WAEC SSCE - Chemistry - 2001

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Detalhes da Resposta

Detalhes da Resposta

Hard water is water that contains dissolved minerals such as calcium and magnesium. These minerals can provide health benefits and contribute to the formation of strong bones, which is an advantage. However, the other options listed in the question are disadvantages of hard water. When hard water reacts with soap, it forms scum that reduces the effectiveness of cleaning. Furring of kettles is the buildup of mineral deposits that reduces efficiency and increases energy consumption. Blockage of water pipes can lead to reduced water flow and increased maintenance costs.

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To calculate the mass of sodium hydroxide in 5.00 dm³ of a 0.125 mol dm^-3 solution, we need to use the formula: mass = molarity x volume x molecular weight where: - molarity = 0.125 mol dm^-3 (given) - volume = 5.00 dm³ (given) - molecular weight of NaOH = 40 g mol^-1 (given) Substituting the values into the formula, we get: mass = 0.125 mol dm^-3 x 5.00 dm³ x 40 g mol^-1 mass = 25.0 g Therefore, the mass of sodium hydroxide in 5.00 dm³ of a 0.125 mol dm^-3 solution is 25.0 g. The correct option is (d).

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Sugars are carbohydrates that can be broken down by hydrolysis, which involves breaking down the glycosidic bond between the sugar units. Hydrolysis of sugars can occur by the action of different agents. Mineral acids and strong bases can both catalyze the hydrolysis of sugars, but they require high temperatures to work efficiently. Enzymes, on the other hand, are biological catalysts that can catalyze the hydrolysis of sugars at room temperature, with high specificity and efficiency. Enzymatic hydrolysis of sugars involves the action of specific enzymes that recognize and break down specific types of glycosidic bonds. Oxidants, which are substances that can accept electrons in a chemical reaction, are not typically used to hydrolyze sugars.

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Detalhes da Resposta

The presence of unpaired electrons in an atom of a transition metal gives rise to paramagnetism. Paramagnetism is a type of magnetism that occurs when certain materials are placed in an external magnetic field. When a transition metal atom has unpaired electrons in its outermost energy level, it can become magnetized and attracted to an external magnetic field. This is because the unpaired electrons can align their spins with the external magnetic field, causing the atom to become magnetized. Therefore, the presence of unpaired electrons in an atom of a transition metal is responsible for its paramagnetic behavior.

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When a photographic film is exposed to sunlight, the silver halide grains in the film emulsion are converted to metallic silver. This process involves the reduction of silver ions (Ag⁺) to neutral silver atoms (Ag) by the electrons (e^-) produced by the exposure to light. Therefore, the correct reaction that represents the change that occurs when a photographic film is exposed to sunlight is: Ag⁺ + e^- → Ag

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A mixture of calcium chloride and calcium trioxocarbonate (IV) in water can be separated by filtration. Filtration is a separation technique that separates solids from liquids using a porous material or a filter. Calcium trioxocarbonate (IV) is insoluble in water while calcium chloride is soluble in water. When the mixture is passed through a filter, calcium trioxocarbonate (IV) will be left behind as a residue while the filtrate will contain calcium chloride in water. Therefore, filtration can be used to separate the two components of the mixture.

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In the given reaction, copper is oxidized because its oxidation state increases from 0 in Cu to +2 in Cu(NO3)2. Oxidation is defined as the loss of electrons or an increase in oxidation state. Here, copper loses electrons and its oxidation state increases, hence it is oxidized. Therefore, the correct answer is copper.

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When NH₄Cl is heated with an alkali, it undergoes a chemical reaction known as the Hofmann's degradation reaction, which produces ammonia gas (NH₃). The reaction is represented as: NH₄Cl + NaOH → NaCl + H₂O + NH₃ Therefore, the correct answer is NH₃.

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CuSO₄ 5H₂O can be obtained from an aqueous solution of copper (ll) tetraoxosulphate (VI) by crystallization. Crystallization is a process of separating a solid product from a liquid by allowing the liquid to evaporate under controlled conditions. In this case, an aqueous solution of copper (II) tetraoxosulphate (VI) can be heated to form a saturated solution. The solution is then cooled slowly to allow the formation of CuSO₄ 5H₂O crystals. These crystals can be collected by filtration to obtain the desired product. Therefore, the correct option is "crystallization".

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The hydrogenation of butene involves the addition of hydrogen gas to the double bond of butene molecule. This reaction is a type of addition reaction, where two reactants combine to form a single product. The product of the hydrogenation of butene is butane. This is because the addition of hydrogen to the double bond of butene saturates the carbon atoms, and forms a single bond between them, resulting in a straight-chain alkane. Therefore, the correct answer is "butane".

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Detalhes da Resposta

Gastric juice is a digestive fluid secreted by the stomach. It is primarily composed of hydrochloric acid, which makes it very acidic with a pH value of about 2. Additionally, gastric juice contains enzymes such as pepsin, which is responsible for breaking down proteins. Therefore, it can be concluded that gastric juice contains enzymes and is acidic. It is not a mixture of substances, and it does not have a pH value of 7, which is neutral.

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During the electrolysis of copper sulfate (CuSO₄), the copper anode loses mass as copper atoms are oxidized to form copper ions that enter the solution. This means that the reaction at the anode is the dissolution of the copper electrode. Therefore, the correct option is "dissolution of copper electrode."

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The correct answer is AI ₂X ₃. This is because when the atom of an element X combines with aluminium (₁₃AI), it forms an ionic compound. The formula for ionic compounds is determined by balancing the charges of the ions involved. Aluminium has a charge of 3+ while the atom of element X has a charge of 2-. To balance the charges, two atoms of element X are required for every three aluminium atoms. The resulting formula for the compound is AI ₂X ₃.

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From the balanced chemical equation, we know that 2 moles of NH₃ will produce 3 moles of H₂. Therefore, to calculate the volume of H₂ produced, we need to first find the number of moles of NH₃ in 100cm³: 1 mole of any gas at room temperature and pressure (RTP) occupies a volume of 24dm³ (24000cm³). Therefore, the number of moles of NH₃ in 100cm³ can be calculated as follows: 100cm³ = 100/1000 = 0.1dm³ 0.1dm³ of NH₃ at RTP = 1/24 moles (since 24dm³ = 1 mole) Therefore, the number of moles of NH₃ in 100cm³ = (1/24) x 0.1 = 0.00417 moles (to 3 significant figures) Since 2 moles of NH₃ produce 3 moles of H₂, we can calculate the number of moles of H₂ produced as follows: Number of moles of H₂ = (3/2) x 0.00417 = 0.00625 moles (to 3 significant figures) Finally, we can use the ideal gas law to calculate the volume of H₂ produced: PV = nRT where P is the pressure, V is the volume, n is the number of moles, R is the gas constant, and T is the temperature in Kelvin. Since the temperature and pressure are constant, we can write: V_H2 / n_H2 = V_NH3 / n_NH3 where V_H2 and n_H2 are the volume and number of moles of H₂, and V_NH3 and n_NH3 are the volume and number of moles of NH₃. Substituting the values we have calculated: V_H2 / 0.00625 = 0.1 / (1/24) V_H2 = 0.00625 x (1/0.0417) x 24 = 150cm³ Therefore, the volume of hydrogen produced is 150cm³.

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A substance is said to be hygroscopic if it absorbs moisture from the atmosphere without dissolving. Hygroscopic substances have the ability to attract and retain water molecules from the air around them, which can lead to their physical properties being altered. This can include changes in texture, shape, and size. Some examples of hygroscopic substances include salt, sugar, and certain types of paper.

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Detalhes da Resposta

When a strip of iron is placed in an aqueous solution of CuSO₄, a visible change is observed. This is because copper ions in the CuSO₄ solution gain electrons from the iron strip and are reduced to metallic copper, which is deposited on the iron strip. At the same time, iron atoms on the strip lose electrons and are oxidized to iron(II) ions, which dissolve in the solution. The overall reaction is: Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s) The visible change observed is the deposition of metallic copper on the iron strip, which appears as a reddish-brown coating. This process is known as a displacement reaction, where a more reactive metal (in this case, iron) displaces a less reactive metal (copper) from its salt solution.

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Detalhes da Resposta

Detalhes da Resposta

Detalhes da Resposta

One of the products of combustion of pentane in excess air is carbon (IV) oxide. During combustion, pentane reacts with oxygen from the air to produce carbon dioxide and water. The balanced chemical equation for the combustion of pentane is: C5H12 + 8O2 → 5CO2 + 6H2O This means that for every molecule of pentane that reacts, five molecules of carbon dioxide and six molecules of water are produced. Since carbon dioxide is a type of carbon (IV) oxide, it is one of the products of combustion of pentane in excess air.

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Acidic industrial waste can be treated with lime. Lime is an alkaline substance that can be used to neutralize acidic waste by a process called neutralization. In this process, the lime reacts with the acid to form a salt and water, resulting in a neutral pH. This neutralization reaction is an important step in the treatment of industrial waste, as acidic waste can be harmful to the environment and living organisms. Brine, water, and ethanol are not commonly used for treating acidic waste.

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The accumulation of waste plastics in the soil and the environment is mainly due to the fact that plastic materials are non-biodegradable. This means that they cannot be broken down by natural biological processes, unlike organic materials such as paper or food waste. Plastics are made up of long chains of polymers that do not readily decompose, and thus persist in the environment for a long time, causing pollution. Plastics are also typically insoluble in water, making it difficult for them to dissolve and be removed by water-based processes. While some plastics are flammable, they are not easily affected by heat in the sense of breaking down or decomposing quickly, and may actually release harmful gases when burned.

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The function of H₂SO₄ in the given reaction is that of a dehydrating agent. The concentrated sulfuric acid (H₂SO₄) is used in this reaction to remove the water (H₂O) molecule from the reactant molecule C₁₂H₂₂O₁₁ (sucrose). The removal of a water molecule from a molecule is known as dehydration. Therefore, in this reaction, H₂SO₄ acts as a dehydrating agent, which removes water from the reactant molecule and helps to form carbon (C) and water (H₂O) as products.

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The equation P_Total = F₁ + P₂ + P ₃ + .........P_n' is an expression of Dalton's law. Dalton's law states that the total pressure of a mixture of gases is equal to the sum of the partial pressures of the individual gases in the mixture. In other words, the total pressure exerted by a mixture of gases is equal to the sum of the pressures that each gas would exert if it were present alone in the same container at the same temperature.

Detalhes da Resposta

Detalhes da Resposta

Detalhes da Resposta

When preparing a simple salt, one of the major factors to consider is its solubility in water. This is because the method used for preparation should be able to dissolve the necessary reactants in water to form a solution that can be further processed to yield the desired salt. The solubility of a salt in water determines the concentration of the reactants in the solution and affects the rate and completeness of the chemical reaction. Hence, solubility plays a critical role in selecting a suitable method for preparing a simple salt.

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Detalhes da Resposta

Carbon dating is a method used to determine the age of organic materials, such as bones and teeth. This method is based on the fact that carbon-14, a radioactive isotope of carbon, decays at a known rate over time. As living organisms take in carbon-14 during their lifetime, the amount of carbon-14 in their bones and teeth can be used to determine the approximate age of the sample. Therefore, of the options given, only the age of a human skeleton can be determined by carbon-dating. The other features such as height, mass, and race cannot be determined by carbon-dating.

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Detalhes da Resposta

Feasible reactions between two metals occur when the metal higher in the reactivity series displaces the metal lower in the reactivity series from its compound. Based on the given order of reactivity (P>Q>R>S), feasible reactions will occur between P and Q, R and S, and P and S. Therefore, the only feasible reaction from the options provided is: Q + S⁺ → S + Q⁺

Detalhes da Resposta

Detalhes da Resposta

Metallic bonding is responsible for the unique properties of metals, such as their malleability, ductility, and conductivity. It occurs due to the attraction between the positively charged atomic nuclei and the delocalized cloud of electrons surrounding them. In metallic bonding, the valence electrons are not held tightly by any one atom but are free to move throughout the metal lattice. This allows for the sharing of electrons between many atoms, creating a cohesive and stable structure. Therefore, the correct answer is "Attraction between the atomic nuclei and cloud of electrons."

Detalhes da Resposta

Elements which belong to the same group in the Periodic Table are characterized by the presence of the same number of outermost electrons in the respective atoms. The groups are the columns in the periodic table, and elements within the same group have similar chemical and physical properties. This is because they have the same number of valence electrons, which are the electrons involved in chemical bonding. As a result, they tend to react similarly with other elements and form similar types of compounds. Therefore, the correct option is (b): presence of the same number of outermost electrons in the respective atoms.

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Among the given compounds, CH₄ is covalent. Covalent compounds are formed when two or more non-metal atoms share electrons to form a molecule. CH₄ is methane, which is a simple organic compound made up of one carbon and four hydrogen atoms. Both carbon and hydrogen are non-metals, and they share electrons to form four covalent bonds, resulting in a stable molecule. The other three compounds, CaCI₂, MgO, and NaN, are all ionic compounds, which are formed by the transfer of electrons between a metal and a non-metal.

Detalhes da Resposta

Detalhes da Resposta

The gas evolved when a sample of gastric juice reacts with sodium trioxocarbonate (IV) can be determined by identifying the reaction taking place. Gastric juice contains hydrochloric acid (HCl), and when it reacts with sodium trioxocarbonate (IV) (Na₂CO₃), carbon dioxide (CO₂) gas is evolved. This is because the reaction between hydrochloric acid and sodium trioxocarbonate (IV) produces carbon dioxide, sodium chloride, and water as products. The balanced equation for this reaction is: 2HCl + Na₂CO₃ → CO₂ + NaCl + H₂O Therefore, the correct answer is CO₂.

Detalhes da Resposta

In AI(NO₃)₃, the compound has a total charge of 0 because aluminum has a charge of +3 and nitrate (NO₃) has a charge of -1. Since there are three nitrate ions, their total charge is -3. Therefore, the oxidation number of nitrogen can be calculated as follows: AI(NO₃)₃ Aluminum = +3 Oxygen (O) in NO₃ = -2 Nitrogen (N) in NO₃ = x The sum of the oxidation numbers must equal the charge of the compound (0). +3 + 3(-2) + 3x = 0 -6 + 3x = 0 3x = 6 x = +2 Therefore, the oxidation number of nitrogen (N) in AI(NO₃)₃ is +5.

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Out of the given options, tetrachloromethane is the substance that decomposes when an electric current is passed through it. Tetrachloromethane, also known as carbon tetrachloride, is a covalent compound with the chemical formula CCl₄. It is a non-conductor of electricity in its pure form, but when it is mixed with small amounts of impurities, it becomes a conductor of electricity. When an electric current is passed through it, tetrachloromethane undergoes electrolysis, which breaks down the C-Cl bonds and forms various products, including chlorine gas and phosgene gas. On the other hand, glucose solution, zinc rod, and hydrochloric acid do not decompose when an electric current is passed through them. Glucose is a covalent compound that does not dissociate into ions in solution, so it cannot undergo electrolysis. Zinc is a metal and a good conductor of electricity, but it does not undergo any significant chemical reaction when an electric current is passed through it. Hydrochloric acid is an aqueous solution of hydrogen chloride, which dissociates into H⁺ and Cl^- ions in solution. However, it is not a good conductor of electricity because the concentration of ions is relatively low.

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Nylon is the polymer that contains nitrogen. Nylon is a synthetic polymer that contains amide linkages in its structure. These amide linkages are formed between the amine group (-NH2) of one monomer and the carboxylic acid group (-COOH) of another monomer, resulting in the formation of a long chain polymer with repeating units of -(NH-[CH2]n-CO)-. The presence of the amide group in nylon gives it unique properties such as high strength and elasticity.

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If 10 cm³ of distilled water is added to 10 cm³ of an aqueous salt solution, the total volume of the resulting solution is 20 cm³. However, the amount of solute (the salt) in the solution remains constant, while the amount of solvent (the water) increases. Therefore, the concentration of the solution decreases since the same amount of solute is now present in a larger volume of solution. So, the correct option is: the concentration decreases.

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The given equation is a chemical equation that represents a reaction between silver ion (Ag⁺) and chloride ion (CI^-) to form silver chloride (Ag⁺CI^-). The value of enthalpy change (∆H) is given as -65.7 kJmol^-1, which indicates that the reaction is exothermic, i.e., heat is evolved during the reaction. Thus, the correct option is "a solid is formed and heat is evolved."

Detalhes da Resposta

In the given equation, ∆H represents the Electron affinity. Electron affinity is the energy change that occurs when an atom in the gas phase gains an electron to form a negative ion. The negative value of ∆H in the equation indicates that energy is released when an electron is added to the chlorine atom to form a chloride ion.

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Detalhes da Resposta

The conclusion from Rutherford's alpha-scattering experiment is that atoms are mostly empty space with a small, dense, positively charged nucleus at the center. This was determined by observing that alpha particles, which are positively charged, were mostly able to pass through the gold foil with little to no deflection, but occasionally were deflected at large angles. This led to the conclusion that most of the atom's mass and positive charge is concentrated in a small, dense nucleus at the center, with the electrons orbiting around it.

Detalhes da Resposta

Detalhes da Resposta

The pair of reagents that react to produce hydrogen is: Magnesium and dilute tetraoxosulphate (VI) acid. When magnesium is added to dilute tetraoxosulphate (VI) acid, hydrogen gas is produced as one of the products. This is a classic example of a single-displacement reaction, where the more reactive metal (magnesium) displaces the less reactive hydrogen from the acid. The other options do not produce hydrogen gas upon reaction.

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(a) Two substances for each purpose:

(b)(i) Two alloys that contain lead: solder (tin and lead) and type metal (lead, tin and antimony).

(b)(ii) On bubbling H₂S into a solution of Pb(NO₃)₂, a black precipitate is observed. This is lead(II) sulfide, PbS, which is insoluble and black:

(b)(iii) Metal M is tin.

• I. The phenomenon (existing in two different solid forms) is allotropy (the change from white to grey tin is known as tin pest).
• II. The temperature of 18 °C, at which one form changes into the other, is the transition temperature.

(c)(i) Action of heat:

I. On silver trioxonitrate(V):

II. On ammonium trioxocarbonate(IV):

(c)(ii) Completed table:

Detalhes da Resposta

(a)(i) An acid anhydride is an oxide which reacts with water to form an acid. For example:

\[\mathrm{SO_3(g)+H_2O(l)\rightarrow H_2SO_4(aq)}\]

Thus, \(\mathrm{SO_3}\) is an acid anhydride.

(ii) Three chemical properties of hydrochloric acid are:

1. It reacts with metals above hydrogen in the electrochemical series to form a chloride salt and hydrogen gas:
   \[\mathrm{Zn(s)+2HCl(aq)\rightarrow ZnCl_2(aq)+H_2(g)}\]
2. It neutralises bases or alkalis to form a chloride salt and water:
   \[\mathrm{HCl(aq)+NaOH(aq)\rightarrow NaCl(aq)+H_2O(l)}\]
3. It reacts with carbonates or hydrogencarbonates to produce carbon(IV) oxide, water and a chloride salt:
   \[\mathrm{Na_2CO_3(aq)+2HCl(aq)\rightarrow 2NaCl(aq)+H_2O(l)+CO_2(g)}\]

(b)(i) \(\mathrm{H_2SO_4}\) is dibasic because it has two replaceable hydrogen ions. It can therefore form an acid salt, for example \(\mathrm{NaHSO_4}\), and a normal salt, for example \(\mathrm{Na_2SO_4}\). \(\mathrm{HNO_3}\) is monobasic, having only one replaceable hydrogen ion, so it forms only normal salts such as \(\mathrm{NaNO_3}\).

(ii) Concentrated \(\mathrm{H_2SO_4}\) is an oxidising acid. It reacts with both iron and copper, including copper which is below hydrogen in the electrochemical series. For example:

\[\mathrm{Cu(s)+2H_2SO_4(conc)\rightarrow CuSO_4(aq)+SO_2(g)+2H_2O(l)}\]

Dilute \(\mathrm{H_2SO_4}\) behaves as an ordinary acid. Iron, being above hydrogen in the electrochemical series, displaces hydrogen from it, whereas copper, being below hydrogen, does not:

\[\mathrm{Fe(s)+H_2SO_4(aq)\rightarrow FeSO_4(aq)+H_2(g)}\]

(iii) Zinc dust has a much larger surface area than zinc granules of the same mass. Hence, more zinc particles are in contact with the acid at a time, giving a faster reaction and more vigorous effervescence.

(c)(i) Activation energy is the minimum energy which reacting particles must possess for collisions between them to result in a chemical reaction.

(ii) Energy profile for \(\mathrm{A+B\rightarrow C+D}\), where \(\Delta H=+x\ \mathrm{kJ\,mol^{-1}}\):

(iii) Mineral acids such as hydrochloric acid, nitric acid and tetraoxosulphate(VI) acid are strong acids and are fully ionised in dilute aqueous solution. Their reaction with sodium hydroxide has the same net ionic equation:

\[\mathrm{H^+(aq)+OH^-(aq)\rightarrow H_2O(l)}\]

Since the same process, the formation of one mole of water, occurs in each case, the heat of neutralisation is approximately constant, about \(-57\ \mathrm{kJ\,mol^{-1}}\).

(d)(i) \(\mathrm{Q_{(l)}}\) has the higher entropy. In the liquid state, the particles have greater freedom of movement and a more random arrangement than in the solid state.

(ii) A decrease in temperature favours the reverse reaction, that is, the conversion of \(\mathrm{Q_{(l)}}\) to \(\mathrm{Q_{(s)}}\). The forward reaction is melting and is endothermic, so lowering the temperature favours the exothermic reverse change.

Detalhes da Resposta

(a) Characteristic properties

(i) Electrovalent (ionic) compounds:

• High melting and boiling points.
• Conduct electricity when molten or dissolved in water.
• Usually soluble in water but insoluble in organic solvents.

(ii) Alpha particles:

• Positively charged (helium nuclei, 2 protons and 2 neutrons).
• Deflected by electric and magnetic fields (toward the negative plate).
• Low penetrating power (stopped by a sheet of paper) but strongly ionising.

(iii) Catalysts:

• Alter the rate of a reaction without being used up.
• Needed only in small amounts and are chemically unchanged at the end.
• Are specific and can be poisoned by impurities.

(b) Silicon

(i) Electronic configuration of Si (Z = 14): \(1s^2\,2s^2\,2p^6\,3s^2\,3p^2\), i.e. 2, 8, 4. It belongs to Group IV (Group 14).

(ii) Bonding between silicon and oxygen in SiO₂: covalent (a giant covalent structure).

(iii) Number of silicon atoms:

Moles of Si \( = \dfrac{5.72 \times 10^{-3}}{28} = 2.043 \times 10^{-4}\ \text{mol} \)

Number of atoms \( = 2.043 \times 10^{-4} \times 6.02 \times 10^{23} = \mathbf{1.23 \times 10^{20}\ atoms} \)

(c) Element X (same group as Na but more reactive)

(i) X is a reducing agent. Being more electropositive than sodium, it loses its outer electron even more readily, so it reduces other species while being oxidised itself.

(ii) Store X under paraffin oil (kerosene) to keep it away from air and moisture.

(iii) If dropped into cold water coloured with red litmus, X would float and dart about on the surface, melt into a shining ball, hiss and give off a gas (hydrogen) which may ignite; the red litmus turns blue as an alkaline hydroxide is formed.

(iv) \[ X_2O + 2HCl \to 2XCl + H_2O \]

(v) The salts of X are likely to be white (colourless), as for other Group I salts.

Detalhes da Resposta

None

Detalhes da Resposta

(a)(i) Electrolytic purification of copper

Impure copper is made the anode and pure copper is the cathode. Both are immersed in acidified copper(II) sulfate solution and connected to a direct-current supply. Pure copper is deposited on the cathode, while insoluble impurities settle as anode mud.

(ii) Electrode reactions

Anode (oxidation): \[\mathrm{Cu_{(s)} \rightarrow Cu^{2+}_{(aq)} + 2e^-}\]

Cathode (reduction): \[\mathrm{Cu^{2+}_{(aq)} + 2e^- \rightarrow Cu_{(s)}}\]

(iii) The Daniell cell is an electrochemical (galvanic) cell because it converts chemical energy from a spontaneous redox reaction into electrical energy.

(iv) In the Leclanché cell, \(\mathrm{MnO_2}\) acts as a depolariser. It removes hydrogen formed at the carbon electrode, thereby preventing polarisation and a fall in voltage.

(b)(i)

Balancing oxygen atoms gives \(y=4\), and balancing charge gives \(x=5\):

\[\mathrm{MnO_4^- + 8H^+ + 5e^- \rightarrow Mn^{2+} + 4H_2O}\]

Therefore, \(\boxed{x=5}\) and \(\boxed{y=4}\).

(ii) Let the oxidation state of Mn be \(z\):

\[z+4(-2)=-1\]

\[z=+7\]

Hence, the oxidation state of Mn in \(\mathrm{MnO_4^-}\) is \(\boxed{+7}\).

(c)(i) Factors affecting selective discharge of ions

1. Position of the ions in the electrochemical series.
2. Concentration of the ions in the electrolyte.
3. Nature of the electrode.

(ii) Faraday's second law of electrolysis

When the same quantity of electricity passes through different electrolytes, the masses of substances liberated or deposited are proportional to their chemical equivalent masses.

(iii) Calculation

Since the voltameters are connected in series, the same quantity of electricity passes through both solutions.

For silver:

\[\mathrm{Ag^+ + e^- \rightarrow Ag}\]

\[\text{Moles of Ag deposited}=\frac{0.780}{108}=7.222\times10^{-3}\ \text{mol}\]

One mole of Ag requires one mole of electrons. Therefore:

\[\text{Moles of electrons}=7.222\times10^{-3}\ \text{mol}\]

I. Mass of copper deposited

\[\mathrm{Cu^{2+}+2e^-\rightarrow Cu}\]

\[\text{Moles of Cu}=\frac{7.222\times10^{-3}}{2}=3.611\times10^{-3}\ \text{mol}\]

\[\text{Mass of Cu}=3.611\times10^{-3}\times63.5=0.229\ \text{g}\]

\[\boxed{\text{Mass of copper deposited}=0.229\ \text{g}}\]

II. Quantity of electricity and time of flow

\[Q=nF=(7.222\times10^{-3})(96500)=696.9\ \text{C}\]

\[\boxed{Q\approx697\ \text{C}}\]

Since \(Q=It\):

\[t=\frac{Q}{I}=\frac{696.9}{0.200}=3484.5\ \text{s}\]

\[\boxed{t\approx3.48\times10^3\ \text{s}=58.1\ \text{min}}\]

Detalhes da Resposta

(a) Preparing pure MgSO₄ from MgO

Warm some dilute tetraoxosulphate(VI) acid in a beaker and add magnesium oxide (a base) a little at a time, stirring, until no more dissolves (excess MgO ensures all the acid is used up). Filter to remove the unreacted MgO. Evaporate the filtrate until saturated (to the point of crystallisation), allow to cool so that MgSO₄·7H₂O crystallises, then filter off and dry the crystals between filter papers.

\[ MgO + H_2SO_4 \to MgSO_4 + H_2O \]

(b) Water

(i) Two sources of water pollution: industrial effluents/waste; sewage or domestic waste (also agricultural run-off, oil spillage).

(ii) Oxygen is more soluble in water than nitrogen, so the dissolved air has a higher proportion of oxygen than atmospheric air; when this air is expelled by boiling, it is richer in oxygen (about 33% O₂) than ordinary air (about 21% O₂).

(iii) Coagulant: alum (aluminium sulphate).

(c) Chlorine

(i) Two physical properties: it is a greenish-yellow gas with a choking, pungent smell (also: denser than air, moderately soluble in water, poisonous).

(ii) \[ 2Fe + 3Cl_2 \to 2FeCl_3 \]

(iii) Chlorine is preferred to sulphur(IV) oxide because chlorine bleaches by oxidation, giving a permanent effect, whereas SO₂ bleaches by reduction, which is temporary (the colour returns on exposure to air).

(d) Mass of bleaching powder

\[ CaOCl_2 + 2HCl \to CaCl_2 + H_2O + Cl_2 \]

Convert 400 cm³ of Cl₂ at 25°C (298 K) and \(1.20 \times 10^5\ \text{N m}^{-2}\) to s.t.p. (273 K, \(1.01 \times 10^5\ \text{N m}^{-2}\)) using \( \dfrac{P_1V_1}{T_1} = \dfrac{P_2V_2}{T_2} \):

\[ V_2 = \frac{1.20\times10^5 \times 0.400 \times 273}{298 \times 1.01\times10^5} = 0.435\ \text{dm}^3 \]

Moles of Cl₂ \( = \dfrac{0.435}{22.4} = 1.94 \times 10^{-2}\ \text{mol} \)

From the equation, 1 mol CaOCl₂ gives 1 mol Cl₂, so moles of CaOCl₂ \( = 1.94 \times 10^{-2}\ \text{mol} \).

Molar mass of CaOCl₂ \( = 40 + 16 + 71 = 127\ \text{g mol}^{-1} \)

Mass \( = 1.94 \times 10^{-2} \times 127 = \mathbf{2.47\ g} \)

(The stated "standard pressure = 1.01 × 10⁶" is taken as 1.01 × 10⁵ N m^-2, the true standard pressure.)