Problem 104 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

State with balanced equations, what happens when (i) Silver is treated with hot concentrated sulphuric acid. (ii) Ammonium dichromate is heated. (iii) Hydrogen sulphide is passed through a solution of potassium permanganate acidified with dilute sulphuric acid.

Step-by-Step Solution

Answer

1) Silver reacts with concentrated H₂SO₄ to form Ag₂SO₄ and SO₂. 2) Ammonium dichromate decomposes into Cr₂O₃, N₂, and H₂O. 3) H₂S with KMnO₄ in acidic solution forms MnSO₄, sulfur, and water.

Step 1: Reaction of Silver with Hot Concentrated Sulfuric Acid

When silver (Ag) reacts with hot concentrated sulfuric acid (H₂SO₄), silver sulfate (Ag₂SO₄) is formed along with sulfur dioxide (SO₂) gas, and water (H₂O). The balanced chemical equation for this reaction is:\[2Ag + 2H_2SO_4 (conc) ightarrow Ag_2SO_4 + 2H_2O + SO_2 \]

Step 2: Heating Ammonium Dichromate

When ammonium dichromate \((NH_4)_2Cr_2O_7\) is heated, it decomposes to produce chromium(III) oxide (Cr₂O₃), nitrogen gas (N₂), and water vapor (H₂O). The balanced chemical equation is:\[(NH_4)_2Cr_2O_7 ightarrow Cr_2O_3 + N_2 + 4H_2O\]

Step 3: Reaction of Hydrogen Sulfide with Potassium Permanganate in Acidic Solution

When hydrogen sulfide (H₂S) is passed through a solution of potassium permanganate (KMnO₄) acidified with dilute sulfuric acid (H₂SO₄), manganese sulfate (MnSO₄), sulfur (S), and water (H₂O) are produced. The balanced chemical equation is:\[2KMnO_4 + 3H_2S + 2H_2SO_4 ightarrow 2MnSO_4 + K_2SO_4 + 3S + 4H_2O\]

Key Concepts

Redox Reactions

In the realm of chemical reactions, redox reactions—or oxidation-reduction reactions—hold a significant place. These reactions involve the transfer of electrons between two species.

The term "redox" comes from the two fundamental processes occurring simultaneously:

• Oxidation: The loss of electrons by a molecule, atom, or ion.
• Reduction: The gain of electrons by a molecule, atom, or ion.

In the example of hydrogen sulfide (\( \text{H}_2\text{S} \)) reacting with potassium permanganate (\( \text{KMnO}_4 \)), we observe a redox reaction. Here, hydrogen sulfide is oxidized to sulfur, as it loses electrons (oxidation), and the manganese in potassium permanganate is reduced to manganese sulfate as it gains electrons (reduction).

It is crucial to identify both oxidation and reduction processes in any redox reaction as they always occur together. Redox reactions are balanced not just for atoms but also for charge. Make sure electrons gained and lost are the same in a perfectly balanced redox equation.

Stoichiometry

Stoichiometry is the mathematical backbone of chemistry, providing us with the tools to quantify relationships in chemical reactions. It helps us determine the exact proportions of reactants and products involved to ensure a balanced and correct reaction equation.

When silver reacts with sulfuric acid, stoichiometry is used to balance the equation:\[2\text{Ag} + 2\text{H}_2\text{SO}_4 \rightarrow \text{Ag}_2\text{SO}_4 + 2\text{H}_2\text{O} + \text{SO}_2\]To facilitate this, one must ensure that the number of atoms for each element is equal on both sides of the reaction equation. Stoichiometry involves working with coefficients that represent moles, ensuring the law of conservation of mass is adhered to, meaning no atoms are lost or gained in the process.

Practical steps in stoichiometry include:

• Identifying and writing down the chemical reactions, making sure they're correctly balanced.
• Calculating the moles of reactants and products using their molar masses.
• Using mole ratios derived from the balanced equation to find out how much of each substance is required or produced.

Chemical Equations

Chemical equations are the symbolic representation of chemical reactions. They provide a concise way to express what happens when substances transform into different substances.

Each chemical equation consists of reactants—the starting materials—and products—the resulting substances. Writing a chemical equation involves not only knowing the substances involved but also how they interact, break apart, or come together to form new compounds.For example, when ammonium dichromate (\((NH_4)_2Cr_2O_7\)) decomposes upon heating, it forms chromium(III) oxide (\(Cr_2O_3\)), nitrogen gas (\(N_2\)), and water (\(H_2O\)). The balanced chemical equation is given as:\[(NH_4)_2Cr_2O_7 \rightarrow Cr_2O_3 + N_2 + 4H_2O\]

A well-formed chemical equation must be balanced:

• This means the number of each type of atom must be the same on both sides of the equation.
• It must respect the conservation of mass principle.

Balanced equations are essential for stoichiometry, helping to quantify reactants' measures and products in a chemical process.