WAEC SSCE - Chemistry - 2013

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The separation of petroleum fractions depends on the differences in their boiling points. Petroleum is a mixture of hydrocarbons with different boiling points, ranging from gases to heavy liquids. In order to separate these different components, petroleum is heated to vaporize the liquid, and then the vapor is cooled to condense the different fractions. The fractions are then collected at different points in the cooling process, based on their boiling points. This process of separation is known as fractional distillation, and it is the most common method for separating petroleum fractions. The boiling point of each fraction depends on the length of the hydrocarbon chain and the degree of branching or saturation, which determine the strength of the intermolecular forces and the thermal stability of the molecules. Therefore, the boiling points of the fractions increase with increasing molecular weight and decreasing volatility.

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The variable that is a measure of the average kinetic energy of the molecules of a gas is temperature. Temperature is a measure of the average kinetic energy of the particles in a substance, including gas molecules. The higher the temperature, the higher the kinetic energy of the gas molecules, and the faster they move. Therefore, temperature is a crucial variable to consider when dealing with gases, as it affects their behavior, such as their pressure and volume.

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The instrument used in detecting the presence of radiation is the Geiger-Muller counter. Radiation refers to the emission of energy in the form of particles or waves. This can include gamma rays, beta particles, and alpha particles, among others. The Geiger-Muller counter is a device that is used to detect and measure ionizing radiation, such as alpha and beta particles, gamma rays, and X-rays. The Geiger-Muller counter works by using a gas-filled tube that contains a wire electrode. When ionizing radiation enters the tube, it ionizes the gas atoms, which then cause a cascade of electrons to move toward the wire electrode. This generates a pulse of electrical current that is detected and counted by the device. The number of pulses detected per unit time is proportional to the intensity of the radiation present. Out of the given options, the only instrument used in detecting the presence of radiation is the Geiger-Muller counter. - Cathode ray tube: This instrument is used to generate and manipulate a focused beam of electrons in a vacuum. It is commonly used in televisions and computer monitors. - Mass spectrometer: This instrument is used to determine the mass of individual atoms and molecules in a sample by ionizing them and separating them based on their mass-to-charge ratio. It is commonly used in chemistry and physics research. - X-ray tube: This instrument is used to generate X-rays by accelerating electrons and colliding them with a metal target. X-rays are commonly used in medical imaging. Therefore, the correct answer is the Geiger-Muller counter.

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The statement that is correct about nuclear reactions is "takes place inside the nucleus." A nuclear reaction involves changes in the nucleus of an atom, where protons and/or neutrons can be added or removed. This process can result in the formation of a new element or the release of energy in the form of radiation. These reactions can occur spontaneously or be induced by high-energy particles such as neutrons or protons. Unlike chemical reactions, nuclear reactions are not affected by temperature or pressure and involve only particles within the nucleus, such as protons and neutrons, but not electrons.

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When heat is absorbed during a chemical reaction, the reaction is said to be endothermic. Endothermic reactions are those that absorb energy from their surroundings in the form of heat, resulting in a decrease in temperature of the surroundings. In an endothermic reaction, the reactants have lower energy than the products, and the difference in energy is absorbed from the surroundings as heat. Examples of endothermic reactions include the process of melting ice, the reaction between baking soda and vinegar, and the reaction between ammonium nitrate and water.

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The main function of limestone in the blast furnace is to remove impurities. When heated, limestone decomposes to form calcium oxide (CaO), which reacts with the impurities (mainly silica and alumina) to form slag. The slag, being less dense than molten iron, floats on top of it and can be easily removed. This process is known as fluxing, and it helps to purify the molten iron. In addition, the limestone also helps to regulate the temperature in the furnace and provides the necessary basicity for the chemical reactions to occur. Therefore, the correct option is: "remove impurity".

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The solubility of a solid in a given solvent refers to the maximum amount of the solid that can be dissolved in a specific amount of the solvent at a given temperature and pressure. The factors that affect the solubility of a solid in a given solvent are the nature of the solute, nature of the solvent, pressure, and temperature. - Nature of solute: The chemical and physical properties of the solute, such as its molecular size, polarity, and shape, can affect its solubility in a given solvent. For example, a polar solute is more likely to dissolve in a polar solvent, while a nonpolar solute is more likely to dissolve in a nonpolar solvent. - Nature of solvent: The chemical and physical properties of the solvent, such as its polarity and boiling point, can affect its solubility with a specific solute. - Pressure: The solubility of a gas in a liquid is directly proportional to the pressure applied to the system. This is known as Henry's Law. However, the solubility of solids in liquids is not significantly affected by pressure changes. - Temperature: The solubility of most solids in liquids increases as the temperature increases, because higher temperatures increase the kinetic energy of both the solvent molecules and the solute particles. However, for some solutes, such as calcium sulfate, the solubility decreases as the temperature increases. Therefore, the factor that does not affect the solubility of a solid in a given solvent is pressure, as it only significantly affects the solubility of gases in liquids, not solids in liquids.

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The number of isomers that C₃H₆Cl₂ can have depends on the possible arrangements of the atoms in the molecule. First, let's consider the number of possible structural isomers that can be formed. Structural isomers are molecules with the same molecular formula but with different arrangements of their atoms. For C₃H₆Cl₂, there are two possible structural isomers: 1,1-dichloropropane and 2,2-dichloropropane. Next, we can consider the possibility of stereoisomers, which are molecules with the same molecular formula and the same structural arrangement of atoms, but with different orientations of atoms in space. However, in C₃H₆Cl₂, there are no chiral centers (carbon atoms with four different substituents), so there are no possible stereoisomers. Therefore, the total number of isomers for C₃H₆Cl₂ is 2

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The aqueous solution which has pH > 7 is Na\(_2\)CO\(_3(aq)\). pH is a measure of the acidity or basicity of an aqueous solution. A pH of 7 indicates neutrality, while a pH greater than 7 indicates basicity or alkalinity. The pH of a solution is determined by the concentration of hydrogen ions (H\(^+\)) in the solution. In the case of Na\(_2\)CO\(_3(aq)\), it is a basic compound because it has a carbonate ion, which can react with water to form hydroxide ions (OH\(^-\)). The hydroxide ions increase the concentration of hydroxide ions in the solution, which makes the solution basic. The chemical reaction that occurs when Na\(_2\)CO\(_3(aq)\) dissolves in water is: Na\(_2\)CO\(_3(aq)\) + H\(_2\)O → 2Na\(^+\) + CO\(_3^{2-}\) + 2OH\(^-\) The presence of hydroxide ions in the solution makes it basic, and the higher the concentration of hydroxide ions, the higher the pH of the solution. Therefore, Na\(_2\)CO\(_3(aq)\) is the aqueous solution which has pH > 7.

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To determine the empirical formula of a hydrocarbon compound, we need to know the percentage composition of each element in the compound. In this case, we are given that the compound contains 92.3% carbon, which means that the remaining percentage (100 - 92.3 = 7.7%) must be hydrogen. To calculate the empirical formula, we need to convert the percentages to mole ratios. Assuming a 100 g sample of the compound, we have 92.3 g of carbon and 7.7 g of hydrogen. The next step is to convert the masses of each element to moles. We use the molar masses of carbon and hydrogen (12.0 g/mol and 1.00 g/mol, respectively) to calculate the number of moles of each element: - Moles of carbon = 92.3 g / 12.0 g/mol = 7.69 mol - Moles of hydrogen = 7.7 g / 1.00 g/mol = 7.7 mol To get the simplest whole-number ratio of carbon to hydrogen, we divide each number of moles by the smallest number of moles (7.69 mol in this case): - Carbon: 7.69 mol / 7.69 mol = 1.00 - Hydrogen: 7.7 mol / 7.69 mol = 1.00 Therefore, the empirical formula of the hydrocarbon compound is CH, which corresponds to.

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The bond responsible for the high boiling point of water is the hydrogen bond. A hydrogen bond is a type of dipole-dipole interaction that occurs when a hydrogen atom bonded to an electronegative atom (such as oxygen or nitrogen) in one molecule is attracted to an electronegative atom in another molecule. In water, the hydrogen atoms of one water molecule are attracted to the oxygen atom of another water molecule through hydrogen bonds, creating a network of intermolecular attractions. These hydrogen bonds require a large amount of energy to break, resulting in a high boiling point for water compared to similar-sized molecules with only London dispersion or dipole-dipole interactions.

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Evaporation leads to decrease in volume of a liquid in an open container. This is because during evaporation, the molecules at the surface of the liquid gain enough kinetic energy to overcome the intermolecular forces of attraction and escape into the atmosphere as vapour. As a result, the total number of molecules in the liquid decreases and the volume of the liquid decreases as well. This process continues until the rate of evaporation equals the rate of condensation, at which point the liquid reaches equilibrium with the surrounding atmosphere.

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The correct statement about fine chemical is: it is produced in relatively small amounts. Fine chemicals are complex, pure chemicals produced in smaller quantities than bulk chemicals. They are used as building blocks in the manufacture of various chemical products such as pharmaceuticals, agrochemicals, flavors and fragrances, and specialized materials. Their production involves precise and sophisticated processes that ensure a high degree of purity and quality. Therefore, the other options are incorrect, as fine chemicals are not necessarily injurious to health, they have a high degree of purity, and they may or may not be stored for a long time, depending on their properties and intended use.

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In the given reaction, C\(_2\)H\(_2\) and H\(_2\) react to form C\(_2\)H\(_6\). The molecular formula of C\(_2\)H\(_2\) is C\(_2\)H\(_2\) and that of H\(_2\) is H\(_2\). The molecular formula of C\(_2\)H\(_6\) is C\(_2\)H\(_6\). Therefore, we can write the balanced chemical equation as: C\(_2\)H\(_2\) + 2H\(_2\) → C\(_2\)H\(_6\) We can see that for every one mole of C\(_2\)H\(_2\) used in the reaction, two moles of H\(_2\) are required. Therefore, the value of y in the equation is 2. So, the correct option is (C) 2.

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Alkaline earth metals can be found in group II of the periodic table. Group II is also known as the alkaline earth metals group, which consists of beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra). These elements are called alkaline earth metals because their oxides and hydroxides are alkaline in nature, and they form basic solutions when dissolved in water.

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The rate of diffusion is inversely proportional to the molar mass of the gas, which means that the lighter the gas, the faster it will diffuse. Therefore, to arrange the given gases in increasing order of diffusion rate, we need to arrange them in decreasing order of their molar mass. Let's calculate the molar mass of each gas: - SO₂: 32 + 2(16) = 64 - O₂: 2(16) = 32 - NH₃: 14 + 3(1) = 17 - H₂: 2(1) = 2 - H₂S: 2 + 32 = 34 - NH₂: 14 + 2(1) = 16 - CO₂: 12 + 2(16) = 44 - N₂O: 2(14) + 16 = 44 - NO₂: 14 + 2(16) = 46 - CO₂: 12 + 2(16) = 44 Now, let's arrange the gases in decreasing order of their molar mass: - H₂, NH₃, O₂, SO₂ - NH₂, H₂S, O₂, SO₂ - CO₂, N₂O, O₂, SO₂ - CO₂, N₂, NO₂, NH₃ Therefore, the correct answer is the first option: SO₂ < O₂ < NH₃ < H₂.

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In the periodic table, all elements within the same group have the same number of valence electrons. The periodic table is a systematic arrangement of elements based on their atomic structure. Elements in the same group have the same number of valence electrons, which are the electrons in the outermost energy level of the atom that are involved in chemical reactions. The number of valence electrons determines the chemical properties of an element and its ability to bond with other elements to form compounds. For example, the elements in Group 1 (also called the alkali metals), such as lithium, sodium, and potassium, all have one valence electron. This makes them highly reactive and gives them similar chemical properties. Similarly, the elements in Group 7 (also called the halogens), such as fluorine, chlorine, and bromine, all have seven valence electrons, which also gives them similar chemical properties. In contrast, elements in the same period have the same number of energy levels or shells, but they do not have the same number of valence electrons. For example, oxygen and neon are in the same period, but oxygen has six valence electrons while neon has eight. Therefore, it can be concluded that all elements within the same group have the same number of valence electrons.

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The halogen that is liquid at room temperature is Bromine. Bromine is a reddish-brown liquid with a boiling point of 58.8°C, which is lower than room temperature (typically around 20-25°C). This means that bromine is in its liquid state at room temperature. In contrast, fluorine and chlorine are gases at room temperature, while iodine is a solid. Bromine is the only halogen that is a liquid at room temperature.

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The pressure exerted by a gas is a function of the frequency of collision between gaseous molecules. When gas molecules move, they collide with one another and with the walls of their container. These collisions create a force that is distributed over the area of the container, which is the pressure of the gas. The more often these collisions occur, the higher the pressure of the gas. Therefore, the pressure of a gas is directly proportional to the frequency of collision between its molecules. The total volume of the gas, the speed of the gaseous molecules, and the mass of each gaseous molecule can affect the frequency of collision between gaseous molecules, but they are not the primary factors that determine the pressure of the gas.

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The molar mass of KOH (potassium hydroxide) is K + O + H = 39.0 + 16.0 + 1.00 = 56.0 g/mol. To find the amount of KOH needed to yield 6.5g, we divide 6.5 by the molar mass of KOH: 6.5g / 56.0 g/mol = 0.116 mol Since the concentration of KOH is given as 0.25 moldm^-3, we can use the formula: concentration (mol/dm³) = amount of substance (mol) / volume (dm³) to find the volume of the solution required: 0.25 mol/dm³ = 0.116 mol / volume (dm³) Solving for volume, we get: volume (dm³) = 0.116 mol / 0.25 mol/dm³ = 0.464 dm³ = 464.30 cm³ Therefore, the answer is 464.30 cm³.

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Bronze is an alloy that is primarily composed of copper (Cu) with the addition of other metals to give it specific properties. The exact composition of bronze can vary depending on its intended use, but typically it contains a mixture of copper with one or more additional metals. Out of the given options, only one contains copper as a component, and that is "Cu and X," where X represents the other metal. - Cu and Mg: This mixture is not bronze because magnesium (Mg) is not typically added to copper to create bronze. - Cu and Sn: This mixture is commonly referred to as tin bronze and is often used for making musical instruments, statues, and other decorative objects. - Cu and Zn: This mixture is commonly referred to as brass and is often used for making musical instruments, door knobs, and other household items. - Cu and Pb: This mixture is not commonly used to make bronze, but it can be used to create other alloys such as leaded bronze. Therefore, the correct answer is "Cu and Sn," meaning that bronze is a mixture of copper and tin.

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The question is asking us to identify the particle that the mobile valence electrons in a metallic solid are attracted to. In a metallic solid, the valence electrons are not tightly bound to any particular atom, but rather are free to move throughout the entire solid. These electrons form a "sea" of negatively charged particles that is dispersed throughout the solid. The positive charges in a metallic solid are located in the nuclei of the atoms that make up the solid. Since opposite charges attract each other, the mobile valence electrons are attracted to these positively charged nuclei. Therefore, the correct answer is: "positively charged nuclei".

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Rare gases, also known as noble gases, are stable because they have a complete outermost shell of electrons. In other words, they have octet structures, meaning they have eight electrons in their outermost energy level, with the exception of helium, which has two electrons in its outermost energy level. This full outer shell makes them highly unreactive, since they have no need to gain or lose electrons to achieve a more stable configuration. Therefore, rare gases do not readily form chemical bonds with other elements, making them very stable.

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The electron configuration of an element indicates the distribution of its electrons among the energy levels and orbitals. In this case, the element has 2 electrons in the 1s orbital, 2 electrons in the 2s orbital, 6 electrons in the 2p orbital, 2 electrons in the 3s orbital, and 1 electron in the 3p orbital. Based on this configuration, we can determine that the element belongs to the p-block of the periodic table, specifically to period 3 and group 3, which contains the elements Boron (B), Aluminum (Al), Gallium (Ga), Indium (In), and Thallium (Tl).

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The major product formed by the reaction between ethanoic acid and aqueous sodium hydroxide is sodium ethanoate, which is a salt. When ethanoic acid reacts with aqueous sodium hydroxide, the acid-base neutralization reaction occurs, and the products are sodium ethanoate (the salt) and water. Sodium hydroxide is a strong base, and ethanoic acid is a weak acid, so the reaction favors the formation of the salt. The reaction can be represented by the following chemical equation: CH\(_3\)COOH + NaOH → CH\(_3\)COONa + H\(_2\)O Therefore, the major product of the reaction is sodium ethanoate, and the other product is water.

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When an atom gains electrons, it becomes negatively charged, because electrons have a negative charge. The number of electrons gained is equal to the difference between the number of protons and the number of electrons. In this case, the atom of element X gains two electrons, so its charge will be -2. The symbol for an ion with a charge of -2 is X ^2-. Therefore, the correct answer is.

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The compound that crystallizes without water of crystallization is sodium chloride (NaCl). When a compound crystallizes, it can trap water molecules within the crystal lattice, forming a hydrated compound. These water molecules are called water of crystallization. However, NaCl is an ionic compound that does not have water of crystallization. This is because ionic compounds do not have a defined molecular structure that can incorporate water molecules. NaCl is formed by the ionic bond between sodium and chloride ions. Therefore, when it crystallizes, it does not include any water molecules in the crystal structure. On the other hand, the other compounds listed have a defined molecular structure that can incorporate water molecules in the crystal lattice, forming hydrated compounds. For example, magnesium sulfate (MgSO₄) can crystallize with 7 water molecules in the lattice, forming MgSO₄·7H₂O. Sodium carbonate (Na₂CO₃) can crystallize with 10 water molecules, forming Na₂CO₃·10H₂O. Iron(II) sulfate (FeSO₄) can crystallize with 7 water molecules, forming FeSO₄·7H₂O.

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Noble gas molecules are held together by van der Waals forces. These forces are weak attractive forces that exist between atoms or molecules due to temporary fluctuations in electron density. In noble gas molecules, the van der Waals forces are the result of the temporary dipoles that form due to the random motion of the electrons in the atoms. These temporary dipoles induce similar dipoles in nearby atoms, leading to weak attractive forces that hold the atoms together in a solid or a liquid state.

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The term "diatomic" refers to an element that can exist as a molecule composed of two atoms. Out of the options given, only oxygen is diatomic, existing as O₂. Sodium exists as a single atom (Na), iron as a solid metal, and neon as a noble gas with single atoms. Therefore, the correct answer is Oxygen.

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The gas law which describes the relationship between volume and temperature is Charles' law. According to Charles' law, the volume of a gas is directly proportional to its temperature (in Kelvin) at constant pressure and amount of gas. As the temperature of a gas increases, its volume also increases proportionally, and as the temperature decreases, the volume also decreases proportionally. This law is commonly expressed as V/T = constant or V1/T1 = V2/T2, where V is the volume, T is the temperature, and the subscripts 1 and 2 refer to two different conditions.

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To calculate the percentage by mass of calcium in Ca(OCl)2, we need to first find the molar mass of the compound, which is the sum of the atomic masses of its elements. Molar mass of Ca(OCl)2 = (1 × atomic mass of Ca) + (2 × atomic mass of O) + (2 × atomic mass of Cl) = (1 × 40.0) + (2 × 16.0) + (2 × 35.5) = 142.0 g/mol Next, we need to find the mass of calcium in one mole of Ca(OCl)2. Mass of calcium = (1 × atomic mass of Ca) = 40.0 g/mol Finally, we can calculate the percentage by mass of calcium in Ca(OCl)2 by dividing the mass of calcium by the molar mass of the compound and multiplying by 100. Percentage by mass of calcium in Ca(OCl)2 = (mass of calcium / molar mass of Ca(OCl)2) × 100 = (40.0 / 142.0) × 100 = 28.2% (rounded to one decimal place) Therefore, the correct answer is (A) 28.0%.

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The cells that produce electrical energy from chemical reactions are called electrochemical cells. In an electrochemical cell, a chemical reaction takes place, and the energy released by the reaction is converted into electrical energy. Out of the given options, the lead-acid battery, dry cell, and Daniell cell are all electrochemical cells that produce electrical energy from chemical reactions. The electrolytic cell, on the other hand, is used to drive a non-spontaneous chemical reaction using electrical energy. Therefore, the correct answer is (B) l, ll, and III only.

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The molecule which has a linear shape is CO₂. CO₂ has a linear shape because it consists of two identical oxygen atoms bonded to a central carbon atom by double bonds. The carbon atom is located in the center of the molecule and the oxygen atoms are arranged symmetrically on either side. This symmetrical arrangement of atoms results in a linear shape for the molecule.

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Thermoplastic materials soften and melt on heating. This means that they can be heated, molded into different shapes, and cooled to form a solid. Unlike thermosetting plastics, which harden irreversibly on heating and cannot be remolded, thermoplastics can be remolded multiple times without any change in their chemical structure. Therefore, the correct statement about thermoplastic materials is that they soften and melt on heating.

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The material classified as a non-biodegradable pollutant is plastic. Plastic does not decompose naturally and takes hundreds of years to break down into smaller fragments, which can persist in the environment and cause harm to wildlife and ecosystems. On the other hand, animal hide, paper, and wood are biodegradable materials that can be broken down naturally by microorganisms in the environment.

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Ethene is the raw material used in the plastic industry. Ethene is a colorless gas with a sweet smell and is a type of hydrocarbon that is produced from crude oil and natural gas. It is used as a feedstock in the production of many chemicals, including polyethylene, which is the most widely used plastic in the world. The polymerization of ethene produces polyethylene, which can be molded into various shapes and forms to produce plastic products. Methane is also a hydrocarbon, but it is not commonly used as a raw material in the plastic industry. Calcium and hydrogen are not hydrocarbons and are not used as raw materials in plastic production.

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The common reaction to all hydrocarbons is combustion. Combustion is a chemical reaction between hydrocarbons and oxygen, which results in the production of carbon dioxide and water. This reaction is common to all hydrocarbons regardless of their structure, functional group, or size. Therefore, the correct answer is combustion.

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(a)(i) The standard electrode potential of an element is the potential difference developed when the element is in contact with a solution of its ions of concentration \(1\ \text{mol dm}^{-3}\) at 298 K and 1 atmosphere, measured relative to the standard hydrogen electrode (which is taken as zero).

(a)(ii) Three factors affecting the discharge of ions during electrolysis: the position of the ion in the electrochemical series (relative ease of discharge), the concentration of the ion in solution, and the nature of the electrode used.

(a)(iii) Two functions of a salt bridge: it completes the electrical circuit by allowing ions to flow between the two half-cells, and it maintains electrical neutrality in the half-cells (preventing charge build-up).

(b) Electrolysis of \(CuSO_4\) with copper electrodes

Copper is deposited at the cathode: \(Cu^{2+} + 2e^- \to Cu\). At the anode the copper electrode itself dissolves: \(Cu \to Cu^{2+} + 2e^-\). Copper is therefore transferred from anode to cathode, the anode loses mass while the cathode gains mass, and the blue colour of the solution stays constant because the \(Cu^{2+}\) concentration is unchanged (this is the basis of electro-refining).

(c) Mass of copper deposited

From \(Cu^{2+} + 2e^- \to Cu\), \(2 \times 96500\ \text{C}\) deposits 64 g of copper.

About 0.14 g of copper is deposited.

(d)(i) Three characteristics of a catalyst: it is chemically unchanged in mass and composition at the end of the reaction; it is needed only in a small amount; and it is specific in its action (a given catalyst works for a particular reaction).

(d)(ii) Manufacturing processes using the metals as catalysts:

• I. Iron: the Haber process (manufacture of ammonia).
• II. Nickel: the hydrogenation of vegetable oils (manufacture of margarine).
• III. Platinum: the Ostwald process (catalytic oxidation of ammonia in the manufacture of nitric acid); it is also used in the Contact process.

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(a)(i) A chemical industry is an industry that uses chemical reactions and processes to convert raw materials into useful finished products on a large (commercial) scale.

(a)(ii) Three factors to consider in siting a chemical industry: availability of raw materials, a cheap and adequate source of energy/power and water, and good transport and nearness to the market (availability of labour and safe waste disposal are also acceptable).

(b)(i) Extraction of tin

Tin is obtained from its ore cassiterite, \(SnO_2\). The ore is first concentrated by washing (froth flotation/gravity), then reduced with carbon (coke) in a furnace: \[SnO_2 + 2C \to Sn + 2CO\] The molten tin is run off and refined.

(b)(ii) Two uses of tin: tin-plating (coating) of steel to make food cans, and making alloys such as solder and bronze.

(c)(i) Constituents of cement: limestone (\(CaCO_3\)), clay/shale (aluminosilicates), and a little gypsum (with silica and iron oxide).

(c)(ii) Mortar (slaked lime, sand and water) sets by absorbing carbon(IV) oxide from the air, forming hard calcium trioxocarbonate(IV): \[Ca(OH)_2 + CO_2 \to CaCO_3 + H_2O\]

(d)(i) Pollution is the introduction of harmful (unwanted) substances into the environment, making it unclean and injurious to living things.

(d)(ii) Two air pollutants: carbon(II) oxide (\(CO\)) and sulphur(IV) oxide (\(SO_2\)) (oxides of nitrogen and unburnt hydrocarbons are also acceptable).

(e) \(N_{2(g)} + 3H_{2(g)} \rightleftharpoons 2NH_{3(g)};\ \Delta H = -92.37\ \text{kJ}\). This is the exothermic Haber synthesis of ammonia; a high yield is favoured by low temperature and high pressure, while an iron catalyst is used to reach equilibrium quickly. (No specific sub-question is stated for part (e).)

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(a)(i) Hygroscopic

A hygroscopic substance is one which absorbs moisture (water vapour) from the atmosphere without dissolving in it or forming a solution.

(a)(ii) Two differences between physical and chemical changes

(a)(iii) Charles' law using the kinetic theory of gases

At constant pressure, when the temperature of a fixed mass of gas is increased, the gas molecules gain kinetic energy and move faster. The molecules move farther apart, causing the gas to expand. Hence, the volume of the gas increases directly with its absolute temperature.

(b)(i) Order of increasing boiling points

\[ CO_2 < CS_2 < NaH \]

Both \(CO_2\) and \(CS_2\) are simple molecular substances held together by van der Waals' forces. The forces in \(CS_2\) are stronger because its molecules are larger and more polarizable than those of \(CO_2\). Sodium hydride, \(NaH\), is an ionic compound with strong electrostatic forces of attraction between its ions; therefore, it has the highest boiling point.

(b)(ii) Reaction of chlorine with cold dilute sodium hydroxide

\[ Cl_{2(g)} + 2NaOH_{(aq)} \rightarrow NaCl_{(aq)} + NaOCl_{(aq)} + H_2O_{(l)} \]

(c)(i) Balanced equation for the reaction

\[ Mg_{(s)} + 2HCl_{(aq)} \rightarrow MgCl_{2(aq)} + H_{2(g)} \]

(c)(ii)

I. Mass of pure magnesium in the sample

Volume of hydrogen gas evolved \(= 8.6\text{ dm}^3\).

\[ n(H_2)=\frac{8.6}{22.4}=0.384\text{ mol} \]

From the equation, \(1\) mole of \(Mg\) produces \(1\) mole of \(H_2\).

\[ n(Mg)=0.384\text{ mol} \]

\[ \text{Mass of pure }Mg = 0.384 \times 24.0 = 9.21\text{ g} \]

II. Percentage purity of the magnesium sample

\[ \%\,\text{purity}=\frac{\text{mass of pure magnesium}}{\text{mass of impure sample}}\times100 \]

\[ =\frac{9.21}{15.0}\times100=61.4\% \]

III. Number of ions produced

The \(0.384\) mol of magnesium forms \(0.384\) mol of \(MgCl_2\).

\[ MgCl_2 \rightarrow Mg^{2+} + 2Cl^- \]

Therefore, \(1\) mole of \(MgCl_2\) produces \(3\) moles of ions.

\[ \text{Moles of total ions}=3 \times 0.384=1.152\text{ mol} \]

\[ \text{Number of ions}=1.152 \times 6.02\times10^{23} \]

\[ =6.94\times10^{23}\text{ ions} \]

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(a)(i) The Group VII elements are commonly called the halogens.

(a)(ii) The hydrides of the first two elements (fluorine and chlorine) are hydrogen fluoride, \( HF \) and hydrogen chloride, \( HCl \).

(a)(iii) Three chemical properties of Group VII elements

• They are strong oxidizing agents (their oxidizing power decreases down the group).
• They react with metals to form salts (metal halides), for example \( 2Na + Cl_2 \rightarrow 2NaCl \).
• They react with hydrogen to form hydrogen halides, and a higher halogen displaces a lower one from its salt solution.

(b) Completed table

Using mass number \( = \) protons \( + \) neutrons, and for an ion the electrons \( = \) protons \( - \) (positive charge):

• \( W^{2+} \): neutrons 12, mass 24, so protons \( = 24 - 12 = 12 \); electrons \( = 12 - 2 = 10 \).
• \( X^{2+} \): protons 8, mass 16, so neutrons \( = 16 - 8 = 8 \); electrons \( = 8 - 2 = 6 \).
• \( Y \): electrons 13 (neutral, so protons 13), mass 27, so neutrons \( = 27 - 13 = 14 \).
• \( Z \): electrons 11 (neutral, so protons 11), neutrons 12, so mass number \( = 11 + 12 = 23 \).

(c)(i) I. Nuclear fission: the splitting of a heavy nucleus into two lighter nuclei of comparable mass, with the release of a large amount of energy. II. Nuclear fusion: the combination of two light nuclei to form a heavier nucleus, with the release of a large amount of energy.

(c)(ii) Fission is used in nuclear power stations to generate electricity (and in the atomic bomb). Fusion is the source of the energy of the sun and stars (and of the hydrogen bomb).

(d)(i) The three radiations are alpha, beta and gamma radiation.

(d)(ii) I. Increasing penetrating power: alpha \( < \) beta \( < \) gamma. II. Increasing ionizing power: gamma \( < \) beta \( < \) alpha.

Détails de la réponse

(a)(i) I. Normal salt: a salt formed when all the replaceable hydrogen ions of an acid have been completely replaced by a metal ion (or the ammonium ion), for example \( Na_2SO_4 \). II. Acid salt: a salt formed when only part of the replaceable hydrogen ions of an acid has been replaced by a metal ion, so it still contains replaceable hydrogen, for example \( NaHSO_4 \).

(a)(ii)

I. Normal salt: \[ H_2SO_4 + 2NaOH \rightarrow Na_2SO_4 + 2H_2O \]

II. Acid salt: \[ H_2SO_4 + NaOH \rightarrow NaHSO_4 + H_2O \]

(b)(i) An acid-base indicator is a substance (usually a weak organic acid or base) that shows different colours in acidic and in basic media, and so is used to detect the end point of a titration.

(b)(ii) Completed table

(b)(iii) I. Strong acid against strong base: methyl orange or phenolphthalein. II. Strong acid against weak base: methyl orange. III. Weak acid against strong base: phenolphthalein.

(c) Mass of baking soda producing 10 g of \( CO_2 \)

Molar mass of \( NaHCO_3 = 23 + 1 + 12 + (3 \times 16) = 84\ \text{g mol}^{-1} \); molar mass of \( CO_2 = 12 + 32 = 44\ \text{g mol}^{-1} \).

From the equation, \( 84\ g \) of \( NaHCO_3 \) gives \( 44\ g \) of \( CO_2 \). Let \( x \) be the mass needed for \( 10\ g \) of \( CO_2 \):

Mass of baking soda = 19.1 g.

(d) A given mass of sodium hydroxide pellets cannot be used to prepare a standard solution because sodium hydroxide is deliquescent and also absorbs carbon(IV) oxide from the air; its mass therefore changes and is unreliable, so it is not a primary standard.

Détails de la réponse

(a)(i) Two characteristics of a homologous series

• All members are represented by the same general molecular formula.
• Successive members differ from one another by a CH2 unit (relative mass 14).

(They also share the same functional group, show a steady gradation in physical properties, and have similar chemical properties.)

(a)(ii) The compound CH3(CH2)2CH3 is butane, molecular formula C4H10.

I. Homologous series: the alkanes (saturated aliphatic hydrocarbons).

II. Possible isomers. The formula C4H10 has only two possible structural (chain) isomers; a third isomer of this formula cannot be drawn. The two are:

1. CH3-CH2-CH2-CH3
2. CH3-CH(CH3)-CH3  i.e. a central carbon bearing three methyl groups and one hydrogen.

III. Names: (1) butane (n-butane); (2) 2-methylpropane (isobutane). No third isomer exists for C4H10.

(b) & (c) Major products and their names

(d) The two compounds shown

X is (CH3)3COH and Y is a benzene ring bearing -COOH, i.e. C6H5COOH.

(i) IUPAC names: X = 2-methylpropan-2-ol (2-methyl-2-propanol); Y = benzoic acid (benzenecarboxylic acid).

(ii) Chemical test for the functional group:

• X (hydroxyl, -OH): add solid phosphorus(V) chloride, PCl₅; misty white fumes of hydrogen chloride are evolved, confirming the -OH group. (Alternatively a piece of sodium gives effervescence of hydrogen.)
• Y (carboxyl, -COOH): add aqueous sodium hydrogentrioxocarbonate(IV), NaHCO₃; brisk effervescence of carbon dioxide occurs, and the gas turns lime water milky, confirming the -COOH group.

(e) Empirical and molecular formula (using C = 12.0, H = 1.00, O = 16.0; the third figure in the data is O, not H).

Divide each percentage by the atomic mass:

\[C:\ \frac{70.57}{12.0}=5.88,\qquad H:\ \frac{5.90}{1.00}=5.90,\qquad O:\ \frac{23.53}{16.0}=1.47\]

Divide through by the smallest ratio (1.47):

\[C:\ \frac{5.88}{1.47}=4,\qquad H:\ \frac{5.90}{1.47}=4,\qquad O:\ \frac{1.47}{1.47}=1\]

(i) Empirical formula = C4H4O.

Empirical formula mass \(=4(12.0)+4(1.00)+16.0 = 68\).

\[n=\frac{\text{molar mass}}{\text{empirical mass}}=\frac{136}{68}=2\]

(ii) Molecular formula = (C4H4O)2 = C8H8O2.