Q.Describe the preparation of potassium dichromate from iron chromite ore. What is the effect of increasing pH on a solution of potassium dichromate?
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🔒 Start your 14-day free trial to unlock the full solution →Concept understanding — Inorganic Synthesis
Inorganic Synthesis – What It Really Means
Imagine you want to build a house. You need bricks, cement, steel, and a plan to put them together. Inorganic synthesis is exactly that — but for making chemical compounds that do not contain carbon-hydrogen bonds (the domain of organic chemistry). You take simple starting materials (elements or simple compounds) and, through a controlled chemical reaction, build a more complex inorganic product.
The intuition is simple: you are a chemist-craftsman. You decide what to make, choose the right ingredients, set the right conditions (temperature, pressure, solvent, time), and then isolate the pure product. The "synthesis" part is the entire journey from idea to pure substance.
The Precise Statement
Inorganic synthesis is the branch of chemistry concerned with the design, planning, and execution of chemical reactions to prepare inorganic compounds — including metals, alloys, coordination complexes, main-group compounds, solid-state materials, and nanomaterials — with controlled purity, structure, and properties.
It is not just "mixing chemicals." It involves:
- Choosing the correct starting materials (precursors) — often simple salts, oxides, or elements.
- Selecting a reaction method — solid-state heating, solution precipitation, electrochemical deposition, sol-gel, hydrothermal, etc.
- Controlling reaction conditions — temperature, pressure, pH, concentration, atmosphere (inert gas, air, vacuum).
- Purifying the product — recrystallization, distillation, sublimation, chromatography.
- Characterising the product — proving you actually made what you intended (X-ray diffraction, spectroscopy, elemental analysis).
A Concrete Example: Making Copper(II) Sulfate Pentahydrate
You want to make the familiar blue crystal, CuSOX4⋅5HX2O.
Intuition: You have copper metal (a wire) and dilute sulfuric acid. Copper does not react with dilute acid directly — you need an oxidising agent. So you add nitric acid or simply heat copper with concentrated sulfuric acid.
Reaction:
Cu+2HX2SOX4(conc⋅)CuSOX4+SOX2+2HX2O
Then you evaporate the solution carefully. Blue crystals of CuSOX4⋅5HX2O appear.
What you did: You synthesised an inorganic compound from elemental copper and an acid. You controlled the concentration, temperature, and evaporation rate. You then filtered and dried the crystals.
Why It Matters
Inorganic synthesis is the foundation of:
- Catalysts (e.g., Pt on alumina for car exhausts)
- Electronic materials (silicon wafers, gallium arsenide for LEDs)
- Medicinal compounds (cisplatin for cancer therapy)
- Pigments (titanium dioxide white, Prussian blue)
- Batteries (lithium cobalt oxide electrodes)
Without inorganic synthesis, modern technology would not exist.
A Common Misconception …
Why this formula?
Inorganic Synthesis: Why the Key Formulae Hold
Inorganic synthesis is the branch of chemistry concerned with the preparation of inorganic compounds — from simple salts to complex coordination compounds, organometallics, and solid-state materials. The key formulae in this field are not arbitrary; they arise from fundamental principles of stoichiometry, thermodynamics, kinetics, and coordination chemistry.
Let’s break down the reasoning behind the most important formulae.
1. The Yield Formula: Why It’s Not Just “Product/Reactant”
The most basic formula in any synthesis is:
Percentage Yield=Theoretical YieldActual Yield×100%
Why this holds:
- Theoretical yield is calculated from the limiting reagent — the reactant that runs out first. This is based on the law of conservation of mass and the stoichiometric coefficients from the balanced chemical equation.
- Actual yield is always less than theoretical because of:
- Side reactions (competing pathways)
- Incomplete reactions (equilibrium limitations)
- Loss during purification (filtration, crystallization, etc.)
- The formula is a ratio because yield is a fractional measure of efficiency — it tells you how much of the maximum possible product you actually obtained.
Key insight: The formula works only if you correctly identify the limiting reagent. For example, in the synthesis of FeClX3 from Fe and ClX2, if you have 1 mol Fe and 2 mol ClX2, Fe is limiting (1:1.5 stoichiometry), so theoretical yield is based on Fe.
2. The Atom Economy Formula: Why It Measures “Greenness”
Atom Economy=Sum of Molecular Masses of All ReactantsMolecular Mass of Desired Product×100%
Why this holds:
- This formula was introduced by Barry Trost (1991) to quantify how much of the starting materials ends up in the product.
- It is not a yield — it’s a theoretical maximum based on the balanced equation. It assumes 100% yield.
- The denominator includes all reactants (including solvents if they are consumed, but usually only stoichiometric reagents).
- A high atom economy (e.g., 100% for addition reactions like A+BC) means less waste. A low atom economy (e.g., substitution reactions with leaving groups) means more byproducts.
Example: In the synthesis of NaCl from Na and ClX2:
2Na+ClX2→2NaCl
Atom economy = 2×22.99+70.902×58.44×100%=100% — because all atoms end up in the product.
3. The Solubility Product and Precipitation: Why Ksp Controls Synthesis
For a sparingly soluble salt like AgCl:
AgCl(s)AgX+(aq)+ClX−(aq)
Ksp=[AgX+][ClX−]
Why this holds:
- Ksp is an equilibrium constant derived from the law of mass action. It applies only to saturated solutions.
- In synthesis, you use Ksp to predict whether a precipitate will form when mixing solutions. If the ion product Q=[AgX+][ClX−] exceeds Ksp, precipitation occurs.
- The formula is temperature-dependent (because ΔG∘=−RTlnKsp). So you must control temperature to control precipitation.
Reasoning: The equilibrium constant arises from the balance between the lattice energy (holding the solid together) and the hydration energy (stabilizing ions in solution). A very small Ksp means the solid is very stable — useful for gravimetric synthesis.
4. The Coordination Number and Ligand Field Stabilization Energy (LFSE)
For an octahedral complex, the LFSE is:
LFSE=(−0.4×nt2g+0.6×neg)Δo
Why this holds:
- This formula comes from crystal field theory (CFT). In an octahedral field, the five d orbitals split into two sets: the lower-energy t2g (three orbitals) and the higher-energy eg (two orbitals).
- The splitting energy Δo is the energy difference between these sets.
- Electrons fill the t2g orbitals first (Hund’s rule), and each electron in t2g stabilizes the complex by −0.4Δo relative to the barycenter (average energy). Each electron in eg destabilizes by +0.6Δo.
- The formula explains why certain coordination numbers are preferred: for example, [Co(HX2O)X6]X2+ (high-spin d7) has LFSE = −0.8Δo, while [CoClX4]X2− (tetrahedral) has a smaller LFSE — so the octahedral form is more stable. …
Concept: Inorganic Synthesis — extraction and conversion of chromite ore into dichromate via oxidative fusion.
Preparation from iron chromite (FeCr2O4):
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Fusion: The ore is fused with sodium carbonate in the presence of air (or oxygen) at high temperature.
4FeCr2O4+8Na2CO3+7O2→8Na2CrO4+2Fe2O3+8CO2
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Leaching & Acidification: The fused mass is leached with water to extract soluble sodium chromate. The solution is then acidified with concentrated H2SO4 to convert chromate to dichromate:
2Na2CrO4+H2SO4→Na2Cr2O7+Na2SO4+H2O
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Conversion to potassium salt: Sodium dichromate is treated with KCl (or K2CO3) to precipitate the less soluble potassium dichromate:
Na2Cr2O7+2KCl→K2Cr2O7+2NaCl
Effect of increasing pH on K2Cr2O7 solution:
In water, the dichromate ion exists in equilibrium with chromate: …
Potassium dichromate is prepared from chromite ore via alkaline oxidation (fusion with Na₂CO₃/air) followed by acidification and metathesis with KCl. Increasing pH shifts the dichromate–chromate equilibrium toward chromate, turning the solution from orange to yellow.
Why this approach works
Chromite ore (FeCr2O4) is a mixed oxide of iron(II) and chromium(III). The key challenge is to oxidise chromium from the +3 state to the +6 state, then isolate it as a pure dichromate salt. Direct acid leaching won’t work because Cr(III) is stable in acid — you need a strong oxidising environment under alkaline conditions first. The fusion with sodium carbonate and air does exactly that: it converts insoluble Cr(III) into water-soluble sodium chromate, while iron remains behind as insoluble iron(III) oxide.
The second part — the effect of pH — is a classic example of Le Chatelier’s principle applied to the dichromate–chromate equilibrium. Understanding this equilibrium is essential for controlling the colour and oxidising power of dichromate solutions in volumetric analysis.
Step-by-step preparation
1. Fusion of the ore
Finely powdered chromite ore is mixed with sodium carbonate and a small amount of quicklime (CaO, to prevent caking) and roasted in a reverberatory furnace at about 1000–1200 °C with a current of air. The reaction is:
4FeCr2O4+8Na2CO3+7O2⟶8Na2CrO4+2Fe2O3+8CO2
The chromium(III) in the ore is oxidised to chromium(VI) as yellow sodium chromate. Iron(III) oxide remains as a solid residue.
A common mistake is to write the product as Na2Cr2O7 directly. The fusion product is always chromate, not dichromate — dichromate forms only after acidification.
2. Leaching and filtration
The fused mass is cooled and treated with hot water. Sodium chromate dissolves, while Fe2O3 and other insoluble impurities are filtered off. The yellow filtrate contains Na2CrO4.
3. Conversion to dichromate
The sodium chromate solution is acidified with concentrated sulfuric acid. The chromate ions dimerise to dichromate:
2CrO42−+2H+⇌Cr2O72−+H2O
The solution turns from yellow to orange, indicating the formation of dichromate. Sodium dichromate is obtained on crystallisation.
4. Metathesis to potassium dichromate
Sodium dichromate is more soluble than potassium dichromate. A concentrated solution of Na2Cr2O7 is treated with potassium chloride (or potassium sulfate). The less soluble K2Cr2O7 crystallises out:
Na2Cr2O7+2KCl⟶K2Cr2O7+2NaCl
The orange crystals are filtered, washed with a little cold water, and dried.
The metathesis works because K2Cr2O7 has a steep solubility curve — it is much less soluble in cold water than Na2Cr2O7. Cooling the mixture maximises the yield.
Effect of increasing pH on a solution of potassium dichromate
An aqueous solution of K2Cr2O7 contains the equilibrium: …
Method: Alkaline Oxidative Fusion (followed by acidification)
This is the standard industrial route to convert chromite ore (FeCr2O4) into soluble dichromate.
Steps for Preparation
- Fusion (Roasting) with Soda Ash and Air Finely powdered chromite ore is mixed with sodium carbonate (Na2CO3) and a small amount of lime (CaO, to keep the mixture porous). The mixture is roasted in a reverberatory furnace at 1100–1200 °C in a current of air. Reaction:
4FeCr2O4+8Na2CO3+7O2→8Na2CrO4+2Fe2O3+8CO2
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Leaching
The fused mass is cooled and treated with hot water. Sodium chromate (Na2CrO4, yellow) dissolves, while iron(III) oxide (Fe2O3) remains as insoluble residue and is filtered off.
-
Conversion to Dichromate (Acidification)
The yellow sodium chromate solution is treated with concentrated sulfuric acid to lower the pH. This shifts the chromate-dichromate equilibrium toward dichromate:
2CrO42−+2H+⇌Cr2O72−+H2O
The solution turns **orange** due to formation of sodium dichromate ($\text{Na}_2\text{Cr}_2\text{O}_7$).
4. Double Decomposition (to get Potassium Salt)
Sodium dichromate is more soluble than potassium dichromate, so treating the sodium salt with a potassium salt precipitates the less soluble potassium dichromate. A hot concentrated solution of Na2Cr2O7 is treated with KCl (or K2CO3):
Na2Cr2O7+2KCl→K2Cr2O7+2NaCl …
Here is a breakdown of the common mistakes students make on this specific inorganic synthesis question, along with how to avoid them.
1. Mistake: Writing the wrong formula for the ore
Students often confuse chromite with other ores or write incorrect formulas (e.g., FeCrOX4 or CrX2OX3 alone).
- The correct formula: FeCrX2OX4 (Iron(II) chromite). It is a mixed oxide of FeO and CrX2OX3.
- How to avoid: Memorize the ore formula as a spinel structure (FeX2+CrX23+OX4). Remember it contains both iron and chromium in a 1:2 ratio.
2. Mistake: Forgetting the role of air (oxygen) in the fusion step
Many students write the fusion reaction as just heating with NaX2COX3, forgetting that oxygen from air is the actual oxidizing agent.
- The correct reaction:
4FeCrX2OX4+8NaX2COX3+7OX2→8NaX2CrOX4+2FeX2OX3+8COX2
- How to avoid: Always include OX2 as a reactant. The purpose of heating in a reverberatory furnace is to maximize contact with air. Without oxygen, the chromium(III) cannot be oxidized to chromium(VI).
3. Mistake: Skipping the leaching and filtration step
Students jump directly from fusion to acidification, forgetting that the yellow sodium chromate (NaX2CrOX4) must be separated from the insoluble red-brown iron(III) oxide (FeX2OX3).
- The correct sequence: Fusion → Cool → Leach with water → Filter → Acidify the yellow filtrate.
- How to avoid: Remember that FeX2OX3 is insoluble in water, while NaX2CrOX4 is soluble. Filtration is essential to remove the iron impurity before converting to dichromate.
4. Mistake: Confusing the conversion of chromate to dichromate
Students often write the acidification step incorrectly, either forgetting the equilibrium or writing the wrong products.
- The correct equilibrium:
2CrOX4X2−+2HX+⇌CrX2OX7X2−+HX2O
(Yellow) → (Orange)
- How to avoid: Understand this is a pH-dependent equilibrium, not a simple precipitation. Adding acid (HX+) shifts the equilibrium to the right (orange dichromate). Adding base (increasing pH) shifts it left (yellow chromate).
5. Mistake: Getting the effect of increasing pH wrong
This is the most common conceptual error. Students often think increasing pH makes the solution more orange (dichromate) because they associate dichromate with "stronger" conditions.
- The correct effect: Increasing pH (adding base) removes HX+ from the solution. The equilibrium shifts left, converting orange dichromate back to yellow chromate.
CrX2OX7X2−+2OHX−→2CrOX4X2−+HX2O
- How to avoid: Remember the Le Chatelier's principle logic:
- Acidic pH → favors CrX2OX7X2− (orange)
- Basic/neutral pH → favors CrOX4X2− (yellow)
- Increasing pH = more basic = yellow color dominates. …
Showing the 12 most recent of 23 on this concept.
- CBSE 2025Set ANNUAL1 markQ.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 (MnO2) is fused with KOH in presence of air (or an oxidising agent like KNO3):
2MnO2+4KOH+O2fuse2K2MnO4+2H2O
…
- CBSE 2025Set ANNUAL1 markQ.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:
Na2Cr2O7+2KCl→K2Cr2O7+2NaCl
…
- CBSE 2024Set 56/2/11 markMCQQ.When MnO2 is fused with KOH in air, it gives : (A) KMnO4 (B) K2MnO4 (C) Mn2O7 (D) Mn2O3
›Reveal solutionSolution
Fusing MnO2 with KOH in air oxidises Mn(IV) to Mn(VI), forming the green manganate ion MnO42−. The product is potassium manganate, K2MnO4, 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. MnO2 is already a common starting material for manganese chemistry. When you fuse it with KOH, you create a melt rich in OH− ions. Air (O2) acts as the oxidising agent, pulling electrons away from manganese. The Mn(IV) in MnO2 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.
-
Identify the starting oxidation state. In MnO2, 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.
-
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 (O2) — a good oxidising agent.
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Predict the likely product. In alkaline conditions, manganese can exist in several oxidation states. The +6 state, as the manganate ion MnO42−, is well-known and stable in basic solution. The +7 state, as permanganate MnO4−, is more stable in acidic or neutral conditions. Here, the basic melt favours the manganate. Also, O2 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 KNO3 or KClO3).
-
Write the balanced chemical equation. The reaction is:
2MnO2+4KOH+O2→2K2MnO4+2H2O
Check: Mn goes from +4 to +6 (loss of 2 electrons per Mn). O2 goes from 0 to −2 (gain of 4 electrons per O2). Two Mn atoms lose 4 electrons total, exactly balancing the gain by one O2 molecule. The KOH provides the potassium ions and the oxygen for the water. …
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- CBSE 2024Set ANNUAL1 markMCQQ.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). …
- CBSE 2023Set ANNUAL1 markMCQQ.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 …
- CBSE 2022Set M1 markQ.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 …
- CBSE 2022Set ANNUAL1 markMCQQ.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 …
- CBSE 2020Set ANNUAL1 markQ.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 Cu2+, 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:
Cu2+(aq)+M→Cu+M2+(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 Cu2+ to Cu:
Cu2++Fe→Cu+Fe2+ …
- CBSE 2020Set ANNUAL1 markQ.Complete the reaction XeF₆ + H₂O ⟶ ? + 2HF .
›Reveal solutionSolution
One molecule of water partially hydrolyses XeF6 to XeOF4, 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).
XeF6+H2O→XeOF4+2HF
Here one O atom replaces two F atoms, and the two displaced F combine with the two H of water to give 2HF.
…
- CBSE 2019Set ANNUAL1 markQ.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.
…
- CBSE 2019Set ANNUAL1 markMCQQ.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 …
- CBSE 2019Set ANNUAL1 markMCQQ.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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