is oxygen, which is harmless. In the extraction of sodium the gas discharged at the anode is chlorine, which is poisonous and pollutes the environment. Hence the sodium process releases a toxic gas whereas the aluminium process does not.

(b)(i) The reaction is redox because chromium is reduced (its oxidation number falls from +6 in Cr₂O₇^2- to +3 in CrCl₃) while chlorine is oxidized (from -1 in HCl to 0 in Cl₂); oxidation and reduction occur together.

(ii) Half equations
Reduction: Cr₂O₇^2- + 14H⁺ + 6e^- → 2Cr³⁺ + 7H₂O
Oxidation: 2Cl^- → Cl₂ + 2e^-

(iii) Overall:
K₂Cr₂O₇ + 14HCl → 2KCl + 2CrCl₃ + 7H₂O + 3Cl₂

(c) Time = 1 h 30 min = 5400 s, current = 6 A, Al³⁺ + 3e^- → Al.