Problem 101 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following equations are correctly formulated here? (1) \(4 \mathrm{Sn}+10 \mathrm{HNO}_{3}\) (dilute) \(\longrightarrow 4 \mathrm{Sn}\left(\mathrm{NO}_{3}\right)_{2}\) \(+\mathrm{NH}_{4} \mathrm{NO}_{3}+\) \(3 \mathrm{H}_{2} \mathrm{O}\) (2) \(3 \mathrm{Zn}+8 \mathrm{HNO}_{3}\) (very dilute) \(\longrightarrow 3 \mathrm{Zn}\left(\mathrm{NO}_{3}\right)_{2}\) \(+2 \mathrm{NO}+4 \mathrm{H}_{2} \mathrm{O}\) (3) \(3 \mathrm{Cu}+8 \mathrm{HNO}_{3}(\) dilute \() \longrightarrow 3 \mathrm{Cu}\left(\mathrm{NO}_{3}\right)_{2}\) \(+2 \mathrm{NO}+4 \mathrm{H}_{2} \mathrm{O}\) (4) \(\mathrm{As}+3 \mathrm{HNO}_{3}\) (dilute) \(\longrightarrow \mathrm{H}_{3} \mathrm{AsO}_{3}+3 \mathrm{NO}_{2}\) (a) 1,2 and 3 (b) 1,3 and 4 (c) 2,3 and 4 (d) 1,2 and 4

Step-by-Step Solution

Answer

Correctly formed equations are 1, 2, and 3; option (a) is correct.

Step 1: Analysis of Equation 1

First, check if the atoms and charges are balanced in the equation: \(4 \mathrm{Sn} + 10 \mathrm{HNO}_{3} \longrightarrow 4 \mathrm{Sn}\left(\mathrm{NO}_{3}\right)_{2} + \mathrm{NH}_{4} \mathrm{NO}_{3} + 3 \mathrm{H}_{2} \mathrm{O}\). Calculating each element: Sn (4 on both sides), N (10 on the left, 10 on the right since \(4 \times 2 + 1 + 3 = 10\)), O (30 on left, 30 on right via distribution in products), H (10 on the left, 10 on the right side). Equation 1 is balanced.

Step 2: Analysis of Equation 2

Check that 3 Zn + 8 HNO3 -> 3 Zn(NO3)2 + 2 NO + 4 H2O is balanced. Evaluate elements: Zn (3 on each side), N (8 on the left side, exists as 6 in Zn(NO3)2 and 2 in NO, totaling 8), O (24 on both sides from distribution of HNO3 and products), H (8 on each side). Equation 2 is balanced.

Step 3: Analysis of Equation 3

Check 3 Cu + 8 HNO3 -> 3 Cu(NO3)2 + 2 NO + 4 H2O for balance. Elements: Cu (3 on each side), N (8 on the left and right, where the distribution requires recalculation of charges), O balances as apparent previously. This equation balances in terms of atoms and charges, thus it is correctly formulated.

Step 4: Analysis of Equation 4

Finally, examine As + 3 HNO3 -> H3AsO3 + 3 NO2. Verify each element: As (1 on each side), N (3 on each side), O (9 on both sides from 3 HNO3 & product combinations), and H (3 on both sides from water molecules and product combinations). Equation 4 is balanced.

Key Concepts

Redox Reactions

In redox reactions, which stands for reduction-oxidation reactions, there is a transfer of electrons among reactants. These reactions can seem complex, but they're just exchanges where one substance gets oxidized (loses electrons) and another gets reduced (gains electrons).
Oxidation involves an increase in oxidation state, while reduction involves a decrease. A helpful mnemonic is "OIL RIG," where Oxidation Is Loss and Reduction Is Gain of electrons.

• Consider the first equation: Sn is oxidized from 0 in Sn to +2 in Sn(NO₃)₂, while N in NO₃⁻ is reduced, signifying its lower oxidation state in NH₄NO₃.
• The second equation is similar, where Zn is oxidized to form Zn(NO₃)₂.
• Despite similar patterns, keep in mind that the balancing of electrons is vital to ensure mass and charge balance.

This electron play is often challenging. However, identifying changes in oxidation states helps decode the electron exchanges.

Stoichiometry

Stoichiometry is like the math of chemistry, ensuring equations reflect the conservation of mass.
This principle is key, especially in chemical balancing where the number of atoms for each element must be equal on both sides of an equation.

• A quantitative analysis ensures atoms are neither created nor destroyed, only rearranged.
• In equation 1, all Sn, N, and H atoms are accounted perfectly, ensuring mass conservation.
• Similarly, equation 2 maintains a balance with the formula unit calculations keeping zinc and nitrogen atoms in proportion.

Getting stoichiometry right requires practice. It involves comparing the moles of reactants and products, mastering the mole concept, and constructing balanced equations.

Inorganic Chemistry

Inorganic Chemistry covers the study of inorganic and mineral compounds, distinct from the organic realm which deals with carbon-based compounds.
Understanding reactions like the ones given requires familiarity with some central concepts in this field, such as acidity, basicity, and compound formation.

• Reactions involving nitric acid (HNO₃), a common component in these equations, are a part of inorganic chemistry's extensive study of acids and bases.
• The formation of nitrates, as seen in products such as Zn(NO₃)₂ and Sn(NO₃)₂, is typical in inorganic reactions.
• The intricate interplay of metals, such as copper and zinc, reflects their behavior in oxidation states and compound formation.

These topics are fundamental for understanding the vast range of chemical interactions in inorganic chemistry.