WAEC SSCE - Chemistry - 2019

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The name of a organic compound is based on its molecular structure, which includes the arrangement of its carbon and hydrogen atoms. The compound CH\(_3\)C(CH\(_3\))CH\(_2\)CH\(_2\)CH\(_3\) has a total of seven carbon atoms, which form a straight chain. The first carbon atom has three methyl groups (CH\(_3\)) attached to it, while the second carbon atom has only one methyl group attached to it. Based on these characteristics, the name of the compound is 2,2-dimethylpentane. The prefix "2,2-dimethyl" indicates that there are two methyl groups attached to the second carbon atom, and the suffix "pentane" indicates that there are five carbon atoms in the chain.

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When quicklime (calcium oxide, CaO) dissolves in water, heat is evolved. This process is known as slaking of lime. When CaO is added to water, it undergoes a vigorous reaction in which it combines with water (H2O) to form calcium hydroxide [Ca(OH)2]. The balanced chemical equation for this reaction is: CaO + H2O -> Ca(OH)2 + heat The heat is released because the reaction is exothermic. This means that the products (Ca(OH)2) have a lower energy than the reactants (CaO and H2O). The excess energy is released in the form of heat. The heat evolved during the slaking of lime makes it useful in a variety of applications, such as in construction and in the treatment of water and wastewater. It can also be used as a source of heat in some industrial processes. Therefore, when quicklime dissolves in water, heat is evolved.

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The best indicator for the reaction between NaOH (sodium hydroxide) and HCOOH (formic acid) is phenolphthalein. Phenolphthalein is a pH indicator that changes color based on the acidity of the solution. In an acidic solution, it is colorless, while in a basic (alkaline) solution, it turns pink or red. This makes it ideal for monitoring the progress of a reaction between an acid and a base. As the reaction between NaOH and HCOOH proceeds and the concentration of hydroxide ions increases, the solution will become more basic, causing the phenolphthalein to change color, indicating that the reaction is complete.

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The compound that is formed by the oxidation of ethanol is acetic acid, which has the chemical formula CH\(_3\)CO\(_2\)H. Ethanol, which has the chemical formula C\(_2\)H\(_5\)OH, can be oxidized by various oxidizing agents, such as potassium dichromate or acidified potassium permanganate, to form acetic acid. During the oxidation process, the two carbon atoms in ethanol are oxidized to form a carbonyl group (C=O) in acetic acid, and two hydrogen atoms are removed to form a carboxyl group (-COOH). The other options listed do not correspond to the oxidation product of ethanol. C\(_2\)H\(_4\)CO\(_2\)H is oxalic acid, which is not derived from ethanol. C\(_2\)H\(_5\)CO\(_2\)H is ethanoic acid, which is another name for acetic acid and is the correct answer. CH\(_3\)OH is methanol, which is not an oxidation product of ethanol. Finally, CH\(_3\)CO\(_2\)H is also acetic acid, but it is not an oxidation product of ethanol since it is already in its fully oxidized form.

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Molecular mass is the sum of the atomic masses of all the atoms in a molecule. To determine which pair of components have the same molecular mass, we need to calculate the molecular mass of each component and compare them. The molecular mass of each component is calculated by adding up the atomic masses of all the atoms in the molecule. - Ethene: C\(_2\)H\(_4\) = (2 x 12.0) + (4 x 1.0) = 28.0 - Ethyne: C\(_2\)H\(_2\) = (2 x 12.0) + (2 x 1.0) = 26.0 - Ethane: C\(_2\)H\(_6\) = (2 x 12.0) + (6 x 1.0) = 30.0 - Carbon (ii) Oxide: CO = (1 x 12.0) + (1 x 16.0) = 28.0 - Carbon (iv) Oxide: CO\(_2\) = (1 x 12.0) + (2 x 16.0) = 44.0 From the calculations above, we can see that ethene and carbon (ii) oxide have the same molecular mass of 28.0.

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Stoichiometry is the branch of chemistry that deals with the quantitative relationships between reactants and products in a chemical reaction. It is based on the law of conservation of mass, which states that in a chemical reaction, the total mass of the reactants must equal the total mass of the products. This means that in a chemical reaction, the number of atoms of each element involved must be conserved. This can be used to calculate the amounts of reactants and products needed to produce a given reaction, and to predict the products that will result from a given reaction. The law of conservation of mass is the fundamental principle upon which stoichiometry is based. Therefore, stoichiometry is based on the law of conservation of mass.

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Vinegar is an aqueous solution of ethanoic acid.

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Oxidation potential is a measure of the tendency of a substance to lose electrons and get oxidized (or become positively charged). A higher oxidation potential means that the substance is more likely to lose electrons, while a lower oxidation potential means that it is less likely to lose electrons. In the case of Mg and Al, Mg has a higher oxidation potential of +2.37 volts compared to Al's +1.66 volts. This means that Mg is more likely to lose electrons and get oxidized, while Al is less likely to do so. Therefore, Mg will replace Al in a reaction. In other words, Mg will take the place of Al, meaning that Mg will be oxidized while Al will remain in the solution.

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Rutherford's alpha particle scattering experiment was conducted by bombarding a thin gold foil with alpha particles. The experiment led to the conclusion that most of the alpha particles passed through the gold foil with little or no deflection, but some were scattered at various angles. Rutherford deduced that the positive charge of an atom is concentrated in a tiny nucleus at the center of the atom, which is why most of the alpha particles passed through the gold foil with little or no deflection. Therefore, one of the deductions from Rutherford's alpha particle scattering experiment is that the nuclei of atoms are positively charged.

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A catalyst is a substance that speeds up the rate of a chemical reaction without being consumed in the process. When a catalyst is added to a system, the activation energy of the reaction remains unchanged, but the rate of the reaction increases. The activation energy of a reaction is the minimum amount of energy that must be supplied to the reactants for the reaction to occur. The addition of a catalyst does not change this energy requirement, but it can provide an alternative reaction pathway with a lower activation energy, allowing the reaction to proceed more easily and quickly. The rate of the reverse reaction, which is the rate at which the products of the reaction revert back to the reactants, may also be affected by the addition of a catalyst, but this depends on the specific reaction and the type of catalyst being used. The heat of reaction, which is the change in enthalpy (heat content) that occurs during a chemical reaction, is also not affected by the addition of a catalyst. This is because a catalyst only speeds up the reaction rate, but does not change the energy required for the reaction to occur. Therefore, the addition of a catalyst to a system with constant temperature and pressure would affect the rate of the reaction, but would not affect the activation energy, rate of the reverse reaction, or heat of reaction.

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If the value of \(\Delta H\) is positive for a reaction, it means that the reaction is endothermic. \(\Delta H\) stands for the change in enthalpy, which is a measure of the amount of heat absorbed or released during a chemical reaction. A positive value of \(\Delta H\) indicates that the reaction absorbs heat from the surroundings. This means that the reaction requires an input of energy in order to proceed. Endothermic reactions are those that absorb heat from the surroundings. In an endothermic reaction, the products have a higher energy than the reactants. The energy is absorbed from the surroundings, so the surroundings become cooler. For example, melting ice is an endothermic process because it absorbs heat from the surroundings. On the other hand, exothermic reactions release heat to the surroundings, and have a negative \(\Delta H\) value. These reactions are often spontaneous and occur without the need for an external energy input. For example, burning wood is an exothermic process because it releases heat to the surroundings. Therefore, if the value of \(\Delta H\) is positive for a reaction, it means that the reaction is endothermic.

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The reactant in the equation is C\(_2\)H\(_2\), which is the chemical formula for ethyne (also known as acetylene). The products are formed by adding hydrogen gas (H\(_2\)) to the ethyne molecule. When ethyne reacts with hydrogen, it undergoes a reduction reaction and forms ethene (also known as ethylene) as the first product, which still has a carbon-carbon double bond, and then ethane, which is a saturated hydrocarbon and has only single bonds between carbon atoms, as the second product. Therefore, the products X and Y, respectively, are ethene and ethane. So, the correct option is "ethene and ethane".

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The periodic table is an arrangement of elements according to their atomic number, which is the number of protons in the nucleus of an atom. The elements are arranged in order of increasing atomic number in rows and columns, with elements in the same row (period) having the same number of electron shells and elements in the same column (group/family) having similar chemical and physical properties due to their similar electronic configurations. The periodic table provides a systematic way to organize and study the properties of elements, allowing chemists and scientists to predict the behavior of elements and their compounds. The atomic number is a fundamental property of an element that determines its identity and properties, so it is the most important factor in arranging the elements in the periodic table. Therefore, the correct option is "Atomic number".

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Electrolysis is NOT used for the separation of mixtures. Electrolysis is a process in which an electric current is passed through a solution or molten compound to decompose it into its constituent elements or ions. It is used for the extraction of metals from their ores and for the purification of some chemicals. On the other hand, the other three methods mentioned - chromatography, crystallization, and distillation - are commonly used for the separation of mixtures. - Chromatography is a technique used to separate different components of a mixture based on their properties such as size, polarity, and solubility. It is used in fields such as chemistry, biochemistry, and environmental science. - Crystallization is the process of forming solid crystals from a solution, melt, or gas. It is used to purify solids and to separate a mixture of solids into individual components. - Distillation is a process that separates the components of a mixture based on their boiling points. It is used to purify liquids, to separate a mixture of liquids into individual components, and to remove impurities from a liquid. Therefore, electrolysis is NOT used for the separation of mixtures, while the other three methods - chromatography, crystallization, and distillation - are commonly used.

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Biotechnology is applied in the treatment of sewage, treatment of water, and purification of chemicals. Biotechnology is the application of living organisms or their products to improve human health and the environment. In the case of sewage treatment, biotechnology is used to degrade organic matter and remove pathogens from wastewater, making it safe for discharge into the environment. This can be done using microbes such as bacteria and fungi that break down organic matter through processes such as aerobic and anaerobic digestion. Similarly, in water treatment, biotechnology can be used to remove pollutants and pathogens from water sources to make them safe for drinking or other uses. This can involve the use of living organisms such as bacteria or algae to remove contaminants or the use of biological processes such as bioremediation to degrade pollutants. In addition, biotechnology can be applied in the purification of chemicals. This can involve the use of enzymes or other biological molecules to catalyze chemical reactions or the use of microbes to produce chemicals such as antibiotics or enzymes. Purification of metals, on the other hand, typically involves physical and chemical methods such as smelting, leaching, and electrolysis, and does not typically involve biotechnology.

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Of the given options, calcium is the metal that reacts slowly with cold water. Calcium reacts with water, producing hydrogen gas and calcium hydroxide. The reaction is exothermic but the heat produced is not sufficient to ignite the hydrogen gas. The other metals listed in the options - iron, silver, and potassium - are more reactive with water than calcium. Iron reacts with steam, while silver and potassium react vigorously with cold water. Therefore, the correct option is "Calcium".

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The process by which alkanoic acid reacts reversibly with alkanols is known as esterification. During this chemical reaction, a molecule of alkanoic acid and a molecule of an alcohol combine to form an ester and a molecule of water. This reaction is reversible, which means that the products can react with each other to produce the starting materials. Esters are a group of organic compounds that are commonly found in fragrances, flavors, and plastics. They have a characteristic fruity or flowery smell, and they are responsible for the flavor and aroma of many fruits and flowers. Esterification is an important industrial process used to produce a wide variety of products such as perfumes, flavors, and plastics.

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The solubility of a salt in a solution is the maximum amount of the salt that can dissolve in the solution at a given temperature to form a homogeneous mixture. It is typically expressed as the concentration of the salt in the solution, in units of moles per liter (mol/L) or grams per liter (g/L). To determine the solubility of a salt, you need to know the mass of the salt that dissolves in a given volume of solution at a given temperature. In this case, the saturated solution of the salt contains 0.28 g of the salt in 100 cm\(^3\) of the solution, which means that 0.28 g of the salt dissolves in 100 cm\(^3\) of the solution. To convert the mass of the salt to moles, you need to know the molecular weight of the salt. Given that the molecular weight of the salt is 56.0 g, 0.28 g of the salt is equal to 0.28/56.0 = 0.005 moles of the salt. The concentration of the salt in the solution is then given by the number of moles of the salt divided by the volume of the solution, which is 0.005 moles/100 cm\(^3\) = 0.05 mol/L. Therefore, the solubility of the salt X at 25ºC is 0.05 mol/L or 0.05 M.

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The organic compound with the lowest boiling point is (CH\(_3\))\(_3\). This is because it is a gas at room temperature and has a very low molecular weight. As the molecular weight of the organic compound increases, the boiling point also increases. Therefore, C\(_4\)H\(_10\) has a higher boiling point than (CH\(_3\))\(_3\), C\(_5\)H\(_12\) has a higher boiling point than C\(_4\)H\(_10\), and C\(_6\)H\(_14\) has the highest boiling point of all.

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The group to which elements belong in the periodic table is determined by the number of valence electrons. Valence electrons are the electrons in the outermost shell (valence shell) of an atom that are involved in chemical bonding. The group number of an element in the periodic table corresponds to the number of valence electrons in its atom. For example, elements in Group 1 (also known as the alkali metals) have one valence electron, elements in Group 2 (also known as the alkaline earth metals) have two valence electrons, and so on. Elements in the same group have similar chemical and physical properties due to their similar valence electron configurations. Therefore, the correct option is "valence electrons".

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Kipps apparatus can be used to prepare H2S. Kipps apparatus is a type of apparatus used in chemistry laboratories to prepare and store gases. It consists of a glass bulb that is connected to a tap and a gas generator. The bulb can be filled with a solution of a substance that will react with a base to produce a gas, such as H2S. To prepare H2S, the bulb is filled with a solution of sulfur and an acid, which will react to produce H2S gas. The gas can then be collected and stored in the bulb, and the tap can be used to release the gas as needed. Overall, Kipps apparatus is a useful tool for preparing and storing gases, and is commonly used in chemistry and other scientific fields to produce and store gases like H2S for research and experimentation.

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The pair of substances that has the highest heat of neutralization is the one composed of a strong acid and a strong base. When an acid reacts with a base, they undergo a neutralization reaction, in which an acid donates a proton (H+) to a base to form water and a salt. The heat released during this reaction is called the heat of neutralization. Strong acids and bases are highly reactive, meaning that they will release a large amount of heat when they neutralize each other. This is because they completely dissociate in water to form ions, making their reaction highly exothermic. On the other hand, weak acids and bases have a lower degree of dissociation and are less reactive, meaning that they will release less heat when they neutralize each other. This is because only a small fraction of the weak acid or base molecules will react, and the reaction will not be as exothermic as that of strong acids and bases. Therefore, the highest heat of neutralization will be observed in the reaction between a strong acid and a strong base.

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The compound that has coordinate bonds in its structure is Al\(_2\)Cl\(_6\). A coordinate bond (also known as a dative bond) is a type of covalent bond in which both electrons in the bond are donated by one atom. In Al\(_2\)Cl\(_6\), each aluminum atom shares its three valence electrons with three chlorine atoms, and each chlorine atom shares one of its electrons with the aluminum atom. This results in a compound with two aluminum atoms that are both surrounded by six chlorine atoms, and each aluminum atom has six shared electrons, three from its own valence electrons and three from the chlorine atoms' lone pairs of electrons. MgCl\(_2\), AgCl, and NaCl do not have coordinate bonds in their structures. MgCl\(_2\) and NaCl have ionic bonds, where the electrons are transferred from one atom to another. AgCl has a polar covalent bond, where the electrons are shared unequally between the two atoms.

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Non-polar substances are those that have an equal distribution of electrons and therefore have no positive or negative poles. An example of a non-polar substance is Hydrogen Bromide. On the other hand, polar substances have a positive and negative pole, meaning that the electrons are not evenly distributed. Examples of polar substances are Water and Ammonia. Bromine, like Hydrogen Bromide, is a non-polar substance.

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The bleaching action of chlorine is through the process of oxidation. Chlorine is a strong oxidizing agent, which means that it is able to remove electrons from other molecules. In the presence of water, chlorine reacts with water molecules to form hypochlorous acid, which then breaks down into hypochlorite ions and hydrogen ions. The hypochlorite ions are responsible for the bleaching action. When hypochlorite ions come into contact with a colored molecule, such as a dye or a pigment, they oxidize the molecule by removing electrons. This changes the chemical structure of the molecule, making it colorless. This is why chlorine is such an effective bleaching agent. Oxidation is a chemical process in which a molecule loses electrons. This process is opposite to reduction, in which a molecule gains electrons. Chlorine is a strong oxidizing agent because it has a high tendency to attract electrons from other molecules. This makes it useful in a variety of applications, including water treatment and bleaching.

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Among the options given, H\(_2\)SO\(_4\) (sulfuric acid) can be classified as a heavy chemical. Heavy chemicals refer to chemicals that are produced in large quantities and are used as raw materials in various industries, such as fertilizers, pharmaceuticals, plastics, and dyes. Sulfuric acid is one of the most widely produced chemicals in the world, and it is used in the production of fertilizers, detergents, and other industrial chemicals. Therefore, H\(_2\)SO\(_4\) can be considered a heavy chemical. CaOCl\(_2\) (calcium hypochlorite), AgNO\(_3\) (silver nitrate), and CaO (calcium oxide) are also chemicals but they are not typically classified as heavy chemicals because they are not produced in the same large quantities as sulfuric acid and are not as widely used as raw materials in various industries.

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The number of electrons in the 3d orbital of \(_{24}\)Cr is 5. In the electronic configuration of chromium (Cr), the atomic number 24 indicates that there are 24 electrons present in the atom. The electronic configuration of Cr is: 1s\(^2\) 2s\(^2\) 2p\(^6\) 3s\(^2\) 3p\(^6\) 4s\(^1\) 3d\(^5\). The electronic configuration tells us that there are five electrons present in the 3d orbital of Cr. This is because the 3d orbital can hold up to 10 electrons, and in the case of Cr, only five of those slots are occupied by electrons. Therefore, the number of electrons in the 3d orbital of \(_{24}\)Cr is 5.

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Calcium and magnesium belong to the same group in the periodic table because they have the same number of valence electrons in their outermost shell. They both have two valence electrons, which makes them very similar in terms of their chemical behavior. This similarity is also reflected in their properties, such as their ability to form cations and their tendency to form colorless salts. Therefore, the correct option is "have the same number of valence electrons".

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A desiccator is not used in volumetric analysis. A desiccator is a container used for preserving moisture-sensitive items by creating a dry and airtight environment. It is commonly used in laboratory settings to store chemicals and other materials that are sensitive to moisture. On the other hand, a pipette and a burette are essential apparatus in volumetric analysis. A pipette is a laboratory tool used to measure and dispense precise volumes of liquid, while a burette is a long, graduated glass tube with a precisely adjustable valve used for adding a solution of known volume to a reaction mixture. A conical flask is also commonly used in volumetric analysis. It is a type of laboratory flask with a cone-shaped body and a narrow neck. It is used to mix and store solutions, as well as to hold reactions while they are being observed.

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The oxidation state of chromium in the Cr\(_2\)O\(_7)\(-2\) ion is +6. The oxidation state of an element refers to the number of electrons it has lost or gained in a chemical reaction. In the Cr\(_2\)O\(_7)\(-2\) ion, there are two chromium atoms, each with a different oxidation state. To determine the oxidation state of chromium in the Cr\(_2\)O\(_7)\(-2\) ion, we need to consider the overall charge of the ion, which is -2. Oxygen has an oxidation state of -2, so the seven oxygen atoms in the ion contribute a total oxidation state of -14. We also know that the oxidation state of the entire Cr\(_2\)O\(_7)\(-2\) ion is -2, so the two chromium atoms must contribute a total oxidation state of +12 to balance out the negative charge. Since there are two chromium atoms, each chromium atom must have an oxidation state of +6. This is because +6 + +6 = +12, which balances out the -2 charge on the ion. Therefore, the oxidation state of chromium in the Cr\(_2\)O\(_7)\(-2\) ion is +6.

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The pH of water is a measure of its acidity, with a lower pH indicating a more acidic solution and a higher pH indicating a more basic solution. When the pH of water changes from 7 to 6, it means that the water has become more acidic. Carbon dioxide (CO\(_2\)) is known to dissolve in water and form carbonic acid, which is a weak acid. This weak acid will increase the acidity of the water, causing its pH to decrease. Oxygen and nitrogen are not acidic gases and would not cause the pH of the water to change. Carbon(II) oxide, also known as carbon monoxide, is a toxic gas that can interfere with the ability of the blood to transport oxygen, but it is not an acidic gas and would not cause the pH of the water to change. Therefore, the gas that caused the change in the pH of the water from 7 to 6 is carbon dioxide (CO\(_2\)).

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A solid with higher melting and boiling points is likely to be an electrovalent compound, also known as an ionic compound. Electrovalent compounds are made up of positively charged ions (cations) and negatively charged ions (anions) held together by electrostatic attractions. These attractions are strong and require a significant amount of energy to break, which is why ionic compounds have higher melting and boiling points compared to covalent compounds, which are held together by weaker intermolecular forces. Covalent compounds consist of molecules with covalent bonds between atoms, where atoms share electrons to form a bond. Covalent compounds typically have low melting and boiling points, as the intermolecular forces are weak. A dative compound, also known as a coordinate compound, contains a coordinate bond where one atom donates both electrons to the bond. These types of compounds can have varying melting and boiling points depending on the strength of the bond. Non-metals can form both covalent and ionic compounds, so their identity alone does not determine the melting and boiling points of a solid. Therefore, the correct option is "An Electrovalent compound".

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The pH of a solution is a measure of its acidity, with a lower pH indicating a more acidic solution and a higher pH indicating a more basic solution. The pH scale ranges from 0 to 14, with 7 being neutral. Of the options given, hydrochloric acid has the lowest pH because it is a strong acid. Ethanoic acid (also known as acetic acid) is a weak acid, so its pH is higher than that of hydrochloric acid. Sodium chloride is a salt and does not contribute to the acidity of a solution, so its pH would be neutral (7). Sodium hydrogen trioxocarbonate (IV), also known as sodium carbonate, is a basic compound, so its pH would be higher than 7, meaning it is not as acidic as the other options. Therefore, hydrochloric acid would have the lowest pH of the solutions listed.

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When excess chlorine is mixed with ethane at room temperature, the product formed is 1,2-dichloroethane. This is because ethane reacts with chlorine in the presence of ultraviolet light or heat to form a mixture of 1,2-dichloroethane and 1,1,2,2-tetrachloroethane. However, in the absence of light or heat, only 1,2-dichloroethane is produced as the major product.

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The empirical formula of a compound is the simplest whole-number ratio of atoms in the compound. To find the empirical formula, we need to convert the percent composition by mass of each element to its corresponding number of moles, and then divide each number by the smallest number of moles to find the smallest whole-number ratio. Given the following percent composition by mass: - Carbon: 72% = 72 g / (12.0 g/mol) = 6.0 mol - Hydrogen: 12% = 12 g / (1.0 g/mol) = 12.0 mol - Oxygen: 16% = 16 g / (16.0 g/mol) = 1.0 mol Dividing each number of moles by the smallest number of moles (1.0 mol for oxygen), we get: - Carbon: 6.0 mol / 1.0 mol = 6.0 - Hydrogen: 12.0 mol / 1.0 mol = 12.0 - Oxygen: 1.0 mol / 1.0 mol = 1.0 Rounding each number to the nearest whole number, we get: - Carbon: 6.0 - Hydrogen: 12.0 - Oxygen: 1.0 The smallest whole-number ratio of atoms in the compound is C\(_6\)H\(_12\)O, which is the empirical formula of the compound.

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NH\(_3\) is acting as a reducing agent in the given reaction. A reducing agent is a substance that donates electrons, and as a result, it causes the reduction of another substance in a chemical reaction. In the given reaction equation, NH\(_3\) donates electrons to Cl\(_2\), which gets reduced to HCl. At the same time, NH\(_3\) gets oxidized to N\(_2\). Therefore, NH\(_3\) acts as a reducing agent in the given reaction. It is important to note that a reducing agent always gets oxidized in a chemical reaction as it donates electrons to another substance. Similarly, an oxidizing agent always gets reduced as it accepts electrons from another substance.

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In a mixture of gases that do not react chemically together, the pressure of each individual gas is called its partial pressure. When different gases are mixed together in a container, each gas behaves independently of the others. The total pressure exerted by the mixture of gases is equal to the sum of the partial pressures of each gas present in the mixture. This is known as Dalton's law of partial pressures. Mathematically, this can be represented as: Total pressure = Partial pressure of gas A + Partial pressure of gas B + ... + Partial pressure of gas N The partial pressure of each gas in the mixture depends on its mole fraction, which is the ratio of the number of moles of that gas to the total number of moles of all gases present in the mixture. The partial pressure of a gas can be calculated using the following formula: Partial pressure of gas A = Mole fraction of gas A x Total pressure Therefore, in a mixture of gases that do not react chemically together, the pressure of the individual gas is known as its partial pressure, and it can be calculated using the above formula.

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(a)(i) Dalton's atomic theory: matter is made of tiny indivisible particles called atoms; atoms of the same element are identical in mass and properties, and differ from those of other elements; atoms combine in simple whole-number ratios to form compounds; atoms are neither created nor destroyed in a chemical reaction (which is only a rearrangement of atoms).

(ii) Because whole atoms combine in simple whole-number ratios, when two elements form more than one compound the masses of one element that combine with a fixed mass of the other must be in a simple whole-number ratio, which is the law of multiple proportions.

(b)(i) Three products: margarine (by hydrogenation), soap (by saponification), and glycerol (by hydrolysis/saponification).

(ii)

• Hydrogenation (to margarine): unsaturated oil \(+ H_2 \xrightarrow{Ni} \) hardened (saturated) fat.
• Saponification (to soap): \((C_{17}H_{35}COO)_3C_3H_5 + 3NaOH \rightarrow 3C_{17}H_{35}COONa + C_3H_5(OH)_3\)
• Hydrolysis (to glycerol): \((C_{17}H_{35}COO)_3C_3H_5 + 3H_2O \rightarrow 3C_{17}H_{35}COOH + C_3H_5(OH)_3\)

(c) This is a diagonal relationship: beryllium and aluminium lie diagonally in the periodic table and have similar charge-to-radius ratios (similar polarizing power and electronegativity), so their properties resemble each other rather than those of magnesium.

(d) \(I_2 + 2Na_2S_2O_3 \rightarrow 2NaI + Na_2S_4O_6\)

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Cracking of a Hydrocarbon

A hydrocarbon with formula C<sub>10</sub>H<sub>22</sub> is cracked to produce C<sub>6</sub>H<sub>14</sub> and another hydrocarbon, P.

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(a)(i) Three methods: neutralization (acid + alkali/base); action of an acid on a metal; precipitation (double decomposition).

(ii) One equation for each:

• Neutralization: \(NaOH + HCl \rightarrow NaCl + H_2O\)
• Acid on metal: \(Zn + H_2SO_4 \rightarrow ZnSO_4 + H_2\)
• Precipitation: \(AgNO_3 + NaCl \rightarrow AgCl + NaNO_3\)

(iii) Two uses of sodium trioxocarbonate(IV), Na<sub>2</sub>CO<sub>3</sub>: manufacture of glass; softening of hard water (also soap/detergent and paper manufacture).

(b) Purifying copper by electrolysis: use the impure copper as the anode and a thin strip of pure copper as the cathode, dipped in acidified copper(II) tetraoxosulphate(VI) solution as electrolyte. On passing current, the anode dissolves (\(Cu \rightarrow Cu^{2+} + 2e^-\)) and pure copper is deposited on the cathode (\(Cu^{2+} + 2e^- \rightarrow Cu\)). Impurities collect below the anode as anode sludge.

(c)(i) Take the solubility (g per 100 g water) as the mass dissolved at each temperature.

Sodium chloride: deposited \(= 39.0 - 36.3 = 2.7\) g.
Percentage of the dissolved salt deposited \(= \dfrac{2.7}{39.0} \times 100 = \mathbf{6.9\%}\).

Silver trioxonitrate(V): deposited \(= 952.0 - 297.0 = 655.0\) g.
Percentage of the dissolved salt deposited \(= \dfrac{655.0}{952.0} \times 100 = \mathbf{68.8\%}\).

(ii) Silver trioxonitrate(V) can be purified more efficiently by crystallization, because a far larger fraction of it (about 68.8%) crystallizes out on cooling than for sodium chloride (about 6.9%).

Maelezo ya Majibu

Solvay Process and Chemical Reactions

(a) Functions in the Solvay Process:

• Limestone (CaCO3):
  • Function: Source of calcium oxide (CaO) for the regeneration of ammonia.
  • Equation: CaCO3 (g) -> CaO (s) + CO2 (g) (Limestone decomposes to lime and carbon dioxide)
• Ammonia (NH3):
  • Function:
    • Reacts with calcium and chloride ions in brine to form ammonium chloride (NH4Cl).
    • Makes the solution alkaline, favoring the precipitation of calcium carbonate.
  • Equation: Not directly involved in a reaction, but participates in the overall process.

(b) Fermentation and Shattered Bottle:

(i) Glucose Fermentation:

C6H12O6 (aq) -> 2C2H5OH (aq) + 2CO2 (g) (Glucose ferments to ethanol and carbon dioxide)

(ii) Shattered Bottle:

Fresh palm wine undergoes fermentation by yeast. During fermentation, yeast consumes sugar and produces carbon dioxide gas. In a tightly sealed bottle, the CO2 gas gets trapped and builds up pressure. As pressure increases, the glass bottle can no longer withstand the force and shatters.

(c) Metal Reactivity:

(i) Increasing Reactivity: K > Na > Fe > Cu

(ii) Reacting with Cold Water: K, Na (These alkali metals react vigorously with water)

(iii) Colored Salts: Cu (Copper forms characteristic colored salts, like blue copper sulfate)

(d) Redox Reactions and Half Reactions:

(i) Redox Reaction:

A redox reaction is a type of chemical reaction where there is a transfer of electrons between the participating atoms or ions. Oxidation is the loss of electrons, and reduction is the gain of electrons.

(ii) Identifying Redox Reactions:

• Redox: (II), (V)
• Not Redox: (I), (III), (IV)

(iii) Explanation for Redox Reactions:

• (II) AgCl + 2NH3 -> [Ag(NH3)2]Cl:
  • Silver (Ag) loses an electron to become Ag+ (oxidation).
  • Nitrogen (N) in ammonia gains an electron to form the complex ion [Ag(NH3)2]+ (reduction).
• (V) 2FeCl3 + 2KI -> 2FeCl2 + 2KCl + I2:
  • Iron (Fe) in FeCl3 loses an electron to become Fe2+ in FeCl2 (oxidation).
  • Iodine (I) gains an electron to form I2 (reduction).

(iv) Balanced Half Reactions (Examples):

• Oxidation Half Reaction (from reaction II):

                Ag -> Ag+ + e-

• Reduction Half Reaction (from reaction V):

               I- -> I2 + e- (Note: I- needs to be balanced with Cl- from the reactant side)

Maelezo ya Majibu

(a) Conditions for cracking long-chain hydrocarbons to give more gasoline

A high temperature (about 400 - 600 °C) together with a catalyst (aluminosilicate / zeolite, i.e. catalytic cracking). Purely thermal cracking uses high temperature and high pressure.

(b) Two reasons metallic objects are electroplated

• To protect the metal from corrosion / rusting.
• To improve the appearance (make it bright and decorative). (Also: to resist wear and increase durability.)

(c)(i) Why calcium oxide cannot dry hydrogen chloride gas

Calcium oxide is a basic oxide, and hydrogen chloride is an acidic gas; the two react chemically (CaO + 2HCl → CaCl2 + H2O) instead of the CaO merely removing moisture, so the gas would be absorbed and lost.

(c)(ii) One drying agent for HCl gas: concentrated tetraoxosulphate(VI) acid (conc. H<sub>2</sub>SO<sub>4</sub>). (Anhydrous calcium chloride is also acceptable.)

(d) Conc. HNO<sub>3</sub> added to iron(II) tetraoxosulphate(VI) and heated

Concentrated trioxonitrate(V) acid is a strong oxidizing agent. It oxidizes the pale-green Fe2+ ion to the yellow (brown-yellow) Fe3+ ion, which accounts for the colour change from pale green to yellow. In being reduced, the acid liberates nitrogen(IV) oxide, the brown gas observed.

\[3Fe^{2+} + 4H^{+} + NO_3^{-} \longrightarrow 3Fe^{3+} + NO\uparrow + 2H_2O\]

(with concentrated acid the brown gas evolved is NO<sub>2</sub>).

(e)(i) Zinc oxide with dilute acid and with alkali

\[ZnO + H_2SO_4 \longrightarrow ZnSO_4 + H_2O\]

\[ZnO + 2NaOH \longrightarrow Na_2ZnO_2 + H_2O\]

(sodium zincate is formed with the alkali).

(e)(ii) Property shown: zinc oxide is amphoteric (it reacts with both acids and bases).

(f) Isotopes 3517Cl and 3717Cl

• Similarity: both have the same atomic (proton) number, 17, hence the same number of electrons and the same chemical properties.
• Difference: they have different mass numbers (35 and 37), i.e. different numbers of neutrons (18 and 20).

(g) Two products when chlorine water is exposed to sunlight

Chlorine water (HOCl + HCl) decomposes in sunlight, 2HOCl → 2HCl + O2, giving hydrogen chloride (hydrochloric acid) and oxygen.

(h) For the reaction 2H2S + SO2 → 3S + 2H2O

Assigning oxidation numbers to sulphur: it is −2 in H<sub>2</sub>S, +4 in SO<sub>2</sub>, and 0 in the free sulphur produced.

• Species undergoing oxidation: H<sub>2</sub>S - the sulphur in it rises from −2 to 0 (loses electrons).
• Oxidizing agent: SO<sub>2</sub> - the sulphur in it falls from +4 to 0 (it is reduced, so it oxidizes the H<sub>2</sub>S).

(i) Carbon-12 scale

It is the scale of relative atomic masses on which one atom of the carbon-12 isotope is assigned a mass of exactly 12.000 units, and the mass of every other atom is measured relative to one-twelfth of the mass of a carbon-12 atom.

(j) Two properties of a chemical system in equilibrium

• It is dynamic: the forward and backward reactions continue at equal rates.
• Its macroscopic properties (concentrations, colour, pressure) remain constant, and it is attained only in a closed system.

Maelezo ya Majibu

(a)(i) Hardness of water is the property of water that prevents it from readily forming lather (foam) with soap, caused by dissolved calcium and magnesium salts.

(ii) Boil a measured sample of the water to remove temporary hardness, then filter. Determine (by soap-lather titration) the soap needed to give a lasting lather with the unboiled sample (total hardness) and with the boiled sample (permanent hardness). The permanent hardness is the value obtained with the boiled sample; the difference gives the temporary hardness.

(iii) Any two: it wastes soap and forms scum; it deposits scale (fur) in kettles, boilers and pipes, which wastes fuel and can block pipes.

(b)(i) Reagents: potassium (or sodium) trioxonitrate(V), KNO<sub>3</sub>, and concentrated tetraoxosulphate(VI) acid.

(ii) Conditions: gentle heating, using all-glass apparatus (the acid attacks cork and rubber).

(iii) Concentrated HNO<sub>3</sub> with:

• Sulphur: it oxidizes sulphur to tetraoxosulphate(VI) acid. \(S + 6HNO_3 \rightarrow H_2SO_4 + 6NO_2 + 2H_2O\)
• Aluminium: no reaction; aluminium is rendered passive because a protective oxide layer forms on its surface.

(c) One amphoteric oxide: aluminium oxide, Al<sub>2</sub>O<sub>3</sub> (also ZnO or PbO).

Maelezo ya Majibu

(a) The Contact process. The scheme reads: X(gas) + Y(solid) \(\xrightarrow{I}\) Sulphur(IV) oxide; Oxygen + Sulphur(IV) oxide \(\xrightarrow{II}\) Sulphur(VI) oxide \(\xrightarrow{III}\) Oleum \(\xrightarrow{IV}\) Concentrated H<sub>2</sub>SO<sub>4</sub>.

(i) X and Y. The first step burns a solid in a gas to give SO<sub>2</sub>, so X is oxygen (air) and Y is sulphur.

(ii) Balanced equations.

Process I: \[ S_{(s)} + O_{2(g)} \rightarrow SO_{2(g)} \]

Process II: \[ 2SO_{2(g)} + O_{2(g)} \rightleftharpoons 2SO_{3(g)} \]

Process III (SO<sub>3</sub> absorbed in concentrated H<sub>2</sub>SO<sub>4</sub> to form oleum): \[ SO_{3(g)} + H_2SO_{4(l)} \rightarrow H_2S_2O_{7(l)} \]

Process IV (oleum diluted with water): \[ H_2S_2O_{7(l)} + H_2O_{(l)} \rightarrow 2H_2SO_{4(l)} \]

(iii) Catalyst in process II: vanadium(V) oxide, V<sub>2</sub>O<sub>5</sub> (platinum can also be used).

(iv) Le Chatelier's principle. Process II is exothermic in the forward direction. By Le Chatelier's principle, raising the temperature shifts the equilibrium position in the endothermic (backward) direction so as to absorb the added heat. This reduces the yield of SO<sub>3</sub>, so a high temperature does not favour the forward reaction; a moderate temperature (about 450 C) is used instead.

(v) Two uses of SO<sub>2</sub>:

• As a bleaching agent (for wood pulp, straw, silk).
• As a food preservative and fumigant/disinfectant (it also serves as the starting gas for making H<sub>2</sub>SO<sub>4</sub>).

(b) Volume of unused oxygen. From \(2H_{2(g)} + O_{2(g)} \rightarrow 2H_2O_{(g)}\), H<sub>2</sub> reacts with O<sub>2</sub> in the ratio 2 : 1 by volume.

\[ \text{O}_2\ \text{needed} = \frac{1}{2} \times 40 = 20\ \text{cm}^3 \]

\[ \text{Unused O}_2 = 30 - 20 = \mathbf{10\ cm^3} \]

(c) Heating calcium trioxocarbonate(IV).

\[ CaCO_{3(s)} \rightarrow CaO_{(s)} + CO_{2(g)} \]

Moles of CaCO<sub>3</sub>: molar mass \(= 40.0 + 12.0 + (3\times16.0) = 100\ g\,mol^{-1}\).

\[ n = \frac{1.0}{100} = 0.01\ \text{mol} \]

Mass of residue (CaO): molar mass of CaO \(= 40.0 + 16.0 = 56\ g\,mol^{-1}\).

\[ m_{CaO} = 0.01 \times 56 = \mathbf{0.56\ g} \]

Volume of CO<sub>2</sub> at s.t.p.: \(n_{CO_2} = 0.01\ mol\).

\[ V = 0.01 \times 22.4 = 0.224\ dm^3 = \mathbf{224\ cm^3} \]

Volume at 15 C and 760 mmHg: pressure is unchanged (760 mmHg = s.t.p. pressure), so use \(\dfrac{V_1}{T_1} = \dfrac{V_2}{T_2}\) with \(T_1 = 273\ K,\ T_2 = 273 + 15 = 288\ K\).

\[ V_2 = 0.224 \times \frac{288}{273} = \mathbf{0.236\ dm^3\ (\approx 236\ cm^3)} \]

Maelezo ya Majibu

(a)(i) \(SO_2 + Cl_2 + 2H_2O \rightarrow H_2SO_4 + 2HCl\) (sulphur dioxide is oxidized to tetraoxosulphate(VI) acid).

(ii) Add barium chloride solution followed by dilute HCl: a white precipitate of BaSO<sub>4</sub> (insoluble in the acid) confirms the tetraoxosulphate(VI) ion.

(iii) Concentrated H<sub>2</sub>SO<sub>4</sub> with:

• Oxalic acid: it dehydrates the acid. \((COOH)_2 \rightarrow CO + CO_2 + H_2O\)
• Copper (hot): it oxidizes copper. \(Cu + 2H_2SO_4 \rightarrow CuSO_4 + SO_2 + 2H_2O\)

(iv) With oxalic acid it acts as a dehydrating agent; with copper it acts as an oxidizing agent.

(b)(i) Water is called a universal solvent because it dissolves a greater number of substances than any other solvent, owing to its polar nature.

(ii) Add the liquid to white anhydrous copper(II) tetraoxosulphate(VI): it turns blue if water is present (or anhydrous cobalt(II) chloride turns from blue to pink).

(c)(i) The major components of synthesis gas are carbon(II) oxide and hydrogen.

(ii) It burns completely and cleanly to give carbon(IV) oxide and water, producing no soot or sulphurous fumes.

(d) From destructive distillation of coal: (i) solid: coke; (ii) liquid: coal tar (or ammoniacal liquor); (iii) gas: coal gas.

(e)(i) \(C + O_2 \rightarrow CO_2\)

(ii) Carbon(IV) oxide: (I) physical: it is a colourless gas, denser than air; (II) chemical: it turns lime water milky and does not support combustion.

Maelezo ya Majibu

(a) The stability of an atom's electron configuration is determined by the number of electrons in the outermost energy level. Helium has two electrons in its outermost energy level, which is the maximum number it can hold, so its electron configuration is stable. Beryllium, on the other hand, has four electrons in its outermost energy level, which is not stable, so it tends to lose or gain electrons to achieve stability.

(b)(i) An atom of hydrogen would have one electron, while an atom of lithium would have no electrons after forming an ionic bond.

(b)(ii) When hydrogen forms an ionic bond, it loses its single electron to become a positively charged ion (H+). Lithium, with three electrons in its outermost energy level, readily donates one electron to become a positively charged ion (Li+). This electron donation allows both elements to achieve a stable electron configuration.

(c) Two factors to consider when siting a chemical industry are access to raw materials and transportation infrastructure.

(d) Two advantages of using a catalyst instead of high temperatures in chemical reactions are that it reduces the energy required for the reaction and it can increase the selectivity of the reaction, meaning that it can produce more of the desired product and less of unwanted byproducts.

(e) To calculate the mass of turpentine that would completely burn in 21.3 g of chlorine, we need to use stoichiometry. First, we need to calculate the number of moles of chlorine present:

21.3 g Cl2 x (1 mol Cl2/71 g Cl2) = 0.3 mol Cl2

Next, we need to use the balanced chemical equation to determine the number of moles of turpentine needed to react with 0.3 mol Cl2:

8 mol Cl2 / 1 mol turpentine

0.3 mol Cl2 x (1 mol turpentine / 8 mol Cl2) = 0.0375 mol turpentine

Finally, we can use the molar mass of turpentine to convert the number of moles to mass:

0.0375 mol turpentine x 136 g/mol = 5.1 g turpentine

Therefore, 5.1 g of turpentine would completely burn in 21.3 g of chlorine.

(f) Cracking is the process of breaking down large hydrocarbons into smaller ones by breaking the carbon-carbon bonds in the molecules. This is typically done by heating the hydrocarbons to high temperatures in the presence of a catalyst.

(g) Two factors that may influence the value of electron affinity are the atomic radius and the effective nuclear charge. A smaller atomic radius and a higher effective nuclear charge will increase the attraction between the nucleus and electrons, leading to a higher electron affinity.

(h) Carbohydrates are a group of biomolecules that include sugars, starches, and cellulose. They are made up of carbon, hydrogen, and oxygen atoms and serve as a source of energy for living organisms.

(i) Simple sugars are monosaccharides, meaning they consist of a single sugar molecule, while starch is a polysaccharide, meaning it is made up of multiple

Maelezo ya Majibu

(a)(i) Laboratory preparation and collection of dry chlorine gas

Chlorine is prepared by heating manganese(IV) oxide with concentrated hydrochloric acid:

\[MnO_2(s)+4HCl(aq)\rightarrow MnCl_2(aq)+Cl_2(g)+2H_2O(l)\]

The gas is passed through saturated sodium chloride solution to remove hydrogen chloride, then through concentrated sulphuric acid to dry it. It is collected by downward delivery, that is, by upward displacement of air, because chlorine is denser than air and dissolves in water.

(ii) Uses of chlorine

1. It is used to disinfect and sterilize drinking water.
2. It is used as a bleaching agent for paper pulp and textiles.

(b) Preparation of hydrogen from water gas

Water gas, a mixture of carbon(II) oxide and hydrogen, is mixed with steam and passed over heated iron(III) oxide or chromium(III) oxide catalyst at about \(450^\circ\mathrm{C}\). The carbon(II) oxide is converted to carbon(IV) oxide, producing more hydrogen:

\[CO(g)+H_2O(g)\rightarrow CO_2(g)+H_2(g)\]

The carbon(IV) oxide is removed by absorption in water under pressure or in aqueous sodium hydroxide, potassium hydroxide, or calcium hydroxide. The gas remaining is hydrogen.

(c)(i) The chief ore of aluminium is bauxite, \(Al_2O_3\cdot 2H_2O\).

(ii) The ore is purified to obtain alumina and to remove impurities, such as iron(III) oxide and silica, which would contaminate the aluminium and may poison the electrodes.

(iii) The electrodes used are carbon (graphite) electrodes.

(iv) Cryolite, \(Na_3AlF_6\), is added to lower the melting point of alumina, thereby reducing the energy required for electrolysis. It also increases the electrical conductivity of the electrolyte.

(d) Three products obtained directly from the destructive distillation of coal are:

1. coke;
2. coal tar;
3. coal gas.

Ammoniacal liquor is also obtained.