WAEC SSCE - Chemistry - 2005

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The factor that does not affect the rate of reaction of CaCO₃ with HCI is the solubility of CaCO₃. When CaCO₃ reacts with HCI, it produces carbon dioxide, water, and calcium chloride. The rate of this reaction is affected by several factors, such as temperature, concentration of HCI, and surface area of CaCO₃. However, the solubility of CaCO₃ does not affect the rate of reaction because it is a solid and does not dissolve in the reaction mixture. In other words, the reaction rate depends on the number of collisions between CaCO₃ and HCI molecules, and the solubility of CaCO₃ does not influence this collision process. Therefore, it is safe to say that the solubility of CaCO₃ does not affect the rate of reaction of CaCO₃ with HCI.

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The correct term that describes an element which can exist in two or more forms in the same physical state is allotropy. Allotropy is a type of property that some elements have, where they can exist in different structural forms known as allotropes. Each allotrope has different physical and chemical properties, even though they are made up of the same atoms. For example, carbon is an element that exhibits allotropy. It can exist in several different allotropes, including diamond, graphite, and fullerenes. Diamond is a hard, transparent substance used in jewelry and cutting tools, while graphite is a soft, dark substance used in pencils and lubricants. Both diamond and graphite are made up of carbon atoms, but their structures are different, which results in their different physical and chemical properties. Isotopy refers to the property of having the same atomic number but a different atomic mass due to a different number of neutrons. Structural isomerism refers to the property of having the same chemical formula but different structural arrangements. Variable valency refers to the property of having different valences, or combining capacities, depending on the chemical reaction.

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The molecule with a linear structure is CO2. A molecule's structure depends on the arrangement of atoms in the molecule and the bonds between them. In CO2, there are two oxygen atoms bonded to a central carbon atom. The bond between each oxygen atom and the carbon atom is double-bonded, which means they share two pairs of electrons. This double bond arrangement forces the molecule to be linear in shape. In contrast, NH3 and CH4 have a tetrahedral shape due to the presence of lone pairs on the central atom, which cause the atoms to spread out. Water (H2O) also has a bent shape because of its lone pairs and its two single bonds.

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A salt that absorbs moisture from the atmosphere without forming a solution is said to be hygroscopic. Hygroscopic substances have a strong affinity for water molecules and can absorb water vapor from the surrounding atmosphere. This can cause the substance to become damp or even dissolve in the absorbed water. In contrast, efflorescent substances release water molecules into the surrounding atmosphere and can become dry and powdery. Deliquescent substances can absorb so much water that they dissolve and form a solution. Insoluble substances cannot dissolve in water and will not absorb moisture from the atmosphere.

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To determine the mass of copper deposited, we need to use the concept of Faraday's law of electrolysis. This law states that the amount of substance produced at an electrode is directly proportional to the quantity of electricity passed through the electrolyte. The quantity of electricity is measured in coulombs (C) and is related to the number of moles of electrons (n) by the equation: Q = nF where Q is the quantity of electricity, n is the number of moles of electrons, and F is the Faraday constant, which is equal to 96,485 C/mol. In this case, we have 4.0 moles of electrons, so the quantity of electricity is: Q = nF = 4.0 mol x 96,485 C/mol = 385,940 C The reaction involves the reduction of Cu2+ ions to Cu metal, which requires two electrons per ion. So, for every two electrons that pass through the electrolyte, one Cu atom is produced. This means that the number of moles of Cu produced is half the number of moles of electrons: n(Cu) = 0.5 x n(e-) = 0.5 x 4.0 mol = 2.0 mol The molar mass of copper is 63.55 g/mol, so the mass of copper produced is: m = n x M = 2.0 mol x 63.55 g/mol = 127.1 g Therefore, the mass of copper deposited is approximately 127.1 g. The closest option to this value is which is 128 g.

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When ice changes into water, it goes through a process called melting. During melting, the ice absorbs heat from the surroundings without increasing its temperature. This is because the heat energy is used to break the bonds between the molecules of ice, which are held together by strong forces. The heat energy that is absorbed during melting is called the "heat of fusion". It is the amount of heat energy required to change a solid into a liquid, or vice versa, without any change in temperature. In the case of ice melting, the heat of fusion is absorbed from the surroundings, which keeps the temperature of the ice constant at 0°C (32°F) until all of the ice has melted. Therefore, the reason why the temperature of ice remains the same during the melting process is that the heat gained is being used to separate the molecules and break the bonds between them, rather than increasing the temperature of the ice.

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The process involved in the reaction represented by the given equation is hydrolysis. In this reaction, a compound (AICI3) reacts with water, and as a result, it breaks down into two products, AI(OH)3 and HCI. This reaction is called hydrolysis because water is used to break down the compound. Specifically, the AICI3 is split into AI(OH)3 and HCI through the addition of water molecules, which causes the chemical bonds in AICI3 to break. The resulting AI(OH)3 is a solid precipitate that falls out of the solution, while the HCI is a gas that is released into the air.

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Détails de la réponse

An alpha-particle emission is characterized by the nucleus losing two protons and two neutrons. An alpha particle is a type of radiation that consists of two protons and two neutrons, which is equivalent to the nucleus of a helium atom. During alpha-particle emission, a nucleus will eject an alpha particle from its structure. This process causes the nucleus to lose two protons and two neutrons, which changes its atomic number and mass number. This type of radioactive decay occurs in heavy nuclei that are unstable due to having too many protons and neutrons, and it allows the nucleus to become more stable by reducing the number of particles in its structure.

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Détails de la réponse

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The most likely oxidation state of element X in its binary compounds can be determined by looking at its position in the periodic table and its electron configuration. Element X has an atomic number of 16, which places it in group 16 (also known as the oxygen group) of the periodic table. This group includes elements such as oxygen, sulfur, and selenium. The elements in this group have six valence electrons, meaning they need two more electrons to complete their outermost shell and attain a stable octet configuration. Based on this information, the most likely oxidation state for element X in its binary compounds would be -2. This is because in binary compounds, the element will typically gain two electrons to achieve a stable octet configuration. Therefore, it will have a charge of -2 to balance the positive charge of the other element in the compound. So, option -2 is the correct answer.

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The statement that "Molecules move with the same speed" is not a postulate of the kinetic theory of gases. According to the kinetic theory of gases, gas molecules are in constant random motion, and they move with different speeds and in different directions. The average kinetic energy of the gas molecules is directly proportional to the temperature of the gas. This means that at higher temperatures, the gas molecules will have a higher average speed. The other postulates of the kinetic theory of gases are that intermolecular forces between gas molecules are negligible, and collisions between gas molecules are elastic, which means that no energy is lost during the collision. These postulates help to explain the macroscopic properties of gases, such as their pressure, temperature, and volume, in terms of the behavior of their constituent molecules.

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The activation energy of a chemical reaction is the amount of energy that needs to be supplied to start the reaction. It is an energy barrier that must be overcome for a reaction to occur. There are different ways to alter the activation energy of a reaction, and one of them is by using a catalyst. A catalyst is a substance that lowers the activation energy of a reaction, making it easier for the reaction to occur. Catalysts do not change the overall energy change of the reaction, but they make it happen faster and with less energy input. Another way to alter the activation energy is by changing the temperature. When the temperature is increased, the particles of the reactants move faster, and they collide with more energy, which makes it more likely for the reaction to occur. This increase in kinetic energy of the particles can overcome the activation energy barrier, and the reaction can proceed. On the other hand, adding a reducing agent or applying high pressure does not directly alter the activation energy of a reaction. A reducing agent is a substance that donates electrons to another chemical species, while high pressure can affect the equilibrium of a reaction. However, neither of these methods directly changes the energy barrier required to start the reaction. In summary, the activation energy of a reaction can be altered by using a catalyst or changing the temperature, but not by adding a reducing agent or applying high pressure.

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The mass number of an element is the sum of its protons and neutrons. In this case, the element has 15 protons and 20 neutrons, so the mass number would be: Mass number = protons + neutrons Mass number = 15 + 20 Mass number = 35 Therefore, the correct option is 35. To summarize, the mass number of an element can be found by adding the number of protons and neutrons in the nucleus of an atom.

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The volume occupied by a given mass of gas depends on its temperature and pressure. This is because gas molecules are constantly moving and colliding with each other and with the walls of their container. When the temperature of a gas increases, the molecules move faster and collide with each other more frequently, which increases the pressure of the gas. Similarly, when the pressure of a gas increases, the molecules are pushed closer together, which decreases the volume of the gas. Therefore, temperature and pressure are the main factors that determine the volume occupied by a given mass of gas.

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Détails de la réponse

Sucrose is a non-reducing sugar. A reducing sugar is a sugar that has a free aldehyde or ketone functional group, which can reduce certain chemicals such as Benedict's solution or Fehling's solution. When a reducing sugar is heated with these chemicals, they are oxidized and reduce the chemical reagent, leading to the formation of a colored precipitate. Sucrose is a disaccharide composed of glucose and fructose linked by a glycosidic bond. It does not have a free aldehyde or ketone functional group, so it cannot act as a reducing sugar. In contrast, glucose, fructose, and maltose all have a free aldehyde or ketone functional group, so they can act as reducing sugars. Therefore, among the options given, sucrose is the non-reducing sugar.

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Fats are classified as "alkanoates". Alkanoates are a type of organic compound consisting of a long chain of carbon atoms (called an alkyl group) and a carboxyl group (-COO-) at one end. Fats are made up of glycerol molecules (a type of alcohol) and three alkanoic acids (also known as fatty acids) that are attached to the glycerol backbone, forming a molecule known as a triglyceride. Therefore, fats are classified as alkanoates because they contain these alkyl groups and carboxyl groups.

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The substance that is an alloy of lead is called soft solder. Soft solder is a type of metal alloy that is commonly used to join together two or more metal pieces by melting the solder and allowing it to flow into the joint. Soft solder typically contains a combination of lead and tin, although other metals may also be added to achieve specific properties. The reason why soft solder is considered an alloy of lead is because lead is one of the main components in the mixture. The lead in the soft solder helps to lower the melting point of the alloy, making it easier to work with and allowing it to flow more easily into the joint. It is worth noting that while soft solder is commonly used in various applications such as plumbing, electronics, and jewelry-making, it is also important to handle it with care since lead is a toxic metal.

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The method suitable for preparing insoluble salts is double decomposition. Double decomposition is a chemical reaction in which two ionic compounds exchange ions to form two new compounds. This reaction can occur when one of the products is insoluble in water, leading to the formation of an insoluble salt. In the process of double decomposition, the cations of one compound switch places with the cations of the other compound, resulting in the formation of two new compounds. If one of these new compounds is insoluble in water, it will precipitate out of the solution as a solid. The insoluble compound can then be collected through filtration and washed with water to remove any remaining soluble impurities. Thermal decomposition involves breaking down a compound into simpler substances by heating it. Oxidation involves adding oxygen or removing hydrogen from a compound. Neutralization is a reaction between an acid and a base to form a salt and water. These methods are not suitable for preparing insoluble salts. Therefore, double decomposition is the suitable method for preparing insoluble salts.

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Ethyne (also known as acetylene) can undergo polymerization, addition, and substitution reactions. However, it does not undergo esterification reactions. Polymerization involves the combination of many ethyne molecules to form a long-chain polymer. Addition reactions involve the addition of atoms or groups to the double bond of ethyne to form a saturated compound. Substitution reactions involve the replacement of one or more hydrogen atoms in ethyne with another atom or group. Esterification, on the other hand, involves the reaction between an alcohol and a carboxylic acid to form an ester. Ethyne does not have a functional group that can react with carboxylic acids or alcohols to form esters, so it cannot undergo esterification reactions.

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Détails de la réponse

Biotechnology is not applied in the Electroplating of metals process. Biotechnology is the use of living organisms or biological processes for industrial, medical, or environmental applications. In the manufacture of drugs, biotechnology is used to produce medications such as insulin, vaccines, and antibodies. In the treatment of domestic sewage, biotechnology is used to break down organic matter in wastewater using microorganisms. In the production of alcoholic beverages, biotechnology is used to ferment sugars into alcohol using yeast. However, in electroplating, a process used to coat one metal with another by passing an electric current through a solution containing ions of the coating metal and the metal to be coated, biotechnology is not involved. This process involves chemical and electrical reactions, but not biological ones.

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The main difference between a primary cell and a secondary cell is that the primary cell cannot be recharged, while the secondary cell can be recharged. A primary cell is a battery that generates electrical energy through an irreversible chemical reaction. Once the reactants are used up, the cell cannot produce electricity anymore and must be replaced. On the other hand, a secondary cell, also known as a rechargeable battery, can be recharged by reversing the chemical reactions that occur during discharge. Both primary and secondary cells are types of electrochemical cells, which convert chemical energy into electrical energy through an oxidation-reduction reaction. They both contain two electrodes and an electrolyte, which allows the flow of charged particles between the electrodes.

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When ethyne, also known as acetylene, reacts with an acidic solution of potassium tetraoxomanganate (VII), also known as potassium permanganate, a chemical reaction occurs. The purple colour of the potassium permanganate solution fades away, and the solution becomes colourless. This happens because the potassium permanganate oxidizes the ethyne, which breaks down into carbon dioxide and water. This reaction converts the potassium permanganate into manganese(II) ions, which are colourless, hence the colour of the solution changes from purple to colourless. Therefore, the correct answer is "colourless."

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Détails de la réponse

The substance X reacts with manganese (IV) oxide when heated, and produces oxygen gas. Among the options given, the only substance that fits this description is KCIO₃, also known as potassium chlorate. When potassium chlorate is heated with manganese (IV) oxide, the following chemical equation takes place: 2KClO₃ → 2KCl + 3O₂ In this equation, manganese (IV) oxide acts as a catalyst, which means it helps to speed up the reaction without being consumed in the process. The result is the production of oxygen gas, which is released as a byproduct. Therefore, the answer to this question is KCIO₃ (potassium chlorate).

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The correct option to predict the type of bonding in HCl is electronegativity difference. Bonding occurs when atoms share or transfer electrons to achieve a more stable electron configuration. The electronegativity of an atom is its ability to attract electrons towards itself in a chemical bond. When two atoms with different electronegativity values come together, the atom with higher electronegativity will pull the electrons closer to itself, creating a polar covalent bond. In the case of HCl, hydrogen has an electronegativity value of 2.1, while chlorine has an electronegativity value of 3.0. The difference between these two values is 0.9, which falls in the range of a polar covalent bond. To summarize, the electronegativity difference between two atoms is a measure of their tendency to form a bond and the type of bond that will form.

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Détails de la réponse

The major product formed when C₂H₅OH reacts with CH₃COOH is CH₃COOC₂H₅. This reaction is a type of esterification, where an alcohol and a carboxylic acid react to form an ester and water. In this case, the ethyl alcohol (C₂H₅OH) reacts with acetic acid (CH₃COOH) to form ethyl acetate (CH₃COOC₂H₅) and water (H₂O) as a byproduct. The reaction takes place due to the acidic nature of the carboxylic acid, which donates a proton (H⁺) to the hydroxyl group (-OH) of the alcohol, forming water and a carboxylate ion. The carboxylate ion then reacts with the electrophilic carbon atom of the carboxylic acid, leading to the formation of the ester. To summarize, the reaction between C₂H₅OH and CH₃COOH leads to the formation of CH₃COOC₂H₅ and H₂O as a byproduct, making CH₃COOC₂H₅ the major product of the reaction.

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Détails de la réponse

Among the given compounds, only NH4Cl dissolves readily in water. This is because NH4Cl is an ionic compound, composed of ammonium (NH4+) and chloride (Cl-) ions. When NH4Cl is added to water, the water molecules surround the ions, separating them from each other and forming a solution. This process is called hydration or solvation. On the other hand, BaSO4, CuSO3, and AgCl are insoluble or only slightly soluble in water. BaSO4 and AgCl are both ionic compounds, but they have very low solubility in water. CuSO3 does not exist as a stable compound under normal conditions, and it is usually synthesized as a complex or in a solution of a different solvent. Therefore, the compound that dissolves readily in water among the given options is NH4Cl.

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None

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(a)(i) Two amorphous forms of carbon: wood charcoal and soot (lamp-black). (Coke and carbon black are also acceptable.)

(ii) Why graphite is a lubricant but diamond is not: in graphite the carbon atoms are arranged in flat parallel layers held together by weak forces, so the layers slide easily over one another, giving graphite its slippery, lubricating property. In diamond every carbon atom is joined to four others by strong covalent bonds in a rigid three-dimensional lattice with no layers to slide, so diamond is hard and cannot act as a lubricant.

(iii) Laboratory preparation of a dry sample of carbon(IV) oxide: marble chips (calcium trioxocarbonate(IV)) are placed in a flask and dilute hydrochloric acid is run onto them through a thistle funnel whose stem dips below the acid. The carbon(IV) oxide evolved is passed first through water to remove any hydrogen chloride gas, then through concentrated tetraoxosulphate(VI) acid to dry it, and the dry gas is finally collected in a gas jar by upward delivery (downward displacement of air), since carbon(IV) oxide is denser than air.

\( \text{CaCO}_{3(s)} + 2\text{HCl}_{(aq)} \rightarrow \text{CaCl}_{2(aq)} + \text{H}_2\text{O}_{(l)} + \text{CO}_{2(g)} \)

(b)(i) I. Soil pollutant: pesticides (or excess fertilizers, polythene, oil spillage). II. Water pollutant: sewage/domestic waste (or factory effluent, crude oil). III. Air pollutant: sulphur(IV) oxide (or carbon(II) oxide, oxides of nitrogen, chlorofluorocarbons).

(ii) The major industrial use of sulphur(IV) oxide is in the manufacture of tetraoxosulphate(VI) acid, \( \text{H}_2\text{SO}_4 \), by the Contact process.

(c)(i) The molecules of petrol are held together by only very weak intermolecular forces of attraction. At room temperature the molecules already possess enough kinetic energy to overcome these weak forces, so they escape readily from the liquid surface into the vapour state; hence petrol is volatile.

(ii) Two criteria for purity: a pure substance has a sharp, constant melting point and a fixed, constant boiling point. (A fixed density or refractive index is also acceptable.)

(iii) I. Krypton: used in photographic flash lamps and fluorescent (fluorescent tube) lighting. II. Argon: used to provide an inert atmosphere in gas-filled electric light bulbs and in welding.

(d)(i) I. The colourless solution contains zinc tetraoxosulphate(VI), ZnSO₄.
II. Ionic equation: \( \text{Zn}_{(s)} + \text{Cu}^{2+}_{(aq)} \rightarrow \text{Zn}^{2+}_{(aq)} + \text{Cu}_{(s)} \).
III. On adding a few drops of sodium hydroxide solution, a white gelatinous precipitate of zinc hydroxide, \( \text{Zn(OH)}_2 \), is formed (which redissolves in excess sodium hydroxide).

(d)(ii) Volume of CO₂ produced from 5.3 g of Na₂CO₃:

\[ \text{Na}_2\text{CO}_3 + 2\text{HNO}_{3(aq)} \rightarrow 2\text{NaNO}_{3(aq)} + \text{CO}_{2(g)} + \text{H}_2\text{O}_{(l)} \]

Molar mass of \( \text{Na}_2\text{CO}_3 = (2\times23) + 12 + (3\times16) = 106 \ \text{g mol}^{-1} \).

Amount of \( \text{Na}_2\text{CO}_3 = \dfrac{5.3}{106} = 0.05 \ \text{mol} \).

From the equation, 1 mol \( \text{Na}_2\text{CO}_3 \) gives 1 mol \( \text{CO}_2 \), so amount of \( \text{CO}_2 = 0.05 \ \text{mol} \).

Volume at s.t.p. \( = 0.05 \times 22.4 = \mathbf{1.12 \ dm^3} \).

Détails de la réponse

(i) Structure of 2-chloro-2-methylpropane: a central carbon carries three methyl groups and one chlorine atom, (CH₃)₃C-Cl. Displayed:

CH₃
    |
CH₃ - C - Cl
    |
CH₃

(ii) Compound X is CH₃COOC₂H₅.

• I. Functional group: the ester (alkanoate) group, -COO-.
• II. IUPAC name: ethyl ethanoate.
• III. Homologous series: the alkanoates (esters).
• IV. Two compounds from which X is formed: ethanoic acid and ethanol.
• V. One physical characteristic: X has a sweet, fruity, pleasant smell (it is a colourless volatile liquid).

(b)(i) Two products of the fractional distillation of petroleum: petrol (gasoline) and kerosene.

(ii) One use of each:

• Petrol: fuel for car and vehicle (petrol) engines.
• Kerosene: fuel for heating and lighting, and as aviation (jet) fuel.

(iii) One disadvantage of crude oil production: it causes oil spillage and environmental pollution.

(c)(i) Monomer of protein: the amino acid.

(c)(ii) Molecular formula from empirical formula CHO₂, molar mass 90. Empirical formula mass \(= 12 + 1 + 2(16) = 45\).

Molecular formula \(= (CHO_2)_2 = \) C₂H₂O₄.

(d) Reaction scheme (ethanol / ethene route):

• (i) Reagents: Stage II requires concentrated H₂SO₄; Stage V requires hydrogen gas.
• (ii) Product Q of the reaction in Stage IV is 1,2-dibromoethane.
• (iii) Conditions for Stage III: high temperature, high pressure and a catalyst.
• (iv) Names of the processes: Stage I - fermentation; Stage II - dehydration; Stage III - polymerisation; Stage V - hydrogenation.

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None

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(a)(i) Graham's law of diffusion

At constant temperature and pressure, the rate of diffusion of a gas is inversely proportional to the square root of its density (or relative molecular mass).

(a)(ii) Relative molecular mass of gas Y

Rate of oxygen \( = \dfrac{100}{4} = 25\ \text{cm}^3\text{s}^{-1} \); rate of Y \( = \dfrac{50}{3} = 16.67\ \text{cm}^3\text{s}^{-1} \). Relative molecular mass of \( O_2 = 2 \times 16 = 32 \).

Squaring both sides: \( 2.25 = \dfrac{M_Y}{32} \), so \( M_Y = 2.25 \times 32 = 72 \).

Relative molecular mass of gas Y = 72.

(b) \( X + 2Y_{(g)} \rightleftharpoons XY_{2(g)} \), \( \Delta H = -52\ \text{kJ mol}^{-1} \) (the forward reaction is exothermic).

(i) An increase in temperature favours the endothermic (backward) direction, so the yield of \( XY_2 \) decreases.

(ii) A decrease in pressure shifts the equilibrium towards the side with the greater number of gas molecules. The left side has 3 gas molecules \( (X + 2Y) \) while the right has 1 \( (XY_2) \); the position of equilibrium therefore moves backward (to the left) and the yield of \( XY_2 \) decreases.

(iii) A catalyst: I. has no effect on the position of equilibrium; II. lowers the activation energy of both the forward and backward reactions equally.

(c)(i) Solubility of solids compared with gases in liquids

• The solubility of most solids increases with a rise in temperature, whereas the solubility of gases decreases with a rise in temperature.
• Pressure has almost no effect on the solubility of solids, but the solubility of gases increases markedly with an increase in pressure.

(c)(ii) Physical property used for separation

• I. Kerosene and petrol: difference in boiling points (fractional distillation).
• II. Calcium trioxocarbonate(IV) and potassium chloride: difference in solubility in water (\( CaCO_3 \) insoluble, \( KCl \) soluble); dissolve, then filter.
• III. Ammonium chloride and sodium chloride: difference in the ability to sublime (\( NH_4Cl \) sublimes); sublimation.

(d)(ii) Basicity of the acids

Basicity is the number of replaceable hydrogen ions produced per molecule of the acid on ionization.

Only the hydrogen of the \( -COOH \) group in ethanoic acid is replaceable, so its basicity is 1.

(d)(iii) Anhydrous and hydrated salts

An anhydrous salt contains no water of crystallization, while a hydrated salt contains a fixed number of molecules of water of crystallization chemically combined with it (for example \( CuSO_4 \) is anhydrous while \( CuSO_4 \cdot 5H_2O \) is hydrated).

Détails de la réponse

(a) Hardness of water

• (i) Temporary hardness and permanent hardness.
• (ii) Temporary hardness: calcium hydrogen trioxocarbonate (IV), Ca(HCO₃)₂. Permanent hardness: calcium tetraoxosulphate (VI), CaSO₄.
• (iii) Removal of temporary hardness (by boiling):
  Ca(HCO₃)_2(aq) → CaCO_3(s) + H₂O_(l) + CO_2(g)
  Removal of permanent hardness (by adding washing soda):
  CaSO_4(aq) + Na₂CO_3(aq) → CaCO_3(s) + Na₂SO_4(aq)
• (iv) Hard water forms an insoluble scum with soap and does not readily form a lather, so it wastes soap.

(b)(i) pH values

• I. Glucose solution: equal to 7.
• II. Chlorine water: less than 7.
• III. Lime water: greater than 7.
• IV. Sour milk: less than 7.

(b)(ii) Differences

• I. An acidic oxide reacts with bases only to form salt and water, while an amphoteric oxide reacts with both acids and bases to form salt and water.
• II. A concentrated acid contains little or no water (a high proportion of acid), while a dilute acid contains a large amount of water (a low proportion of acid).
• III. In a normal salt all the ionizable hydrogen of the acid has been replaced by a metal, while in an acid salt only part of the ionizable hydrogen has been replaced (it still contains replaceable hydrogen).

(c)(i) Mass of FeSO₄ from 0.5 mole of Fe

From Fe + H₂SO₄ → FeSO₄ + H₂, the mole ratio Fe : FeSO₄ is 1 : 1, so moles of FeSO₄ = 0.5 mol.
Molar mass of FeSO₄ = 56 + 32 + 64 = 152 g mol^-1.
Mass = 0.5 × 152 = 76 g.

(c)(ii) Rhombic sulphur and monoclinic sulphur.

(d)(i) The damp starch-iodide paper turns blue-black.

(d)(ii) Chlorine is a stronger oxidizing agent than iodine; it oxidizes the iodide ions on the paper to iodine, and the liberated iodine turns the starch blue-black.

(d)(iii) With excess chlorine, ammonia gives nitrogen trichloride (NCl₃) and hydrogen chloride: NH₃ + 3Cl₂ → NCl₃ + 3HCl.