WAEC SSCE - Chemistry - 2015

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The atomic number of an element tells us the number of protons in its nucleus. For an electrically neutral atom, the number of electrons is equal to the number of protons. The electronic configuration of an atom with atomic number 13 is: 1s²2s²2p⁶3s²3p¹. The electronic configuration tells us that there are three occupied energy levels or orbits, namely the first, second, and third energy levels. The first energy level has a maximum of 2 electrons, the second energy level has a maximum of 8 electrons, while the third energy level can hold a maximum of 18 electrons. Therefore, an atom with atomic number 13 has 3 energy levels, which are occupied by 13 electrons. So the answer is 3.

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The correct statement about intermolecular distances and cohesive forces between gases is "both negligible." The intermolecular distances between gas particles are very large compared to their size, so the cohesive forces between them are negligible. As a result, gases tend to expand to fill any available space and have no definite shape or volume. The kinetic energy of gas particles is much greater than the attractive forces between them, and as a result, they move freely and randomly in all directions.

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Argon is the only monoatomic gas among the given options. Monoatomic gases are those that exist as individual atoms, not as molecules. Argon, with atomic number 18, exists as individual atoms and doesn't form molecules. Chlorine, nitrogen, and oxygen are all diatomic gases, meaning that they exist as molecules consisting of two atoms of the same element bonded together. Chlorine has the formula Cl₂, nitrogen has the formula N₂, and oxygen has the formula O₂.

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The Van der Waals' forces are dominant intermolecular forces in Chlorine. Van der Waals' forces are weak intermolecular forces that exist between molecules due to temporary changes in electron density. These forces arise due to the attraction between the positively charged nucleus of one molecule and the negatively charged electrons of another molecule. Chlorine is a non-polar molecule, which means it has no net dipole moment. Van der Waals' forces are the strongest intermolecular forces acting between non-polar molecules, and as Chlorine is non-polar, it is the dominant intermolecular force acting between Chlorine molecules.

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The empirical formula of the compound is CH₂O, which indicates that for every carbon atom there are two hydrogen atoms and one oxygen atom. The molecular mass of the compound is 90, which is the sum of the atomic masses of all the atoms in the molecule. Let's calculate the molecular formula of the compound using this information. The empirical formula mass of CH₂O is: (1 x 12.0) + (2 x 1.00) + (1 x 16.0) = 30.0 g/mol To calculate the molecular formula, we need to divide the molecular mass by the empirical formula mass: Molecular formula mass / Empirical formula mass = n 90.0 / 30.0 = 3 Therefore, the molecular formula of the compound is three times the empirical formula: C₃H₆O₃ So, the correct option is C3H6O3.

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The complete hydrogenation of benzene involves adding hydrogen gas (H2) to the benzene ring under the right conditions. This reaction is known as hydrogenation and it can be catalyzed by a metal catalyst, such as platinum or nickel. During the reaction, the double bonds in the benzene ring are broken and each carbon atom in the ring is bonded to two hydrogen atoms. The product of the reaction is cyclohexane, a six-membered ring hydrocarbon with single bonds between all the carbon atoms. Therefore, the correct answer is (b) Cyclohexane.

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Starch is a complex carbohydrate made up of many glucose units linked together. In order to convert it to glucose, the process of hydrolysis is required. Hydrolysis is a chemical reaction that involves breaking down a compound by adding water molecules. In the case of starch, enzymes can be used to break down the long chains of glucose molecules into individual glucose molecules by adding water. This process is used in the production of glucose syrup and other sweeteners, which are used in a variety of food products. The other options listed - condensation, dehydration, and fermentation - do not involve the breakdown of starch into glucose.

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The correct option is: They have the same number of valence electrons. Alkaline earth metals are the elements in Group 2 of the periodic table. They have similar chemical properties because they have the same number of valence electrons, which is two. The valence electrons of an element are the electrons in the outermost shell of the atom, and they determine the chemical properties of the element. Because alkaline earth metals have two valence electrons, they have similar reactivity and form similar types of chemical bonds. For example, they react with nonmetals to form ionic compounds with a 2+ charge. So, their similar chemical properties are due to the fact that they all have two valence electrons.

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The correct answer is (c) NaHSO₄. An acid salt is a salt that is derived from the partial neutralization of an acid, resulting in a substance that has both acidic and basic properties. NaHSO₄ is an example of an acid salt because it is derived from the partial neutralization of sulfuric acid (H₂SO₄) with sodium hydroxide (NaOH). It has acidic properties because it can donate a hydrogen ion (H+) in water, and it has basic properties because it can accept a hydrogen ion (H+) in the presence of a strong acid. The other options are not acid salts. CH₃COOONa is a salt of a weak acid (acetic acid) and a strong base (sodium hydroxide), and therefore it is a basic salt. Mg(OH)Cl is a salt of a strong base (magnesium hydroxide) and a strong acid (hydrochloric acid), and therefore it is a neutral salt. (NH₄)₂SO₄ is a salt of a weak base (ammonia) and a strong acid (sulfuric acid), and therefore it is an acidic salt.

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The balanced chemical equation is: C2H4(g) + 3O2(g) -> 2CO2(g) + 2H2O(g) From the equation, we can see that 1 mole of C2H4 produces 2 moles of CO2. Therefore, 0.05 moles of C2H4 will produce: 2 x 0.05 = 0.1 moles of CO2 Now we need to find the volume of 0.1 moles of CO2 at s.t.p. The molar volume of gas at s.t.p. is 22.4 dm3. So, the volume of 0.1 moles of CO2 at s.t.p. will be: 0.1 x 22.4 = 2.24 dm3 Therefore, the volume of CO2 produced at s.t.p when 0.05 moles of C2H4 was burnt in O2 is 2.24 dm3. Hence, the correct option is (b) 2.24 dm3.

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The substance responsible for the sour taste of unripe orange is an organic acid. Organic acids are a class of compounds that have at least one carboxyl group (-COOH) in their molecular structure. The most common organic acid found in oranges is citric acid. When an orange is unripe, it contains more citric acid, giving it a sour taste. As the fruit ripens, the citric acid content decreases, and the fruit becomes sweeter. Therefore, the correct option is alkanoic acid.

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Bakelite is a thermosetting polymer. Explanation: Thermosetting polymers are those that undergo a chemical reaction when heated and become irreversibly hard and rigid. Bakelite, which is also known as phenolic resin, is a thermosetting polymer that is synthesized by the reaction of phenol and formaldehyde. Once the reaction is complete, the resulting material is in a hard, infusible state that cannot be melted or reshaped by heating. On the other hand, nylon, polypropene, and polystyrene are thermoplastic polymers, which can be melted and reshaped upon heating.

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The correct answer is Sc³⁺. Transition metal ions are often colored because their electrons can absorb light energy and jump to higher energy levels. When the electrons drop back down to their original energy levels, they release the energy as visible light, which we see as color. The color of the complex depends on the arrangement of the ligands around the metal ion. Scandium (Sc) has only one electron in its outermost shell, which it loses to form the Sc³⁺ ion. Since it has an empty d-orbital, it cannot absorb light in the visible region, and therefore it is colorless in aqueous solution. On the other hand, Cr³⁺, Fe³⁺, and Mn³⁺ ions have partially filled d-orbitals, which allow them to absorb light in the visible region and give them characteristic colors in aqueous solution. Cr³⁺ is violet, Fe³⁺ is yellow, and Mn³⁺ is pink.

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The molar mass of oxygen gas (O₂) is 32 g/mol. Therefore, 16.0 g of oxygen gas will contain one-half the number of moles as its mass in grams. Number of moles = mass ÷ molar mass Number of moles of O₂ in 16.0 g = 16.0 g ÷ 32 g/mol = 0.5 mol Since one mole of any substance contains Avogadro's number of particles (atoms or molecules), the number of oxygen molecules in 0.5 mol of O₂ is: Number of molecules = Avogadro's number x Number of moles Number of molecules = 6.02 x 10²³ x 0.5 = 3.01 x 10²³ Therefore, the correct option is: 3.01 x 10²³.

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The shape of a molecule of water is non-linear. A water molecule consists of two hydrogen atoms and one oxygen atom. The oxygen atom is more electronegative than the hydrogen atoms, so it pulls the shared electrons towards itself, creating a partial negative charge on the oxygen and partial positive charges on the hydrogen atoms. This results in a bent or non-linear shape of the water molecule, where the oxygen atom is at the center, and the two hydrogen atoms are positioned at an angle of about 104.5 degrees with respect to each other.

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The correct statement is "Oxidation occurs at the anode in an electrochemical cell." In an electrochemical cell, there are two half-cells: the anode and the cathode. At the anode, oxidation takes place, meaning that electrons are lost by the species being oxidized. These electrons travel through an external circuit to the cathode, where reduction takes place, and the electrons are gained by the species being reduced. This flow of electrons through the external circuit generates an electric current. The salt bridge is used to maintain electrical neutrality by allowing the flow of ions between the half-cells. The aqueous solution is where the species being oxidized and reduced are dissolved.

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In the given reaction equation, CuO reacts with H₂ to form Cu and H₂O. Here, CuO is reduced to Cu and H₂ is oxidized to H₂O. So, the substance that is oxidized in the reaction is H₂.

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The shape of a graphite crystal is hexagonal. Graphite is a form of carbon that consists of layers of carbon atoms arranged in a hexagonal lattice. The layers are held together by weak intermolecular forces called Van der Waals forces. Each carbon atom is bonded to three neighboring carbon atoms in a trigonal planar arrangement, resulting in a flat hexagonal structure. This gives graphite its characteristic layered structure, which makes it useful as a lubricant and as the "lead" in pencils.

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The process that does not take place in domestic water treatment is neutralization. Domestic water treatment involves several processes to make water safe for human consumption. Chlorination is a process that involves adding chlorine to water to kill harmful microorganisms. Flocculation is the process of adding a chemical called a flocculant to water to help clump together small particles and impurities, making it easier to remove them through sedimentation. Sedimentation is a process in which the water is left to settle, allowing heavy particles and impurities to sink to the bottom. Neutralization, on the other hand, is a chemical process that involves the reaction of an acid with a base to form water and a salt. This process is not typically used in domestic water treatment, as the pH of the water is usually adjusted through other means such as adding lime or carbon dioxide.

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The correct answer is saponification. When concentrated sodium hydroxide is added to palm oil, the triglyceride esters present in palm oil react with the sodium hydroxide to form glycerol and sodium salts of fatty acids. This process is called saponification, and the resulting product is soap. Therefore, saponification is the correct answer.

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C₂H₆ and C₄H₁₀ belong to the same homologous series, which is the series of alkanes. The homologous series refers to a group of organic compounds that have similar structures and chemical properties, and the same functional group. They have a general formula, with the same difference in the number of carbon and hydrogen atoms between consecutive members. The alkanes are a homologous series of hydrocarbons with the general formula C_nH_2n+2. Therefore, C₂H₆ and C₄H₁₀ have different numbers of carbon atoms, but they both have only single bonds between the carbon atoms and hydrogen atoms.

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The gas given off when ethanol reacts with sodium is hydrogen. This is because sodium is a more reactive metal than hydrogen and it will displace the hydrogen in ethanol, forming sodium ethoxide and hydrogen gas. The reaction can be represented as follows: C2H5OH + Na → C2H5ONa + H2 So, during this reaction, hydrogen gas is evolved as a product.

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To determine the number of unpaired electrons in an atom of an element, we need to know its electron configuration. The electron configuration of an element describes how its electrons are distributed in energy levels or orbitals. The element Q has an atomic number of 8, which means it has 8 electrons in its neutral state. To determine its electron configuration, we can use the periodic table. Q is in the second period and the 6th group (also known as group 16 or the oxygen group), which means it has 6 valence electrons. The electron configuration for Q is: 1s² 2s² 2p⁴ To determine the number of unpaired electrons, we need to look at the electron configuration and identify how many electrons occupy each orbital. In the case of Q, there are two electrons in the 2s orbital and four electrons in the 2p orbitals. The 2p orbitals each have one electron with the same spin, which means they are unpaired. Therefore, Q has 2 unpaired electrons. The answer is (A) 2.

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The general gas equation, also known as the ideal gas law, was derived from Boyle's and Charles' laws. Boyle's law states that the pressure of a gas is inversely proportional to its volume, while Charles' law states that the volume of a gas is directly proportional to its temperature. The general gas equation combines these two laws along with Avogadro's law, which states that equal volumes of gases under the same conditions of temperature and pressure contain the same number of molecules. The equation is PV = nRT, where P is pressure, V is volume, n is the number of moles, R is the gas constant, and T is temperature in Kelvin.

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The rate of diffusion of gases is inversely proportional to their molar mass. Therefore, the gas with the lowest molar mass will diffuse the most rapidly. The molar mass of each gas is: - Cl₂: 2 x 35.5 = 71.0 g/mol - SO₂: 32.0 + 2 x 16.0 = 64.0 g/mol - CH₄: 12.0 + 4 x 1.0 = 16.0 g/mol - C₂H₆: 2 x 12.0 + 6 x 1.0 = 30.0 g/mol Therefore, methane (CH₄) has the lowest molar mass and will diffuse most rapidly among the given options.

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A substance that dissolves readily in organic solvent would be a covalent compound. This is because organic solvents are usually nonpolar, which means that they dissolve substances that are also nonpolar or have weak polar characteristics. Covalent compounds are formed by the sharing of electrons between atoms and do not have strong electrostatic forces of attraction between their constituent atoms. This weak intermolecular attraction allows the organic solvent to penetrate and dissolve the covalent compound. In contrast, ionic compounds have strong electrostatic forces of attraction between their ions, which makes them insoluble in organic solvents. Covalent compounds typically have low melting points and do not conduct electricity in their solid or molten states because they do not have free-moving charged particles.

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A secondary alkanol is an alcohol where the carbon atom attached to the hydroxyl (-OH) group is bonded to two other carbon atoms. In other words, it has two alkyl groups attached to the carbon atom with the -OH group. Out of the options given, 2-methylbutan-2-ol and 3-methylpentan-2-ol have the required structure to be classified as secondary alkanols. Therefore, the correct answer is either 2-methylbutan-2-ol or 3-methylpentan-2-ol.

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Protein and fat are both organic compounds that can undergo hydrolysis when boiled with an aqueous solution of sodium hydroxide. Polythene, on the other hand, is a synthetic polymer and is not susceptible to hydrolysis under such conditions. Therefore, the correct answer is (c) i and ii.

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The given reaction equation represents the reduction of sulfate ion (SO₄^2-) to hydrogen sulfide (H₂S) using two hydrogen ions (H⁺) and some number of electrons (e^-). In order to balance the charges on both sides of the equation, the number of electrons must be equal to the positive charges on the left side. On the left side, there are 2 hydrogen ions (2H⁺) with a charge of +2 and one sulfate ion (SO₄^2-) with a charge of -2. Thus, the total charge on the left side is 0. On the right side, there is one water molecule (H₂O) with a charge of 0 and one sulfate ion (SO₄^2-) with a charge of -2. Thus, the total charge on the right side is also -2. Since the charges on both sides must be equal, the number of electrons (y) involved in the reaction must be equal to the difference between the charges on both sides, which is 2. Therefore, the correct answer is y = 2.

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Solubility is practically applied in solvent extraction. Solubility refers to the ability of a substance to dissolve in a given solvent. Solvent extraction is a process that uses solubility to separate a specific substance from a mixture. In this process, a substance is dissolved in a solvent that selectively dissolves the desired compound, leaving behind unwanted impurities. This technique is widely used in the chemical, pharmaceutical, and mining industries for the extraction and purification of various compounds.

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The negatively charged particle in an atom is the electron. Electrons are subatomic particles that orbit around the nucleus of an atom in shells or energy levels. They have a negative charge, which is equal in magnitude to the positive charge of a proton. Electrons are responsible for chemical bonding and the formation of ions in chemical reactions.

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The bond formed when ammonia reacts with hydrogen ion to form ammonium ion is a dative bond. In this reaction, the lone pair of electrons on the nitrogen atom of the ammonia molecule is donated to the hydrogen ion (H+) to form the ammonium ion (NH4+). This type of bond is also called a coordinate covalent bond, as both electrons in the bond come from the same atom (in this case, the nitrogen atom of the ammonia molecule).

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The solubility of a compound in water is determined by its ability to dissociate into ions in the water. Compounds that dissociate completely into ions are more soluble in water than those that do not dissociate as much. Looking at the given compounds, CaCl₂ dissociates into three ions (Ca²⁺ and two Cl^-) in water and NaCl dissociates into two ions (Na⁺ and Cl^-). CaSO₄ dissociates into two ions (Ca²⁺ and SO₄^2-) but it is less soluble than CaCl₂ because the SO₄^2- ion is larger and more polar than the Cl^- ion, so it has a stronger attraction to the Ca²⁺ ion, making it more difficult to dissociate. Na₂SO₄ dissociates into three ions (two Na⁺ and SO₄^2-) but it is also less soluble than NaCl because of the larger, more polar SO₄^2- ion. Therefore, CaSO₄ is the least soluble compound in water because it has a weaker ability to dissociate into ions in water compared to the other given compounds.

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Identify each element from its atomic number and mass number.

(a)(i)

• I. Halogen: J (fluorine), a Group 7 element.
• II. Most likely attracted by a magnet: Y (chromium), a transition metal.
• III. Belongs to Group 1: X (potassium), configuration 2,8,8,1.
• IV. Readily forms an ion with a double negative charge: R (sulfur), which gains 2 electrons to form S^2-.

(a)(ii) J is a Group 7 non-metal and X is a Group 1 metal, so they form an ionic (electrovalent) bond (potassium transfers one electron to fluorine, giving K⁺ and F^-).

(a)(iii) Neutrons in Q:

(a)(iv) R (S^2-) combines with X (K⁺). Balancing charges gives K₂S.

(a)(v) The element that exists as a diatomic molecule is J (fluorine, F₂).

(a)(vi) The d-block element is Y (chromium).

(b)(i) An atomic orbital is a region in space around the nucleus of an atom where the probability of finding an electron is highest.

(b)(ii) I. Three postulates of Dalton's atomic theory:

1. All matter is made up of tiny, indivisible particles called atoms.
2. Atoms of the same element are identical in mass and chemical properties, while atoms of different elements differ.
3. Atoms combine in simple whole-number ratios to form compounds, and atoms are neither created nor destroyed in a chemical reaction.

II. Two limitations of the theory:

1. The atom is not indivisible; it is made up of smaller particles (protons, neutrons and electrons).
2. Atoms of the same element are not always identical in mass, because of the existence of isotopes.

(b)(iii) In the solid state, sodium chloride has a giant ionic lattice. Na⁺ and Cl^- ions are arranged alternately in a face-centred cubic structure, each Na⁺ surrounded by 6 Cl^- ions and each Cl^- surrounded by 6 Na⁺ ions (6:6 coordination), held together by strong electrostatic forces of attraction.

(c) Carbon burnt at 0.50 g per second for 30 minutes.

Total mass of carbon burnt:

Moles of carbon:

(i) Balanced equation:

(ii) I. From the equation, 1 mol C gives 1 mol CO₂, so 75 mol C gives 75 mol CO₂.

II. From the equation, 1 mol C uses 1 mol O₂, so:

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(a)(i) Two gases for the fountain experiment: ammonia (NH₃) and hydrogen chloride (HCl).
(ii) The property is their very high solubility in water.

(b)(i) I. Solubility is the maximum amount of a solute that dissolves in a given amount of solvent at a stated temperature to form a saturated solution.
II. A saturated solution is one that contains the maximum amount of solute it can dissolve at a given temperature, in contact with undissolved solute.

(ii) Two factors affecting solubility of a solid in a liquid: temperature; the nature of the solute and solvent.

(iii) Mass per dm³ \(= 10.2 \times \dfrac{1000}{15.4} = 662.3\ \text{g dm}^{-3}\).
Solubility \(= \dfrac{662.3}{331} = \mathbf{2.0\ mol\ dm^{-3}}\).

(c)(i) I. P = Fe²⁺ (iron(II)), Q = Fe³⁺ (iron(III)).
II. \(4Fe^{2+} + 4H^+ + O_2 \rightarrow 4Fe^{3+} + 2H_2O\)

(ii) Test for purity of water: pure water boils at exactly 100 °C (and freezes at 0 °C) at 1 atmosphere; impurities raise the boiling point.

(iii) I. Boiling temporary hard water removes the hardness (it decomposes the hydrogentrioxocarbonate(IV) to insoluble trioxocarbonate(IV), softening the water). II. Adding sodium trioxocarbonate(IV) crystals to permanent hard water softens it by precipitating the Ca²⁺/Mg²⁺ ions as insoluble trioxocarbonate(IV).

(iv) \(Ca(HCO_3)_2 \xrightarrow{heat} CaCO_3 + H_2O + CO_2\)

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(a)(i) Characteristics of a homologous series

• Members have the same general formula.
• Successive members differ by a \(\ce{-CH2-}\) unit.
• They have the same functional group and therefore similar chemical properties.
• They show a gradual change in physical properties, such as boiling point and density, as molecular mass increases.

(ii) Examples are alkanes and alkanols (alcohols).

(b) Compound A is ethanol, because a two-carbon alcohol reacts with ethanoic acid to form a sweet-smelling ester.

(i) The functional group in A is the hydroxyl group, \(\ce{-OH}\).

(ii) Structure of A, ethanol

(iii)

\[\ce{CH3COOH + CH3CH2OH <=>[conc. H2SO4][heat] CH3COOCH2CH3 + H2O}\]

(iv) B is ethyl ethanoate.

(c)(i)

\[\ce{HC#CH + 2Br2 -> CHBr2CHBr2}\]

(ii) The IUPAC name of the product is 1,1,2,2-tetrabromoethane.

(iii) Conditions for cracking

• High temperature, usually about \(500\text{ to }700\,^\circ\mathrm{C}\).
• Use of a suitable catalyst, such as silica-alumina or zeolite.

(d)(i) Preparation of ethanol from starch

1. Warm starch with malt containing the enzyme diastase. The starch is hydrolysed to maltose:
   \[\ce{(C6H10O5)_{n} + \tfrac{n}{2}H2O ->[diastase] \tfrac{n}{2}C12H22O11}\]
2. Add maltase to convert maltose to glucose:
   \[\ce{C12H22O11 + H2O ->[maltase] 2C6H12O6}\]
3. Add yeast and keep the mixture warm, about \(30\,^{\circ}\mathrm{C}\), under anaerobic conditions. Zymase in the yeast ferments glucose to ethanol and carbon(IV) oxide:
   \[\ce{C6H12O6 ->[zymase] 2C2H5OH + 2CO2}\]
4. Distil the fermented liquid to obtain ethanol.

(ii) Two properties of starch

• It is a white, tasteless solid and is insoluble in cold water.
• When warmed with water, it forms a colloidal paste or solution.

(iii) Starch does not reduce Fehling's solution because it has no free reducing carbonyl group.

(iv) Add iodine solution to the sample. A blue-black colour confirms the presence of starch.

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(a) Esterification is the reaction between a carboxylic (alkanoic) acid and an alcohol (alkanol), in the presence of a little concentrated H₂SO₄, to form an ester (alkanoate) and water.

(b) Two properties of plastic: it is light (low density) and a poor conductor of heat and electricity (also resistant to corrosion, mouldable, non-biodegradable).

(c) Duralumin contains aluminium, copper, magnesium and manganese.

(d)(i) A raw material is a naturally occurring substance from which useful products are manufactured.
(ii) A primary product is the product obtained directly (first) from processing a raw material.

(e) Charles' law: the volume of a fixed mass of gas is directly proportional to its absolute (Kelvin) temperature at constant pressure. \(\dfrac{V}{T} = \text{constant}\).

(f) Four protective items: safety goggles; laboratory coat (overall); hand gloves; nose/face mask (also fume cupboard, safety boots).

(g) Two uses of ammonia: manufacture of fertilizers; manufacture of trioxonitrate(V) acid (also as a refrigerant).

(h)(i) The process is cracking.
(ii) Ethanol + sodium gives sodium ethoxide and hydrogen gas. \(2C_2H_5OH + 2Na \rightarrow 2C_2H_5ONa + H_2\)

(i) Sulphur in H₂SO₄: \(2(+1) + S + 4(-2) = 0 \Rightarrow S = +6\).

(j)(i) NaClO₃: sodium trioxochlorate(V).
(ii) CuSO₄·5H₂O: copper(II) tetraoxosulphate(VI) pentahydrate.