WAEC SSCE - Chemistry - 1998 (Objective)

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The spontaneity of a chemical reaction is determined by the change in the free energy of the system. Free energy is the energy available to do work, and a chemical reaction is spontaneous when the change in free energy is negative. In other words, the products of the reaction have lower free energy than the reactants, and the reaction can occur without the input of additional energy. Concentration, temperature, pressure, and volume can affect the spontaneity of a reaction, but the driving force behind the spontaneity is the change in free energy.

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A positive reaction to Fehling's test indicates the presence of reducing sugars. Fehling's test is a chemical test used to detect the presence of reducing sugars such as glucose, fructose, and maltose in a solution. The test involves adding Fehling's reagent, which is a mixture of copper sulfate and potassium tartrate, to the solution being tested and heating it. If reducing sugars are present, they will react with the copper ions in the Fehling's reagent, causing a red precipitate of copper(I) oxide to form. Therefore, the presence of a red precipitate after the Fehling's test indicates the presence of reducing sugars in the solution.

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Rusting of iron is an example of a redox reaction. It involves the oxidation of iron, where it loses electrons and reacts with oxygen to form iron oxide. The process is also accompanied by reduction, where oxygen gains electrons to form hydroxide ions. The overall reaction is: Fe + 2H2O + O2 → Fe(OH)2 2Fe(OH)2 + 1/2O2 → Fe2O3.H2O (rust) Therefore, rusting of iron involves both oxidation and reduction, which is the basis of redox reactions.

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The two metallic ions commonly present in hard water are Mg²⁺ and Ca²⁺. Hard water is water that contains high levels of dissolved minerals, such as calcium and magnesium. These minerals come from the ground and rock formations that the water flows through. When hard water is heated or evaporates, it leaves mineral deposits behind that can build up in pipes, appliances, and other surfaces. Mg²⁺ and Ca²⁺ ions are the most common minerals that contribute to the hardness of water. They form insoluble precipitates with soaps, reducing their lathering ability, and can also cause scaling in pipes and boilers. Therefore, it is important to remove or reduce these ions from water for many domestic and industrial applications.

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When alkanols react with sodium, the gas evolved is hydrogen. This is because sodium is more reactive than hydrogen and reacts with alkanols to liberate hydrogen gas. The reaction is as follows: R-OH + Na → R-ONa + 1/2H2 where R represents the alkyl group. The reaction is a redox reaction, in which sodium is oxidized and alkanols are reduced. The hydrogen gas evolved can be tested using the 'squeaky pop' test, which involves holding a lighted splint near the gas. The hydrogen gas ignites with a squeaky pop sound due to its high reactivity with oxygen.

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An aqueous solution is acidic if it contains more H₃O- ions than OH- ions. This is because the concentration of H₃O- ions determines the acidity or basicity of a solution. In an acidic solution, there is an excess of H₃O- ions, which can donate a proton (H+) to other substances, making it corrosive and able to change the color of litmus to red. A high pH value indicates that the solution is basic, not acidic. A bitter taste is not a reliable indicator of acidity, as many substances can have a bitter taste without being acidic.

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When Cr^2- is oxidized to CrO₄^2-, it gains oxygen atoms and loses electrons. To determine how many electrons are lost, we need to look at the change in oxidation state of chromium in the reaction. In Cr^2-, the oxidation state of chromium is +2. In CrO₄^2-, the oxidation state of chromium is +6. Therefore, chromium has lost 4 electrons in the reaction (going from +2 to +6). So, the correct answer is 4 electrons.

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In the given reaction, sulphur (IV) oxide (SO₂) acts as an oxidizing agent. It oxidizes hydrogen sulphide (H₂S) gas to form solid sulfur (S) and water (H₂O). The oxidation state of sulphur in SO₂ is +4, while in H₂S, it is -2. During the reaction, sulphur (IV) oxide is reduced to sulphur (S) with a decrease in its oxidation state, while hydrogen sulphide is oxidized to sulphur (S) with an increase in its oxidation state. Therefore, sulphur (IV) oxide acts as an oxidizing agent in the given reaction.

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To solve this problem, we can use the formula: mass of substance deposited = (current × time × molar mass) ÷ (Faraday’s constant × number of electrons transferred) First, we need to find the number of moles of copper deposited by dividing the charge passed by the Faraday’s constant: Q = I × t = 1.0 A × 965 s = 965 C n = Q ÷ (Faraday’s constant × number of electrons transferred) n = 965 C ÷ (96500 C/mol × 2) n = 0.005 mol Next, we can use the number of moles to calculate the mass of copper using its molar mass: mass = n × molar mass mass = 0.005 mol × 63.5 g/mol mass = 0.318 g Therefore, the mass of copper deposited by the current is 0.318 g. The correct option is (a) 0.318g.

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The method suitable for the preparation of an insoluble salt is the Double decomposition method. In this method, two soluble salts are mixed together to form an insoluble salt, which is then separated from the mixture through filtration. This method is particularly useful for preparing salts that are insoluble in water, as it does not require the use of a precipitating agent.

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Hexane has the highest boiling point among the given options. Boiling point increases with increasing molecular size and branching decreases boiling point. Hexane has the highest molecular size and the least branching compared to the other options, making it have the highest boiling point.

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Aluminium is extracted from bauxite. Bauxite is an ore that is rich in aluminium oxide (Al2O3). The process of extracting aluminium involves the following steps: first, the bauxite ore is purified to remove any impurities, then it is converted into alumina (Al2O3) by the Bayer process. Next, the alumina is electrolyzed in molten cryolite (Na3AlF6) to produce pure aluminium metal. The process of extracting aluminium is energy-intensive and requires large amounts of electricity.

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Out of the given options, dilute sodium chloride solution is the one that will produce oxygen and hydrogen during its electrolysis using platinum electrodes. During electrolysis, the ions present in the solution are attracted to the electrodes and undergo reduction or oxidation, depending on their charge. In the case of dilute sodium chloride solution, it contains Na+ and Cl- ions. At the cathode (negative electrode), the Na+ ions are attracted and undergo reduction to form sodium atoms and hydrogen gas: 2Na+ + 2e- → 2Na 2H+ + 2e- → H2(g) At the anode (positive electrode), the Cl- ions are attracted and undergo oxidation to form chlorine gas and release oxygen gas: 2Cl- → Cl2(g) + 2e- 2H2O → O2(g) + 4H+ + 4e- Since platinum electrodes are inert, they do not participate in the chemical reactions and do not affect the electrolysis process.

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Benzene is the only compound that can undergo both addition reactions and substitution reactions. This is because of its unique structure, which consists of a ring of six carbon atoms with alternating double bonds. The double bonds in benzene are considered to be "delocalized," meaning that the electrons involved in the bonds are spread out over the entire ring rather than being localized between specific carbon atoms. This delocalization makes benzene relatively stable and resistant to addition reactions, but it also makes it susceptible to substitution reactions. In a substitution reaction, one or more of the hydrogen atoms in the benzene ring can be replaced with another atom or group of atoms. In an addition reaction, on the other hand, two molecules combine to form a single molecule, usually with the addition of a double or triple bond. Because benzene already has a stable ring structure with delocalized double bonds, it is not easily able to undergo addition reactions, but it can undergo substitution reactions.

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Petrol consists mainly of hydrocarbons. Hydrocarbons are organic compounds made up of carbon and hydrogen atoms only. In the case of petrol, these hydrocarbons are typically made up of between 5 and 12 carbon atoms per molecule, with some additional small amounts of other compounds. When petrol is burned in an engine, the hydrocarbons react with oxygen in the air to produce energy, carbon dioxide, and water vapor.

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Metals are said to be malleable because they can be beaten into thin sheets without breaking. This is due to the nature of metallic bonding, where positively charged metal ions are surrounded by a sea of delocalized electrons that are free to move around. When a force is applied to a metal, these delocalized electrons act as a lubricant between the metal ions, allowing them to slide past each other and change shape without breaking. This property is important for the shaping of metals into various forms and structures for use in construction, manufacturing, and other applications.

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Substances which absorb water from the atmosphere without dissolving in it are said to be hygroscopic. This means that they have the ability to attract and hold water molecules from the surrounding environment due to their physical and chemical properties. Examples of hygroscopic substances include salt, sugar, and silica gel. Hygroscopic substances are commonly used as desiccants, which are materials that are used to absorb moisture from the air and keep products dry. They are also used in certain industrial processes to remove water from various substances.

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The device used in the laboratory for intermittent production of gases without heating is Kipp's apparatus. The Kipp's apparatus is a laboratory equipment used for generating gases such as hydrogen, carbon dioxide, and sulfur dioxide, etc. without heating. It is made up of a glass bottle with three parts: a bulb-shaped bottom, a cylindrical middle section, and a funnel-shaped top. The middle section contains a tube with a tap or stopcock, while the bottom is filled with the reactants needed to generate the gas. When the tap is opened, water or another liquid enters the bottom bulb and reacts with the solid to generate the gas, which then bubbles up and is collected in the top funnel. Kipp's apparatus is often used for qualitative analysis in the laboratory.

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The products of the reaction between CH₃CH ₂COOH (propanoic acid) and CH₃OH (methanol) are CH ₃CH₂COOCH₃ (methyl propanoate) and H₂O (water). This is an esterification reaction where the carboxylic acid (propanoic acid) reacts with an alcohol (methanol) in the presence of an acid catalyst (such as sulfuric acid) to form an ester (methyl propanoate) and water. The general equation for this reaction is: R-COOH + R'-OH ⇌ R-COOR' + H₂O In this specific case, the reaction between propanoic acid and methanol produces methyl propanoate and water as the products.

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The liquid product of the destructive distillation of coal is ammoniacal liquor. This is because during the destructive distillation of coal, coal is heated in the absence of air to produce several by-products such as coke, coal tar, and coal gas. Ammoniacal liquor is a by-product of coal gas production. It is a brown liquid that contains ammonia, carbon dioxide, and other impurities. The ammonia in ammoniacal liquor is usually recovered through a process called scrubbing, which involves passing the gas through water to dissolve the ammonia.

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The compound that is used as a gaseous fuel is CH _3-CH _2-CH _2-CH ₃. This is the chemical formula for butane, a hydrocarbon that is commonly used as a fuel in lighters and camping stoves. Butane is a gas at room temperature and pressure, which makes it ideal for use as a gaseous fuel. It burns cleanly and efficiently, producing mainly carbon dioxide and water vapor as byproducts.

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Dehydration of ethanol produces ethene. During the reaction, a molecule of water is eliminated from ethanol, and the remaining atoms combine to form ethene. This is an example of an elimination reaction, where a small molecule is removed from a larger molecule to form a new product. The reaction is typically carried out at high temperatures in the presence of a catalyst such as concentrated sulfuric acid or alumina. Ethene is an important industrial chemical that is used as a feedstock for the production of many other organic chemicals.

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Pipe-borne water is usually chlorinated in order to kill harmful bacteria in the water. Chlorine is a strong oxidizing agent that can destroy the cell walls and enzymes of bacteria, viruses and other microorganisms that might be present in the water. By killing these microorganisms, chlorination helps to prevent the spread of waterborne diseases and make the water safe for consumption.