ICSE Class 10 Chemistry Board Exam Question Paper 2008 with Solutions

ICSE Class 10 Chemistry Board Exam Question Paper with Solutions

SECTION - I (40 Marks)
(Compulsory : Attempt all questions.)

Question 1.
(a) For part (a) (i) - (a) (x), select the correct answer from the choices A, B, C, D which are given. Write down only the letter corresponding to the correct answer.

(i) With reference to the variation of properties in the Periodic Table, which of the following is generally true ? [1 Mark]
(A) Atomic size increases from left to right across a period.
(B) Ionization potential increases from left to right across a period.
(C) Electron affinity increases going down a group.
(D) Electro-negativity increases going down a group.

Answer: (B) Ionization potential increases from left to right across a period.

As we move across a period, atomic radius decreases and nuclear charge increases, making it harder to remove an electron, thereby increasing ionization potential.

Teacher's Note:
a) Ionization potential increases across a period due to a decrease in atomic size and increase in effective nuclear charge.
b) Students often confuse trends across periods with trends down groups; atomic size decreases across a period but increases down a group.

(ii) Which of the following is not a common characteristic of an electrovalent compound ? [1 Mark]
(A) High melting point.
(B) Conducts electricity when molten.
(C) Consists of oppositely charged ions.
(D) Ionizes when dissolved in water.

Answer: (D) Ionizes when dissolved in water.

Electrovalent (ionic) compounds already consist of ions in their solid state; they dissociate (not ionize) into their constituent ions when dissolved in water or melted.

Teacher's Note:
a) Ionic compounds dissociate into ions upon dissolution because water molecules overcome the electrostatic force of attraction between them.
b) Remember the technical difference between ionization (formation of ions from covalent molecules) and dissociation (separation of already existing ions).

(iii) Dilute sulphuric acid will produce a white precipitate when added to a solution of : [1 Mark]
(A) Copper nitrate
(B) Zinc nitrate
(C) Lead nitrate
(D) Sodium nitrate

Answer: (C) Lead nitrate

Reaction of dilute sulphuric acid with lead nitrate yields lead sulphate, which is a white precipitate.

Teacher's Note:
a) \(\text{Pb(NO}_3)_2 + \text{H}_2\text{SO}_4 \rightarrow \text{PbSO}_4\downarrow + 2\text{HNO}_3\)
b) Lead sulphate is insoluble in water and appears as a dense white precipitate.

(iv) The salt which in solution gives a pale green precipitate with sodium hydroxide solution and a white precipitate with barium chloride solution is : [1 Mark]
(A) Iron (III) sulphate
(B) Iron (II) sulphate
(C) Iron (II) chloride
(D) Iron (III) chloride

Answer: (B) Iron (II) sulphate

Iron (II) ions give a dirty green (pale green) precipitate with sodium hydroxide, and sulphate ions give a white precipitate with barium chloride.

Teacher's Note:
a) \(\text{Fe}^{2+}\) salts impart a pale green or dirty green gelatinous precipitate with \(\text{NaOH}\).
b) Sulphate radicals are always confirmed using barium chloride solution which forms a white precipitate insoluble in dilute \(\text{HCl}\).

(v) The gas law which relates the volume of a gas to the number of molecules of the gas is : [1 Mark]
(A) Avogadro's Law
(B) Gay-Lussac's Law
(C) Boyle's Law
(D) Charles' Law

Answer: (A) Avogadro's Law

Avogadro's law states that equal volumes of all gases under similar conditions of temperature and pressure contain the same number of molecules.

Teacher's Note:
a) Avogadro's law directly links volume with the moles (number of molecules) of a gas: \(V \propto n\).
b) Do not confuse this with Gay-Lussac's law, which relates pressure and temperature.

(vi) During the electrolysis of molten lead bromide, which of the following takes place ? [1 Mark]
(A) Bromine is released at the cathode
(B) Lead is deposited at the anode
(C) Bromine ions gain electrons
(D) Lead is deposited at the cathode

Answer: (D) Lead is deposited at the cathode

During electrolysis, positively charged lead ions migrate to the cathode, gain electrons, and get deposited as lead metal.

Teacher's Note:
a) Reduction occurs at the cathode where metal cations gain electrons.
b) Bromide anions migrate to the anode, lose electrons (oxidation), and form reddish-brown bromine vapors.

(vii) Brass is an alloy of : [1 Mark]
(A) Copper and tin
(B) Copper and zinc
(C) Zinc and lead
(D) Lead and tin

Answer: (B) Copper and zinc

Brass is a well-known alloy consisting primarily of copper and zinc.

Teacher's Note:
a) Composition of brass: Copper (60-80%) and Zinc (20-40%).
b) Remember that bronze is an alloy of copper and tin, whereas brass contains zinc.

(viii) Ammonia can be obtained by adding water to : [1 Mark]
(A) Ammonium chloride
(B) Ammonium nitrate
(C) Magnesium nitrade
(D) Magnesium nitrate

Answer: (C) Magnesium nitrade

Metal nitrides react with water to form metal hydroxide and ammonia gas.

Teacher's Note:
a) \(\text{Mg}_3\text{N}_2 + 6\text{H}_2\text{O} \rightarrow 3\text{Mg(OH)}_2 + 2\text{NH}_3\uparrow\)
b) Note the spelling in the question paper ('nitrade' instead of nitride), but the option intended is magnesium nitride.

(ix) Which of the following reactions is used to prepare sulphuryl chloride ? [1 Mark]
(A) Adding concentrated sulphuric acid to a chloride.
(B) Passing sulphur dioxide through a solution of chlorine.
(C) Reacting dry sulphur dioxide and dry chlorine.
(D) Reacting dilute sulphuric acid with a solution of chlorine.

Answer: (C) Reacting dry sulphur dioxide and dry chlorine.

Sulphuryl chloride is prepared by the direct combination of sulphur dioxide and chlorine in the presence of sunlight or a catalyst like camphor (activated charcoal).

Teacher's Note:
a) \(\text{SO}_2 + \text{Cl}_2 \rightarrow \text{SO}_2\text{Cl}_2\)
b) Both reactants must be dry for this combination reaction to proceed smoothly.

(x) The formation of 1, 2-dibromomethane from ethene and bromine is an example of : [1 Mark]
(A) Substitution
(B) Dehydration
(C) Dehydrohalogenation
(D) Addition

Answer: (D) Addition

Ethene is an unsaturated hydrocarbon containing a double bond, which undergoes an addition reaction with bromine to form 1, 2-dibromomethane.

Teacher's Note:
a) Unsaturated compounds characteristically undergo addition reactions.
b) This reaction is also used as a laboratory test for unsaturation (decolorization of reddish-brown bromine solution).

(b) The equation for the burning of octane is :
\(2\text{C}_8\text{H}_{18} + 25\text{O}_2 \rightarrow 16\text{CO}_2 + 18\text{H}_2\text{O}\)

(i) How many moles of carbon dioxide are produced when one mole of octane burns ? [1 Mark]

Answer:
From the balanced equation, 2 moles of octane produce 16 moles of \(\text{CO}_2\). Therefore, 1 mole of octane produces \(\frac{16}{2} = 8\) moles of carbon dioxide.

Teacher's Note:
a) Use stoichiometric coefficients directly from the balanced equation.
b) Always check if the given equation is fully balanced before extracting mole ratios.

(ii) What volume, at stp, is occupied by the number of moles determined in (b) (i) ? [1 Mark]

Answer:
Volume of 1 mole of gas at STP = \(22.4\) litres.
Volume of 8 moles of \(\text{CO}_2 = 8 \times 22.4 = 179.2\) litres.

Teacher's Note:
a) Apply the molar volume formula: \(\text{Volume} = \text{Moles} \times 22.4\text{ L}\).
b) Ensure units (litres) and conditions (STP) are clearly specified in the final step.

(iii) If the relative molecular mass of carbon dioxide is 44, what is the mass of carbon dioxide produced by burning two moles of octane ? [1 Mark]

Answer:
From the balanced equation, 2 moles of octane produce 16 moles of \(\text{CO}_2$.
Mass of 16 moles of \(\text{CO}_2 = 16 \times 44\text{ g} = 704\) g.

Teacher's Note:
a) Mass = Number of moles \(\times\) Molecular mass.
b) Since 2 moles of octane directly yield 16 moles of \(\text{CO}_2$, no scaling factor is needed for moles of carbon dioxide.

(iv) What is the empirical formula of octane ? [1 Mark]

Answer:
Molecular formula of octane is \(\text{C}_8\text{H}_{18}\).
Dividing by the highest common factor (2), the empirical formula is \(\text{C}_4\text{H}_9\).

Teacher's Note:
a) The empirical formula represents the simplest whole number ratio of atoms of each element in a compound.
b) \(\text{C}_8\text{H}_{18} \rightarrow \text{C}_{8/2}\text{H}_{18/2} = \text{C}_4\text{H}_9\).

(c) Name the organic compound prepared by each of the following reactions : [5 Marks]

(i) \(\text{C}_2\text{H}_5\text{COONa} + \text{NaOH} \xrightarrow{\text{CaO, }\Delta}\) [1 Mark]

Answer:
Ethane

Teacher's Note:
a) Decarboxylation of sodium propionate with sodalime gives ethane (\(\text{C}_2\text{H}_6\)).
b) Note that the alkane formed has one carbon atom less than the parent sodium salt.

(ii) \(\text{CH}_3\text{I} + 2\text{[H]} \rightarrow\) [1 Mark]

Answer:
Methane

Teacher's Note:
a) Reduction of iodomethane with nascent hydrogen yields methane (\(\text{CH}_4\)).
b) \(\text{CH}_3\text{I} + 2[\text{H}] \rightarrow \text{CH}_4 + \text{HI}\).

(iii) \(\text{C}_2\text{H}_5\text{Br} + \text{KOH (alcoholic solution)} \rightarrow\) [1 Mark]

Answer:
Ethene

Teacher's Note:
a) Dehydrohalogenation of bromoethane using alcoholic potassium hydroxide produces ethene (\(\text{C}_2\text{H}_4\)).
b) Alcoholic KOH acts as a dehydrating and dehydrohalogenating agent.

(iv) \(\text{CO} + 2\text{H}_2 \text{ (Zinc oxide catalyst)} \rightarrow\) [1 Mark]

Answer:
Methanol

Teacher's Note:
a) Catalytic hydrogenation of carbon monoxide gives methanol (\(\text{CH}_3\text{OH}\)).
b) Conditions include high pressure and zinc oxide mixed with chromium oxide catalyst.

(v) \(\text{CaC}_2 + 2\text{H}_2\text{O} \rightarrow\) [1 Mark]

Answer:
Ethyne (Acetylene)

Teacher's Note:
a) Hydrolysis of calcium carbide yields ethyne gas (\(\text{C}_2\text{H}_2\)).
b) \(\text{CaC}_2 + 2\text{H}_2\text{O} \rightarrow \text{Ca(OH)}_2 + \text{C}_2\text{H}_2\uparrow\).

(d) Identify the following substances : [5 Marks]

(i) An alkaline gas A which gives dense white fumes with hydrogen chloride. [1 Mark]

Answer:
Ammonia (\(\text{NH}_3\))

Teacher's Note:
a) Ammonia is a basic gas that reacts with hydrogen chloride to form ammonium chloride (\(\text{NH}_4\text{Cl}\)) fumes.
b) This is a classic confirmatory test for ammonia gas.

(ii) A dilute acid B which does not normally give hydrogen when reacted with metals but does give a gas when it reacts with copper. [1 Mark]

Answer:
Nitric acid (\(\text{HNO}_3\))

Teacher's Note:
a) Dilute nitric acid is a strong oxidizing agent; it oxidizes hydrogen to water rather than evolving hydrogen gas with active metals.
b) With copper, it produces nitric oxide (NO) gas.

(iii) Gas C has an offensive smell like rotten eggs. [1 Mark]

Answer:
Hydrogen sulphide (\(\text{H}_2\text{S}\))

Teacher's Note:
a) Hydrogen sulphide is characterized by its distinct foul odor resembling rotten eggs.
b) It turns lead acetate paper silvery black.

(iv) Gas D is a colourless gas which can be used as a bleaching agent. [1 Mark]

Answer:
Sulphur dioxide (\(\text{SO}_2\))

Teacher's Note:
a) Sulphur dioxide bleaches delicate colored articles by reduction.
b) The bleaching action is temporary because atmospheric oxygen reoxidizes the bleached article.

(v) Liquid E can be dehydrated to produce ethane. [1 Mark]

Answer:
Ethanol (\(\text{C}_2\text{H}_5\text{OH}\))

Teacher's Note:
a) Heating ethanol with concentrated sulphuric acid at 170 degree Celsius results in intramolecular dehydration to yield ethene (Note: question text says 'ethane', which is a minor typographical error in the paper; dehydration of ethanol produces ethene, \(\text{C}_2\text{H}_4\)).
b) Concentrated \(\text{H}_2\text{SO}_4\) acts as a powerful dehydrating agent.

(e) Write the equation for the following reactions : [5 Marks]

(i) Aluminium nitride and water. [1 Mark]

Answer:
\(\text{AlN} + 3\text{H}_2\text{O} \rightarrow \text{Al(OH)}_3 + \text{NH}_3\)

Teacher's Note:
a) Metal nitrides react with water to yield corresponding hydroxides and ammonia gas.
b) Ensure proper balancing of water molecules and ammonia produced.

(ii) Calcium carbide and water. [1 Mark]

Answer:
\(\text{CaC}_2 + 2\text{H}_2\text{O} \rightarrow \text{Ca(OH)}_2 + \text{C}_2\text{H}_2\)

Teacher's Note:
a) This is the standard industrial and laboratory preparation method for ethyne (acetylene).
b) Calcium hydroxide is formed alongside acetylene gas.

(iii) Ethene and water (steam). [1 Mark]

Answer:
\(\text{C}_2\text{H}_4 + \text{H}_2\text{O} \xrightarrow{\text{H}_3\text{PO}_4, \text{300}^\circ\text{C}, \text{atm}} \text{C}_2\text{H}_5\text{OH}\)

Teacher's Note:
a) Catalytic hydration of ethene produces ethanol.
b) Phosphoric acid is typically used as a catalyst under high temperature and pressure conditions.

(iv) Sulphur dioxide and water. [1 Mark]

Answer:
\(\text{SO}_2 + \text{H}_2\text{O} \rightarrow \text{H}_2\text{SO}_3\)

Teacher's Note:
a) Sulphur dioxide dissolves in water to form sulphurous acid.
b) This proves that \(\text{SO}_2\) is the anhydride of sulphurous acid.

(v) Bromoethane and an aqueous solution of sodium hydroxide. [1 Mark]

Answer:
\(\text{C}_2\text{H}_5\text{Br} + \text{NaOH (aq)} \rightarrow \text{NaBr} + \text{C}_2\text{H}_5\text{OH}\)

Teacher's Note:
a) Aqueous alkali causes nucleophilic substitution, replacing the halogen atom with a hydroxyl group to form an alcohol.
b) Contrast this with alcoholic KOH which causes elimination (dehydrohalogenation).

(f) Here is an electrode reaction :
\(\text{Cu} \rightarrow \text{Cu}^{2+} + 2e\)

(i) At which electrode (anode or cathode) would such a reaction take place ? Is this an example of oxidation or reduction ? [1 Mark]

Answer:
Anode; Oxidation.

Teacher's Note:
a) Loss of electrons always takes place at the anode, which is defined as oxidation.
b) Remember the mnemonic 'An Ox' (Anode is Oxidation).

(ii) A solution contains magnesium ions (\(\text{Mg}^{2+}\)) iron (II) ions (\(\text{Fe}^{2+}\)) and copper ions (\(\text{Cu}^{2+}\)). On passing an electric current through this solution which ions will be the first to be discharged at the cathode ? Write the equation for the cathode reaction. [2 Marks]

Answer:
Copper ions (\(\text{Cu}^{2+}\)) will be discharged first.
Cathode reaction: \(\text{Cu}^{2+} + 2e \rightarrow \text{Cu}\).

Teacher's Note:
a) Ions lower in the electrochemical series (reactivity series) get discharged preferentially at the respective electrodes.
b) Copper is much lower in the series than magnesium and iron, hence it gets reduced and deposited first.

(iii) Why is carbon tetrachloride, which is a liquid, a non-electrolyte ? [2 Marks]

Answer:
Carbon tetrachloride is a covalent compound consisting of molecules and does not contain free ions or charged particles capable of carrying an electric current when in the liquid state.

Teacher's Note:
a) Presence of free mobile ions or delocalized electrons is mandatory for electrical conductivity in liquids.
b) Since \(\text{CCl}_4\) is held by covalent bonds with no ions, it is a non-electrolyte.

(g) What are the terms defined in (g) (i) - (v) below ? [5 Marks]

(i) A bond formed by a shared pair of electrons, each bonding atom contributing one electron to the pair. [1 Mark]

Answer:
Covalent bond

Teacher's Note:
a) Mutual sharing of electrons between two non-metal atoms results in a covalent bond.
b) If both electrons come from a single atom, it is called a coordinate covalent (dative) bond.

(ii) A bond formed by a shared pair of electrons with both electrons coming from the same atom. [1 Mark]

Answer:
Coordinate bond (or Dative bond)

Teacher's Note:
a) Formed when one atom donates an electron pair (lone pair) to another atom deficient in electrons.
b) Example: Ammonium ion (\(\text{NH}_4^+\)) contains a coordinate bond between \(\text{NH}_3\) and \(\text{H}^+\).

(iii) A salt containing a metal ion surrounded by other ions or molecules. [1 Mark]

Answer:
Complex salt

Teacher's Note:
a) Complex salts contain a central metal atom or ion coordinated to a group of surrounding molecules or anions called ligands.
b) Example: Tetraamminecopper(II) sulphate, \(\text{[Cu(NH}_3\)_4]\text{SO}_4\).

(iv) A base which is soluble in water. [1 Mark]

Answer:
Alkali

Teacher's Note:
a) All alkalis are bases, but not all bases are alkalis.
b) Alkalis release hydroxyl ions (\(\text{OH}^-\)) as the only negative ions in aqueous solution.

(v) A reaction in which the hydrogen of an alkane is replaced by another element like chlorine. [1 Mark]

Answer:
Substitution reaction

Teacher's Note:
a) Characteristic reaction of saturated hydrocarbons (alkanes) in the presence of diffuse sunlight.
b) Results in successive replacement of hydrogen atoms by halogen atoms.

SECTION - II (40 Marks)
(Answer any four questions from this section)

Question 2.
Copy and complete the following table relating to important industrial processes. Output refers to the product of the process not the intermediate steps. [5 Marks]

[Figure: Incomplete table with columns Name of process, Inputs, Catalyst, Equation for catalysed reaction, and Output for Haber Process, Contact Process, and Ostwald's Process]

Teacher's Note:
a) Industrial processes are high-scoring topics; students must memorize inputs, exact catalysts, conditions, and balanced equations.
b) Ensure equilibrium arrows are correctly marked for reversible reactions like Haber and Contact processes.

Question 3.
The following equations refer to the Periodic Table :

(a) (i) Name the first and last element in period 2. [2 Marks]

Answer:
First element: Lithium (Li)
Last element: Neon (Ne)

Teacher's Note:
a) Period 2 begins with Lithium (Group 1) and ends with Neon (Group 18).
b) Hydrogen is in period 1, so period 2 starts with Li.

(ii) What happens to the atomic size of elements moving from top to bottom of a group ? [1 Mark]

Answer:
Atomic size increases.

Teacher's Note:
a) Moving down a group, a new principal energy shell is added successively.
b) This addition outweighs the increasing nuclear charge, leading to an increase in atomic radius.

(iii) Which of the elements has the greatest electron affinity among the halogens ? [1 Mark]

Answer:
Chlorine

Teacher's Note:
a) Although fluorine is the most electronegative, chlorine has the highest electron affinity (electron gain enthalpy) due to small electron-electron repulsion in its larger 3p orbital compared to the compact 2p orbital of fluorine.
b) This is a standard exception frequently tested in board exams.

(iv) What is the common feature of the electronic configurations of the elements in group 7 ? [1 Mark]

Answer:
They all have 7 valence electrons in their outermost shell.

Teacher's Note:
a) Elements in the same group share the same number of valence electrons, giving them similar chemical properties.
b) Group 7 (Halogens) elements require 1 electron to complete their octet.

(b) Supply the missing word from those in the brackets (Do not write out the sentence).

(i) If an element has a low ionization energy then it is likely to be ........................ (metallic / non-metallic). [1 Mark]

Answer:
metallic

Teacher's Note:
a) Metals have a tendency to lose electrons easily, which corresponds to low ionization energy.
b) Non-metals have high ionization energy and high electronegativity.

(ii) If an element has seven electrons in its outermost shell then it is likely to have the ........................ (largest / smallest) atomic size among all the elements in the same period. [1 Mark]

Answer:
smallest

Teacher's Note:
a) Halogens (group 17) lie towards the far right of a period where nuclear charge is maximum.
b) Strong nuclear pull shrinks the electron cloud, resulting in the smallest atomic size in that period.

(c) (i) The metals of Group 2 from top to bottom are : Be, Mg, Ca, Sr, Ba. Which of these metals will form ions most readily and why ? [2 Marks]

Answer:
Barium (Ba) will form ions most readily because it has the largest atomic size and lowest ionization potential in Group 2, making it easiest to lose its valence electrons.

Teacher's Note:
a) Metallic character and tendency to form positive ions increase down a group.
b) Lower ionization energy implies electrons are held less tightly by the nucleus.

(ii) What property of an element is measured by electronegativity ? [1 Mark]

Answer:
The relative tendency of an atom to attract the shared pair of electrons towards itself in a covalent bond.

Teacher's Note:
a) Electronegativity is a dimensionless property applicable to bonded atoms.
b) Fluorine is assigned the highest electronegativity value of 4.0 on the Pauling scale.

Question 4.

(a) Distinguish between the saturated hydrocarbon ethane and the unsaturated hydrocarbon ethene by drawing their structural formulae. [2 Marks]

Answer:
Ethane (\(\text{C}_2\text{H}_6\)) contains a single covalent bond between carbon atoms, whereas ethene (\(\text{C}_2\text{H}_4\)) contains a double covalent bond between carbon atoms.

Teacher's Note:
a) Ensure all carbon-hydrogen and carbon-carbon bonds are clearly indicated with single or double lines.
b) Tetravalency of carbon and monovalency of hydrogen must be maintained.

(b) Addition reactions and substitution reactions are types of organic reactions. Which type of reaction is shown by :
(i) ethane
(ii) ethene [2 Marks]

Answer:
(i) Ethane: Substitution reactions.
(ii) Ethene: Addition reactions.

Teacher's Note:
a) Saturated hydrocarbons undergo substitution because all valence bonds are fully satisfied.
b) Unsaturated hydrocarbons readily undergo addition reactions across their multiple bonds.

(c) (i) Write the equation for the complete combustion of ethane. [2 Marks]

Answer:
\(2\text{C}_2\text{H}_6 + 7\text{O}_2 \rightarrow 4\text{CO}_2 + 6\text{H}_2\text{O}\)

Teacher's Note:
a) Combustion reactions of hydrocarbons produce carbon dioxide and water vapor along with a large amount of heat.
b) Balance carbons first, then hydrogens, and finally oxygens.

(ii) Using appropriate catalysts, ethane can be oxidized to an alcohol, an aldehyde and an acid. Name the alcohol, aldehyde and acid formed when ethane is oxidized. [2 Marks]

Answer:
Alcohol: Ethanol (\(\text{C}_2\text{H}_5\text{OH}\))
Aldehyde: Ethanal (\(\text{CH}_3\text{CHO}\))
Acid: Ethanoic acid (\(\text{CH}_3\text{COOH}\))

Teacher's Note:
a) Controlled stepwise oxidation of ethane (or ethanol) yields alcohol, then aldehyde, and finally carboxylic acid.
b) Note IUPAC nomenclature: Ethanol (alcohol), Ethanal (aldehyde), Ethanoic acid (acid).

(d) (i) Why is pure acetic acid known as glacial acetic acid ? [1 Mark]

Answer:
Pure anhydrous acetic acid freezes at \(16.6^\circ\text{C}\), which is slightly below room temperature, forming ice-like crystalline solids on cooling. Hence, it is called glacial acetic acid.

Teacher's Note:
a) 'Glacial' refers to its ice-like appearance upon solidification.
b) This is a classic descriptive term in organic chemistry.

(ii) What type of compound is formed by the reaction between acetic acid and an alcohol ? [1 Mark]

Answer:
Ester

Teacher's Note:
a) Reaction of a carboxylic acid with an alcohol in the presence of concentrated \(\text{H}_2\text{SO}_4\) is called esterification.
b) Esters are characterized by their sweet fruity smell.

Question 5.

(a) (i) A compound has the following percentage composition by mass : carbon 14.4%, hydrogen 1.2% and chlorine 84.5%. Determine the empirical formula of this compound. Work correct to 1 decimal place. (\(H = 1; C = 12; Cl = 35.5\)). [3 Marks]

Answer:

Empirical formula = \(\text{CHCl}_2\)

Teacher's Note:
a) Always set up the calculation table with columns: Element, Percentage, Atomic Mass, Relative Number of Atoms, Simplest Atomic Ratio, and Simplest Whole Number Ratio.
b) Round off intermediate division values carefully to one decimal place as specified.

(ii) The relative molecular mass of this compound is 168, so what is its molecular formula ? [2 Marks]

Answer:
Empirical formula mass of \(\text{CHCl}_2 = 12 + 1 + (2 \times 35.5) = 12 + 1 + 71 = 84\).
\(n = \frac{\text{Relative molecular mass}}{\text{Empirical formula mass}} = \frac{168}{84} = 2\).
Molecular formula = \((\text{Empirical formula})_n = (\text{CHCl}_2)_2 = \text{C}_2\text{H}_2\text{Cl}_4\).

Teacher's Note:
a) Molecular formula = \(n \times (\text{Empirical formula})\).
b) Calculate \(n\) accurately by dividing the given molecular mass by the calculated empirical formula mass.

(iii) By what type of reaction could this compound be obtained from ethyne ? [1 Mark]

Answer:
Addition reaction

Teacher's Note:
a) Addition of chlorine across the triple bond of ethyne yields tetrachloroethane (\(\text{C}_2\text{H}_2\text{Cl}_4\)).
b) Halogenation of alkynes is an electrophilic addition reaction.

(b) From the equation :
\(\text{C} + 2\text{H}_2\text{SO}_4 \rightarrow \text{CO}_2 + 2\text{H}_2\text{O} + 2\text{SO}_2\)
Calculate :
(i) The mass of carbon oxidized by \(49\text{ g}\) of sulphuric acid (\(C = 12\); relative molecular mass of sulphuric acid = \(98\)). [2 Marks]

Answer:
From the balanced equation:
\(2 \times 98\text{ g}\) (\(196\text{ g}\)) of \(\text{H}_2\text{SO}_4\) oxidizes \(12\text{ g}\) of carbon.
Therefore, \(49\text{ g}\) of \(\text{H}_2\text{SO}_4\) will oxidize = \(\frac{12 \times 49}{2 \times 98} = \frac{12 \times 49}{196} = 3\text{ g}\) of carbon.

Teacher's Note:
a) Use stoichiometric mass relationships directly from the balanced chemical equation.
b) Always write the unit alongside the final numerical answer.

(ii) The volume of sulphur dioxide measured at stp. liberated at the same time. Volume occupied by 1 mole of a gas at stp is \(22.4\text{ dm}^3\). [2 Marks]

Answer:
From the equation:
\(2 \times 98\text{ g}\) of \(\text{H}_2\text{SO}_4\) liberates \(2 \times 22.4\text{ litres}\) of \(\text{SO}_2\).
Therefore, \(49\text{ g}\) of \(\text{H}_2\text{SO}_4\) will liberate = \(\frac{2 \times 22.4 \times 49}{2 \times 98} = \frac{22.4 \times 49}{98} = 11.2\text{ litres}\) (or \(\text{dm}^3\)) of \(\text{SO}_2\).

Teacher's Note:
a) 1 mole of any gas occupies \(22.4\text{ dm}^3\) at STP.
b) Apply unitary method or direct proportionality based on moles from the equation.

Question 6.

(a) The following is a sketch of an electrolytic cell used in the extraction of aluminium : [5 Marks]

[Figure: Sketch of Hall-Heroult electrolytic cell showing carbon electrodes A connected to positive terminal, carbon lining B acting as cathode, and electrolyte C containing alumina dissolved in molten cryolite and fluorspar]

(i) What is the substance of which the electrodes A and B are made ? [1 Mark]

Answer:
Graphite (or carbon).

Teacher's Note:
a) Anode A consists of multiple carbon rods suspended in the electrolyte.
b) Cathode B is the carbon lining at the bottom of the iron tank.

(ii) At which electrode (\(A\) or \(B\)) is the aluminium formed ? [1 Mark]

Answer:
Electrode B (Cathode).

sTeacher's Note:
a) Positively charged aluminium ions (\(\text{Al}^{3+}\)) migrate to the cathode (B) and get reduced to molten aluminium metal.
b) Molten aluminium collects at the bottom of the tank and is tapped off periodically.

(iii) What are the two aluminium compounds in the electrolyte C ? [1 Mark]

Answer:
Alumina (\(\text{Al}_2\text{O}_3\)) and Cryolite (\(\text{Na}_3\text{AlF}_6\)) [also Fluorspar, \(\text{CaF}_2\)].

Teacher's Note:
a) Pure alumina is a bad conductor of electricity and has a very high melting point (\(2050^\circ\text{C}\)).
b) Cryolite and fluorspar are added to lower the melting point and improve conductivity.

(iv) Why is it necessary for electrode B to be continuously replaced ? [2 Marks]

Answer:
The oxygen liberated at the anode reacts with the carbon of the anode rods at high temperatures, burning them away to form carbon dioxide. Hence, anode A (not cathode B) must be continuously replaced. Note: The question paper mentions electrode B, but scientifically it is electrode A (anodes) that burns away.

Teacher's Note:
a) \(\text{C} + \text{O}_2 \rightarrow \text{CO}_2\) causes the carbon anodes to get consumed gradually.
b) Students should note the typographical error in the original question paper regarding electrode identification.

(b) Making use only of substances chosen from those given below :
Dilute sulphuric acid, sodium carbonate, zinc, sodium sulphite, lead, calcium carbonate
give the equations for the reactions by which you could obtain :
(i) hydrogen
(ii) sulphur dioxide
(iii) carbon dioxide
(iv) zinc carbonate (two steps required) [5 Marks]

(i) hydrogen [1 Mark]

Answer:
\(\text{Zn} + \text{H}_2\text{SO}_4 \rightarrow \text{ZnSO}_4 + \text{H}_2\uparrow\)

Teacher's Note:
a) Active metals like zinc react with dilute acids to evolve hydrogen gas.
b) Ensure correct valency of zinc and sulphate.

(ii) sulphur dioxide [1 Mark]

Answer:
\(\text{Na}_2\text{SO}_3 + \text{H}_2\text{SO}_4 \rightarrow \text{Na}_2\text{SO}_4 + \text{H}_2\text{O} + \text{SO}_2\uparrow\)

Teacher's Note:
a) Action of dilute acid on a metal sulphite releases sulphur dioxide gas.
b) \(\text{SO}_2\) has a suffocating smell of burning sulphur.

(iii) carbon dioxide [1 Mark]

Answer:
\(\text{Na}_2\text{CO}_3 + \text{H}_2\text{SO}_4 \rightarrow \text{Na}_2\text{SO}_4 + \text{H}_2\text{O} + \text{CO}_2\uparrow\)

Teacher's Note:
a) Action of acid on metal carbonates liberates brisk effervescence of carbon dioxide.
b) Carbon dioxide turns lime water milky.

(iv) zinc carbonate (two steps required) [2 Marks]

Answer:
Step 1: \(\text{Zn} + \text{H}_2\text{SO}_4 \rightarrow \text{ZnSO}_4 + \text{H}_2\)
Step 2: \(\text{ZnSO}_4 + \text{Na}_2\text{CO}_3 \rightarrow \text{ZnCO}_3\downarrow + \text{Na}_2\text{SO}_4\)

Teacher's Note:
a) Precipitation reaction between zinc sulphate and sodium carbonate yields insoluble zinc carbonate.
b) Both steps must be written clearly to secure full marks for a two-step preparation.

Question 7.

(a) (i) What is the property of concentrated sulphuric acid which allows it to be used in the preparation of hydrogen chloride and nitric acid ? [1 Mark]

Answer:
It is non-volatile.

Teacher's Note:
a) Concentrated sulphuric acid has a high boiling point compared to hydrochloric and nitric acids.
b) Hence, it can displace more volatile acids from their respective salts.

(ii) What property of hydrogen chloride is demonstrated when it is collected by downward delivery (upward displacement) ? [1 Mark]

Answer:
Hydrogen chloride is heavier than air (its vapor density is 18.25 compared to 14.4 for air).

Teacher's Note:
a) Heavier gases are collected by downward delivery (upward displacement of air).
b) Always justify collection methods based on relative vapor density.

(iii) Why is hydrogen chloride not collected over water ? [1 Mark]

Answer:
Because hydrogen chloride is extremely soluble in water.

Teacher's Note:
a) One volume of water dissolves about 450 volumes of \(\text{HCl}\) gas.
b) Water would rush into the delivery apparatus, causing breakage.

(iv) What is the property of nitric acid which allows it to react with copper ? [1 Mark]

Answer:
It is a strong oxidizing agent.

Teacher's Note:
a) Nitric acid oxidizes unreactive metals like copper into copper nitrate while getting reduced itself.
b) Hydrogen gas is never evolved when metals react with nitric acid.

(v) What property of concentrated sulphuric acid is in action when sugar turns black in its presence ? [1 Mark]

Answer:
It is a strong dehydrating agent.

Teacher's Note:
a) Concentrated \(\text{H}_2\text{SO}_4\) removes chemically combined hydrogen and oxygen atoms in the 2:1 ratio from sugar (\(\text{C}_{12}\text{H}_{22}\text{O}_{11}\)), leaving behind a spongy mass of carbon (charring).

(b) Write the equations for the following reactions : [5 Marks]

(i) Dilute nitric acid and copper. [1 Mark]

Answer:
\(3\text{Cu} + 8\text{HNO}_3\text{ (dil.)} \rightarrow 3\text{Cu(NO}_3)_2 + 4\text{H}_2\text{O} + 2\text{NO}\uparrow\)

Teacher's Note:
a) Reaction with dilute nitric acid yields nitric oxide (NO) gas.
b) Ensure stoichiometric coefficients are correctly balanced.

(ii) Dilute sulphuric acid and barium chloride. [1 Mark]

Answer:
\(\text{H}_2\text{SO}_4 + \text{BaCl}_2 \rightarrow \text{BaSO}_4\downarrow + 2\text{HCl}\)

Teacher's Note:
a) Barium sulphate is formed as a dense white precipitate insoluble in dilute \(\text{HCl}\).
b) This serves as the standard confirmatory test for sulphate radicals.

(iii) Dilute hydrochloric acid and sodium thiosulphate. [1 Mark]

Answer:
\(\text{Na}_2\text{S}_2\text{O}_3 + 2\text{HCl} \rightarrow 2\text{NaCl} + \text{H}_2\text{O} + \text{SO}_2 + \text{S}\downarrow\)

Teacher's Note:
a) This reaction produces sulphur dioxide gas and colloidal sulphur (yellow turbidity).
b) It is also used to study the rate of chemical reactions.

(iv) Dilute hydrochloric acid and lead nitrate solution. [1 Mark]

Answer:
\(\text{Pb(NO}_3)_2 + 2\text{HCl} \rightarrow \text{PbCl}_2\downarrow + 2\text{HNO}_3\)

Teacher's Note:
a) Lead chloride precipitates out as a white solid which is soluble in hot water.
b) Lead chloride distinguishes itself from other white precipitates by dissolving upon heating and reappearing on cooling.

(v) Dilute sulphuric acid and sodium sulphide. [1 Mark]

Answer:
\(\text{Na}_2\text{S} + \text{H}_2\text{SO}_4 \rightarrow \text{Na}_2\text{SO}_4 + \text{H}_2\text{S}\uparrow\)

Teacher's Note:
a) Metal sulphides react with dilute acids to evolve hydrogen sulphide gas with its characteristic rotten egg smell.
b) Always indicate gas evolution arrows (\(\uparrow\)) and precipitate arrows (\(\downarrow\)) appropriately.