WAEC SSCE - Chemistry - 2020

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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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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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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 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 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 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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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 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 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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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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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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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 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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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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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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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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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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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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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 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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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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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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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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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 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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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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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 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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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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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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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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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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a)

(i) The two gases that can be used to perform the fountain experiment are hydrogen and helium.

(ii) The aim of the fountain experiment is to demonstrate the properties of a gas and the behavior of a gas in a closed container.

(iii) The fountain experiment involves filling a container with a gas, such as hydrogen or helium, and then releasing the gas into the air. The gas rises into the air and creates a fountain-like effect, which demonstrates the properties of a gas.

b)

(i) Two chemical industries are the petroleum industry and the pharmaceutical industry.

(ii) The effects of a chemical industry on the community in which it is sited can include air pollution, water pollution, and negative impacts on the health of local residents.

c)

(i) Three products of the destructive distillation of coal are coal gas, coal tar, and coke.

(ii) Coal gas can be used as a fuel, while coke can be used as a fuel and in the production of iron and steel.

d)

(i) Two substances responsible for hardness in water are calcium and magnesium.

(ii) Two methods for the removal of hardness in water are ion exchange and reverse osmosis.

(iii) Two advantages of hard water are that it can improve the flavor of coffee and tea, and it can provide beneficial minerals for the body.

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Indicator = Methyl Orange

Volume of the base used = 25.00cm3 ${}^3$

24.80+24.70+24.90

Average Titre = 1st+2nd+3rd3 $\frac{1𝑠𝑡+2𝑛𝑑+3𝑟𝑑}{3}$

= 24.80+24.70+24.903 $\frac{24.80+24.70+24.90}{3}$

= 24.80cm3

Alternatively 2 concordant titres can be used to calculate average titre 

Equation of thereaction; Na2 ${}_2$CO3 ${}_3$.XH2 ${}_2$O + 2HCl(aq) ${}_{(𝑎𝑞)}$ → $\to$ 2Nacl(aq) ${}_{(𝑎𝑞)}$ → $\to$ 2Nacl(aq) ${}_{(𝑎𝑞)}$ + CO2(aq) ${}_{2(𝑎𝑞)}$ + (X + 1)H2 ${}_2$O(l) ${}_{(𝑙)}$

CAVACBVB=nAnB $\frac{{𝐶}_{𝐴}{𝑉}_{𝐴}}{{𝐶}_{𝐵}{𝑉}_{𝐵}}=\frac{{𝑛}_{𝐴}}{{𝑛}_{𝐵}}$

CA ${}_{𝐴}$ = Molar concentration of HCl(aq) ${}_{(𝑎𝑞)}$ in moldm3 ${}^3$

VA ${}_{𝐴}$ = Volume of acid used in cm3 ${}^3$ = 24.80cm3 ${}^3$

CB ${}_{𝐵}$ = Molar concentration of Na2 ${}_2$Cu3 ${}_3$ . xH2 ${}_2$O in molddm3 ${}^3$

nA ${}_{𝐴}$ = 2

nB ${}_{𝐵}$ = 1

(b)(i) Concentration of C in moldm−3 ${}^{-3}$ = ?

From the equation reaction;

CAVACBVB=nAnB $\frac{{𝐶}_{𝐴}{𝑉}_{𝐴}}{{𝐶}_{𝐵}{𝑉}_{𝐵}}=\frac{{𝑛}_{𝐴}}{{𝑛}_{𝐵}}$

CA ${}_{𝐴}$ = 0.200 molddm−3 ${}^{-3}$ 

CB ${}_{𝐵}$ = ? 

VA ${}_{𝐴}$ = 24.80 cm

VB ${}_{𝐵}$ = 25.00 cm3 ${}^3$ 

Substitution of known values 

0.200×24.80CB×25.00=21 $\frac{0.200\times 24.80}{{𝐶}_{𝐵}\times 25.00}=\frac{2}{1}$

CB ${}_{𝐵}$ = 1×0.200×24.802×25.00 $\frac{1\times 0.200\times 24.80}{2\times 25.00}$

CB ${}_{𝐵}$ = 0.0992 moldm−3 ${}^{-3}$ 

(ii) Concentration of C in gdm3 ${}^3$ = ?

500 cm2 ${}^2$ → $\to$ 14.3 g

1000cm3 ${}^3$ → $\to$ 14.3500 $\frac{14.3}{500}$ x 1000g

= 28.6 dm−3 ${}^{-3}$

(iii) Molar mass of Na2 ${}_2$CO3 ${}_3$ . XH2 ${}_2$O

Molar conc in moldm−3 ${}^{-3}$ = conc. in gdm−3molar mass $\frac{{\text{conc. in gdm}}^{-3}}{\text{molar mass}}$ 

Molar mass gmol−1 ${}^{-1}$ = conc in g dm−3molar conc.in moldm−3 $\frac{{\text{conc in g dm}}^{-3}}{{\text{molar conc.in moldm}}^{-3}}$

28.6gdm−30.0992moldm−3 $\frac{28.6𝑔𝑑{𝑚}^{-3}}{0.0992𝑚𝑜𝑙𝑑{𝑚}^{-3}}$

= 288.3065

= 288 gmol−1 ${}^{-1}$

(iv) Value of x in Na2 ${}_2$CO3 ${}_3$ . xH2 ${}_2$O?

[H = 1.0, C = 12.0, O = 16.0, Na = 23.0]

N2 ${}_2$CO3 ${}_3$ . xH2 ${}_2$O = 288

2(23) + 12 + 3 (16) + x(2(1) + 16) = 288

46 + 12 +48 +18x = 288

106 + 18x = 288

18x = 288 - 106

18x = 182

x = 18218 $\frac{182}{18}$

x = 10.11

x = 10

Maelezo ya Majibu

(a)(i) Faraday's first law of electrolysis states that the amount of a substance produced at an electrode during electrolysis is directly proportional to the quantity of electricity passed through the electrolyte.

(a)(ii) Strong electrolytes completely dissociate into ions in solution, while weak electrolytes only partially dissociate into ions.

(b) One chemical property of ethyne is its ability to undergo addition reactions.

(c)(i) Unsaturated hydrocarbons are hydrocarbons that contain at least one double or triple bond between carbon atoms.

(c)(ii) CH3 + CH3OH → (CH3)2O is the balanced equation for the reaction.

(c)(iii) The major product formed in this reaction is dimethyl ether.

(d) One way to increase the rate of esterification is by using a catalyst.

(e) Using the equation Zn + H2SO4 → ZnSO4 + H2, the number of molecules of hydrogen gas produced can be calculated by first determining the number of moles of Zn used. The molar mass of Zn is 65.0 g/mol, so 3.75 g of Zn is equal to 0.0577 moles. From the balanced equation, it can be seen that one mole of Zn produces one mole of H2, so the number of moles of H2 produced is also 0.0577. Using Avogadro's number (6.02 x 10^23), the number of molecules of H2 produced is 3.47 x 10^22.

(f) One effect of global warming is an increase in the frequency and severity of extreme weather events such as hurricanes, floods, and heat waves.

(g)(i) Equation A represents a redox process.

(g)(ii) In Equation A, Pb is reduced from +2 to 0, while H2S is oxidized from -2 to 0.

(h)(i) Two of the main concepts of Bohr's model of the atom are the idea that electrons exist in discrete energy levels around the nucleus, and that electrons can jump between energy levels by absorbing or emitting energy in the form of photons.

(h)(ii) One limitation of Bohr's model is that it only works for single-electron systems, such as hydrogen. Another limitation is that it doesn't explain the chemical behavior of atoms beyond their electron configuration.

(i) Three factors that could influence the equilibrium position of a reversible reaction are changes in temperature, changes in pressure (for reactions involving gases), and changes in concentration of reactants or products.

(j) When calcium trioxocarbonate(iv) powder is added to separate equimolar solutions of hydrochloric acid and ethanoic acid, one similarity in the observation is the production of effervescence or the release of gas. One difference in the observation is that the gas produced in the hydrochloric acid solution is hydrogen gas, while the gas produced in the ethanoic acid solution is carbon dioxide gas.

Maelezo ya Majibu

(a)(i) An energy profile diagram can be drawn as follows:

Reactants (100 kJ) ----> Activated complex (210 kJ) ----> Products (54 kJ)

(a)(ii)
I. The activation energy of the reaction can be calculated as the difference in energy between the activated complex and the reactants, which is 210 kJ - 100 kJ = 110 kJ.
II. The enthalpy change, ΔH, of the reaction can be calculated as the difference in energy between the products and the reactants, which is 100 kJ - 54 kJ = 46 kJ.

(a)(iii) The balanced equation for the reaction cannot be determined with the information given.

(a)(iv) The reason for not being able to determine the balanced equation is because the information about the chemical formulas of the reactants, products, and the coefficients in the balanced equation are missing.

(a)(v) Based on the enthalpy change of the reaction, ΔH = 46 kJ, the reaction is exothermic.

(b)(i) The electron configuration for the elements can be determined as follows:

Element E: 1s² 2s² 2p³
Element F: 1s² 2s² 2p⁵
Element G: 1s² 2s² 2p⁶ 3s² 3p⁶
Element H: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s¹

(b)(ii)
I. The elements that are metals are element E and G.
II. The element most likely to form an ion with a charge of +3 is element H.
III. Element H belongs to group VII on the periodic table.
IV. The formula of the compound formed between F and G can be determined by combining the elements in a ratio that results in a neutral compound. Given that F has 9 electrons and G has 12 electrons, the formula of the compound can be determined as FG₂, or FG, with a charge of 2-.

(c) Isotopy refers to the existence of isotopes, which are atoms of the same element that have the same number of protons but a different number of neutrons in their nucleus.

(d) The three building blocks of matter are protons, neutrons, and electrons. Protons and neutrons make up the nucleus of an atom and are made up of even smaller particles called quarks. Electrons orbit the nucleus and are much lighter than the protons and neutrons. These three particles make up all matter in the universe and determine the chemical and physical properties of the elements.

Maelezo ya Majibu

(a)

1. (i) The salt E is likely to be Zinc sulfide (ZnS).
2. (ii) The equation for the decomposition of Zinc sulfide (ZnS) is:
   2 ZnS (s) -> 2 Zn (s) + S₈ (g)
3. (iii) When the residue is allowed to cool, a white solid would be observed. This solid would be Zinc (Zn).

(b)

To prepare 250cm³ of 0.2 mol dm³ H₂SO₄ from 150 cm³ of a 1.0 mol dm³ stock solution of the acid, you would need to dilute the stock solution. You can do this by adding 150 cm³ of the 1.0 mol dm³ stock solution to an equal amount of water, which would give you 300 cm³ of a 0.5 mol dm³ solution. Then, you could add 50 cm³ more of water to the solution, which would give you a final volume of 350 cm³ of a 0.2 mol dm³ solution of H₂SO₄.

(c)

The aqueous solution of Al₂(SO₄)₃ has a neutral effect on litmus paper. It does not turn red litmus paper blue or blue litmus paper red, indicating that it is a neutral solution.