WAEC SSCE - Chemistry - 2008

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Halogens are a group of nonmetal elements consisting of fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and astatine (At). Among the given options, the statement that is not true of halogens is "their ionic radii decrease down the group". In reality, as we move down the group, the ionic radii of halogens increase due to the addition of a new energy level in each successive element, which results in a larger atomic radius. The other statements are true of halogens: they exist in different physical states (F₂ and Cl₂ are gases, Br₂ is a liquid, and I₂ is a solid), they exist as diatomic molecules, and their melting and boiling points increase down the group due to stronger van der Waals forces between larger molecules.

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Bronze is mainly used in making statues. Bronze is an alloy of copper, which is a good conductor of heat and electricity, and tin, which strengthens copper. It is widely used in sculpture due to its strength, ductility, and resistance to corrosion. Bronze also has a low melting point, which allows for easier casting and shaping of the statue. In addition to being used for statues, bronze is also used in making bells, musical instruments, and decorative items.

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The mass spectrometer is an instrument used to determine the mass-to-charge ratio of ions. It works by ionizing a sample, accelerating the ions in an electric and/or magnetic field, and then separating the ions based on their mass-to-charge ratio. Therefore, it can be used to measure the mass of an ion, which can represent the mass of a molecule or atom. Therefore, the correct option is "an atom."

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To solve the problem, we need to use the formula: C₁V₁ = C₂V₂ Where C₁ is the initial concentration, V₁ is the initial volume, C₂ is the final concentration, and V₂ is the final volume. Plugging in the values we know: C₁ = 0.20 mol dm^-3 V₁ = 5.00 cm³ = 0.005 dm³ V₂ = 250 cm³ = 0.250 dm³ Solving for C₂: C₂ = (C₁V₁)/V₂ C₂ = (0.20 mol dm^-3)(0.005 dm³)/(0.250 dm³) C₂ = 0.004 mol dm^-3 Therefore, the concentration of the resulting solution is 0.004 mol dm^-3. Answer: 0.004 mol dm^-3

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The correct answer is "saponification." Saponification is a chemical reaction that occurs when a fat or oil is hydrolyzed with a strong base, such as potassium hydroxide (KOH) or sodium hydroxide (NaOH), to form soap and glycerol. In the case of groundnut oil, when it reacts with potassium hydroxide, it breaks down into soap (potassium salt of fatty acids) and glycerol. The soap produced is water-soluble and can be used for cleaning or as a surfactant.

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In dilute HNO_3(aq), phenolphthalein indicator will be colorless. Phenolphthalein is a pH indicator that changes color depending on the pH of the solution it is in. In acidic solutions (pH less than 7), phenolphthalein is colorless. HNO_3(aq) is a strong acid, so it will completely dissociate in water to form H⁺ ions, resulting in a low pH and a colorless phenolphthalein indicator.

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The boiling point of an organic compound is determined by the strength of intermolecular forces present between its molecules. These intermolecular forces are influenced by the molecular mass and polarity of the compound. The larger the molecular mass and polarity of the molecule, the stronger the intermolecular forces and thus the higher the boiling point. Among the given compounds, III has the largest molecular mass and also the maximum number of polar C-O bonds. Therefore, it has the highest boiling point. IV has a branched structure which decreases its surface area of contact and hence the intermolecular forces of attraction are weaker, thus it has the lowest boiling point among the given options. Therefore, the correct order of increasing boiling point would be: IV < II < I < III. Therefore, is the correct answer.

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Détails de la réponse

The answer is: "forms an oxide film over its surface which prevents corrosion". This is because when aluminium is exposed to air, it reacts with oxygen to form a thin layer of aluminium oxide on its surface. This oxide layer is tightly bound to the aluminium and acts as a barrier, protecting the underlying metal from further oxidation or corrosion. This makes aluminium a good roofing material, as it can resist the effects of weather and other corrosive elements over time.

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Alpha decay occurs when a nucleus emits an alpha particle consisting of two protons and two neutrons, which reduces the atomic number by 2 and the mass number by 4. In this case, atom Q has an atomic number of 88 and a mass number of 234. After it decays by alpha emission, it will lose two protons and two neutrons, so the atomic number will be 88 - 2 = 86, and the mass number will be 234 - 4 = 230. Therefore, the correct answer is (a) 86 and 230.

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The collision theory proposes that reactants must collide with a certain minimum amount of energy to form products. In other words, for a chemical reaction to occur, the reactant molecules must collide with sufficient energy and in the correct orientation. If the collision has insufficient energy, the reactants will bounce off each other and not form any products. Therefore, the higher the frequency of successful collisions between reactant molecules, the higher the rate of reaction. The collision theory also suggests that not all collisions are effective; only those that have sufficient energy and correct orientation will lead to product formation.

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When the temperature of a gas is increased, the kinetic energy of the gas molecules increases. This means that the gas molecules move around faster and collide with the walls of the container more frequently and with greater force. This increase in the frequency and force of the collisions between the gas molecules and the walls of the container results in an increase in pressure. Therefore, an increase in temperature of a gas in a fixed volume results in an increase in pressure due to an increase in the number of collisions between the gas molecules and the walls of the container.

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Under the same conditions of temperature and pressure, gases with higher molar mass diffuse more slowly than those with lower molar mass. This is because heavier gas molecules have a slower average speed compared to lighter gas molecules. Therefore, among the given options, chlorine (CI) with a molar mass of 35 g/mol has the highest molar mass and hence the lowest rate of diffusion. In contrast, hydrogen (H) with a molar mass of 1 g/mol has the lowest molar mass and hence the highest rate of diffusion. Neon (Ne) and oxygen (O) have molar masses of 20 g/mol and 16 g/mol, respectively, and therefore have diffusion rates between those of hydrogen and chlorine.

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The high solubility of ethanol in water is due to its hydrogen bonding. Ethanol is a polar molecule, which means it has a partially positive end and a partially negative end. Water is also a polar molecule and it can form hydrogen bonds with ethanol molecules. These hydrogen bonds allow ethanol to dissolve readily in water, resulting in a high solubility. The other options are not correct because they do not explain why ethanol is soluble in water.

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To find the molecular formula of the hydrocarbon, we need to first determine its empirical formula. The empirical formula represents the simplest whole number ratio of atoms present in a compound. From the given data, we know that the hydrocarbon contains 88.9% carbon. This means that the remainder of the compound is made up of hydrogen. To calculate the empirical formula, we assume a 100g sample of the hydrocarbon. Therefore, the sample would contain 88.9g of carbon and 11.1g of hydrogen. Next, we convert the mass of each element to the number of moles using their respective atomic masses. The atomic mass of carbon is 12 g/mol, and that of hydrogen is 1 g/mol. The number of moles of carbon can be calculated as follows: moles of carbon = mass of carbon/atomic mass of carbon moles of carbon = 88.9g/12 g/mol moles of carbon = 7.41 mol Similarly, the number of moles of hydrogen can be calculated as follows: moles of hydrogen = mass of hydrogen/atomic mass of hydrogen moles of hydrogen = 11.1g/1 g/mol moles of hydrogen = 11.1 mol The empirical formula is then calculated by dividing the number of moles of each element by the smallest number of moles obtained in the previous step. This gives the simplest whole number ratio of the elements. The empirical formula for the hydrocarbon is therefore: C : H = 7.41 mol : 11.1 mol Dividing by 7.41 gives: C : H = 1 : 1.5 Therefore, the empirical formula for the hydrocarbon is CH_1.5. We can round this up to CH₂. The molecular formula can be determined by multiplying the empirical formula by a whole number n, which represents the number of empirical formula units in the compound. However, since we don't have any additional information about the hydrocarbon, we cannot determine the molecular formula with certainty. Therefore, the answer is (B) CH₂, which represents the empirical formula of the hydrocarbon.

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A mixture of kerosene and diesel oil can be separated by distillation. This is because the two liquids have different boiling points, and thus can be separated by heating the mixture to a temperature at which one of the liquids boils but the other remains a liquid. The vapor of the boiling liquid can then be collected and condensed back into a liquid, leaving the other liquid behind. In this case, kerosene has a lower boiling point than diesel oil, so heating the mixture will cause the kerosene to vaporize and be collected separately.

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The solubility of a substance is the amount of the substance that can be dissolved in a given amount of solvent at a particular temperature. In this question, we are given that 100 cm³ of a saturated solution of KCIO₃ at 40^oC was evaporated to recover 14 g of the salt. This means that the remaining solution was no longer saturated and all of the excess KCIO₃ had precipitated out. To find the solubility of KCIO₃ at 40^oC, we can use the formula: Solubility = (mass of solute recovered) / (volume of solution before evaporation) The mass of KCIO₃ recovered is given as 14 g, and the volume of solution before evaporation is 100 cm³ or 0.1 dm³. We can calculate the number of moles of KCIO₃ using the given molar mass of KCIO₃: Number of moles = (mass of KCIO₃) / (molar mass of KCIO₃) = 14 / 122.5 = 0.114 moles Now, we can calculate the solubility of KCIO₃: Solubility = (mass of solute recovered) / (volume of solution before evaporation) = (0.114 moles) / (0.1 dm³) = 1.14 mol dm^-3 Therefore, the solubility of KCIO₃ at 40^oC is 1.14 mol dm^-3. The correct option is: - 1.14 mol dm^-3

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Elements in the same group of the periodic table have the same number of valence electrons. This is because elements in the same group have the same outermost electron configuration, which determines their chemical properties. For example, all elements in group 1 have one valence electron, which makes them highly reactive and likely to lose that electron to form a cation with a charge of +1. Similarly, all elements in group 17 have 7 valence electrons, which makes them highly reactive and likely to gain one electron to form an anion with a charge of -1. Therefore, elements in the same group have similar chemical behavior, but different atomic sizes and electron shell numbers.

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An aqueous solution of CaCl₂ is a neutral solution. Calcium chloride (CaCl₂) is a salt composed of a cation, calcium (Ca²⁺), and an anion, chloride (Cl^-). When dissolved in water, the ions dissociate and become surrounded by water molecules. The dissociation of CaCl₂ does not result in the formation of any H⁺ or OH^- ions, so the solution does not have any excess of either acid or base. Hence, the solution remains neutral.

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Chemicals that are produced in small quantities and with very high degree of purity are called fine chemicals. Fine chemicals are usually produced for specific applications or purposes, such as pharmaceuticals, agrochemicals, or specialty chemicals. They are different from bulk chemicals, which are produced in large quantities and are typically less pure. Fine chemicals are also different from heavy and light chemicals, which are produced in large quantities and have a wide range of industrial and commercial applications.

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When ethanol is heated with excess concentrated tetraoxosulphate (VI) acid (also known as sulfuric acid), the organic product formed is ethene. During the reaction, the concentrated sulfuric acid acts as a dehydrating agent, removing a water molecule from the ethanol molecule. This results in the formation of ethene, which is an unsaturated hydrocarbon with a double bond between two carbon atoms. The reaction can be represented by the following equation: CH3CH2OH + H2SO4 → CH2=CH2 + H2O + H2SO4 So, the correct answer is ethene.

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The balanced equation for the reaction between KOH and HCl is: KOH + HCl → KCl + H₂O From the equation, we can see that the mole ratio of KOH to HCl is 1:1. This means that for every 1 mole of HCl, 1 mole of KOH is required to neutralize it. To find the volume of 0.25 mol dm^-3 KOH needed to neutralize 40 cm³ of 0.10 mol dm^-3 HCl, we need to use the formula: n_acid V_acid = n_base V_base where n is the number of moles and V is the volume in dm³. Rearranging the formula to isolate V_base, we get: V_base = (n_acid V_acid) / n_base Substituting the values we have: n_acid = 0.10 mol dm^-3 (concentration of HCl) V_acid = 40 cm³ = 0.04 dm³ (volume of HCl) n_base = 0.25 mol dm^-3 (concentration of KOH) V_base = (0.10 mol dm^-3 x 0.04 dm³) / 0.25 mol dm^-3 = 0.016 dm³ = 16 cm³ Therefore, the volume of 0.25 mol dm^-3 KOH needed to completely neutralize 40 cm³ of 0.10 mol dm^-3 HCl is 16 cm³. The answer is (d) 16cm³.

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Electron affinity refers to the energy released when an electron is added to an atom in the gas phase. The greater the electron affinity of an atom, the more energy is released when an electron is added to it. Based on this definition, the electron affinity of the elements listed can be arranged in the following order: F ₂ < Cl₂ < Br₂ < I ₂ Therefore, the correct arrangement in order of increasing electron affinity would be: I ₂ < Br₂ < CI₂ < F ₂

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The number 45₂₁X represents an atom of an element with atomic number 21. The atomic number of an element represents the number of protons in its nucleus. Since the atom is neutral, the number of protons in its nucleus is equal to the number of electrons surrounding the nucleus. To determine the number of neutrons, we can use the formula: number of neutrons = mass number - atomic number The mass number of the atom is not given, so we cannot directly calculate the number of neutrons. However, we know that for a particular element, the number of protons plus the number of neutrons is equal to the mass number. Since the atomic number of the element is 21, we can look up its atomic mass on the periodic table, which is approximately 45. Therefore, the number of neutrons in the atom is: number of neutrons = mass number - atomic number number of neutrons = 45 - 21 number of neutrons = 24 Therefore, the correct answer is 24.

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The equation that could be used to represent the standard enthalpy of formation of glucose is the second option: 6CO_2(g) + 6H₂O _(l) → C₆H₁₂O_6(s) + 6O _2(g) This is because the standard enthalpy of formation of a compound is the enthalpy change that occurs when one mole of the compound is formed from its constituent elements, with all substances in their standard states. In this case, glucose is formed from carbon dioxide, water, and oxygen gas, which are the standard states of their respective elements. This equation shows the balanced chemical reaction for the formation of glucose from these elements and the resulting enthalpy change.

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In a dry Leclanche cell, manganese (IV) oxide acts as a depolarizer. The depolarizer helps to remove hydrogen gas, which is produced at the negative electrode when the cell is in use, by reacting with it to form water. Manganese (IV) oxide is a strong oxidizing agent and it reacts with the hydrogen gas, making it unavailable to react with the cell components, thereby preventing polarization of the cell. This ensures that the cell maintains a steady voltage output over a long period of time.

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Chlorofluorocarbons (CFCs) cause depletion of the ozone layer in the atmosphere. CFCs are organic compounds that contain chlorine, fluorine, and carbon. When CFCs are released into the atmosphere, they rise up to the stratosphere, where they are broken down by ultraviolet radiation from the sun. This breakdown releases chlorine atoms, which react with ozone molecules to form oxygen gas and chlorine monoxide. Chlorine monoxide then reacts with another ozone molecule to form more oxygen gas and another chlorine atom. This cycle continues, with each chlorine atom destroying many thousands of ozone molecules. The reduction of the ozone layer can lead to increased levels of harmful ultraviolet radiation reaching the Earth's surface, which can have negative effects on human health and the environment.

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Water molecules are held together by hydrogen bonds. A hydrogen bond is a type of weak chemical bond that occurs between a hydrogen atom of one molecule and an electronegative atom of another molecule, such as oxygen or nitrogen. In the case of water, the hydrogen atoms are attracted to the oxygen atoms of neighboring water molecules, forming hydrogen bonds. This results in the unique properties of water, such as its high boiling and melting points, and its ability to dissolve a wide range of substances.

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First, we need to calculate the number of moles of zinc used in the reaction. Moles of Zn = mass ÷ molar mass Moles of Zn = 3.25 g ÷ 65 g/mol Moles of Zn = 0.05 mol From the balanced equation, we know that 1 mole of Zn produces 1 mole of H₂ gas. Therefore, the number of moles of H₂ gas produced is also 0.05 mol. At s.t.p (standard temperature and pressure), one mole of any gas occupies 22.4 dm³. So, the volume of hydrogen gas produced at s.t.p is: Volume = number of moles × molar gas volume at s.t.p Volume = 0.05 mol × 22.4 dm^-3/mol Volume = 1.12 dm³ Therefore, the answer is 1.12 dm³.

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Diamond does not conduct electricity because it has no free valence electrons. In diamond, all carbon atoms are bonded to four other carbon atoms in a tetrahedral arrangement, forming a giant covalent molecule. Each carbon atom has no free valence electron to move around and conduct electricity. Thus, diamond is an insulator and does not allow electricity to pass through it.

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The molar mass of magnesium is 24 g mol^-1 (as given). This means that the mass of one mole of magnesium atoms is 24 g. We are given the number of atoms of magnesium: 6.02 x 10²⁴ atoms. To find the mass of these atoms, we need to convert the number of atoms to moles, and then multiply by the molar mass of magnesium. Number of moles of magnesium atoms = (6.02 x 10²⁴ atoms) / (6.02 x 10²³ atoms mol^-1) = 10 mol Mass of 10 moles of magnesium atoms = 10 mol x 24 g mol^-1 = 240 g Therefore, the mass of 6.02 x 10²⁴ atoms of magnesium is 240g. Answer: 240g

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Alkenes undergo addition reactions. Addition reactions occur when atoms or groups of atoms are added to the carbon-carbon double bond of the alkene, resulting in the formation of a single product with a single bond. The double bond in the alkene is converted into a single bond, and the added groups are attached to the carbon atoms that were originally involved in the double bond. This type of reaction is commonly observed in organic chemistry, and it is important in the synthesis of a wide range of compounds, including polymers, plastics, and pharmaceuticals. Examples of addition reactions of alkenes include hydrogenation, halogenation, and hydration.

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(a) Redox in terms of electron transfer

• (i) I. Oxidizing agent: a substance that accepts (gains) electrons from another species during a reaction, and is itself reduced.
• II. Reducing agent: a substance that donates (loses) electrons to another species during a reaction, and is itself oxidized.
• (ii) Carbon acting as a reducing agent: \[2CuO + C \to 2Cu + CO_2\] Here carbon removes oxygen from (reduces) copper(II) oxide.
• (iii) The species that reacted with carbon is copper in CuO. Its oxidation number changes from +2 to 0 (a decrease of 2, i.e. it is reduced).

(b) Gas X (vapour density 32, so molar mass \(= 2\times32 = 64\ \text{g mol}^{-1}\)). It reacts with NaOH to give salt and water only (acidic oxide), decolourizes acidified KMnO\(_4\) (it is a reducing agent) and reacts with H\(_2\)S to give sulphur.

• (i) Gas X is sulphur(IV) oxide, SO\(_2\) (molar mass 64).
• (ii) Two properties: it is a colourless gas with a choking, irritating smell; it is acidic and a strong reducing agent (bleaching action; turns moist blue litmus red).
• (iii) Two uses: as a bleaching agent (for straw, paper and wool); as a food preservative/fumigant and in the manufacture of sulphuric acid (contact process).

(c) From sodium, lead(II) iodide, hydrogen, magnesium, oxygen

• (i) Conducts electricity: sodium and magnesium (metals conduct in the solid state).
• (ii) Produced at the cathode during electrolysis of dilute H\(_2\)SO\(_4\): hydrogen.
• (iii) Corresponds to formula A\(_2\): hydrogen (H\(_2\)) and oxygen (O\(_2\)) are diatomic molecules.
• (iv) Alkaline earth metal: magnesium (Group 2).

(d) Salts

• (i) A salt is a compound formed when the ionisable hydrogen ion(s) of an acid are wholly or partly replaced by a metallic ion or the ammonium ion.
• (ii) Two types: normal (neutral) salt and acid salt (also basic salt, double salt, complex salt).
• (iii) Examples: normal salt, e.g. Na\(_2\)SO\(_4\) (sodium sulphate); acid salt, e.g. NaHSO\(_4\) (sodium hydrogen sulphate).

(e) Neutralization of H\(_2\)SO\(_4\) with NaOH

• (i) \[H_2SO_4 + 2NaOH \to Na_2SO_4 + 2H_2O\]
• (ii) From the equation, 1 mole of H\(_2\)SO\(_4\) needs 2 moles of NaOH. Therefore for 0.50 mole of acid: \[0.50 \times 2 = 1.0\ \text{mole of NaOH}\]

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(a) Statements of the rules

• (i) Pauli's Exclusion Principle: No two electrons in the same atom can have the same set of four quantum numbers. In effect, an orbital can hold a maximum of two electrons, and these two must have opposite (paired) spins.
• (ii) Hund's Rule of Maximum Multiplicity: When electrons are being placed in orbitals of equal energy (degenerate orbitals such as the three p or five d orbitals), each orbital is singly occupied first, all with parallel spins, before any orbital receives a second electron. This gives the maximum number of unpaired electrons and the lowest energy arrangement.

(b) Copper ions (Cu has atomic number 29; atom = \([Ar]3d^{10}4s^{1}\)). Electrons are lost from the 4s orbital first, then from 3d.

• (i) I. Cu\(^{+}\): \(1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}3d^{10}\) i.e. \([Ar]3d^{10}\)
• I. Cu\(^{2+}\): \(1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}3d^{9}\) i.e. \([Ar]3d^{9}\)
• (ii) Unpaired electrons: Cu\(^{+}\) (\(3d^{10}\)) has 0 unpaired electrons (all paired); Cu\(^{2+}\) (\(3d^{9}\)) has 1 unpaired electron.
• (iii) The reaction \(2Cu^{+} \to Cu^{2+} + Cu\) is a disproportionation reaction (a redox reaction in which the same species, Cu\(^{+}\), is simultaneously oxidized to Cu\(^{2+}\) and reduced to Cu).
• (iv) A compound of Cu\(^{+}\): copper(I) oxide, Cu\(_2\)O (or copper(I) chloride, CuCl).

(c) Radioactivity

• (i) I. Radiation that penetrates a lead block: gamma (\(\gamma\)) radiation. II. Radiation stopped by thin paper: alpha (\(\alpha\)) radiation.
• (ii) Charges: gamma radiation carries no charge (0); alpha radiation carries a charge of +2 (it is a helium nucleus, \(^{4}_{2}He^{2+}\)).
• (iii) I. Nuclear fusion. II. Nuclear fission. III. Half-life.

(d) Order of increasing ionic size

• I. Li\(^{+}\) < Na\(^{+}\) < K\(^{+}\). Reason: these are ions of the same group with the same charge; going down the group the number of occupied electron shells increases, so the ionic radius increases.
• II. F\(^{-}\) < O\(^{2-}\) < N\(^{3-}\). Reason: these are isoelectronic ions (each has 10 electrons). As the nuclear charge (number of protons) decreases from F (9) to O (8) to N (7), the pull on the fixed number of electrons weakens, so the electron cloud expands and the ionic radius increases.

(e) Percentage composition of P\(_2\)O\(_5\) (phosphorus(V) oxide; P = 31, O = 16)

Molar mass \(= (2\times31) + (5\times16) = 62 + 80 = 142\ \text{g mol}^{-1}\).

\[\%\,P = \frac{62}{142}\times100 = 43.66\%\]

\[\%\,O = \frac{80}{142}\times100 = 56.34\%\]

So P\(_2\)O\(_5\) is about 43.7% phosphorus and 56.3% oxygen.

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(a) Oxides

• (i) Acidic oxides are oxides (usually of non-metals) that react with water to form acids, and react with bases/alkalis to form a salt and water only.
• (ii) Examples: I. acidic oxide CO\(_2\) (or SO\(_2\), SO\(_3\)); II. basic oxide Na\(_2\)O (or CaO, MgO); III. amphoteric oxide Al\(_2\)O\(_3\) (or ZnO, PbO); IV. neutral oxide CO (or NO, H\(_2\)O, N\(_2\)O).

(b) Heat and heat of neutralization

• (i) I. Heat: a form of energy that flows from a body at a higher temperature to a body at a lower temperature. II. Heat of neutralization: the heat change when one mole of water is formed from the neutralization of an acid by a base (alkali) under standard conditions.
• (ii) Illustrative equations: heat change accompanying a reaction, e.g. \[C + O_2 \to CO_2,\ \Delta H = -393\ \text{kJ mol}^{-1}\] heat of neutralization, e.g. \[HCl_{(aq)} + NaOH_{(aq)} \to NaCl_{(aq)} + H_2O_{(l)},\ \Delta H = -57.3\ \text{kJ mol}^{-1}\]
• (iii) Heat released by 14.5 g of butane (C\(_4\)H\(_{10}\), molar mass \(= 48+10 = 58\ \text{g mol}^{-1}\); heat of combustion given as 5877 kJ per mole). Amount of butane \(= \dfrac{14.5}{58} = 0.25\ \text{mol}\). Heat evolved \(= 0.25 \times 5877 = 1469.25\ \text{kJ}\). So about 1469 kJ of heat is released (combustion is exothermic).

(c) Sulphur

• (i) Two allotropes: rhombic (alpha) sulphur and monoclinic (beta) sulphur.
• (ii) Difference: rhombic sulphur is the stable form below 96 degrees Celsius, while monoclinic sulphur is stable between 96 degrees Celsius and its melting point (about 119 degrees Celsius); they also differ in crystal shape (rhombic is octahedral, monoclinic is needle-shaped).

(d) Noble gases

• (i) Two characteristics: they are chemically inert/unreactive (stable, filled outer shells); they are monatomic, colourless gases with very low melting and boiling points.
• (ii) Uses: I. He used to fill balloons and airships, and (with oxygen) in breathing mixtures for divers. II. Ar used to provide an inert atmosphere in electric (filament) light bulbs and in welding.

(e) On warming ammonium trioxonitrate(V) (ammonium nitrate) with sodium hydroxide, a colourless gas with a pungent, choking smell (ammonia) is evolved which turns moist red litmus paper blue: \[NH_4NO_3 + NaOH \to NaNO_3 + H_2O + NH_3\uparrow\]

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(a) Covalent bonding

(i) A covalent bond is a bond formed when two atoms contribute and share one or more pairs of electrons.

(ii) Properties of covalent compounds include:

• They are generally non-conductors of electricity.
• They usually have low melting and boiling points.
• They are usually insoluble in water but soluble in non-polar organic solvents.

(iii) and (iv) The dot-and-cross diagrams below show the formation of ammonia and ammonium ion. Dots represent electrons from nitrogen, while crosses represent electrons from hydrogen.

(v)

1. In ammonia, NH₃, the N-H bonds are ordinary covalent bonds.
2. In ammonium ion, NH₄⁺, there are covalent bonds and one co-ordinate (dative covalent) bond formed by donation of the lone pair on nitrogen to H⁺. After formation, the four N-H bonds are equivalent.

(b)

(i) The subatomic particles and their relative masses are:

Subatomic particle	Relative mass
Proton	1
Neutron	1
Electron	\(\dfrac{1}{1840}\)

(ii) Water exists as a solid (ice), liquid (water), and gas (steam or water vapour).

(c) Diffusion

(i) Graham's law states that, at constant temperature and pressure, the rate of diffusion of a gas is inversely proportional to the square root of its molar mass or vapour density.

\[r \propto \frac{1}{\sqrt{M}}\]

(ii) \(M_r(\mathrm{He})=4\), \(M_r(\mathrm{CH_4})=16\), and \(M_r(\mathrm{N_2})=28\). Hence, the order of increasing rate of diffusion is:

\[\boxed{\mathrm{N_2\; <\; CH_4\; <\; He}}\]

This is because gases with smaller molar masses diffuse faster.

(d) Structural formulae

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None

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(a)(i) Hard water is water that does not readily form lather with soap because it contains dissolved calcium or magnesium ions.

(ii) Two substances responsible for hardness in water are:

1. Calcium tetraoxosulphate(VI), \(\mathrm{CaSO_4}\).
2. Calcium hydrogentrioxocarbonate(IV), \(\mathrm{Ca(HCO_3)_2}\).

(iii) Two methods of removing hardness are:

1. Boiling, for water with temporary hardness.
2. Adding sodium trioxocarbonate(IV) (washing soda), \(\mathrm{Na_2CO_3}\), to precipitate the calcium or magnesium ions.

(b)(i) The raw materials used in the Contact process are sulphur (or sulphur dioxide), dry air or oxygen, and water.

(ii) The catalyst-dependent reaction is:

\[2\mathrm{SO_{2(g)}}+\mathrm{O_{2(g)}}\rightleftharpoons2\mathrm{SO_{3(g)}}\]

(iii) The catalyst is vanadium(V) oxide, \(\mathrm{V_2O_5}\).

(c)(i) Two uses of sodium tetraoxosulphate(VI), \(\mathrm{Na_2SO_4}\), are:

1. In the manufacture of glass or paper.
2. In the manufacture of detergents.

(ii) The reaction is efflorescence. The hydrated salt loses water of crystallisation to the atmosphere.

(iii)

Mass of anhydrous \(\mathrm{Na_2CO_3}\) in \(30.0\,\mathrm{cm^3}\) of saturated solution \(=1.80\,\mathrm{g}\).

\[\text{Mass in }1000\,\mathrm{cm^3}=\frac{1000\times1.80}{30.0}=60.0\,\mathrm{g}\]\[M_r(\mathrm{Na_2CO_3})=(2\times23)+12+(3\times16)=106\]\[\text{Solubility}=\frac{60.0}{106}=0.566\,\mathrm{mol\,dm^{-3}}\]

Solubility of \(\mathrm{Na_2CO_3}\) at \(25^\circ\mathrm{C}\) = \(0.566\,\mathrm{mol\,dm^{-3}}\).

(d)(i) An activated complex is a temporary, unstable arrangement of atoms formed when reacting particles collide with sufficient energy, before products are formed.

(ii) A collision may fail to produce a reaction when the colliding particles have energy less than the activation energy.

(iii) Energy profile diagram for the formation of water gas:

The products have a higher enthalpy than the reactants; hence \(\Delta H=+131\,\mathrm{kJ\,mol^{-1}}\) and the reaction is endothermic.