S & P Block Elements MCQs set 3 for AKU Entry Test Chemistry — 20 solved questions.
Q1. When a fresh piece of lithium metal is heated in a stream of pure nitrogen gas, a ruby-red crystalline solid is formed. What is the formula of this product?
Answer: Lithium nitride (Li3N)
Explanation: Lithium is the only alkali metal reacting directly with nitrogen to form Li3N; LiN3 is an azide, not the direct combustion product.
Q2. A strip of magnesium ribbon is ignited in a jar containing atmospheric air. Which two solid products are primarily formed during this combustion process?
Answer: Magnesium oxide and nitride
Explanation: Magnesium reacts with both oxygen and nitrogen in air; MgO is the major product while Mg3N2 forms in smaller amounts simultaneously.
Q3. Magnesium sulfate is readily soluble in water, but Barium sulfate is practically insoluble. What is the primary thermodynamic reason for this decrease in solubility?
Answer: Hydration energy decreases more rapidly than lattice energy
Explanation: Hydration energy decreases faster than lattice energy down the group, making the dissolution of Barium sulfate energetically unfavorable compared to Magnesium sulfate.
Q4. When aluminum metal is added to a concentrated solution of sodium hydroxide, it dissolves with the evolution of hydrogen gas. What is the final complex ion formed?
Answer: Sodium tetrahydroxoaluminate
Explanation: Aluminum is amphoteric and reacts with NaOH to form soluble sodium tetrahydroxoaluminate; Al(OH)3 is an intermediate that dissolves in excess base.
Q5. In a laboratory experiment, chlorine gas is bubbled through a cold, dilute solution of sodium hydroxide. Which two sodium salts are produced in this disproportionation reaction?
Answer: Sodium chloride and sodium chlorate(I)
Explanation: Chlorine reacts with cold, dilute NaOH to form NaCl and NaClO; NaClO3 only forms when the solution is hot and concentrated.
Q6. During the Contact process, sulfur dioxide is converted to sulfur trioxide using a Vanadium pentoxide catalyst. What is the specific role of the catalyst in the intermediate step?
Answer: V2O5 is reduced to V2O4 by SO2
Explanation: V2O5 is reduced to V2O4 by SO2 and then re-oxidized by O2; this redox cycle allows the continuous oxidation of sulfur dioxide.
Q7. When phosphorus pentachloride is treated with a limited amount of water, it undergoes partial hydrolysis. What is the primary phosphorus-containing product formed in this initial stage?
Answer: Phosphorus oxychloride (POCl3)
Explanation: PCl5 reacts with limited water to form POCl3 and HCl; full hydrolysis to H3PO4 only occurs with excess water.
Q8. A mixture of Xenon and Fluorine in a 1:2 ratio is heated in a sealed nickel vessel at 400 degrees Celsius. Which noble gas compound is synthesized?
Answer: Xenon difluoride (XeF2)
Explanation: Xenon reacts with fluorine in a 1:2 ratio at 400 degrees Celsius and 6 atm to produce XeF2; higher ratios produce XeF4 or XeF6.
Q9. Orthoboric acid behaves as a weak monobasic acid in aqueous solution. Which step correctly describes the mechanism of its acidity when it interacts with water?
Answer: It accepts a hydroxyl ion from water
Explanation: Boric acid accepts OH- from water instead of donating H+, making it a Lewis acid; H3BO3 does not dissociate its own protons directly.
Q10. Sodium peroxide is often used as an oxidizing agent. What are the products when sodium peroxide is added to cold water in a controlled environment?
Answer: Sodium hydroxide and hydrogen peroxide
Explanation: Sodium peroxide reacts with water to yield NaOH and H2O2; the peroxide ion O2(2-) is protonated to form hydrogen peroxide.
Q11. When concentrated sulfuric acid is poured onto a sample of common table sugar (sucrose), the sugar turns black and swells up. What process is occurring?
Answer: Dehydration of sucrose to carbon
Explanation: Concentrated H2SO4 acts as a powerful dehydrating agent, removing water elements from sucrose and leaving behind a porous mass of black carbon.
Q12. During a thunderstorm, the high temperature of a lightning bolt provides the activation energy for atmospheric gases to react. What is the first oxide of nitrogen formed?
Answer: Nitric oxide (NO)
Explanation: Nitrogen and oxygen combine only at high temperatures (approx. 3000C) produced by lightning or electric arcs to form Nitric oxide (NO).
Q13. The Hasenclever apparatus is used for the large-scale production of bleaching powder. Which two starting materials are fed into this industrial process?
Answer: Calcium hydroxide and chlorine gas
Explanation: Bleaching powder is produced by passing chlorine over dry slaked lime (calcium hydroxide); CaCO3 is used for CO2 production, not bleaching powder.
Q14. Four different carbonates of Group II elements are heated under identical conditions. Which of these carbonates will require the highest temperature to undergo thermal decomposition?
Answer: Barium carbonate
Explanation: Larger cations stabilize larger anions like carbonate; as you go down Group II, the cation size increases, making the carbonate more thermally stable.
Q15. Aluminum containers can be used to store and transport concentrated nitric acid. What specific mechanism makes the aluminum metal 'passive' in this highly corrosive acid?
Answer: Formation of a protective oxide layer
Explanation: Concentrated HNO3 forms a thin, non-porous layer of Al2O3 on the surface of aluminum, preventing further reaction; HCl would dissolve the metal.
Q16. Silicon tetrachloride reacts violently with water to form silicic acid, whereas Carbon tetrachloride does not react. What structural feature of Silicon allows this mechanism?
Answer: Coordination of water to vacant d-orbitals
Explanation: Silicon has vacant 3d orbitals that allow water to coordinate as a nucleophile; Carbon lacks d-orbitals, making CCl4 resistant to hydrolysis.
Q17. In the industrial production of sodium metal by the Down's process, calcium chloride is added to the electrolyte. What is the primary purpose of this addition?
Answer: To lower the melting point of NaCl
Explanation: In Down's cell, CaCl2 is added to lower the melting point of NaCl from 801 to 600 degrees Celsius, saving energy and preventing Na vaporization.
Q18. In a lead storage battery, several chemical changes occur during the discharge cycle. What reaction takes place at the cathode (positive electrode) during this process?
Answer: Lead(IV) oxide is reduced to Lead(II) sulfate
Explanation: During discharge, PbO2 is reduced to PbSO4 at the cathode while Pb is oxidized to PbSO4 at the anode; both electrodes become coated with PbSO4.
Q19. When sulfur dioxide gas is passed through an acidified solution of potassium dichromate, the orange color changes to green. What is the chemical basis for this change?
Answer: It reduces Cr(VI) to green Cr(III) ions
Explanation: Sulfur dioxide is a reducing agent that reduces Cr(VI) in K2Cr2O7 to green Cr(III) in Cr2(SO4)3; KMnO4 would turn colorless, not green.
Q20. A student is given four different Group II metal oxides. Which of these oxides will react with a hot, concentrated solution of sodium hydroxide to form a salt?
Answer: Beryllium oxide
Explanation: BeO is amphoteric, reacting with both acids and bases; MgO and CaO are purely basic and do not dissolve in NaOH.