WAEC 2024 CHEMISTRY ANSWERS

CHEMISTRY OBJ
01-10: BCBDBACCCB
11-20: CACBDCBAAB
21-30: BBCBBCADAD
31-40: BCADDAACAC
41-50: BCBCDBCADA

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(1a)
An electrochemical cell is a device that converts chemical energy into electrical energy or vice versa.

(1b)
(i) Electrochemical cells generate electricity spontaneously, while electrolytic cells require an external power source.
(ii) Electrochemical cells have a positive electrode (cathode) and a negative electrode (anode), while electrolytic cells have two identical electrodes.
(iii) Electrochemical cells are used for energy storage and conversion, while electrolytic cells are used for electrolysis and metal plating.

(1c)
When powdered zinc is added to a solution of copper(II) tetraoxosulphate(VI) in a beaker, the zinc will displace copper from the solution, forming zinc tetraoxosulphate(VI) and leaving a deposit of copper metal.

(1d)
(i) Zn + CuSO₄ –> ZnSO₄ + Cu

(ii) Redox reaction

(1e)
(i) Hydrogen chloride (HCl)
(ii) Ammonia (NH₃)

(1f)
(i) High temperature
(ii) Presence of catalysts
(iii) High pressure

(1g)
Noble gases are chemically inert because they have a full outer energy level, making it difficult for them to react with other elements.

(1h)
(i) Members have the same general formula
(ii) Members have similar chemical properties
(iii) Members have a gradual increase in physical properties (e.g., boiling point, melting point)
(iv) Members have a constant difference in molecular mass between successive members.

(1i)
Global warming refers to the long-term rise in the average surface temperature of the Earth due to the increasing levels of greenhouse gases in the atmosphere, leading to a warming effect on the planet.

(1j)
One cause of global warming is the increasing levels of carbon dioxide (CO₂) in the atmosphere, primarily due to human activities such as burning fossil fuels (coal, oil, and gas), deforestation, and land-use changes.

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(2ai)
I. Deliquescence: The process by which a solid absorbs moisture from the air and dissolves in it, forming a solution.

II. Efflorescence: The process by which a solid loses moisture from the air and forms a powder or crystals.

(2aii)

I. Deliquescence: Calcium chloride (CaCl₂) is an example of a substance that undergoes deliquescence.

II. Efflorescence: Sodium carbonate (Na₂CO₃) is an example of a substance that undergoes efflorescence.

(2bi)
Ionization energy is the energy required to remove an electron from an atom or ion in its ground state.

(2bii)
The second ionization energy value of sodium is greater than the first ionization energy because the second electron is removed from a positively charged ion (Na⁺), which requires more energy than removing an electron from a neutral atom (Na).

(2ci)
Charles’ Law states that at constant pressure, the volume of a gas is directly proportional to the temperature in Kelvin.

(2cii)

(2ciii)
First, we need to convert the temperatures from Celsius to Kelvin:

21 °C + 273 = 294 K
45 °C + 273 = 318 K

Then, we can use Charles’ Law to find the new volume:

V₁ / T₁ = V₂ / T₂

where V₁ = 150 cm³, T₁ = 294 K, and T₂ = 318 K

Rearranging the equation to solve for V₂, we get:

V2 = V₁ * T₂ / T₁
= 150cm³ * 318K/294K
= 162.24cm³

(2di)

I. Sodium oxide (Na₂O): Na₂O + H₂O —-> 2NaOH

II. Phosphorus(III) oxide (P₂O₃): P₂O₃ + 3H₂O —-> 2HPO3₃

III. Chlorine(VII) oxide (Cl2O7): Cl₂O₇ + H₂O —> 2HClO₄
IV. Magnesium oxide (MgO): MgO + H₂O —> Mg(OH)₂

(2dii)
(i) Mg(OH)₂ (basic)
(ii) NaOH (basic)
(iii) HPO₃ (weakly acidic)
(iv) HClO₄ (strongly acidic)

 

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(3ai)
I. Direct combination of constituent elements:
The direct combination reaction involves lead and chlorine gas.
Pb(s)+Cl₂(g) —> PbCl₂(s)

II. Double decomposition:
The double decomposition reaction can be achieved by reacting lead (II) nitrate with sodium chloride.
Pb(NO₃)₂(aq)+2NaCl(aq) —> PbCl₂(s)+2NaNO₃(aq)

III. Displacement reaction:
The displacement reaction can be achieved by reacting lead with hydrochloric acid.
Pb(s)+2HCl(aq) —> PbCl₂(s)+H₂(g)

(3aii)
I. Direct combination of constituent elements:
Increasing the temperature would increase the rate of reaction. Higher temperatures provide more kinetic energy to the reacting particles, increasing the chances of successful collisions.

II. Double decomposition:
Increasing the concentration of the reactants (lead (II) nitrate and sodium chloride) would increase the rate of reaction. Higher concentrations lead to more frequent collisions between reacting particles.

III. Displacement reaction:
Increasing the surface area of lead by using powdered lead instead of a solid block would increase the rate of reaction. A greater surface area allows more reactant particles to collide simultaneously.

(3bi)
-Increase Pressure: According to Le Chatelier’s Principle, increasing the pressure of the system will favor the formation of
SO₃ since there are fewer moles of gas on the product side (3 moles of reactants vs. 2 moles of products).

-Decrease Temperature: Since the reaction is exothermic (∆H is positive), decreasing the temperature will favor the forward reaction, thus increasing the yield of SO₃

(3bii)
First, determine the limiting reactant by comparing the mole ratio:
2SO₂+O₂–>2SO₃

From the balanced equation:
2 volumes of SO₂ react with 1 volume of O₂

Given:
SO₂= 60 cm³
O₂= 40 cm³

Required O₂=60cm³/2= 30cm³

Since 30cm³ of 02 is required but 40cm³ is available, SO₂ is the limiting reactant.

Volume of SO₃ formed: 60cm³SO₂ —> 60cm³SO₃.

Volume of residual O₂:
Initial O₂ – Reacted O₂ = 40cm³ – 30cm³

Total residual gas volume:
Residual O₂=10cm³

(3biii)
-Oxidation number of sulfur in SO₂: +4
-Oxidation number of sulfur in SO₃: +6

Change in oxidation number:
+6-(+4)= +2

(3c)


Cyclopropane has the molecular formula C3H6 with a ring structure WHILE n-Propane has the molecular formula C3H8 with a linear structure.

(3di)
-Compound X: Propanol (or specifically, 1-propanol)
-Compound Y: Propyl butanoate

(3dii)
hydroxyl group (-OH)

(3diii)
CH₃H₇OH +CH₄H₈O₂ —> C₇H4O₂ +H₂O

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(4a&b)

 

 

 (4c)

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(5)

 

 

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