Q.(a) Write method of preparation of Nitric acid by Ostwald's process with chemical reactions. [3]

Showing the 12 most recent of 23 on this concept.

1. CBSE 2025Set ANNUAL1 mark
   Q.How will you prepare K2MnO4 from pyrolusite? (Give chemical equation only)

   ›Reveal solutionSolution

   Fusion of pyrolusite (MnO2) with KOH in the presence of an oxidising agent (air/O2 or KNO3) gives potassium manganate.

   Pyrolusite ($MnO_2$) is fused with $KOH$ in presence of air (or an oxidising agent like $KNO_3$):

   $2MnO_2 + 4KOH + O_2 \xrightarrow{\text{fuse}} 2K_2MnO_4 + 2H_2O$

   …

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2. CBSE 2025Set ANNUAL1 mark
   Q.How will you prepare Potassium dichromate from Sodium dichromate? (Give chemical equation only)

   ›Reveal solutionSolution

   KCl is added to a solution of sodium dichromate; the less soluble potassium dichromate crystallises out.

   Sodium dichromate solution is treated with potassium chloride:

   $Na_2Cr_2O_7 + 2KCl \to K_2Cr_2O_7 + 2NaCl$

   …

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3. CBSE 2024Set 56/2/11 markMCQ
   Q.When $MnO_2$ is fused with KOH in air, it gives : (A) $KMnO_4$ (B) $K_2MnO_4$ (C) $Mn_2O_7$ (D) $Mn_2O_3$

   ›Reveal solutionSolution

   Fusing $MnO_2$ with KOH in air oxidises Mn(IV) to Mn(VI), forming the green manganate ion $MnO_4^{2-}$. The product is potassium manganate, $K_2MnO_4$, option (B).

   This is a classic example of an oxidation reaction in a fused alkaline medium. The key is to track the oxidation state of manganese and the role of the environment.

   Why this approach works: In solid-state or fused-salt reactions, the strong alkaline medium (KOH) and the oxidising power of atmospheric oxygen work together. $MnO_2$ is already a common starting material for manganese chemistry. When you fuse it with KOH, you create a melt rich in $OH^-$ ions. Air ($O_2$) acts as the oxidising agent, pulling electrons away from manganese. The Mn(IV) in $MnO_2$ cannot stay at +4 in such a strongly oxidising, basic melt — it gets pushed to a higher stable state. The +6 state (manganate) is particularly stable in alkaline conditions, while the +7 state (permanganate) requires even stronger oxidising conditions or a different workup.

   Let’s walk through the reasoning step by step.

   1. Identify the starting oxidation state. In $MnO_2$, oxygen is $-2$ (usual for oxides). Let the Mn oxidation state be $x$. Then $x + 2(-2) = 0$, so $x = +4$. Manganese is in the +4 oxidation state.

   2. Recognise the reaction conditions. “Fused with KOH in air” means:

      • High temperature (fusion) — the mixture is molten.
      • Strongly basic medium — excess KOH provides $OH^-$ ions.
      • Presence of atmospheric oxygen ($O_2$) — a good oxidising agent.

   3. Predict the likely product. In alkaline conditions, manganese can exist in several oxidation states. The +6 state, as the manganate ion $MnO_4^{2-}$, is well-known and stable in basic solution. The +7 state, as permanganate $MnO_4^-$, is more stable in acidic or neutral conditions. Here, the basic melt favours the manganate. Also, $O_2$ is a moderately strong oxidiser — it can take Mn from +4 to +6, but not easily to +7 (that usually requires a stronger oxidant like $KNO_3$ or $KClO_3$).

   4. Write the balanced chemical equation. The reaction is:

   $2 MnO_2 + 4 KOH + O_2 \rightarrow 2 K_2MnO_4 + 2 H_2O$

   Check: Mn goes from +4 to +6 (loss of 2 electrons per Mn). $O_2$ goes from 0 to $-2$ (gain of 4 electrons per $O_2$). Two Mn atoms lose 4 electrons total, exactly balancing the gain by one $O_2$ molecule. The KOH provides the potassium ions and the oxygen for the water. …

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4. CBSE 2024Set ANNUAL1 markMCQ
   Q.The chemical formula of chromite ore is -

   (a) MnO2

   (b) Na2Cr2O4

   (c) FeCr2O4

   (d) Na2CrO4

   ›Reveal solutionSolution

   Chromite ore, the main source of chromium, has the formula FeCr2O4 (iron(II) chromite, a mixed oxide of iron and chromium).

   Chromite crystallises in the spinel structure, in which Fe2+ ions occupy tetrahedral holes and Cr3+ ions occupy octahedral holes of a close-packed oxide lattice, giving the overall formula FeCr2O4 (equivalently FeO.Cr2O3). …

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5. CBSE 2023Set ANNUAL1 markMCQ
   Q.Process of commercial production of nitric acid is

   (a) Haber process

   (b) Ostwald's process

   (c) Contact process

   (d) Deacon's process

   ›Reveal solutionSolution

   Ostwald's process is named specifically for industrial nitric-acid manufacture, distinguishing it from Haber's (ammonia), Contact (sulphuric acid) and Deacon's (chlorine) processes.

   In Ostwald's process, ammonia is catalytically oxidised over a Pt-Rh catalyst to nitric oxide, which is further oxidised to NO2 and then absorbed in water to give nitric acid:

   4NH3 + 5O2 --(Pt/Rh, 500 K, 9 bar)--> 4NO + 6H2O …

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6. CBSE 2022Set M1 mark
   Q.Name the method used for concentration of sulphide ore.

   ›Reveal solutionSolution

   Sulphide ores are concentrated by the froth flotation process.

   The froth flotation process is used to concentrate sulphide ores. The powdered ore is mixed with water and a collector/frother (e.g. pine oil); air is blown through. The sulphide ore particles are preferentially wetted by the oil and rise with t …

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7. CBSE 2022Set ANNUAL1 markMCQ
   Q.Zone refining is used for obtaining ultra pure sample of

   (a) copper

   (b) sodium

   (c) germanium

   (d) zinc

   ›Reveal solutionSolution

   Zone refining purifies a metal based on the difference in solubility of impurities in the molten vs solid state of the metal.

   In zone refining, a mobile induction heater melts a narrow zone of an impure metal rod at one end and moves slowly to the other end. Impurities are more soluble in the molten zone than in the solid, so they get swept along with the moving molten zone and concentrate at one end, which is then cut off. This te …

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8. CBSE 2020Set ANNUAL1 mark
   Q.Iron scraps are advisable and advantageous than zinc scraps for reducing the low grade copper ores. Why?

   ›Reveal solutionSolution

   Iron and zinc both lie above copper in the reactivity series and can reduce $Cu^{2+}$, but iron scrap is far cheaper and more abundant, so it is the economical choice.

   Concept. In hydrometallurgy of copper, a low-grade ore is leached and the copper in solution is displaced by a more reactive metal:

   $Cu^{2+}(aq) + M \rightarrow Cu + M^{2+}(aq)$

   where $M$ must lie above copper in the activity series.

   Reason. Both $Fe$ and $Zn$ are more reactive than $Cu$, so either can reduce $Cu^{2+}$ to $Cu$:

   $Cu^{2+} + Fe \rightarrow Cu + Fe^{2+}$ …

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9. CBSE 2020Set ANNUAL1 mark
   Q.Complete the reaction XeF₆ + H₂O ⟶ ? + 2HF .

   ›Reveal solutionSolution

   One molecule of water partially hydrolyses $XeF_6$ to $XeOF_4$, liberating $2HF$.

   Concept. Xenon hexafluoride is readily hydrolysed. The extent of hydrolysis depends on the amount of water. With a limited amount (1 mole of water), only partial hydrolysis occurs.

   Reaction (partial hydrolysis).

   $XeF_6 + H_2O \rightarrow XeOF_4 + 2HF$

   Here one $O$ atom replaces two $F$ atoms, and the two displaced $F$ combine with the two $H$ of water to give $2HF$.

   …

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10. CBSE 2019Set ANNUAL1 mark
    Q.What is the role of depressant (NaCN) in Froth-Flotation method?

    ›Reveal solutionSolution

    NaCN selectively prevents ZnS from being wetted by the collector oil (by forming a complex on its surface), so ZnS sinks while PbS floats — separating a mixed Pb–Zn sulphide ore.

    Concept: Froth flotation concentrates sulphide ores: pine-oil collectors make the mineral surface hydrophobic so it rises with the froth. When two sulphides are present, a depressant is used to keep one down.

    …

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11. CBSE 2019Set ANNUAL1 markMCQ
    Q.Which of the following noble gases is abundant in air?

    (i) He

    (ii) Ne

    (iii) Ar

    (iv) Kr

    ›Reveal solutionSolution

    Argon is the most abundant noble gas in air.

    Dry air contains about 0.93% argon by volume, whereas neon, helium and krypton are present only in trace amounts (of the order of parts per million). Hen …

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12. CBSE 2019Set ANNUAL1 markMCQ
    Q.Which one is the ore of copper?

    (i) Haematite

    (ii) Chalcopyrite

    (iii) Dolomite

    (iv) Bauxite

    ›Reveal solutionSolution

    Chalcopyrite (CuFeS2) is the ore of copper.

    An ore is a mineral from which a metal is extracted profitably. Chalcopyrite (copper pyrites), CuFeS2, is the principal ore of copper. Haematite (Fe2O3) is an iron ore, dolomite (CaCO3·MgCO3) is a c …

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