WAEC SSCE - Chemistry - 1992

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When iron rusts, it undergoes a redox reaction. This means that iron reacts with oxygen in the presence of water to form hydrated iron(III) oxide, commonly known as rust. This reaction involves the transfer of electrons from the iron to the oxygen, which causes the iron to become oxidized and the oxygen to become reduced. Rust is a reddish-brown substance that weakens and destroys iron over time, which is why it's important to prevent rust from forming on metal objects.

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The molar mass of oxygen is 16 g/mol. This means that 1 mole of oxygen gas, which has a molar mass of 32 g/mol (since oxygen exists as O₂), contains 2 moles of oxygen atoms. We can use Avogadro's number (L = 6.0 X 10²³ mol^-1) to find out the number of oxygen atoms present in 32g of oxygen gas. First, we need to find out how many moles of oxygen gas are present in 32g: moles = mass ÷ molar mass = 32g ÷ 32 g/mol = 1 mole So, 32g of oxygen gas contains 1 mole of O₂. Since 1 mole of O₂ contains 2 moles of oxygen atoms, the number of oxygen atoms present in 32g of oxygen gas is: number of oxygen atoms = 2 moles x L = 2 x 6.0 X 10²³ = 1.2 x 10²⁴ Therefore, there are 1.2 x 10²⁴ oxygen atoms in 32g of the gas. Answer matches this result.

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The product of the reaction between propanoic acid and ethanol is ethylpropanoate. This is because when propanoic acid and ethanol react, they undergo an esterification reaction to form an ester called ethylpropanoate. Esters are organic compounds that are formed from the reaction between an alcohol and a carboxylic acid. In this case, ethanol acts as the alcohol, and propanoic acid acts as the carboxylic acid. The reaction involves the removal of a water molecule, resulting in the formation of the ester. The general reaction can be represented as follows: Carboxylic acid + alcohol → ester + water Propanoic acid + ethanol → ethylpropanoate + water Therefore, the correct answer is ethylpropanoate.

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The correct deduction from the information that a reaction is exothermic is that the reaction vessel gets hotter as the reaction proceeds. This means that energy is released during the reaction, which increases the temperature of the surroundings. The other options are incorrect because: - ∆H for the reaction is negative in an exothermic reaction, not positive. This indicates that the reaction releases heat. - The activation energy of the reaction is not related to whether it is exothermic or endothermic. It is the minimum amount of energy required for a reaction to occur. - The rate of the reaction may or may not increase with time in an exothermic reaction. The rate depends on factors such as concentration, temperature, and catalysts.

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Methanol can be obtained from wood by a process called destructive distillation. In this process, wood is heated in the absence of air or oxygen, which causes it to break down and release volatile compounds. Methanol is one of the compounds released during this process and can be collected by condensation. This method has been used historically for producing methanol, which is also known as wood alcohol, as well as other chemicals such as charcoal, acetic acid, and tar.

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The given formula, (CH₃)₃COH, represents an alkanol with a hydroxyl (-OH) group attached to a carbon atom. To determine whether it is a primary, secondary, or tertiary alkanol, we need to look at the carbon atom to which the -OH group is attached and count the number of carbon atoms that are directly bonded to it. In this case, the carbon atom to which the -OH group is attached is bonded to three other carbon atoms, which means that it is a tertiary carbon. Therefore, the alkanol is a tertiary alkanol. However, the question also gives an option for trihydric alkanol, which means an alkanol with three hydroxyl (-OH) groups attached to it. The given formula has only one hydroxyl group, so it cannot be a trihydric alkanol. Therefore, the correct answer is: tertiary alkanol.

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The difference in the mode of conduction of electricity by metals and aqueous salt solutions is due to the fact that metals have mobile electrons that can move freely and carry an electric current, while aqueous salt solutions have mobile ions (charged particles) that carry an electric current. Therefore, the correct option is: "Electricity is carried by mobile electrons in metals but by ions in aqueous salt solution." In metals, the outermost electrons of the atoms are not tightly bound to their nuclei, which allows them to move freely through the metal structure. When a voltage is applied, these mobile electrons can move in a particular direction, carrying an electric current. In contrast, aqueous salt solutions contain dissolved salts that break up into positively and negatively charged ions. When a voltage is applied, these mobile ions move towards the opposite electrode, carrying an electric current. Therefore, the mode of conduction of electricity in metals and aqueous salt solutions is fundamentally different, with metals conducting electricity via mobile electrons, and aqueous salt solutions conducting electricity via mobile ions.

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The pH scale ranges from 0 to 14, where 0 is the most acidic and 14 is the most alkaline (basic). A pH value of 7 is considered neutral, meaning that the solution is neither acidic nor alkaline. So, a slightly alkaline solution would have a pH value greater than 7. Among the given options, only two values are greater than 7: 8 and 13. A pH value of 8 is slightly alkaline, while a pH value of 13 is strongly alkaline. Therefore, the pH value that is likely to be that of a slightly alkaline solution is 8.

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The maximum number of electrons that can be accommodated in the shell having the principal quantum number 3 is 18. The principal quantum number (n) represents the energy level of an electron in an atom. For a given value of n, there are different subshells with different shapes and orientations. These subshells are represented by the letters s, p, d, and f. For n=3, there are three subshells: 3s, 3p, and 3d. The 3s subshell can hold a maximum of 2 electrons, the 3p subshell can hold a maximum of 6 electrons, and the 3d subshell can hold a maximum of 10 electrons. Therefore, the total number of electrons that can be accommodated in the shell having the principal quantum number 3 is the sum of the maximum number of electrons in each of these subshells: 2 + 6 + 10 = 18 Hence, the maximum number of electrons that can be accommodated in the shell having the principal quantum number 3 is 18.

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The statement "The rate a reaction is proportional to the number of effective collisions occurring per second between the reactants" is associated with Collision Theory. Collision theory explains how chemical reactions occur at the molecular level. It states that for a reaction to occur, the reacting molecules must collide with each other with sufficient energy and proper orientation. Not all collisions lead to a reaction, only those with enough energy and correct orientation can lead to a successful reaction. The rate of a reaction is determined by the frequency of these effective collisions. The more frequently the reactant molecules collide with enough energy and proper orientation, the faster the reaction rate. Therefore, the statement emphasizes that the rate of a chemical reaction is proportional to the number of successful collisions (effective collisions) occurring between the reactants per second.

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The best method to isolate a suspected dye contaminant from a lollipop is paper chromatography. Paper chromatography is a technique used to separate and identify different substances based on their physical and chemical properties. In this technique, a small amount of the sample is placed on a strip of paper and then the paper is placed in a suitable solvent. As the solvent moves up the paper, it carries the different components of the sample along with it. The different components will travel at different rates based on their solubility in the solvent and their attraction to the paper. In the case of a suspected dye contaminant in a lollipop, paper chromatography can be used to isolate and identify the dye by separating it from the other components of the lollipop. The lollipop can be dissolved in a suitable solvent, and a small amount of the resulting solution can be spotted onto a strip of paper. The paper can then be placed in a solvent that will carry the dye and other components up the paper at different rates, allowing the dye to be separated and identified. Fractional distillation, recrystallization, filtration, and evaporation are not suitable methods for isolating a dye contaminant from a lollipop. Fractional distillation is used to separate different components of a liquid mixture based on their boiling points, but it is not suitable for separating a solid dye from a lollipop. Recrystallization is a technique used to purify solid compounds, but it is not suitable for isolating a dye from a lollipop. Filtration is a method used to separate solid particles from a liquid, but it is not suitable for isolating a dye from a lollipop. Evaporation is a process used to remove a solvent from a solution, but it is not suitable for isolating a dye from a lollipop.

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A monobasic acid is an acid that can donate only one hydrogen ion (H+) or proton per molecule in an acid-base reaction. Out of the given acids, all except ethanedioic acid (also known as oxalic acid) are monobasic. Methanoic acid, also known as formic acid, has the formula HCOOH and can donate one hydrogen ion (H+) in an acid-base reaction. Dioxonitrate III acid, also known as nitrous acid, has the formula HNO2 and can donate one hydrogen ion (H+) in an acid-base reaction. Oxochlorate (I) acid, also known as hypochlorous acid, has the formula HClO and can donate one hydrogen ion (H+) in an acid-base reaction. Hydrobromic acid has the formula HBr and can donate one hydrogen ion (H+) in an acid-base reaction. Ethanedioic acid, also known as oxalic acid, has the formula H2C2O4 and can donate two hydrogen ions (H+) in an acid-base reaction, making it a dibasic acid.

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An exothermic reaction is a type of reaction that releases heat into the surroundings. Therefore, the correct deduction from this information is that the reaction vessel gets hotter as the reaction proceeds. This is because the heat energy released from the reaction is absorbed by the surrounding medium, which in this case is the reaction vessel. The other statements are not necessarily true deductions from the given information. For example, the sign of the enthalpy change (∆H) for an exothermic reaction is negative, not positive. Also, the rate of the reaction can either increase or decrease with time, depending on the specific reaction and conditions. The activation energy is a property of the reactants and is not necessarily related to whether the reaction is exothermic or endothermic.

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The energy required to remove the most loosely bound electron from an atom in the gaseous state is known as the ionization energy. This means that it takes a certain amount of energy to remove an electron from an atom, and this energy is specific to each atom. The ionization energy is the minimum amount of energy required to remove an electron from an atom and create a positively charged ion. In simple terms, you can think of an electron in an atom as being held in place by the attraction between the negatively charged electron and the positively charged nucleus of the atom. To remove the electron, you need to overcome this attraction, and that requires energy. The ionization energy is a measure of the amount of energy required to overcome this attraction and remove the electron. Ionization energy is an important concept in chemistry because it determines how easily an atom can form a positive ion, which is an atom that has lost one or more electrons. Elements with low ionization energies tend to form positive ions more readily than elements with high ionization energies.

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The process that involves the decomposition enzymic action is "fermentation". Fermentation is a metabolic process that converts sugar into acids, gases or alcohol in the absence of oxygen. During fermentation, microorganisms such as bacteria or yeast use enzymes to break down the sugar molecules into simpler compounds, releasing energy in the process. This enzymatic action causes the fermentation process to occur. In contrast, esterification is a chemical reaction between an alcohol and a carboxylic acid, which does not involve enzymic action. Dehydration is a chemical process that removes water from a substance, and polymerization is a chemical reaction that combines many small molecules to form a larger molecule. Both of these processes also do not involve enzymic action. Saponification is a chemical reaction between a fat or oil and a strong base, which involves the hydrolysis of ester bonds. Although saponification may involve the action of enzymes, it is not primarily a process that involves enzymic decomposition.

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The mineral that contains fluorine as one of its constituent elements is cryolite. Cryolite is a rare mineral that consists of sodium, aluminum, and fluoride. It is a white or colorless mineral that is found primarily in Greenland. The unique property of cryolite is that it contains a high concentration of fluoride ions, which makes it a valuable industrial material for the production of aluminum. Fluorine is a highly reactive element and is not commonly found in nature in its pure form. However, it is commonly found as a constituent element in minerals such as cryolite. Bauxite, potash alum, kaolin, and mica are minerals that do not contain fluorine as one of their constituent elements, making cryolite the correct answer to the question.

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The compound that is not correct for the reaction of sodium material in the manufacture of plastics is ethane. This is because ethane is an alkane and is not reactive enough to undergo a polymerization reaction, which is necessary for the production of plastics. On the other hand, the other compounds listed - ethyne, monochloroethene, propene, and butadiene - are all capable of undergoing polymerization reactions, which are initiated by the use of sodium material or other catalysts. These reactions involve the joining of smaller molecules, called monomers, to form long chains of polymers, which are the building blocks of plastics. In summary, ethane is not a suitable compound for the production of plastics, whereas the other compounds listed can all undergo polymerization reactions to form plastics.

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The halide that is widely used in photography is silver bromide. Halides are compounds that contain halogen elements like bromine, chlorine, iodine, etc. In photography, halides are used to make light-sensitive materials. Silver bromide is particularly useful because it is sensitive to light and can be used to capture images. When light falls on a film or photographic paper containing silver bromide, the silver ions in the compound are reduced to form silver metal. This forms the basis of photographic development, where the image is created through a chemical reaction between the light-sensitive material and a developing agent. In summary, silver bromide is the halide used in photography because of its light sensitivity and its ability to form a visible image through a chemical reaction with a developing agent.

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The statement that "it is similar to hydrolysis" is not correct about esterification. Esterification is a chemical reaction in which an alcohol and a carboxylic acid react to form an ester and water. It is a slow reaction that is catalyzed by acids and is reversible, meaning that the ester can be hydrolyzed back into the original alcohol and carboxylic acid under appropriate conditions. However, esterification and hydrolysis are opposite reactions. Esterification involves the condensation of an alcohol and a carboxylic acid to form an ester and water, while hydrolysis involves the breaking down of an ester into its constituent alcohol and carboxylic acid through the addition of water. Esters are often responsible for fruity or floral smells and flavors in various foods and perfumes.

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The group that contains entirely linear molecules is (d) CO₂ H₂ N₂. To determine if a molecule is linear, we need to examine its molecular geometry. Linear molecules have a symmetrical arrangement of atoms around a central atom, with a bond angle of 180 degrees. In group (a), H₂NH₃O₂, the molecule contains different atoms (hydrogen, nitrogen, and oxygen) that cause an asymmetrical arrangement and do not have a bond angle of 180 degrees, so it is not linear. In group (b), CO₂NH₃₂, the molecule contains a non-central nitrogen atom that disrupts the symmetry and does not have a bond angle of 180 degrees, so it is not linear. In group (c), H₂CH₄N₂, the molecule contains different atoms (hydrogen, carbon, and nitrogen) that cause an asymmetrical arrangement and do not have a bond angle of 180 degrees, so it is not linear. In group (d), CO₂H₂N₂, both molecules contain only one type of central atom (carbon and nitrogen, respectively) and no lone pairs of electrons, which allows for a symmetrical arrangement of atoms and a bond angle of 180 degrees, making them both linear. In group (e), CH₄O₂CO₂, the molecule contains different atoms (carbon, hydrogen, and oxygen) that cause an asymmetrical arrangement and do not have a bond angle of 180 degrees, so it is not linear. Therefore, the group that contains entirely linear molecules is (d) CO₂H₂N₂.

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A physical reaction is a process that involves changes in the physical properties of a substance, such as shape, size, or state, without any change in its chemical composition. Among the options provided, the process that is a physical reaction is "allotropic change." Allotropic change is a physical reaction because it involves a change in the physical form of an element, but not a change in its chemical composition. Allotropy refers to the ability of an element to exist in different forms, called allotropes, which have different physical properties but the same chemical composition. For example, carbon can exist in the form of diamond, graphite, or charcoal, which all have different physical properties but are chemically identical. In contrast, the other processes listed involve chemical reactions, where there is a change in the chemical composition of the substance. Electrolysis involves the use of an electric current to break down a compound into its constituent elements or ions. Hydrolysis is the reaction of a compound with water to produce a new compound. Neutralization is the reaction between an acid and a base to produce a salt and water. Corrosion is the gradual destruction of a metal by chemical or electrochemical reaction with its environment.

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Vulcanization is a chemical process that involves adding sulfur to natural rubber and heating it. This process hardens the rubber and increases its durability and strength, making it more suitable for use in various applications such as tires and shoe soles. During vulcanization, the sulfur forms cross-linkages between the long chains of rubber molecules, making them more rigid and less elastic. This results in the hardening of the rubber and makes it more resistant to abrasion and tearing. Vulcanization also helps to prevent the growth of fungus on the rubber by making it more resistant to environmental factors such as heat, humidity, and moisture. In summary, vulcanization hardens rubber through cross-linkages and increases its durability, strength, and resistance to environmental factors such as fungus growth.

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(a) Main ore of tin

Cassiterite, which is tin(IV) oxide, SnO₂.

(b) Smelting of purified tin ore

The oxide is reduced by heating with carbon (coke) in a furnace: \[SnO_2 + 2C \rightarrow Sn + 2CO\]

(c) Two alloys of tin

• Solder (tin and lead).
• Bronze (tin and copper). (Pewter and type metal are also tin alloys.)

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(a) Crystalline allotrope stable at room temperature

Rhombic sulphur (alpha-sulphur), which is stable below about 96 °C.

(b)(i) A fuel containing significant sulphur as an impurity

Coal (crude oil / petroleum and diesel are also acceptable).

(ii) One environmental disadvantage

Burning such a fuel releases sulphur(IV) oxide (SO₂), which pollutes the air and reacts with rain water to form acid rain. Acid rain damages buildings, plants, soils and aquatic life, and the gas also causes respiratory problems.

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(a)(i) Uses of aluminium and related properties

1. Cooking utensils: aluminium is a good conductor of heat, is light, and resists attack by food because of its protective oxide coating.
2. Overhead electric cables: aluminium is a good conductor of electricity and has low density, making the cables light.

(a)(ii) Aluminium oxide is amphoteric because it reacts with both acids and bases.

With an acid:

\[Al_2O_3 + 6HCl \rightarrow 2AlCl_3 + 3H_2O\]

With a base:

\[Al_2O_3 + 2NaOH + 3H_2O \rightarrow 2NaAl(OH)_4\]

(b)(i) Electrolytic cell for extracting calcium

(b)(ii) Electrode reactions

At the cathode:

\[Ca^{2+} + 2e^- \rightarrow Ca\]

At the anode:

\[2Cl^- \rightarrow Cl_2 + 2e^-\]

(b)(iii) Calcium fluoride lowers the melting point of calcium chloride. This allows electrolysis to take place at a lower temperature and hence reduces energy required.

(c) The order of reactivity, starting with the least reactive metal, is:

\[\boxed{X < W < Z < Y}\]

Equivalently, in decreasing order of reactivity: \(Y > Z > W > X\).

• Y reacts vigorously with cold water and is therefore the most reactive.
• Z displaces W from a solution of a salt of W, so Z is more reactive than W.
• W reacts with steam but not with cold water.
• X is produced when its trioxonitrate(V) is strongly heated, showing that it is the least reactive.

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(a) Acid salt

An acid salt is a salt formed by the partial replacement of the ionizable (replaceable) hydrogen atoms of a polybasic acid by a metal or ammonium ion, so it still contains replaceable hydrogen. Example: sodium hydrogen tetraoxosulphate(VI), NaHSO₄ (sodium hydrogen carbonate, NaHCO₃, is also acceptable).

(b) Why an all-glass apparatus is used

Concentrated trioxonitrate(V) acid, and its hot vapour, is a strong, corrosive oxidizing agent that attacks and corrodes rubber, cork and metal connections. Glass is chemically unreactive towards the acid, so an all-glass apparatus prevents corrosion, leakage and contamination of the acid.

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(a)(i) Differences between electrovalent and covalent compounds

Electrovalent (ionic) compounds	Covalent compounds
They consist of oppositely charged ions held by strong electrostatic forces.	They consist of discrete molecules formed by sharing electrons.
They conduct electricity when molten or when dissolved in water because their ions are mobile.	They generally do not conduct electricity because they contain no mobile ions.
They usually have high melting and boiling points and are commonly soluble in water.	They usually have relatively low melting and boiling points and are commonly soluble in organic solvents.

(a)(ii) The bond formed when ammonia accepts a hydrogen ion to form ammonium ion is a coordinate (dative covalent) bond.

\[\mathrm{NH_3 + H^+ \longrightarrow NH_4^+}\]

The nitrogen atom donates its lone pair of electrons to \(\mathrm{H^+}\).

(b)(i) The four quantum numbers are:

1. principal quantum number, \(n\);
2. azimuthal or angular momentum quantum number, \(l\);
3. magnetic quantum number, \(m_l\);
4. spin quantum number, \(m_s\).

(b)(ii) The electronic arrangement of an atom with five electrons is \(1s^2\,2s^2\,2p^1\). Therefore, the fifth electron is in a 2p orbital. A 2p orbital is dumb-bell shaped.

(b)(iii) Boron, \(1s^2\,2s^2\,2p^1\), is in Period 2 and Group 13 (Group IIIA) of the Periodic Table.

(c)(i) Entropy is a measure of the degree of disorderliness or randomness of a chemical system.

(c)(ii)

\[\Delta G=\Delta H-T\Delta S\]

\[\Delta G=-710-\left(300\times0.15\right)\]

\[\Delta G=-710-45=-755\ \text{kJ mol}^{-1}\]

(c)(iii) The reaction is spontaneous at \(300\,\text{K}\), because \(\Delta G\) is negative.

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(a)

1. Hydrolysis of simple proteins produces amino acids.
2. Alkaline hydrolysis of fats and oils produces soap (the sodium or potassium salts of fatty acids) and glycerol.

(b)

1. The process is cracking. It is used industrially to convert large, less useful and less demanded hydrocarbon fractions into smaller, more useful fractions such as petrol and alkenes.
2. The broken pieces of clay pot prevent local overheating and provide a medium for more uniform heating in the combustion tube.
3. Pass each gas through bromine water, or expose it to bromine vapour. Hydrocarbon X decolourises the reddish-brown bromine water because it is an unsaturated hydrocarbon (an alkene), whereas methane produces no change.
4. Laboratory preparation of ethene
   Ethene is prepared by dehydrating ethanol with concentrated tetraoxosulphate(VI) acid at about 170°C. The ethene formed is collected over water.

Equation:

\[\mathrm{CH_3CH_2OH\ \xrightarrow[170^\circ C]{conc.\ H_2SO_4}\ CH_2{=}CH_2 + H_2O}\]

(c)

1. There is effervescence as sodium gradually dissolves. Hydrogen gas is evolved and it gives a pop sound with a lighted splint. Sodium propoxide is formed.
   
   \[\mathrm{2CH_3CH_2CH_2OH + 2Na \rightarrow 2CH_3CH_2CH_2ONa + H_2}\]
2. With excess acidified potassium heptaoxodichromate(VI):
   
   • Propan-1-ol is oxidised to propanoic acid, \(\mathrm{CH_3CH_2COOH}\).
   • Propan-2-ol is oxidised to propanone, \(\mathrm{CH_3COCH_3}\).
   The process is oxidation.

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The elements are A (atomic number 8, oxygen), B (atomic number 11, sodium) and C (atomic number 12, magnesium).

(a)(i) Formula of the compound of A and B

Sodium (B) forms Na⁺ and oxygen (A) forms O^2-, so the compound is sodium oxide, Na₂O.

(ii) Elements in the s-block

B (sodium, 1s²2s²2p⁶3s¹) and C (magnesium, 1s²2s²2p⁶3s²) belong to the s-block, because their highest-energy (valence) electron enters an s-orbital. Oxygen (A) is a p-block element (its last electron enters a 2p orbital).

(b) Component elements of bleaching powder

Bleaching powder is calcium oxochlorate(I) chloride, CaOCl₂. Its component elements are calcium, chlorine and oxygen.

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None

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(a) Collecting gases denser than air

Gases that are denser than air are collected by upward delivery (downward displacement of air): the delivery tube is passed to the bottom of an upright gas jar, and the denser gas sinks and fills the jar from the base upwards, pushing the air out.

(b) Gases for the fountain experiment

Two suitable gases are ammonia (NH₃) and hydrogen chloride (HCl).

The physical property that makes them suitable is their very high solubility in water. When a little water enters the flask of gas, the gas dissolves almost instantly, causing a large drop in pressure inside the flask, so more water is forced up rapidly to form a fountain.

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(a) A naturally-occurring acid

Ethanoic acid (found in vinegar). Other acceptable examples are methanoic acid (in ant stings), citric acid (in citrus fruits) and lactic acid (in sour milk).

(b) Molecular formula and name of the acid

\[\text{molar mass} = \frac{44}{0.5} = 88\,g\,mol^{-1}\]

A mono-alkanoic (monocarboxylic) acid has the general formula C_nH_2nO₂, whose molar mass is 14n + 32.

\[14n + 32 = 88 \Rightarrow 14n = 56 \Rightarrow n = 4\]

Molecular formula = C₄H₈O₂ (that is, C₃H₇COOH).

Name of the acid: butanoic acid.

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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 of its molar mass).

\[ r \propto \frac{1}{\sqrt{\rho}} \qquad\text{or}\qquad \frac{r_1}{r_2} = \sqrt{\frac{\rho_2}{\rho_1}} = \sqrt{\frac{M_2}{M_1}} \]

(a)(ii) The equilibrium.

The diagram shows a reversible reaction, heat driving it forward and cooling driving it back:

\[ \text{N}_2\text{O}_{4(g)} \;\underset{\text{cool}}{\overset{\text{heat}}{\rightleftharpoons}}\; 2\text{NO}_{2(g)} \]

(pale yellow \(\rightleftharpoons\) dark brown)

Effect on vapour density as temperature rises: The forward dissociation is endothermic, so raising the temperature shifts the position of equilibrium to the right. One mole of N₂O₄ (molar mass 92) breaks into two moles of NO₂ (molar mass 46 each), so the number of gas particles increases while the total mass is unchanged. The average molar mass of the mixture therefore falls. Since vapour density \( = \dfrac{M}{2} \), the vapour density of N₂O₄ decreases as the temperature is increased.

Effect of cooling on colour: On cooling, the exothermic reverse reaction is favoured, so the equilibrium shifts back towards N₂O₄. The dark brown NO₂ is converted into pale (almost colourless) N₂O₄, so the gases become lighter in colour.

(b)(i) Balloon bursting in the sun. Heat from the sun raises the temperature of the trapped air. The gas molecules gain kinetic energy and move faster, striking the walls more often and more forcefully; the gas also tends to expand. At fixed volume this raises the internal pressure. When the pressure exceeds the elastic limit of the rubber, the balloon bursts.

(b)(ii) No constant boiling point up a mountain. A liquid boils when its saturated vapour pressure equals the external atmospheric pressure. Atmospheric pressure falls with altitude, so at the top of the high mountain the pressure is lower and the liquid boils at a lower temperature, whereas at the base the pressure is higher and the boiling point is higher. The boiling point of a pure liquid is therefore fixed only for a fixed pressure, not for a changing one.

(c)(i) Two gaseous reducing agents: hydrogen (H₂) and carbon monoxide (CO).

(c)(ii) Equations showing the reducing property:

Hydrogen reduces copper(II) oxide to copper:

\[ \text{CuO} + \text{H}_2 \rightarrow \text{Cu} + \text{H}_2\text{O} \]

Carbon monoxide reduces copper(II) oxide (or iron(III) oxide) to the metal:

\[ \text{CuO} + \text{CO} \rightarrow \text{Cu} + \text{CO}_2 \]

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None

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(a) Electrolyte of a named secondary cell

In the lead-acid accumulator, the electrolyte is dilute tetraoxosulphate(VI) acid (dilute sulphuric acid, H₂SO₄). (In the nickel-cadmium cell the electrolyte is potassium hydroxide solution.)

(b) The cell

\[Cu_{(s)} | Cu^{2+}_{(aq)} \parallel Zn^{2+}_{(aq)} | Zn_{(s)}\]

• (i) The vertical double stroke represents the salt bridge (the boundary between the two half-cells).
• (ii) Copper forms the cathode, because it is the less reactive metal, so reduction (gain of electrons) occurs at the copper electrode.
• (iii) To determine the oxidation (electrode) potential of each half-cell, each is coupled to the standard hydrogen electrode (SHE), which is taken as the reference with a potential of zero.