When a chromite ore (A) is fused with sodium carbonate in free excess of air and the product is dissolved in w

NCERT Exemplar · Q43

Q. When a chromite ore (A) is fused with sodium carbonate in free excess of air and the product is dissolved in water, a yellow solution of compound (B) is obtained. After treatment of this yellow solution with sulphuric acid, compound (C) can be crystallised from the solution. When compound (C) is treated with KCl, orange crystals of compound (D) crystallise out. Identify A to D and also explain the reactions.

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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, $\ce{CuSO4·5H2O}$ .

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:

\ce{Cu + 2H2SO4(conc.) -> CuSO4 + SO2 + 2H2O}

Then you evaporate the solution carefully. Blue crystals of $\ce{CuSO4·5H2O}$ 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., $\ce{Pt}$ 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:

\text{Percentage Yield} = \frac{\text{Actual Yield}}{\text{Theoretical Yield}} \times 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 $\ce{FeCl3}$ from $\ce{Fe}$ and $\ce{Cl2}$ , if you have 1 mol Fe and 2 mol $\ce{Cl2}$ , Fe is limiting (1:1.5 stoichiometry), so theoretical yield is based on Fe.

2. The Atom Economy Formula: Why It Measures “Greenness”

\text{Atom Economy} = \frac{\text{Molecular Mass of Desired Product}}{\text{Sum of Molecular Masses of All Reactants}} \times 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 $\ce{A + B -> C}$ ) means less waste. A low atom economy (e.g., substitution reactions with leaving groups) means more byproducts.

Example: In the synthesis of $\ce{NaCl}$ from $\ce{Na}$ and $\ce{Cl2}$ :

$2\ce{Na} + \ce{Cl2} \rightarrow 2\ce{NaCl}$

Atom economy = $\frac{2 \times 58.44}{2 \times 22.99 + 70.90} \times 100\% = 100\%$ — because all atoms end up in the product.

3. The Solubility Product and Precipitation: Why $K_{sp}$ Controls Synthesis

For a sparingly soluble salt like $\ce{AgCl}$ :

\ce{AgCl(s) <=> Ag+(aq) + Cl-(aq)}

K_{sp} = [\ce{Ag+}][\ce{Cl-}]

Why this holds:

$K_{sp}$ is an equilibrium constant derived from the law of mass action. It applies only to saturated solutions.
In synthesis, you use $K_{sp}$ to predict whether a precipitate will form when mixing solutions. If the ion product $Q = [\ce{Ag+}][\ce{Cl-}]$ exceeds $K_{sp}$ , precipitation occurs.
The formula is temperature-dependent (because $\Delta G^\circ = -RT \ln K_{sp}$ ). 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 $K_{sp}$ 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:

\text{LFSE} = \left( -0.4 \times n_{t_{2g}} + 0.6 \times n_{e_g} \right) \Delta_o

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 $t_{2g}$ (three orbitals) and the higher-energy $e_g$ (two orbitals).
The splitting energy $\Delta_o$ is the energy difference between these sets.
Electrons fill the $t_{2g}$ orbitals first (Hund’s rule), and each electron in $t_{2g}$ stabilizes the complex by $-0.4\Delta_o$ relative to the barycenter (average energy). Each electron in $e_g$ destabilizes by $+0.6\Delta_o$ .
The formula explains why certain coordination numbers are preferred: for example, $\ce{[Co(H2O)6]^{2+}}$ (high-spin $d^7$ ) has LFSE = $-0.8\Delta_o$ , while $\ce{[CoCl4]^{2-}}$ (tetrahedral) has a smaller LFSE — so the octahedral form is more stable. …

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The chromite ore (A) is $\text{FeCr}_2\text{O}_4$ . Fusion with $\text{Na}_2\text{CO}_3$ in air oxidises chromium to yellow sodium chromate (B). Acidification converts chromate to orange dichromate (C), and treatment with KCl precipitates orange potassium dichromate (D). The final answer is: A = $\text{FeCr}_2\text{O}_4$ , B = $\text{Na}_2\text{CrO}_4$ , C = $\text{Na}_2\text{Cr}_2\text{O}_7$ , D = $\text{K}_2\text{Cr}_2\text{O}_7$ .

The key to this problem is understanding the stepwise oxidation and conversion of chromium from its ore to a commercially important compound. Chromite is the primary source of chromium, and the entire sequence is a classic industrial route to potassium dichromate.

Let’s walk through the chemistry.

Identify the ore (A). Chromite ore is a mixed oxide of iron(II) and chromium(III), with the formula $\text{FeCr}_2\text{O}_4$ (often written as $\text{FeO} \cdot \text{Cr}_2\text{O}_3$ ). This is the only major chromium ore, so A is unmistakably chromite.
Fusion with sodium carbonate in air. When chromite is fused with $\text{Na}_2\text{CO}_3$ in a free excess of air, a high-temperature oxidation occurs. The purpose is to convert insoluble chromium(III) into soluble chromium(VI). The reaction is:

4\text{FeCr}_2\text{O}_4 + 8\text{Na}_2\text{CO}_3 + 7\text{O}_2 \rightarrow 8\text{Na}_2\text{CrO}_4 + 2\text{Fe}_2\text{O}_3 + 8\text{CO}_2

Here, each chromium(III) ( $\text{Cr}^{3+}$ ) is oxidised to chromium(VI) in the chromate ion ( $\text{CrO}_4^{2-}$ ). The iron(II) is oxidised to iron(III) as $\text{Fe}_2\text{O}_3$ , which is insoluble. The yellow colour of the solution after dissolving the fused mass in water comes from the chromate ion — so compound B is sodium chromate, $\text{Na}_2\text{CrO}_4$ .

Watch out

A common mistake is to think the product is dichromate directly. But in alkaline conditions (due to excess $\text{Na}_2\text{CO}_3$ ), the stable species is chromate, not dichromate. The yellow colour confirms chromate.

Acidification to get compound (C). When the yellow solution of sodium chromate is treated with sulphuric acid, the pH drops. In acidic medium, chromate ions dimerise to form dichromate ions:

2\text{CrO}_4^{2-} + 2\text{H}^+ \rightleftharpoons \text{Cr}_2\text{O}_7^{2-} + \text{H}_2\text{O}

The equilibrium shifts to the right, and the solution turns orange. Upon crystallisation, we obtain sodium dichromate, $\text{Na}_2\text{Cr}_2\text{O}_7 \cdot 2\text{H}_2\text{O}$ (or the anhydrous form). So C is sodium dichromate. …

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Here are the common mistakes students make on this classic inorganic synthesis problem, along with the correct conceptual approach to avoid them.

Mistake 1: Confusing the Ore (A) with a Simple Oxide

The Mistake: Students often write the ore as $Cr_2O_3$ (chromium(III) oxide). While chromite contains $Cr_2O_3$ , the ore itself is a mixed oxide.
Why it’s wrong: The exam expects the mineral formula, not just the metal oxide. Writing $Cr_2O_3$ ignores the iron content, which is crucial for balancing the fusion reaction.
How to Avoid: Memorize the exact formula of the chief ore of chromium: $FeCr_2O_4$ (or $FeO \cdot Cr_2O_3$ ). Think of it as "Iron Chromite."

Mistake 2: Forgetting the Role of Air (Oxygen) in the Fusion

The Mistake: Writing the fusion reaction as a simple acid-base reaction without including $O_2$ $O_{2}$ .
- Wrong: $FeCr_2O_4 + Na_2CO_3 \rightarrow Na_2CrO_4 + ...$
Why it’s wrong: The fusion is an oxidation process. Chromium goes from +3 (in the ore) to +6 (in the product). Without oxygen from the air, this oxidation cannot happen.
How to Avoid: Always write the balanced equation including $O_2$ $O_{2}$ as a reactant. The key is to oxidize $Cr^{3+}$ $C r^{3 +}$ to $Cr^{6+}$ $C r^{6 +}$ .
- Correct: $4FeCr_2O_4 + 8Na_2CO_3 + 7O_2 \rightarrow 8Na_2CrO_4 + 2Fe_2O_3 + 8CO_2$

Mistake 3: Misidentifying the Yellow Solution (B)

The Mistake: Thinking the yellow solution is $Na_2Cr_2O_7$ (sodium dichromate).
Why it’s wrong: The fusion product is dissolved in water. In neutral/alkaline aqueous solution, chromium(VI) exists as the yellow chromate ion ( $CrO_4^{2-}$ ), not the orange dichromate ( $Cr_2O_7^{2-}$ ).
How to Avoid: Remember the pH-dependent equilibrium:
- Alkaline/Neutral: $2CrO_4^{2-}$ (Yellow) $\rightleftharpoons Cr_2O_7^{2-}$ (Orange) + $H_2O$
- Acidic: Equilibrium shifts to the right (orange).
- Therefore, (B) is $Na_2CrO_4$ (Sodium chromate).

Mistake 4: Confusing the Order of Acidification and Crystallization

The Mistake: Thinking that adding $H_2SO_4$ directly gives the orange crystals (D).
Why it’s wrong: Compound (C) in this scheme is the sodium dichromate that crystallises from the acidified solution -- not the sodium sulfate by-product of the same reaction. (C) is then treated with KCl to get (D).
How to Avoid: Trace the steps carefully.
1. Acidify (B): $2Na_2CrO_4 + H_2SO_4 \rightarrow Na_2Cr_2O_7 + Na_2SO_4 + H_2O$ . The solution now contains sodium dichromate.
2. Crystallize (C): The acidified solution is evaporated/crystallised — (C) is $Na_2Cr_2O_7$ (sodium dichromate, usually obtained as the dihydrate), the orange compound the question asks you to identify at this step.
3. Treat with KCl: The remaining solution of $Na_2Cr_2O_7$ $N a_{2} C r_{2} O_{7}$ is treated with KCl. This is a metathesis (double displacement) reaction.
  - $Na_2Cr_2O_7 + 2KCl \rightarrow K_2Cr_2O_7 + 2NaCl$ …

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Same / Similar Concept — real previous-year questions on the same or a closely similar concept, not this exact question.

AHSEC Higher Secondary (HS) Final Examination 2023Set ANNUAL1 mark
$Q. Write the name of the catalyst used during synthesis of Cl2 from HCl by Deacon's process.$
›Reveal solutionSolution

Deacon's process converts HCl gas to Cl₂ by catalytic oxidation with air, using CuCl₂ as the catalyst.

Deacon's process is an industrial method for manufacturing chlorine gas by oxidising hydrogen chloride gas with atmospheric oxygen, avoiding the need for electrolysis:

$4HCl(g) + O_2(g) \xrightarrow[723\,K]{CuCl_2 \text{ (catalyst)}} 2Cl_2(g) + 2H_2O(g)$

…

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AHSEC Higher Secondary (HS) Final Examination 2022Set ANNUAL1 mark
$Q. What happens, when (any three): Reaction of ammonia with excess Cl2 gas?$
›Reveal solutionSolution
The product of NH3 reacting with Cl2 depends on which reagent is in excess: excess NH3 gives N2, but excess Cl2 gives the explosive nitrogen trichloride, NCl3.

Ammonia reacts with chlorine gas, and the product formed depends on the relative amounts of the two reactants:

If NH3 is in EXCESS: 8NH3 + 3Cl2 → N2 + 6NH4Cl (nitrogen gas and ammonium chloride are formed; the excess ammonia neutralises the HCl by-product as NH4Cl). …
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AHSEC Higher Secondary (HS) Final Examination 2022Set ANNUAL1 mark
$Q. What happens, when (any three): Orthophosphoric acid is heated?$
›Reveal solutionSolution

Heating orthophosphoric acid drives off water between two molecules (a condensation reaction), forming pyrophosphoric acid, and further heating can continue this dehydration to metaphosphoric acid.

Orthophosphoric acid (H3PO4) is unstable to prolonged heating: on heating to around 260°C, two molecules of H3PO4 undergo an intermolecular condensation, losing a molecule of water and forming a P-O-P bridge, to give pyrophosphoric acid (H4P2O7):

2H3PO4 —Δ (≈260°C)→ H4P2O7 + H2O

…

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