Author Topic: CORRECT: 2025 NABTEB CHEMISTRY ANSWERS (OBJ & ESSAY) (Read 16042 times)

MANDINHO · « **on:** June 19, 2025, 12:02:05 pm »

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MANDINHO · « **Reply #1 on:** June 20, 2025, 04:00:48 pm »

NABTEB CHEMISTRY ANSWERS

CHEMISTRY OBJ!
01-10: BDCCAABABA
11-20: CCCCDDDCCC
21-30: CBBBADDCDB
31-40: CCAADCBBDB
41-50: ABCAABBDDD

COMPLETED
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INSTRUCTIONS: ANSWER FOUR(4) QUESTIONS ONLY

(1ai)
(i) Diamond
(ii) Graphite

(1aii)
(i) Coal tar - used in making synthetic dyes.
(ii) Ammonia - used in the manufacture of fertilizers.

(1bi)
Alpha particles < Beta particles < Gamma rays.

(1bii)
In nuclear fission, a heavy nucleus splits into two or more lighter nuclei with the release of energy, while in nuclear fusion, two light nuclei combine to form a heavier nucleus with the release of even greater energy.

(1ci)
S(s) + O₂(g) ---> SO₂(g)

(1cii)
1 mole of O₂ produces 1 mole of SO₂
Volume ratio of O₂ to SO₂ = 1:1
Volume of SO₂ produced = 50 cm³
Volume of O₂ used = 50 cm³
Volume of oxygen used up = 50 cm³

(1ciii)
Avogadro’s Law

(1civ)
Avogadro’s Gas Law states that equal volumes of all gases, at the same temperature and pressure, contain the same number of molecules.

(1di)
Heat of combustion: This is the amount of heat evolved when one mole of a substance is completely burned in oxygen under standard conditions.

(1dii)
Heat of reaction: This is the amount of heat absorbed or released during a chemical reaction when the number of moles of reactants react according to the balanced chemical equation.
===========================

(2a)
Collision theory explains the rate of a chemical reaction by stating that chemical reactions occur when reacting particles collide with each other with sufficient energy and proper orientation. The greater the number of effective collisions per unit time, the faster the reaction rate.

(2bi)
Limestone (CaCO₃):
CaCO₃(s) ----> CaO(s) + CO₂(g)

(2bii)
Ammonia (NH₃):
NH₃(g) + H₂O(l) + CO₂ (g) ---> NH₄HCO₃(aq)

NH₄HCO₃(aq) + NaCl(aq) ----> NH₄Cl(aq) + NaHCO₃(s)

(2biii)
Brine (NaCl solution):
NaCl(aq) +NH₃(g) + CO₂(g) + H₂O(l) ----> NaHCO₃(s) + NH₄Cl(aq)

(2ci)
The reaction is a neutralization reaction because it involves the reaction of a base, sodium trioxocarbonate (IV), with an acid, dilute hydrochloric acid, to form a salt, water, and carbon dioxide, thereby neutralizing the acid.

(2cii)
Given:
Molar mass of NaHCO₃
= 23 + 1 + 12 + (16×3)
= 84 g/mol
Molar mass of Na₂CO₃
= (23×2) + 12 + (16×3)
= 106 g/mol
Balanced equation:
2 NaHCO₃ ---> Na₂CO₃ + CO₂ + H₂O
2 mol of NaHCO₃ = (2×84)
= 168 g
1 mol of Na₂CO₃ = 106 g
Mass of Na₂CO₃ from 16.8 g of NaHCO₃ = (106×16./(168)
= 10.6 g

(2d)
(i) Surface area: Increasing the surface area of a solid reactant exposes more particles to collision, thereby increasing the frequency of effective collisions and speeding up the reaction rate.
(ii) Catalyst: A catalyst provides an alternative reaction pathway with lower activation energy, increasing the number of effective collisions and hence speeding up the reaction without being consumed.
(iii) Pressure: Increasing the pressure of gaseous reactants compresses the gas molecules into a smaller volume, increasing their collision frequency and thus the reaction rate.
(iv) Concentration: Increasing the concentration of reactants increases the number of particles per unit volume, resulting in more frequent effective collisions and a faster reaction rate.

(2e)
(i) Oxidation numbers are used to determine the oxidizing and reducing agents in redox reactions by tracking the changes in oxidation states of elements.
(ii) They help in writing and balancing chemical equations, especially redox equations, by ensuring that both mass and charge are conserved.
===========================

(3ai)
Le Chatelier’s principle states that if a system at equilibrium is disturbed by a change in temperature, pressure, or concentration, the system will adjust itself to oppose the change and establish a new equilibrium.

(3aii)
(i) High Pressure: The forward reaction involves a reduction in the number of gas molecules from 4 to 2. Increasing the pressure shifts the equilibrium towards the side with fewer molecules, favouring the production of ammonia.
(ii) Low Temperature: The formation of ammonia is exothermic. Lowering the temperature shifts the equilibrium to the right to produce more heat, thereby increasing the yield of ammonia.
(iii) Removal of Ammonia: Continuously removing ammonia as it forms shifts the equilibrium to the right, favouring more ammonia production to replace what was removed.

(3bi)
Ammonia gas can be tested by exposing it to moist red litmus paper. If ammonia is present, it will turn the red litmus paper blue due to its basic nature. Additionally, ammonia produces white fumes when a glass rod dipped in concentrated hydrochloric acid is brought near it, forming ammonium chloride.

(3bii)
(i) A conductor allows the flow of electric current through the movement of electrons, while an electrolyte conducts electricity through the movement of ions in solution.
(ii) A conductor is usually a solid metal like copper or silver, while an electrolyte is typically a liquid or molten substance like sodium chloride solution.
(iii) A conductor does not undergo any chemical change during conduction, while an electrolyte undergoes chemical decomposition during conduction.
(iv) A conductor can carry electric current in both solid and liquid states, while an electrolyte conducts electricity only in molten or aqueous state.
(v) In a conductor, electric current is carried by free electrons, while in an electrolyte, current is carried by positively and negatively charged ions.

(3ci)
Faraday’s first law of electrolysis states that the mass of a substance deposited or liberated at an electrode during electrolysis is directly proportional to the quantity of electricity passed through the electrolyte.

(3cii)
Given:
Mass of metal deposited = 0.222 g
Current (I) = 0.45 A
Time (t) = 25 minutes
Time (t) = 25 × 60
Time (t) = 1500 s
Valency (n) = 2 (divalent metal)
Faraday’s constant (F) = 96500 C/mol
Mass (m) = (M x I x t)/(n x F)
M = (m x n x F)/(I x t)
M = (0.222 x 2 x 96500)/(0.45 x 1500)
M = (42846)/(675)
M = 63.46
Relative atomic mass = 63.46

(3ciii)
(i) Electroplating of metals to prevent corrosion or improve appearance.
(ii) Extraction of reactive metals such as sodium, potassium, and aluminium from their ores.
(iii) Purification of metals like copper and zinc.
(iv) Production of gases like hydrogen and oxygen from the electrolysis of water.
(v) Manufacturing of chemical compounds like sodium hydroxide and chlorine from the brine.
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(4ai)
Collision Theory: Collision theory states that for a chemical reaction to occur, the reacting particles must collide with each other with sufficient energy and proper orientation.

(4aii)
Activation Energy: Activation energy is the minimum amount of energy that reacting particles must possess during a collision for a chemical reaction to take place.

(4bi)
(Draw the diagram)

(4bii)
(i) Increase the temperature: Raising the temperature increases the kinetic energy of the reacting particles, leading to more frequent and energetic collisions. This increases the rate at which calcium carbonate reacts with hydrochloric acid.
(ii) Increase the surface area of calcium carbonate: Using powdered or finely crushed calcium carbonate exposes more surface area for the acid to act upon, leading to a faster reaction compared to using large chunks.

(4c)
(i) Constant Concentration of Reactants and Products: At equilibrium, the concentrations of reactants and products remain constant over time, although the forward and reverse reactions continue to occur. This indicates that the rates of the forward and reverse reactions are equal.
(ii) Dynamic Nature of the System: Equilibrium is dynamic, not static. This means that both the forward and reverse reactions occur continuously and at the same rate. There is no net change in the amount of substances, but molecules are constantly reacting.
(iii) Equilibrium Can Be Reached from Either Direction: A chemical equilibrium can be established whether the reaction starts with reactants or with products. As long as conditions such as temperature and pressure remain constant, the system will adjust until equilibrium is reached.

(4di)
An increase in pressure causes the equilibrium to shift to the side with fewer gas molecules. The left-hand side has 3 moles of gas (2 SO₂ + 1 O₂), while the right-hand side has 2 moles of SO₃.
Therefore, the equilibrium shifts to the right, favoring the formation of SO₃, to reduce the pressure.

(4dii)
Kc = [SO₃]²/[SO₂]²[O₂]
Where [SO₃], [SO₂], and [O₂] are the equilibrium concentrations of the gases in mol/dm³.

(4ei)
An oxide is a compound formed when an element reacts chemically with oxygen. It consists of one or more oxygen atoms combined with atoms of another element.

(4eii)
(i) Acidic oxide
(ii) Basic oxide
(iii) Amphoteric oxide

(4eiii)
Let the oxidation state of Fe = x
x + 6(-1) = -3
x - 6 = -3
x = -3 + 6
x = +3
Oxidation state of iron = +3
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(5a)
(i) All members have the same general formula.
(ii) Each successive member differs by a CH₂ group.
(iii) Members show similar chemical properties.

(5bi)
The general formula of an alkane series is CₙH₂ₙ₊₂

(5bii)
(i) Methane
Structure: CH₄
(ii) Ethane
Structure: CH₃–CH₃

(5ci)
Mass of H = 7.7 g
Mass of C = 92.3 g
Molar mass of H = 1
Molar mass of C = 12
Mole of H = 7.7 ÷ 1
Mole of H = 7.7
Mole of C = 92.3 ÷ 12
Mole of C = 7.69
Mole ratio H : C = 7.7 : 7.69
Mole ratio = 1 : 1
Empirical formula = CH
Empirical formula mass = 12 + 1
Empirical formula mass = 13
Molecular formula = (78 ÷ 13) × CH
Molecular formula = 6 × CH
Molecular formula = C₆H₆

(5cii)

(5d)
(i) Y will undergo substitution reaction only because it is a saturated hydrocarbon (alkane) that does not react with oxidizing agents like acidified potassium tetraoxomanganate VII.
(ii) X will undergo both addition and substitution reactions because it is an unsaturated hydrocarbon (alkene or alkyne) that decolourizes the oxidizing agent and can react by addition at the double or triple bond and by substitution under certain conditions.
(iii) X will undergo polymerization because unsaturatedits bonds allow the molecules to join together in long chains to form polymers.

(5ei)
(i) Ethanol is converted to ethanoic acid through an oxidation reaction that requires a strong oxidizing agent. Common oxidizing agents used are acidified potassium dichromate or acidified potassium permanganate. These provide the oxygen needed to carry out the oxidation process.
(ii) The mixture is heated under reflux. Refluxing involves boiling the reaction mixture while continuously cooling the vapor so it condenses back into the flask. This setup prevents the loss of ethanol and ensures the reaction proceeds completely to form ethanoic acid.
(iii) An acidic medium is necessary for the oxidizing agent to work effectively. Dilute sulfuric acid is commonly added to provide the acidic environment that supports the oxidation reaction.
(iv) An excess amount of the oxidizing agent is usually added to ensure complete oxidation. Without enough oxidizing agents, the reaction may stop at the intermediate stage, producing ethanal instead of the final product, ethanoic acid.

(5eii)
(i) Name of the reaction: Oxidation of ethanol to ethanoic acid
(ii) Equation of the reaction:
CH₃CH₂OH + 2
---> CH₃COOH + H₂O

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COMPLETED

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Author Topic: CORRECT: 2025 NABTEB CHEMISTRY ANSWERS (OBJ & ESSAY) (Read 16042 times)

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