ICSE Class 10 Chemistry Board Exam Question Paper 2009 with Solutions

ICSE Class 10 Chemistry Board Exam Question Paper 2009 with Solutions

 

SECTION-I (40 Marks)

Attempt all questions from this Section

 

Question 1.

(a) Name the gas evolved in each case (formula is not acceptable). [5 Marks]
(i) The gas produced by the action of concentrated sulphuric acid on sodium chloride.
(ii) The gas produced by the action of dilute nitric acid on copper.
(iii) The gas produced on heating sodium nitrate.
(iv) The gas that burns in oxygen with a green flame.
(v) The gas that can be oxidised to sulphur.

Answer:
(i) Hydrogen chloride gas
(ii) Nitric oxide
(iii) Oxygen (Note: Sodium nitrate on heating decomposes to give sodium nitrite and oxygen gas)
(iv) Ammonia
(v) Hydrogen sulphide

Teacher's Note:
a) Remember that gas names must be written in words, not chemical formulas, as specifically instructed.
b) Be careful with thermal decomposition reactions of nitrates, where alkali metal nitrates like sodium nitrate release oxygen gas.

 

(b) Match the substance A to E listed below with the appropriate description in parts (i) to (v): [5 Marks]
(A) Sulphur (B) Silver chloride (C) Hydrogen chloride (D) Copper (II) sulphate (E) Graphite.
(i) A non-metal which is a good conductor of electricity.
(ii) A covalent compound which behaves like an ionic compound in aqueous solution.
(iii) A compound which is insoluble in cold water but soluble in excess of ammonia solution.
(iv) A pink metal which is deposited at the cathode during the electrolysis of the solution of this salt.
(v) A non-metal which reacts with concentrated nitric acid to form its own acid as one of the product.

Answer:
(i) (E)
(ii) (C)
(iii) (B)
(iv) (D)
(v) (A)

Teacher's Note:
a) Graphite is an allotrope of carbon, which is a non-metal, yet it conducts electricity due to free electrons.
b) Hydrogen chloride is a polar covalent compound that ionises completely in water to form hydronium and chloride ions.

 

(c) For part (c) (i) - (c) (x), select the correct answer from the choices A, B, C and D which are given. Write only the letter corresponding to the correct answer. [10 Marks]

 

(i) Among the period 2 elements the one which has high electron affinity is : [1 Mark]
(A) Lithium
(B) Carbon
(C) Fluorine
(D) Neon

Answer: (C) Fluorine

Fluorine has a high tendency to accept an electron to achieve a stable octet.

Teacher's Note:
a) Electron affinity generally increases across a period from left to right, excluding noble gases.
b) Noble gases like neon have a stable octet and essentially zero electron affinity.

 

(ii) Among the following the one which is composed of all the three kinds of bond (ionic, covalent and coordinate bond) is : [1 Mark]
(A) Sodium chloride
(B) Ammonia
(C) Carbon tetrachloride
(D) Ammonium chloride

Answer: (D) Ammonium chloride

NH₄Cl contains ionic bonds between NH₄⁺ and Cl^-, covalent bonds between N and H, and a coordinate bond between N and H⁺.

Teacher's Note:
a) Ammonium salts are classic examples containing all three types of chemical bonding.
b) Always analyze the constituent ions and molecules carefully to identify coordinate bonds.

 

(iii) Which of the following statements is wrong about alkanes ? [1 Mark]
(A) They are all saturated hydrocarbon.
(B) They can undergo addition as well as substitution reaction.
(C) They are almost non polar in nature.
(D) On complete combustion give out carbon dioxide and water.

Answer: (B) They can undergo addition as well as substitution reaction.

Alkanes are saturated hydrocarbons and undergo substitution reactions, not addition reactions.

Teacher's Note:
a) Unsaturated hydrocarbons (alkenes and alkynes) undergo addition reactions.
b) Saturated hydrocarbons only undergo characteristic substitution reactions like halogenation.

 

(iv) Select the acid which contains four hydrogen atoms in it. [1 Mark]
(A) Formic acid
(B) Sulphuric acid
(C) Nitric acid
(D) Acetic acid

Answer: (D) Acetic acid

The molecular formula of acetic acid is CH₃COOH, which contains a total of 4 hydrogen atoms.

Teacher's Note:
a) Count the atoms in the molecular formula carefully: HCOOH (2), H₂SO₄ (2), HNO₃ (1), CH₃COOH (4).
b) Do not confuse basicity of the acid with the total number of hydrogen atoms present in the molecule.

 

(v) A gas cylinder of capacity of 20 dm³ is filled with gas X the mass of which is 10 g. When the same cylinder is filled with hydrogen gas at the same temperature and pressure the mass of the hydrogen is 2 g, hence the relative molecular mass of the gas is : [1 Mark]
(A) 5
(B) 10
(C) 15
(D) 20

Answer: (B) 10

Vapor density = Mass of x dm³ of gas / Mass of same volume of hydrogen = 10 g / 2 g = 5. Relative molecular mass = 2 × Vapor density = 2 × 5 = 10.

Teacher's Note:
a) Apply Avogadro's law principles where equal volumes of all gases under similar conditions contain equal number of molecules.
b) Molecular mass is always twice the vapor density.

 

(vi) The aqueous solution of the following compounds which contains both ions and molecules is : [1 Mark]
(A) Sulphuric acid
(B) Hydrochloric acid
(C) Nitric acid
(D) Acetic acid

Answer: (D) Acetic acid

Acetic acid is a weak electrolyte and undergoes partial ionisation, so its solution contains both ions and un-ionised molecules.

Teacher's Note:
a) Strong acids like HCl, HNO₃, and H₂SO₄ dissociate completely into ions in aqueous solution.
b) Weak acids exist as a mixture of ions and molecules due to incomplete ionisation.

 

(vii) The metal oxide which can react with acid as well as alkali is : [1 Mark]
(A) Silver oxide
(B) Copper (II) oxide
(C) Aluminium oxide
(D) Calcium oxide

Answer: (C) Aluminium oxide

Aluminium oxide is an amphoteric oxide, hence it reacts with both acids and alkalis to form salt and water.

Teacher's Note:
a) Amphoteric oxides include ZnO, Al₂O₃, and PbO.
b) Basic oxides react only with acids, whereas acidic oxides react only with alkalis.

 

(viii) Carbon dioxide and sulphur dioxide gas can be distinguished by using : [1 Mark]
(A) Moist blue litmus paper
(B) Lime water
(C) Acidified potassium dichromate paper
(D) None of the above.

Answer: (C) Acidified potassium dichromate paper

Sulphur dioxide turns acidified potassium dichromate paper from orange to green, while carbon dioxide has no effect on it.

Teacher's Note:
a) Both gases turn lime water milky, so lime water cannot distinguish them.
b) SO₂ is a strong reducing agent and reduces acidified K₂Cr₂O₇.

 

(ix) The organic compound obtained as the end product of the fermentation of sugar solution is : [1 Mark]
(A) Methanol
(B) Ethanol
(C) Ethane
(D) Methanoic acid

Answer: (B) Ethanol

Fermentation of sugar using invertase and zymase enzymes yields ethanol and carbon dioxide.

Teacher's Note:
a) Ethanol is commercially prepared via the fermentation of molasses.
b) Ensure you remember the specific enzymes involved in the biological breakdown of sugars.

 

(x) A black colour solid which on reaction with dilute sulphuric acid forms a blue coloured solution is : [1 Mark]
(A) Carbon
(B) Manganese (IV) oxide
(C) Lead (II) oxide
(D) Copper (II) oxide

Answer: (D) Copper (II) oxide

CuO is a black solid that reacts with dilute H₂SO₄ to form copper (II) sulphate, which gives a blue-colored solution.

Teacher's Note:
a) Copper salts in aqueous solution typically impart a blue or bluish-green color due to hydrated copper ions.
b) Manganese (IV) oxide is also black, but reacts with concentrated hydrochloric acid, not dilute sulphuric acid, to give chlorine.

 

(d) Write a fully balanced equation for each of the following cases : [5 Marks]
(i) Red lead is warmed with concentrated hydrochloric acid.
(ii) Magnesium metal is treated with dilute hydrochloric acid.
(iii) Lead nitrate is heated in a dry test tube.
(iv) Magnesium nitride is treated with warm water.
(v) Acetic acid is warmed with ethanol in the presence of concentrated sulphuric acid.

Answer:
(i) Pb₃O₄ + 8HCl → 3PbCl₂ + 4H₂O + Cl₂
(ii) Mg + 2HCl → MgCl₂ + H₂ ↑
(iii) 2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂
(iv) Mg₃N₂ + 6H₂O → 3Mg(OH)₂ + 2NH₃ ↑
(v) CH₃COOH + C₂H₅OH Conc. H₂SO₄
→
Δ CH₃COOC₂H₅ + H₂O

Teacher's Note:
a) Make sure equations are balanced with proper physical state symbols or evolution/precipitation arrows where applicable.
b) Esterification requires concentrated sulphuric acid acting as a dehydrating and catalytic agent.

 

(e) Find the odd one out and explain your choice (note : valency is not a criterion) [5 Marks]
(i) Al(OH)₃, Pb(OH)₂, Mg(OH)₂, Zn(OH)₂
(ii) C₅H₈, C₅H₁₀, C₂H₆, CH₄
(iii) Sulphur, Phosphorus, Carbon, Iodine
(iv) Copper, Lead, Zinc, Mercury
(v) Formic acid, Nitric acid, Acetic acid, Propanoic acid.

Answer:
(i) Mg(OH)₂ : It is basic, while the others are amphoteric hydroxides.
(ii) C₅H₈ : It is an alkyne (unsaturated with triple bond), whereas all the rest are saturated hydrocarbons.
(iii) Carbon : It forms a very large number of compounds (catenation property), while the rest do not to the same extensive degree.
(iv) Mercury : It is a liquid metal at room temperature, while all the rest are solid metals.
(v) Nitric acid : It is a mineral (inorganic) acid, while the rest are organic acids.

Teacher's Note:
a) When finding the odd one out, state the precise property possessed by the majority and lacking in the chosen odd one.
b) Ensure you check homology, physical states, and chemical character carefully.

 

(f) Identify the substances, P, Q, R, S and T in each case based on the information given below : [5 Marks]
(i) The deliquescent salt P, turns yellow on dissolving in water, and gives a reddish brown precipitate with sodium hydroxide solution.
(ii) The white crystalline solid Q is soluble in water. It liberates a pungent smelling gas when heated with sodium hydroxide solution.
(iii) The pale green solid R turns reddish brown on heating. Its aqueous solution gives a white precipitate with barium chloride solution. The precipitate is insoluble in mineral acids.
(iv) The reddish brown liquid S is dissolved in water. When Ethyne gas is passed through it, it turns colourless.
(v) The nitrate T does not leave any residue on heating.

Answer:
(i) P is Ferric chloride (FeCl₃).
(ii) Q is Ammonium chloride (NH₄Cl).
(iii) R is Ferrous sulphate (FeSO₄).
(iv) S is Bromine (Br₂ liquid).
(v) T is Ammonium nitrate (NH₄NO₃) or Mercury(II) nitrate.

Teacher's Note:
a) Identify characteristic colors of ions, such as ferric giving a reddish-brown precipitate with NaOH and ferrous sulphate being pale green.
b) Ammonium salts release ammonia gas on warming with caustic soda.

 

(g) (i) Calcium carbide is used for the artificial ripening of fruits. Actually the fruit ripens because of the heat evolved while calcium carbide reacts with moisture. During this reaction calcium hydroxide and acetylene gas is formed. If \( 200\text{ cm}^3 \) of acetylene is formed from a certain mass of calcium carbide, find the volume of oxygen required and carbon dioxide formed during the complete combustion. The combustion reaction can be represented as below :
\( 2\text{C}_2\text{H}_{2(g)} + 5\text{O}_{2(g)} \to 4\text{CO}_{2(g)} + 2\text{H}_2\text{O}_{(g)} \) [5 Marks]
(ii) A gaseous compound of hydrogen and nitrogen contains \( 12.5\% \) hydrogen by mass. Find the molecular formula of the compound if its relative molecular mass is \( 37 \).
[N = 14, H = 1].

Answer:
(i) According to the balanced equation:
\( 2\text{ volumes of }\text{C}_2\text{H}_2 \) require \( 5\text{ volumes of }\text{O}_2 \) and produce \( 4\text{ volumes of }\text{CO}_2 \).
Therefore, for \( 200\text{ cm}^3 \) of acetylene:
Volume of oxygen required = \( \frac{5}{2} \times 200\text{ cm}^3 = 500\text{ cm}^3 \).
Volume of carbon dioxide formed = \( \frac{4}{2} \times 200\text{ cm}^3 = 400\text{ cm}^3 \).

(ii)

Elements	% by Mass	At. Mass	No. of Atoms	Simple ratio
H	12.5	1	12.5	\( 12.5 \div 6.25 = 2 \)
N	87.5	14	6.25	\( 6.25 \div 6.25 = 1 \)

Empirical formula = \( \text{NH}_2 \).
Empirical formula mass = \( 14 + (2 \times 1) = 16 \).
\( n = \frac{\text{Relative molecular mass}}{\text{Empirical formula mass}} = \frac{37}{16} \approx 2.3 \approx 2 \) (Taking nearest whole number or integer factor as standard for such problems).
Molecular formula = \( (\text{NH}_2)_2 = \text{N}_2\text{H}_4 \).

Teacher's Note:
a) Gay-Lussac's Law of combining volumes allows direct stoichiometric conversion of gas volumes under similar conditions of temperature and pressure.
b) For empirical formula calculations, divide the percentage by atomic mass to get relative atoms, then divide by the smallest number to get the simplest whole number ratio.

 

SECTION-II (40 Marks)

(Answer any four questions from this section)

 

Question 2.

(a) Correct the following statements :
For example : 'Chlorine is a bleaching agent'.
Should read : 'Moist chlorine is a bleaching agent'. [5 Marks]
(i) Lead bromide conducts electricity.
(ii) Copper reacts with nitric acid to produce nitrogen dioxide.
(iii) Haematite is the chief ore of aluminium.
(iv) Equal masses of gases under identical conditions contain the same number of molecules.
(v) Hydrochloric acid is prepared in the laboratory by passing hydrogen chloride directly through water.

Answer:
(i) Molten lead bromide conducts electricity.
(ii) Copper reacts with concentrated nitric acid to produce nitrogen dioxide.
(iii) Bauxite is the chief ore of aluminium.
(iv) Equal volumes of gases under identical conditions contain the same number of molecules.
(v) Hydrochloric acid is prepared in the laboratory by passing hydrogen chloride gas through water by inverted funnel arrangement.

Teacher's Note:
a) Read statements carefully to identify missing key modifiers such as 'molten', 'concentrated', or 'inverted funnel'.
b) Avogadro's law refers to equal volumes of gases, not equal masses.

 

(b) Consider the section of the periodic table given below :

Group numbers	IA	IIA	IIIA	IVA	VA	VIA	VIIA	0
	1	2	13	14	15	16	17	18
	Li		D		O	J	Ne
	A	Mg	E	Si	H	K
	B	C		F	G			L

Note : In this table B does not represent boron, C does not represent carbon, F does not represent fluorine, H does not represent hydrogen, K does not represent potassium. You must see the position of the element in the periodic table.

Some elements are given in their own symbol and position in the periodic table, while others are shown with a letter. With reference to the table : [5 Marks]
(i) Which is the most electronegative ?
(ii) How many valence electrons are present in G ?
(iii) Write the formula of the compound between B and H.
(iv) In the compound between F and J, what type of bond will be formed ?
(v) Draw the electron dot structure for the compound formed between C and K.

Answer:
(i) Element J (or fluorine position equivalent in group 17, though Ne is in group 0, J is in group 17 row 2)
(ii) Five (Group 15 elements have 5 valence electrons)
(iii) B₂H (Valency of B is 1, valency of H is 2)
(iv) Covalent bond
(v) [Figure: Electron dot structure showing C releasing two electrons to form C²⁺ and two atoms of K accepting them to form [K]^-₂ ionic compound structure]

Teacher's Note:
a) Electronegativity increases across a period towards the halogen group.
b) Group numbers indicate the number of valence electrons for representative elements.

 

Question 3.

(a) A metal article is to be electroplated with silver. The electrolyte selected is sodium argentocyanide. [5 Marks]
(i) What kind of salt is sodium argentocyanide ?
(ii) Why is it preferred to silver nitrate as an electrolyte ?
(iii) State one condition to ensure that the deposit is smooth, firm and long lasting.
(iv) Write the reaction taking place at the cathode.
(v) Write the reaction taking place at the anode.

Answer:
(i) A complex salt.
(ii) When silver nitrate is used, the deposition of silver is too rapid and non-uniform because it is a strong electrolyte with a high concentration of silver ions. Sodium argentocyanide provides a low concentration of Ag⁺ ions for smooth and even deposition.

(iii) A low current should be passed for a longer time, or the electrolyte should contain a suitable complexing agent.
(iv) Ag⁺ + e^- → Ag
(v) Ag - e^- → Ag⁺

Teacher's Note:
a) Complex salts dissociate to yield complex ions, releasing metal ions at a very slow and controlled rate.
b) At the anode made of pure silver, silver atoms lose electrons to form silver ions entering the solution.

 

(b) The sketch below illustrates the refining of aluminium by Hoope's process.
[Figure: Hoope's electrolytic cell showing three distinct liquid layers: bottom layer of impure aluminium acting as anode, middle layer of molten fluorides serving as electrolyte, and top layer of pure aluminium acting as cathode with graphite rods dipping into it.]
(i) Which of A and B is the cathode and which one is the anode ? [3 Marks]
(ii) What is the electrolyte in the tank ?
(iii) What material is used for the cathode ?

Answer:
(i) A is cathode and B is anode.
(ii) Molten fluorides (mixture of sodium, barium, and aluminium fluorides).
(iii) Carbon rods (or graphite rods) dipping in pure molten aluminium.

Teacher's Note:
a) Hoope's process utilizes differences in densities of three liquid layers to refine aluminium.
b) Impure aluminium forms the bottom dense layer (anode), while pure aluminium forms the top layer (cathode).

 

(c) State the property of the metal being utilized in the following : [2 Marks]

Use of metal	Property
Zinc in Galvanization
Aluminium in Thermite welding

Answer:

Use of metal	Property
Zinc in Galvanization	Zinc forms a protective layer of zinc oxide which prevents rusting of iron.
Aluminium in Thermite welding	Strong affinity for oxygen and releases a large amount of heat on reacting with metal oxides.

Teacher's Note:
a) Galvanization protects iron by sacrificial protection and forming a tough oxide coating.
b) Thermite reaction is highly exothermic due to aluminium's high reactivity towards oxygen.

 

Question 4.

(a) (i) A gas cylinder contains \( 24 \times 10^{24} \) molecules of nitrogen gas. If Avogadro's number is \( 6 \times 10^{23} \) and the relative atomic mass of nitrogen is 14, calculate :
(1) Mass of nitrogen gas in the cylinder.
(2) Volume of nitrogen at STP in \( \text{dm}^3 \).
(ii) Commercial sodium hydroxide weighing 30g has some sodium chloride in it. The mixture on dissolving in water and subsequent treatment with excess silver nitrate solution formed a precipitate weighing 14.3 g. What is the percentage of sodium chloride in the commercial sample of sodium hydroxide ? The equation for the reaction is :
\( \text{NaCl} + \text{AgNO}_3 \to \text{AgCl} + \text{NaNO}_3 \)
[Relative molecular mass of \( \text{NaCl} = 58 \); \( \text{AgCl} = 143 \)]
(iii) A certain gas 'X' occupies a volume of \( 100\text{ cm}^3 \) at S.T.P. and weighs 0.5 g. find its relative molecular mass. [6 Marks]

Answer:
(i) (1) \( 6 \times 10^{23} \) molecules of N₂ weigh \( 2 \times 14 = 28\text{ g} \).
Therefore, \( 24 \times 10^{24} \) molecules of N₂ weigh = \( \frac{28 \times 24 \times 10^{24}}{6 \times 10^{23}} = 1120\text{ g} \).
(2) \( 6 \times 10^{23} \) molecules occupy \( 22.4\text{ dm}^3 \) at STP.
Volume of \( 24 \times 10^{24} \) molecules = \( \frac{22.4 \times 24 \times 10^{24}}{6 \times 10^{23}} = 896\text{ dm}^3 \).

(ii) From the equation:
\( \text{NaCl} + \text{AgNO}_3 \to \text{AgCl} + \text{NaNO}_3 \)
\( 58\text{ g} \) of NaCl gives \( 143\text{ g} \) of AgCl.
Mass of NaCl in sample = \( \frac{58 \times 14.3}{143} = 5.8\text{ g} \).
Percentage of NaCl = \( \frac{5.8}{30} \times 100 = 19.33\% \).

(iii) \( 100\text{ cm}^3 \) of gas X at STP weighs \( 0.5\text{ g} \).
\( 22400\text{ cm}^3 \) of gas X at STP weighs = \( \frac{0.5 \times 22400}{100} = 112\text{ g} \).
Relative molecular mass of gas X is \( 112 \).

Teacher's Note:
a) Use molar volume relations: one mole of any gas occupies \( 22.4\text{ dm}^3 \) or \( 22400\text{ cm}^3 \) at STP.
b) Apply stoichiometric mass ratios directly for precipitation reactions to find percentage purity or composition.

 

(b) Solution A is a strong acid.
Solution B is a weak acid.
Solution C is a strong alkali.
(i) Which solution contains solute molecules in addition to water molecules ? [4 Marks]
(ii) Which solution will give a gelatinous white precipitate with zinc sulphate solution ? The precipitate disappears when an excess of the solution is added.
(iii) Which solution could be a solution of glacial acetic acid ?
(iv) Give an example of a solution which is a weak alkali.

Answer:
(i) Solution B
(ii) Solution C
(iii) Solution B
(iv) Ammonium hydroxide (\text{NH}_4\text{OH})

Teacher's Note:
a) Weak acids and weak bases contain molecules alongside ions due to partial dissociation.
b) Amphoteric hydroxides precipitate with alkalis and dissolve in excess strong alkali.

 

Question 5.

(a) The diagram given below is to prepare Iron (III) chloride in the laboratory :
[Figure: Laboratory setup for preparation of anhydrous iron(III) chloride showing dry chlorine gas passing into a combustion tube containing heated iron wire, connected to a receiver flask with a guard tube B containing a drying agent to protect from moisture.]
(i) What is substance B ? [4 Marks]
(ii) What is the purpose of B ?
(iii) Why is iron (III) chloride to be stored in a closed container ?
(iv) Write the equation for the reaction between iron and chlorine.

Answer:
(i) Anhydrous calcium chloride (\text{CaCl}_2).
(ii) It acts as a drying agent and prevents atmospheric moisture from entering the collection flask.
(iii) Because iron (III) chloride is highly deliquescent and absorbs moisture from the air.
(iv) \( 2\text{Fe} + 3\text{Cl}_2 \to 2\text{FeCl}_3 \)

Teacher's Note:
a) Anhydrous chlorides like FeCl₃ must be protected from moisture to prevent hydrolysis.
b) Direct combination of iron and chlorine produces anhydrous ferric chloride, not ferrous chloride.

 

(b) (i) Write the equation(s) for the reaction(s) to prepare lead sulphate from lead carbonate. [4 Marks]
(ii) Methane is the first member of alkane, when it is treated with excess of chlorine in the presence of diffused sunlight forms carbon tetrachloride. Draw the appropriate structural formula of carbon tetrachloride and state the type of bond present in it.

Answer:
(i) Step 1: \( \text{PbCO}_3 + 2\text{HNO}_3 \to \text{Pb(NO}_3)_2 + \text{H}_2\text{O} + \text{CO}_2 \)
Step 2: \( \text{Pb(NO}_3)_2 + \text{Na}_2\text{SO}_4 \to \text{PbSO}_4\downarrow + 2\text{NaNO}_3 \)
(ii) Structural formula: C atom bonded tetrahedrally to four chlorine atoms via single covalent bonds.
[Figure: Lewis dot or structural representation of CCl₄ showing shared electron pairs between carbon and four chlorine atoms]
Type of bond: Covalent bond.

Teacher's Note:
a) Insoluble salts like lead sulphate are prepared by precipitation (double displacement) method.
b) Carbon tetrachloride is formed by successive substitution of hydrogen atoms in methane by chlorine.

 

(c) Aqueous solution of nickel sulphate contains \( \text{Ni}^{2+} \) and \( \text{SO}_4^{2-} \) ions.
(i) Which ion moves towards the cathode ? [2 Marks]
(ii) What is the product at the anode ?

Answer:
(i) \( \text{Ni}^{2+} \) ions
(ii) Oxygen gas (when inert platinum or carbon electrodes are used, or nickel anode dissolves if active nickel electrode is used).

Teacher's Note:
a) Cations (positively charged ions) always migrate towards the cathode during electrolysis.
b) Specify electrode conditions clearly when discussing oxidation products at the anode.

 

Question 6.

(a) Give one chemical test to distinguish between the following pairs of compounds : [3 Marks]
(i) Zinc sulphate solution and Zinc chloride solution.
(ii) Iron (II) chloride solution and Iron (III) chloride solution.
(iii) Calcium nitrate solution and Calcium chloride solution.

Answer:
(i) On addition of \( \text{BaCl}_2 \) solution, zinc sulphate solution gives a white precipitate of \( \text{BaSO}_4 \), whereas zinc chloride solution does not give any precipitate.
(ii) Iron (II) chloride gives a dirty green precipitate with sodium hydroxide solution, while Iron (III) chloride gives a reddish-brown precipitate with NaOH solution.
(iii) Calcium chloride solution will give a white precipitate with \( \text{AgNO}_3 \) solution, whereas calcium nitrate solution will not give any precipitate with \( \text{AgNO}_3 \).

Teacher's Note:
a) Use selective precipitation reactions involving characteristic anions or cations to distinguish salt solutions.
b) Barium chloride tests confirm sulphate ions, while silver nitrate tests confirm chloride ions.

 

(b) Define the following terms : [3 Marks]
(i) Mole
(ii) Neutralisation
(iii) Ionisation potential

Answer:
(i) Mole : It is the quantity of a substance which contains Avogadro's number (\( 6.022 \times 10^{23} \)) of constituent particles.
(ii) Neutralisation : It is the process by which \( \text{H}^+\) ions of an acid react completely with the \( \text{OH}^-\) ions of a base to give salt and water only.
(iii) Ionisation potential : It is the amount of energy required to remove a valence electron from an isolated gaseous atom of an element.

Teacher's Note:
a) Ensure precise scientific definitions are written using standard terminology.
b) Mention 'isolated gaseous atom' when defining ionization potential or ionization energy.

 

(c) Fill in the blanks with the correct words from the brackets
Generally ionic compounds exist in (i) ............................ (solid / liquid / gas) state.
Melting and boiling points of covalent compounds are generally (ii) .......... (low / high). The general formula for alkane is (iii) .................... (\text{C}_n\text{H}_{2n} / \text{C}_n\text{H}_{2n-2} / \text{C}_n\text{H}_{2n+2}$). For alkynes the general formula is (iv) .......... (\text{C}_n\text{H}_{2n} / \text{C}_n\text{H}_{2n-2} / \text{C}_n\text{H}_{2n+2}$) [4 Marks]

Answer:
(i) solid
(ii) low
(iii) \( \text{C}_n\text{H}_{2n+2} \)
(iv) \( \text{C}_n\text{H}_{2n-2} \)

Teacher's Note:
a) Ionic compounds have strong electrostatic forces of attraction holding ions together in a crystal lattice, resulting in solid states and high melting points.
b) Alkenes follow \( \text{C}_n\text{H}_{2n} \), alkanes follow \( \text{C}_n\text{H}_{2n+2} \), and alkynes follow \( \text{C}_n\text{H}_{2n-2} \).

 

Question 7.

(a) Give chemical equation for : [4 Marks]
(i) The laboratory preparation of methane from sodium acetate.
(ii) The industrial preparation of methanol from water gas.
(iii) The reaction of one mole of ethene with one mole of chlorine gas.
(iv) The preparation of ethyne from 1, 2 - dibromoethane.

Answer:
(i) \( \text{CH}_3\text{COONa} + \text{NaOH} \xrightarrow[\Delta]{\text{CaO}} \text{CH}_4\uparrow + \text{Na}_2\text{CO}_3 \)
(ii) \( \text{CO} + 2\text{H}_2 \xrightarrow[350^{\circ}\text{C}]{\text{ZnO, Cr}_2\text{O}_3} \text{CH}_3\text{OH} \)
(iii) \( \text{CH}_2 = \text{CH}_2 + \text{Cl}_2 \to \text{CH}_2\text{Cl}-\text{CH}_2\text{Cl} \)
(iv) \( \text{CH}_2\text{Br}-\text{CH}_2\text{Br} + 2\text{KOH}_{(alc)} \to \text{CH}\equiv\text{CH} + 2\text{KBr} + 2\text{H}_2\text{O} \)

Teacher's Note:
a) Decarboxylation of sodium acetate with soda-lime produces methane gas.
b) Dehydrohalogenation of vicinal dibromides using alcoholic potassium hydroxide yields alkynes.

 

(b) State how the following conversions can be carried out : [4 Marks]
(i) Ethyl chloride to Ethyl alcohol.
(ii) Ethyl chloride to Ethene.
(iii) Ethene to Ethyl alcohol.
(iv) Ethyl alcohol to Ethyl.

Answer:
(i) By treating ethyl chloride with aqueous KOH.
(ii) By treating ethyl chloride with alcoholic KOH.
(iii) By passing ethene into concentrated \( \text{H}_2\text{SO}_4 \) at \( 80^{\circ}\text{C} \) and high pressure, followed by acid hydrolysis.
(iv) By heating ethyl alcohol with concentrated \( \text{H}_2\text{SO}_4 \) at \( 170^{\circ}\text{C} \).

Teacher's Note:
a) Aqueous alkali gives nucleophilic substitution to form alcohols, while alcoholic alkali causes dehydrohalogenation to form alkenes.
b) Dehydration of ethanol with concentrated sulphuric acid at high temperatures yields ethene.

 

(c) (i) Define isomerism. [2 Marks]
(ii) Give the IUPAC name of the isomer \( \text{C}_4\text{H}_{10} \) which has a branched chain.

Answer:
(i) Isomerism : When two or more compounds having different arrangement of atoms are represented by the same molecular formula, the phenomenon is called isomerism.
(ii) 2 - methylpropane

Teacher's Note:
a) Isomers share identical molecular formulas but exhibit different structural arrangements and physical/chemical properties.
b) The branched-chain isomer of butane (\text{C}_4\text{H}_{10}) is 2-methylpropane (commonly known as isobutane).