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CHEMISTRY-OBJ
01-10: DABBDDBDDD
11-20: BBCBACABCB
21-30: CADBBCCACB
31-40: BBADAABADB
41-50: CACBADDBCA
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ANSWER FOUR(4) QUESTIONS ONLY
(1ai)
Isotopy is a phenomenon where atoms of the same element have the same number of protons (same atomic number) but different numbers of neutrons (different mass numbers).
(1aii)
A molecule of chlorine gas (Cl2) refers to two chlorine atoms chemically bonded together while A mole of chlorine gas refers to Avogadro’s number of molecules of chlorine gas, which is approximately 6.022 ×10²³ molecules.
(1b)
Given:
Lighter isotope mass number = 79
Difference in neutrons = 2
Since the two isotopes are in equal abundance, the average atomic mass can be calculated using the formula:
Average atomic mass = (mass of isotope 1 + mass of isotope 2) / 2
= (79 + 81)/2
= 80
:. Relative atomic mass of the element is 80.
(1ci)
A mixture is a combination of two or more substances that are physically combined and can be separated by physical means.
(1cii)
(I.) A mixture can be separated into its components by physical means, while a compound can only be separated into its elements by chemical reactions.
(II.)The composition of a mixture can vary, while a compound has a fixed composition.
(1ciii)
Separation of the mixture of two liquids, A and B with boiling points of
120°C and 200°C:
Since the two liquids have different boiling points, the separation can be achieved through a process called fractional distillation. In fractional distillation, the mixture is heated, and the component with the lower boiling point (liquid A) will vaporize first. The vapors are then cooled and condensed back into liquid form, resulting in the separation of the two liquids based on their boiling points.
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(2ai)
(2aii)
(2bi)
Element ₁₈C(Argon) will not ionize. This is because it is a noble gas (group 18) and has a complete valence shell, making it chemically inert and unlikely to lose or gain electrons.
(2bii)
For ₁₃A (Aluminum):
Al —> Al⁺ + 3e⁻
For ₁₆B (Sulfur):
S + 2e⁻ —> S²⁻
(2biii)
The most electropositive of the atoms is Aluminium (₁₃A). Its electronic configuration is: 1s² 2s² 2p⁶ 3s² 3p¹
(2ci)
Ionic bonding is the electrostatic force of attraction between positively charged ions (cations) and negatively charged ions (anions) formed by the transfer of one or more electrons from one atom to another.
(2cii)
(2ciii)
(i) MgCl₂ has ionic bonds between magnesium and chlorine ions, while Benzene has covalent bonds between carbon and hydrogen atoms.
(ii) MgCl₂ is typically found as a crystalline solid, while Benzene is a liquid at room temperature.
(iii) MgCl₂, when dissolved in water or melted, conducts electricity due to the presence of free ions, while Benzene does not conduct electricity as it lacks free ions.
(iv) MgCl₂ is soluble in water due to its ionic nature, while Benzene is insoluble in water but soluble in organic solvents like toluene and ether due to its nonpolar covalent structure.
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(3ai)
A saturated solution is a solution that contains the maximum amount of solute that can be dissolved in a given amount of solvent at a specific temperature.
(3aii)
Number of moles of A = Mass of A / Molecular mass
= (50g)/(500g/mol)
= 0.1 mol
Volume of water = 200cm³/1000 = 0.2dm³
Concentration of solute A = Number of moles of A/ Volume of solution in dm³
= 0.1 mol / 0.2dm³
= 0.5 mol/dm³
Since 0.5mol/dm³ is less than the solubility of 0.6mol/dm³, the prepared solution is unsaturated.
(3bi)
I. Sodium carbonate decahydrate (Washing soda) : (Na₂CO₃·10H₂O)
II. Sodium sulfate decahydrate (Glauber’s salt) : (Na₂SO₄·10H₂O)
(3bii)
I. temperature.
II. concentration.
III. particle size.
(3ci)
To prepare nitrogen (IV) oxide (NO₂) in the laboratory, lead(II) nitrate (Pb(NO₃)₂) is used. A small amount of lead(II) nitrate is placed in a dry test tube, which is then connected to a delivery tube leading to a gas collection vessel. The lead(II) nitrate is gently heated using a Bunsen burner. As it decomposes, it forms lead(II) oxide (PbO), nitrogen dioxide (NO₂), and oxygen (O₂). The reaction can be summarized by the equation:
2Pb(NO₃)₂) ===> 2PbO(s) + O₂(g) + 4NO₂(g)
The nitrogen dioxide gas produced is collected over water or by displacement of air.
(3cii)
4Cu(s) + 2NO₂(g) ==> 4CuO(s) + N₂(g)
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(4ai)
An alloy is a substance prepared by adding one or more elements to a base or parent metal to obtain desirable products.
(4aii)
-Stainless steel: FeCrNi (Iron, Chromium, Nickel)
-Carbon steel: FeC (Iron, Carbon)
(4bi)
The extraction of copper from copper pyrite, CuFeS₂.
The ore is crushed and then concentrated by froth flotation. Then it is roasted in a limited supply of air to oxidize the iron and some of the sulphur in the ore to iron(II) oxide and sulphur(IV) oxide respectively.
2CuFeS₂(s) +4O₂(g) —> Cu₂S(s) + 2FeO(s) +3S0₂(g)
Silicon(IV) oxide is added and the mixture is heated in the absence of air to convert the iron(II) oxide to iron(II) trioxosilicate(IV). The latter floats on copper(I) sulphide as slag and is removed.
FeO(s) + SiO₂(s) —> FeSiO₃(s)
The copper(I) sulphide that remains is heated in a controlled amount of air to produce copper with a purity of about 97-99%.
Cu₂S(s) + O₂(g) —> 2Cu(s) + SO2₂(g)
The impure copper is purified by electrolytic refining, where impure copper is used as the anode and pure copper is deposited at the cathode.
(4bii)
(i) Electrical wiring and electronics due to its excellent electrical conductivity.
(ii) Plumbing and roofing materials because of its corrosion resistance and malleability.
(4ci)
Faraday’s second law states that when the same quantity of electricity is passed through different electrolytes, the relative number of moles of the elements discharged are inversely proportional to the charges on the ions of the elements.
(4cii)
(4ciii)
(i) Electrolysis is used to purify metals, such as copper and aluminum.
(ii) It is used to extract metals like aluminum, copper, zinc, and titanium from their ores.
(iii) It is used to coat the surface of an object with a thin layer of a different metal.
(4civ)
(a) At the anode: 4OH⁻(aq) —> 2H₂O(l) + O₂(g) + 4e⁻
(b) At the cathode: 4H⁺(aq) + 4e⁻ —> 2H₂(g)
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(5ai)
Esterification is a chemical reaction in which an acid (typically a carboxylic acid) reacts with an alcohol to form an ester and water.
(5aii)
Saponification is the process by which an ester is hydrolyzed in the presence of a base, typically sodium hydroxide (NaOH) or potassium hydroxide (KOH), to form an alcohol and a salt of the carboxylic acid (soap).
(5bi)
Benzene (C₆H₆) has a higher carbon-to-hydrogen ratio than ethene (C₂H₄). This means that during combustion, benzene has more carbon atoms relative to hydrogen atoms, leading to incomplete combustion and thus the formation of more carbon particles or soot. Ethene, having a lower carbon-to-hydrogen ratio, burns more completely with less soot production.
(5bii)
Ethanoic acid (CH₃COOH) has a higher molecular weight than methanoic acid (HCOOH), which contributes to its higher boiling point. Additionally, ethanoic acid can form stronger intermolecular hydrogen bonds due to the presence of the methyl group, which increases the van der Waals forces. These stronger intermolecular forces require more energy (higher temperature) to break, resulting in a higher boiling point for ethanoic acid.
(5biii)
Ethanoic acid (CH₃COOH) can form hydrogen bonds due to the presence of its carboxyl group, which significantly increases its boiling point. Butane (C₄H₁₀), on the other hand, is a non-polar molecule and only experiences weaker van der Waals forces. These hydrogen bonds in ethanoic acid are much stronger than the van der Waals forces in butane, leading to a higher boiling point for ethanoic acid.
(5ci)
Geometric isomerism, also known as cis-trans isomerism, is a type of stereoisomerism that occurs when molecules have the same molecular formula and sequence of bonded atoms, but differ in the spatial arrangement of their atoms in space.
(5cii)
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