WAEC SSCE - Chemistry - 2020

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One example of a biodegradable pollutant is sewage. Sewage is the wastewater that contains human and household wastes, such as food scraps, soap, and other organic matter. Biodegradable means that the pollutant can be broken down or decomposed by natural processes, such as by bacteria and other microorganisms. In the case of sewage, bacteria in the environment can break down the organic matter and convert it into simpler, less harmful substances like water, carbon dioxide, and other minerals. These natural processes help to reduce the negative impact of sewage on the environment and make it less harmful to living organisms. However, it is important to note that even biodegradable pollutants like sewage can cause harm to the environment and human health if they are not properly treated or disposed of.

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The most suitable process for obtaining water from an aqueous solution of sugar is distillation. Distillation is a process that separates components of a mixture based on their boiling points. In this case, water has a lower boiling point than sugar, so by heating the solution, the water will evaporate and can be collected as a separate liquid. The sugar will remain in the original container as a solid or concentrated solution. Crystallization is a process that separates components of a mixture based on their solubility, but it is not effective for separating water from sugar as both components are soluble in each other. Filtration and decantation are processes used for separating solids from liquids or liquids with different densities, but they are not effective for separating water from sugar as both are liquids with similar densities.

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A polypeptide is a polymer made up of amino acids, which are linked by peptide bonds. Therefore, the substance that is a polypeptide is protein. Starch and glycogen are both polysaccharides, which are polymers made up of monosaccharides, such as glucose. They are not polypeptides. Fats are lipids, which are molecules that are not made up of repeating units of the same type of monomer. They are not polypeptides. Proteins, on the other hand, are made up of long chains of amino acids linked by peptide bonds, forming polypeptides. So, protein is the correct answer to the question.

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The metallic bond is a type of chemical bond that occurs between metal atoms. It arises due to the electrostatic attraction between the positively charged metal ions and the delocalized electrons surrounding them. The strength of the metallic bond depends on various factors, including the size of the metal atom and the number of valence electrons it has. In this case, the correct option is "smaller atomic size." Magnesium has a smaller atomic size than calcium, which means that the distance between the metal ions in magnesium is smaller than that in calcium. The closer the metal ions are to each other, the stronger the metallic bond will be. This is because the attraction between the positively charged metal ions and the negatively charged delocalized electrons is stronger when the ions are closer together. Therefore, since magnesium has a smaller atomic size than calcium, the metallic bond in magnesium is stronger than that in calcium. It is important to note that the option "greater number of valence electrons" is not correct in this case. Although the number of valence electrons can also affect the strength of the metallic bond, it is not the main factor that determines the difference in the strength of the metallic bond between magnesium and calcium.

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To determine the number of electrons in \({31}_{15}\)P\(^{3-}\), we need to first understand the notation. The atomic symbol \({31}_{15}\)P represents an atom of phosphorus with an atomic number of 15, which means it has 15 protons in its nucleus. The superscript 3- indicates that the atom has gained 3 electrons, giving it a net charge of -3. When an atom gains electrons, its number of electrons increases by the same number as the magnitude of the charge. Therefore, to determine the number of electrons in \({31}_{15}\)P\(^{3-}\), we can subtract the charge (-3) from the number of protons (15): Number of electrons = Number of protons - Charge Number of electrons = 15 - (-3) = 18 So \({31}_{15}\)P\(^{3-}\) contains 18 electrons. Therefore, the answer to this question is 18.

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The oxidation state of an element in a compound is the charge that the element would have if all the bonds in the compound were completely ionic. In NaClO\(_3\), the oxidation state of Na is +1 and the oxidation state of O is -2. Since the compound is neutral, the sum of the oxidation states of all the atoms in the compound must be zero. To find the oxidation state of Cl, we can use the equation: (+1) + x + 3(-2) = 0 where x is the oxidation state of Cl. Simplifying the equation gives: x - 5 = 0 x = +5 Therefore, the oxidation state of chlorine in NaClO\(_3\) is +5.

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The solubility of a salt is the maximum amount of the salt that can dissolve in a given amount of solvent at a particular temperature. In this question, we are given that 250 cm\(^3\) of a saturated solution of CuSO\(_4\) at 30°C was evaporated to dryness to obtain 5.0 g of the salt. Since the solution was saturated, all of the CuSO\(_4\) that could dissolve at 30°C was already dissolved in the solution. Therefore, the mass of CuSO\(_4\) obtained from evaporating the solution represents the maximum amount of CuSO\(_4\) that can dissolve in 250 cm\(^3\) of water at 30°C. To find the solubility of CuSO\(_4\) at 30°C, we can use the equation: Solubility = Maximum amount of solute / Volume of solvent The maximum amount of CuSO\(_4\) that can dissolve in 250 cm\(^3\) of water at 30°C is 5.0 g. The volume of solvent is 250 cm\(^3\), or 0.250 L. The molar mass of CuSO\(_4\) is 159.6 g/mol. Therefore, the solubility of CuSO\(_4\) at 30°C is: Solubility = Maximum amount of solute / Volume of solvent = 5.0 g / 0.250 L = 20 g/L To convert the solubility to g/100 mL, we can multiply by 10: Solubility = 20 g/L x 10 = 200 g/100 mL To find the solubility in g/100 mL, we also need to divide by the molar mass of CuSO\(_4\): Solubility = 200 g/100 mL / 159.6 g/mol = 1.253 mol/L Therefore, the solubility of CuSO\(_4\) at 30°C is 1.253 g/100 mL, which is approximately equal to 1.25 g/100 mL. The correct option is 0.125.

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Electronegativity is a measure of the ability of an atom to attract electrons towards itself when it forms a chemical bond with another atom. The electronegativity of an element increases as we move from left to right across a period and from bottom to top in a group in the periodic table. Therefore, in the given options, the element with the highest electronegativity should come last, and the element with the lowest electronegativity should come first in the decreasing order. Out of the given options, the correct arrangement of elements in decreasing order of electronegativity is "P, Si, Al, Mg, Na." Phosphorus (P) has the highest electronegativity among the given elements, followed by silicon (Si), aluminum (Al), magnesium (Mg), and sodium (Na), which has the lowest electronegativity. Hence, the correct answer is option "P, Si, Al, Mg, Na."

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The high solubility of ethanol in water is due to "hydrogen bonding". Hydrogen bonding is a type of intermolecular force that occurs when hydrogen is bonded to a highly electronegative atom such as nitrogen, oxygen, or fluorine. Ethanol contains an -OH group, which is highly polar and can form hydrogen bonds with water molecules. This allows ethanol to dissolve readily in water, even though it is a covalent compound. In fact, ethanol is one of the most soluble organic compounds in water due to its ability to form hydrogen bonds with water molecules. Therefore, the correct answer is "hydrogen bonding".

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The steps to obtain a pure dry sample of AgCl (s) from the mixture include filtering, washing, and drying. In this reaction, AgNO\(_3\) reacts with NaCl to form AgCl, which precipitates out of solution as a solid. To obtain a pure sample of AgCl, it is necessary to separate the solid from any residual reactants or products in the mixture. Filtering can be used to remove the solid AgCl from the mixture. Washing with water can help to remove any residual salt from the surface of the AgCl particles. Finally, drying the solid can help to remove any remaining water, leaving a pure sample of AgCl. So, the answer is filtering, washing, and drying are the steps that can be taken to obtain a pure dry sample of AgCl (s) from the mixture.

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The pair of elements that has the greatest difference in electronegativity is Na and F. Electronegativity is the ability of an atom to attract electrons towards itself in a chemical bond. Fluorine (F) is the most electronegative element in the periodic table, meaning it has the strongest ability to attract electrons. Sodium (Na) is a metal and has a low electronegativity. When sodium and fluorine combine to form an ionic compound, such as NaF, fluorine gains an electron from sodium to form an anion with a negative charge, while sodium becomes a cation with a positive charge. The electronegativity difference between sodium and fluorine is the largest among the options given, making it the pair with the strongest ionic bond. Therefore, Na and F have the greatest difference in electronegativity.

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Incomplete combustion of hydrocarbons occurs when there is not enough oxygen present for complete oxidation. As a result, the hydrocarbon molecule may not be fully broken down and can produce various products. One of the primary products of incomplete combustion is carbon, which can appear as soot or black smoke. This occurs because the carbon in the hydrocarbon molecule is not fully oxidized and instead forms solid particles that are released into the air. Another product that can form is carbon monoxide (CO), which is a toxic gas. Incomplete combustion can result in the formation of CO when there is not enough oxygen present to fully oxidize the carbon in the hydrocarbon molecule. Hydrogen, on the other hand, is not typically a product of incomplete combustion as it is fully oxidized to form water (H2O) when sufficient oxygen is present. Therefore, the correct answer is option (C) I and III only, as incomplete combustion of a hydrocarbon can produce carbon and carbon monoxide.

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The atomic number of an atom is defined as the number of protons in the nucleus of the atom. The mass number of an atom, on the other hand, is the sum of the number of protons and neutrons in the nucleus of the atom. Therefore, the atomic number and mass number of an atom would only be equal if the atom does not contain any neutrons. This is because the mass number of an atom is determined by the number of protons and neutrons in its nucleus, while the atomic number is determined only by the number of protons. If an atom does not contain any neutrons, then its mass number would be equal to the number of protons (i.e., the atomic number). However, most naturally occurring elements have isotopes, which means that they have varying numbers of neutrons in their nuclei. This results in different mass numbers for the same element, while the atomic number remains constant. Therefore, the correct answer is: "the atomic number of an atom would be equal to its mass number if it does not contain neutrons".

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When siting a chemical industry, one of the most important factors to consider is the nearness to raw materials. This is because raw materials are the foundation of the industry, and having easy access to them can reduce transportation costs and ensure a reliable supply chain. Chemical industries require large amounts of raw materials to produce their products, and these materials are often heavy and bulky. Therefore, being situated close to the source of these materials can significantly reduce transportation costs and also ensure that the industry has a reliable supply of materials. In addition to reducing transportation costs and ensuring a reliable supply chain, siting a chemical industry near raw materials can also reduce the environmental impact of the industry. Transporting large amounts of raw materials over long distances can contribute to greenhouse gas emissions and other forms of pollution, which can be minimized by siting the industry near the raw materials. Therefore, the correct answer is nearness to raw materials.

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To find the volume of oxygen required to burn 7.5 dm\(^3\) of methane, we need to balance the chemical equation for the reaction. The balanced equation is: CH\(_4\) \(_g\) + 2O\(_2\) \(_g\) → CO\(_2\) \(_g\) + 2H\(_2\)O\(_g\) This equation tells us that for every molecule of methane that reacts, 2 molecules of oxygen are required. So, for 7.5 dm\(^3\) of methane, we need 2 * 7.5 = 15 dm\(^3\) of oxygen. So, the volume of oxygen required to burn 7.5 dm\(^3\) of methane at s.t.p is 15.0 dm\(^3\).

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A reducing agent is a substance that donates electrons to another substance in a chemical reaction. In other words, a reducing agent is oxidized (loses electrons) while another substance is reduced (gains electrons). Of the substances listed, the one that is not a reducing agent is O\(_2\). This is because oxygen is an oxidizing agent, meaning that it accepts electrons from another substance during a reaction. It does not donate electrons, so it cannot act as a reducing agent.

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Among the given options, calcium hydroxide (Ca(OH)\(_2\)) is not readily soluble in water. When an ionic compound dissolves in water, its constituent ions separate and become surrounded by water molecules in a process called hydration. The solubility of a compound depends on the strength of the attractive forces between the ions in the compound and the water molecules. Calcium hydroxide is an ionic compound composed of calcium ions (Ca\(^{2+}\)) and hydroxide ions (OH\(^-\)). When calcium hydroxide dissolves in water, it undergoes a dissociation reaction, releasing calcium ions and hydroxide ions into the solution: Ca(OH)2 (s) → Ca\(^{2+}\) (aq) + 2OH\(^-\) (aq) However, calcium hydroxide is only slightly soluble in water. This means that it only dissolves to a limited extent in water and forms a suspension or a precipitate. On the other hand, NH\(_4\)OH, NaOH, and KOH are all readily soluble in water, which means they dissolve easily in water and form clear, homogeneous solutions. In summary, calcium hydroxide is not readily soluble in water, while NH\(_4\)OH, NaOH, and KOH are readily soluble in water.

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All of the molecules listed have at least one double covalent bond between two atoms except for water. A covalent bond is a type of chemical bond where atoms share electrons to achieve a stable configuration. A double covalent bond occurs when two pairs of electrons are shared between two atoms. Oxygen has a double covalent bond between two oxygen atoms (O=O). Carbon (IV) oxide, also known as carbon dioxide, has a double covalent bond between one carbon atom and two oxygen atoms (O=C=O). Ethene, also known as ethylene, has a double covalent bond between two carbon atoms (C=C). Water, on the other hand, has two single covalent bonds between one oxygen atom and two hydrogen atoms (H-O-H). It does not have any double covalent bonds. In summary, all the listed molecules have double covalent bonds between two atoms except for water, which has only single covalent bonds.

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The balanced chemical equation is based on the Law of Conservation of Mass. This law states that matter cannot be created or destroyed, but it can change form or be rearranged. In other words, the total mass of the reactants must equal the total mass of the products in a chemical reaction. A balanced chemical equation ensures that the Law of Conservation of Mass is being followed. It means that the same number of atoms of each element is present on both sides of the equation, and the total mass of the reactants is equal to the total mass of the products. For example, consider the equation for the reaction between hydrogen and oxygen to form water: 2H2 + O2 → 2H2O In this equation, there are four hydrogen atoms and two oxygen atoms on the left side of the equation, and the same number of atoms on the right side of the equation. Therefore, this equation is balanced and satisfies the Law of Conservation of Mass. In summary, a balanced chemical equation is based on the Law of Conservation of Mass, which states that the total mass of the reactants must be equal to the total mass of the products in a chemical reaction.

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To find the concentration of the resulting solution after dilution, we need to calculate the amount of solute (Na_2CO_3) in the new solution. The concentration of a solution is given by the amount of solute per unit volume of the solution. The dilution formula is: C1 * V1 = C2 * V2 Where C1 is the initial concentration (0.200 mol dm^-3), V1 is the initial volume (5.0 cm^-3), C2 is the final concentration (what we are trying to find), and V2 is the final volume (250 cm^-3). Plugging in the values: 0.200 * 5.0 = C2 * 250 0.200 * 5.0 / 250 = C2 C2 = 0.004 mol dm^-3 So, the concentration of the resulting solution is 0.004 mol dm^-3.

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The mass of silver deposited can be calculated using the equation: mass = (current x time x atomic weight of silver) / (ionic charge x Faraday constant) In this case, the current is not given, but we are given the total charge passed through the solution, which is 24,125 C. We can assume that this charge was passed at a constant current over a certain amount of time. Therefore, we can rearrange the equation to solve for the current: current = (ionic charge x Faraday constant x mass) / (time x atomic weight of silver) The ionic charge for silver is +1, the Faraday constant is 96,500 C, and the atomic weight of silver is 108 g/mol. Substituting these values into the equation, we get: current = (1 x 96,500 x mass) / (time x 108) We don't know the time, so we can't solve for the current. However, we can use the fact that the current is constant to say that the total charge passed is equal to the current multiplied by the time: charge = current x time Substituting the expression for current into this equation, we get: charge = ((1 x 96,500 x mass) / (time x 108)) x time Simplifying, we get: charge = (96,500 x mass) / 108 Multiplying both sides by 108 and dividing by 96,500, we get: mass = (charge x atomic weight of silver) / 96,500 Substituting the values given in the question, we get: mass = (24,125 x 108) / 96,500 mass = 27 g Therefore, the mass of silver deposited is 27 g

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The pressure of the gas will decrease to \(frac{3}{2}\) P when its volume is increased to 2V at constant temperature. This can be explained using Boyle's law, which states that the pressure of a gas is inversely proportional to its volume, when temperature is held constant. In other words, as the volume of a gas increases, its pressure will decrease proportionally and vice versa. Given that the initial volume and pressure of the gas are V and 3.P, respectively, we can write: P1V1 = P2V2 where P1 is the initial pressure, V1 is the initial volume, P2 is the final pressure, and V2 is the final volume. Substituting the given values, we get: (3.P) V = P2 (2V) Simplifying this equation, we get: P2 = (3.P) V / (2V) P2 = \(frac{3}{2}\) P Therefore, the pressure of the gas will decrease to \(frac{3}{2}\) P when its volume is increased to 2V at constant temperature.

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The statement "Electrons always occupy the lowest empty energy level" is a statement of the Aufbau Principle. This principle explains how electrons fill the available energy levels of an atom, starting with the lowest energy level and moving to higher energy levels. The Aufbau Principle tells us that electrons will occupy the lowest available energy level first before moving to higher energy levels. This means that the first energy level will be filled before the second energy level, and so on. Additionally, each energy level can only hold a certain number of electrons, which is determined by the periodic table. Overall, the Aufbau Principle helps to explain the electron configurations of atoms and how they behave in chemical reactions. By following this principle, we can predict the electron configurations of different elements and understand how they will react with other elements.

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Out of the given options, "Ethylene" is an example of a fine chemical. Fine chemicals are pure chemical substances that are produced in relatively small quantities and with high purity for use in various industrial and scientific applications, such as pharmaceuticals, agrochemicals, and specialty chemicals. Ethylene is an important fine chemical that is widely used in the production of plastics, fibers, and other chemical intermediates. It is also used as a ripening agent for fruits and vegetables and as a fuel in rocket engines. Unlike sodium hydroxide and hydrochloric acid, which are bulk chemicals produced in large quantities for general industrial use, ethylene is a specialty chemical that requires specialized equipment and processes for its production and purification. Ammonia, on the other hand, is a bulk chemical used mainly in fertilizers and other agricultural applications.

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The group VII elements in their combined states are called "halides". Halides are compounds formed when a halogen (a group VII element) reacts with another element to gain an electron and form an anion. Halides are important in many chemical reactions, particularly in organic chemistry and in the production of materials like plastics and pharmaceuticals. Halides are generally very reactive, and some, such as chlorine, are used as disinfectants due to their ability to kill bacteria and other microorganisms. Overall, halides are an important class of compounds that play a critical role in many chemical processes.

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The pair of properties of alkali metals that decreases down the group is "first ionization energy and melting point". First ionization energy refers to the energy required to remove an electron from an atom. As we move down the group in the periodic table, the number of occupied energy levels increases, resulting in an increase in the distance between the nucleus and the outermost electron. This means that the force of attraction between the nucleus and the outermost electron decreases, making it easier to remove the electron. Hence, the first ionization energy decreases down the group. Melting point refers to the temperature at which a solid substance changes into a liquid state. As we move down the group, the size of the atoms increases due to the addition of new electron shells. This results in weaker metallic bonding between the atoms, which requires less energy to overcome, leading to a decrease in melting point down the group. Therefore, the correct pair of properties that decrease down the group for alkali metals is "first ionization energy and melting point".

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The oxide with a giant covalent structure is silicon dioxide (SiO\(_2\)). A giant covalent structure means that each atom in the structure is bonded to neighboring atoms by strong covalent bonds. In the case of SiO\(_2\), each silicon atom is covalently bonded to four oxygen atoms, and each oxygen atom is covalently bonded to two silicon atoms. This creates a 3D network of alternating silicon and oxygen atoms that extends in all directions, forming a giant covalent structure. In contrast, the other oxides listed (Al\(_2\)O\(_3\), Na\(_2\)O, and P\(_4\)O\(_{10}\)) do not have a giant covalent structure. Al\(_2\)O\(_3\) and Na\(_2\)O have ionic structures, where the metal and non-metal atoms are bonded by strong ionic bonds. P\(_4\)O\(_{10}\) has a molecular structure, where the phosphorus and oxygen atoms are bonded by covalent bonds to form discrete P\(_4\)O\(_{10}\) molecules. Overall, the different types of bonding in these oxides result in their varying structures and properties.

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The reactivity of a metal with water depends on its position in the reactivity series. The reactivity series is a list of metals arranged in order of their ability to displace hydrogen from water and acids. The reactivity of metals with water increases from copper to zinc to sodium and potassium. Copper is the least reactive metal in the list, and it does not react with water to produce hydrogen. When a metal reacts with water, it forms a metal hydroxide and hydrogen gas. The more reactive the metal, the more vigorously it reacts with water to produce hydrogen. Sodium and potassium are the most reactive metals, and they react violently with water to produce hydrogen gas. Therefore, the metal that does not react with water to produce hydrogen is copper.

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A molar quantity is a quantity that is related to the number of moles of a substance. A mole is the amount of a substance that contains the same number of entities as there are in 12 grams of carbon-12. Out of the options given, both "Molarity" and "Molar mass" are molar quantities. Molarity is the concentration of a solution, expressed as the number of moles of solute per liter of solution. It is a measure of the amount of solute present in a given volume of solution. Molar mass is the mass of one mole of a substance and is typically expressed in grams per mole (g/mol). It is the sum of the atomic masses of all the atoms in a molecule of the substance. On the other hand, "Mass concentration" and "Molality" are not molar quantities. Mass concentration is the concentration of a solution, expressed as the mass of solute per unit volume of solution. Molality is the concentration of a solution, expressed as the number of moles of solute per kilogram of solvent.

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Neutralization, decomposition, and combustion reactions can all be exothermic, but they don't have to be. An exothermic reaction is one that releases energy in the form of heat. Neutralization reactions occur when an acid and a base react to form a salt and water. Some neutralization reactions can be exothermic because energy is released when the acid and base molecules come together to form a salt and water. Decomposition reactions occur when a compound breaks down into its constituent elements. These reactions can be either endothermic or exothermic, depending on whether energy is absorbed or released during the reaction. Combustion reactions involve a fuel and an oxidizer reacting to form water and carbon dioxide, and these reactions are almost always exothermic because they release energy in the form of heat and light. Therefore, options (I and III only) are correct because neutralization and combustion reactions are usually exothermic. However, option (II) could be endothermic or exothermic, so it cannot be a correct answer. Option (I, II, and III) is also incorrect because option II could be endothermic.

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The rate of a chemical reaction generally increases with temperature. Therefore, the reaction would be slowest at the lowest temperature, which is: calcium trioxocarbonate (IV) chips at 20°C. When calcium trioxocarbonate (IV) reacts with hydrochloric acid, it produces calcium chloride, water, and carbon dioxide gas. The reaction rate depends on the frequency of collisions between the reactant particles. At a lower temperature, the particles move more slowly and collide less frequently, which reduces the reaction rate. Calcium trioxocarbonate (IV) chips will react slower than calcium trioxocarbonate (IV) powder because the chips have less surface area available for the reaction to occur. Powder has a larger surface area, allowing for more collisions to occur between the reactant particles and increasing the reaction rate. Therefore, the slowest reaction would occur when calcium trioxocarbonate (IV) chips are added to dilute hydrochloric acid at 20°C.

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The compound with the lowest boiling point is C\(_4\)H\(_10\). Boiling point is the temperature at which a liquid changes into a gas. The strength of the intermolecular forces between the particles of a substance determines its boiling point. In general, compounds with weaker intermolecular forces have lower boiling points. C\(_4\)H\(_10\) is a simple alkane with weak van der Waals forces between its molecules. CH\(_3\)COOH is a carboxylic acid and has stronger hydrogen bonding between its molecules compared to C\(_4\)H\(_10\). H\(_2\)O has extremely strong hydrogen bonding between its molecules, making it have an even higher boiling point compared to CH\(_3\)COOH. C\(_2\)H\(_5\)OH is an alcohol and also has hydrogen bonding between its molecules, but weaker than H\(_2\)O. Therefore, C\(_4\)H\(_10\) has the lowest boiling point among the given compounds due to its weak intermolecular forces.

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The type of isomerism exhibited by cis and trans isomers is geometrical isomerism. Geometrical isomerism is a type of stereoisomerism where compounds have the same molecular formula and the same sequence of bonded atoms, but differ in the spatial arrangement of their atoms due to restricted rotation around a double bond or a ring structure. Cis-trans isomerism occurs specifically in compounds with double bonds or ring structures, where two different groups or atoms are attached to each carbon of the double bond or ring. In cis isomers, the two identical groups or atoms are on the same side of the double bond or ring, while in trans isomers, they are on opposite sides. For example, in the case of cis-trans isomerism in alkenes, a cis isomer would have the substituents on the same side of the double bond, while a trans isomer would have the substituents on opposite sides of the double bond. This results in different physical and chemical properties, such as melting point, boiling point, and reactivity. Therefore, cis-trans isomerism is a type of geometrical isomerism where compounds have the same molecular formula and bonding sequence but differ in the spatial arrangement of their atoms due to the presence of a double bond or a ring structure.

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The correct statement for a reaction at equilibrium is that the rates of the forward and backward reactions are equal. This means that the reaction is still occurring, but at an equal rate in both the forward and backward directions. Equilibrium is a dynamic state in which the forward and backward reactions are still occurring, but at the same rate. This means that the concentrations of the reactants and products remain constant over time. At equilibrium, the reaction has not gone to completion and the amount of product may not necessarily equal the amount of reactants. It's important to note that equilibrium is a state of balance and not necessarily a state of rest. The reaction is still occurring, but the concentrations of the reactants and products remain constant because the rates of the forward and backward reactions are equal.

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The process that is not exhibited by atoms in order to attain more stable electron configuration is hybridization of orbitals. Atoms can become more stable by gaining or losing electrons to attain a full valence shell, which is the electron configuration of a noble gas. This process is called electron transfer. On the other hand, hybridization of orbitals refers to the mixing of atomic orbitals to form new hybrid orbitals. This process is not directly related to stability in terms of electron configuration, but it may be necessary for atoms to form certain chemical bonds. So, the answer is hybridization of orbitals is not exhibited by atoms in order to attain more stable electron configuration.

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(a)(i) Faraday's first law of electrolysis

The mass of a substance liberated or deposited at an electrode during electrolysis is directly proportional to the quantity of electricity (charge) that passes through the electrolyte.

(a)(ii) Strong versus weak electrolyte

A strong electrolyte is almost completely ionized in aqueous solution, whereas a weak electrolyte is only partially ionized in aqueous solution.

(b) One chemical property of ethyne

Ethyne is unsaturated and undergoes addition reactions, e.g. it decolourizes bromine water; it also burns in air. \(C_2H_2 + 2Br_2 \to C_2H_2Br_4\).

(c)(i) Unsaturated hydrocarbon

A hydrocarbon that contains at least one carbon-carbon double or triple bond.

(c)(ii) and (iii) Esterification

Reading the reaction as an ethanoic acid and methanol esterification:

The major product is the ester methyl ethanoate (\(CH_3COOCH_3\)).

(d) Increasing the rate of esterification

Use a small amount of concentrated tetraoxosulphate(VI) acid as catalyst (also warming the mixture increases the rate).

(e) Number of hydrogen molecules from 3.75 g Zn

Moles of Zn \(= 3.75/65 = 0.0577\,mol\). From the equation 1 mol Zn gives 1 mol \(H_2\), so moles of \(H_2 = 0.0577\,mol\).

(f) One effect of global warming

Melting of polar ice caps leading to a rise in sea level (and flooding of low-lying areas). Other acceptable: climate change, drought, desertification.

(g) Redox analysis

• (i) Equation B (\(H_2 + C_2H_4 \to C_2H_6\)) is the redox process. (In A and C no oxidation number changes.)
• (ii) In B, hydrogen changes from oxidation number 0 (in \(H_2\)) to \(+1\) (in \(C_2H_6\)) so it is oxidized; carbon changes from \(-2\) (in \(C_2H_4\)) to \(-3\) (in \(C_2H_6\)) so it is reduced.

(h)(i) Two concepts of Bohr's model

• Electrons revolve round the nucleus only in certain fixed circular paths (orbits/shells) of definite (quantized) energy.
• An electron does not radiate energy while in a given orbit; it absorbs or emits energy only when it jumps from one energy level to another.

(h)(ii) Limitations of Bohr's model

It successfully explains only the hydrogen (one-electron) atom and fails for atoms with more than one electron; it cannot explain the fine structure/splitting of spectral lines and contradicts the Heisenberg uncertainty principle (an electron cannot have a fixed orbit and definite momentum simultaneously).

(i) Three factors influencing equilibrium position

Concentration of reactants/products, temperature, and pressure (for reactions involving gases). A catalyst does not shift the position of equilibrium.

(j) Calcium trioxocarbonate(IV) with HCl and with ethanoic acid

• (i) Similarity: in both, effervescence occurs and a colourless gas (carbon(IV) oxide) is evolved which turns limewater milky.
• (ii) Difference: the reaction with hydrochloric acid (a strong acid) is faster and more vigorous, while the reaction with ethanoic acid (a weak acid) is slower and less vigorous.

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(a) Compounds A and B

• (i) Relationship: A and B are geometric (cis-trans) isomers. They have the same molecular formula and the same atoms joined in the same order, but differ in the spatial arrangement of the groups about the carbon-carbon double bond.
• (ii) Names: Compound A is trans-1,2-dichloroethene; Compound B is cis-1,2-dichloroethene.
• (iii) Chemical properties: Yes, the chemical properties of A and B are the same.
• (iv) Reason: They contain the same functional group (the carbon-carbon double bond) and the same type of atoms connected in the same way, so they undergo the same chemical reactions.

(b)(i) Two characteristic features of boiling

• Boiling takes place at a specific, constant temperature (the boiling point) at which the vapour pressure of the liquid equals the atmospheric pressure.
• Boiling takes place throughout the whole body of the liquid, with bubbles of vapour forming within it.

(b)(ii) Effect on the boiling point of water

• I. Addition of sodium chloride crystals: it increases the boiling point of the water.
• II. Reduction of atmospheric pressure: it lowers the boiling point of the water.

(b)(iii) One difference between boiling and evaporation

Boiling occurs throughout the whole liquid at a fixed temperature, whereas evaporation occurs only at the surface of the liquid and at all temperatures.

(c)(i) Unsaturated versus saturated solution

An unsaturated solution can still dissolve more solute at a given temperature and contains no undissolved solute, whereas a saturated solution cannot dissolve any more solute at that temperature and is in contact with undissolved solute.

(c)(ii) Making a saturated solution dissolve more solute

• By increasing the temperature of the solution.
• By increasing the volume of the solvent.

(c)(iii) One factor affecting solubility of a solid in a liquid

Temperature (also the nature of the solute and the nature of the solvent).

(d)(i) The mole

The mole is the amount of a substance that contains as many elementary particles as there are atoms in 12 g of carbon-12, that is, \(6.02 \times 10^{23}\) particles.

(d)(ii) Mass of magnesium oxide

\[ MgO + 2HCl \to MgCl_2 + H_2O \]

Moles of HCl \(= \dfrac{25.0}{1000} \times 0.1 = 2.5 \times 10^{-3}\,\text{mol}\).

From the equation, 2 mol HCl react with 1 mol MgO, so moles of MgO \(= \dfrac{2.5 \times 10^{-3}}{2} = 1.25 \times 10^{-3}\,\text{mol}\).

Molar mass of MgO \(= 24.0 + 16.0 = 40.0\).

\[ \text{Mass of MgO} = 1.25 \times 10^{-3} \times 40.0 = 0.05\,\text{g} \]

(e) Three physical properties of metals

• They are malleable and ductile.
• They are lustrous (shiny) and good conductors of heat and electricity.
• They have high melting and boiling points.

Antwortdetails

(a)

(i) Energy profile diagram

(ii)

I. Activation energy, \(E_a\)

\[E_a=\text{energy of activated complex}-\text{energy of reactants}=210-100=110\ \text{kJ mol}^{-1}\]

II. Enthalpy change, \(\Delta H\)

\[\Delta H=H_{\text{products}}-H_{\text{reactants}}=54-100=-46\ \text{kJ mol}^{-1}\]

(iii) \[Q_2(g)+R_2(g)\rightleftharpoons 2QR(g)\]

(iv) The reaction is exothermic.

Reason: The products have lower energy than the reactants and \(\Delta H\) is negative; hence, \(46\ \text{kJ mol}^{-1}\) of energy is released.

(b)(i) Electron configurations

Element	Atomic number	Electron configuration
E	7	\(1s^2\,2s^2\,2p^3\)
F	9	\(1s^2\,2s^2\,2p^5\)
G	12	\(1s^2\,2s^2\,2p^6\,3s^2\)
H	13	\(1s^2\,2s^2\,2p^6\,3s^2\,3p^1\)

(ii)

1. The metals are G and H.
2. The element most likely to form a \(+3\) ion is H.
3. The element in Group VII is F.
4. G forms \(G^{2+}\) and F forms \(F^-\); therefore, the formula of the compound is \(GF_2\).

(c) Isotopy is the phenomenon in which atoms of the same element have the same atomic number but different mass numbers because they contain different numbers of neutrons.

(d) The three building blocks of matter are atoms, molecules and ions.

Antwortdetails

(a)(i) Two gases for the fountain experiment

Ammonia (\(NH_3\)) and hydrogen chloride (\(HCl\)); both are extremely soluble in water.

(a)(ii) Aim of the fountain experiment

To demonstrate the very high solubility of the gas in water.

(a)(iii) Description of the fountain experiment

A dry round-bottomed flask is filled with the dry gas (e.g. ammonia) and fitted with a stopper carrying a jet tube that dips into a beaker of water (containing an indicator such as litmus) and a small dropper of water. A little water is squeezed from the dropper into the flask. The gas dissolves rapidly and almost completely, creating a partial vacuum (reduced pressure) inside the flask. The greater atmospheric pressure on the water in the beaker then forces water up the jet tube, and it spurts into the flask as a fountain. The indicator changes colour (turning blue for ammonia), showing the gas is alkaline.

(b)(i) Two chemical industries

Petrochemical (oil-refining) industry and cement industry. (Also acceptable: fertilizer, brewery, soap and detergent, pharmaceutical industries.)

(b)(ii) Three effects of a chemical industry on its community

• Provision of employment and improved standard of living (positive).
• Provision of social infrastructure such as roads, water and electricity (positive).
• Air and water pollution and health hazards from the disposal of wastes and emission of gases (negative).

(c)(i) Three products of the destructive distillation of coal

Coke, coal gas, coal tar (and ammoniacal liquor).

(c)(ii) One use each of two products

• Coke: used as a fuel and as a reducing agent in the extraction of metals.
• Coal gas: used as a gaseous fuel for heating and lighting. (Coal tar: source of organic chemicals and for surfacing roads.)

(d)(i) Two substances responsible for hardness in water

Calcium and magnesium hydrogencarbonates and sulphates (\(Ca^{2+}\) and \(Mg^{2+}\) salts).

(d)(ii) Two methods of removing hardness

Boiling (for temporary hardness) and addition of washing soda, \(Na_2CO_3\) (also ion exchange or distillation).

(d)(iii) Two advantages of hard water

• It supplies calcium salts needed for the formation of strong bones and teeth.
• It has a pleasant taste and helps prevent lead poisoning by coating lead pipes with an insoluble layer.

Antwortdetails

(a) Laboratory preparation of dry chlorine

• I. Reagents: concentrated hydrochloric acid and manganese(IV) oxide (\(MnO_2\)). (Warm together.)
• II. Drying agent: concentrated tetraoxosulphate(VI) acid (\(H_2SO_4\)).
• III. Mode of collection: upward delivery (downward displacement of air), because chlorine is denser than air.

(i) Equation for preparation:

(iii) Chlorine with hot concentrated NaOH:

(b)(i) Main raw materials for iron extraction in the blast furnace

Iron ore (haematite, \(Fe_2O_3\)), coke (carbon), limestone (\(CaCO_3\)) and hot air (oxygen).

(b)(ii) Reactions in the blast furnace

(b)(iii) The iron obtained directly from the blast furnace is called pig iron (cast iron).

(b)(iv) Pig iron has a relatively low melting point because it contains a high proportion of impurities, chiefly about 4% carbon, which lowers its melting point.

(c)(i) Why SO3 is not dissolved directly in water

Dissolving \(SO_3\) directly in water is violently exothermic and produces a dense, corrosive mist (fog) of tetraoxosulphate(VI) acid that is difficult to condense and escapes. Instead the \(SO_3\) is absorbed in concentrated \(H_2SO_4\) to form oleum, which is later diluted safely.

(c)(ii) \(H_2SO_4\) is regarded as a heavy chemical because it is manufactured and used industrially in very large quantities (tonnage).

(c)(iii) In \(Pb(NO_3)_2 + H_2SO_4 \to PbSO_4 + 2HNO_3\), the tetraoxosulphate(VI) acid acts as an acid (a less volatile acid displacing the more volatile trioxonitrate(V) acid and forming an insoluble sulphate).

(d) Propanol with sodium

(sodium propoxide and hydrogen gas).