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

(c) 2, 3, and 4 are correctly formulated.

Step 1: Analyze Equation 1

Check the conservation of atoms in the reaction: \(4 \mathrm{Sn} + 10 \mathrm{HNO}_{3} \rightarrow 4 \mathrm{Sn( NO}_{3})_{2} + \mathrm{NH}_{4} \mathrm{NO}_{3} + 3 \mathrm{H}_{2} \mathrm{O}\). Tin and nitrogen are balanced, but the total counts of hydrogen and oxygen need verification.

Step 2: Check Hydrogen and Oxygen Atoms in Equation 1

Equation 1: Left has 10H and 30O; Right has 10H (NH4NO3 and 3H2O) and 30O (combined from Sn(NO3)2, NH4NO3, and H2O). The equation is balanced. Equation 1 is correctly formulated.

Step 3: Analyze Equation 2

Check the conservation of atoms in the reaction: \(3 \mathrm{Zn} + 8 \mathrm{HNO}_{3} \rightarrow 3 \mathrm{Zn(NO}_{3})_{2} + 2 \mathrm{NO} + 4 \mathrm{H}_{2} \mathrm{O}\). Balance the zinc, nitrogen, hydrogen, and oxygen atoms.

Step 4: Verify Atom Counts in Equation 2

Equation 2: Left has 6H and 24O while Right has 6H and 24O. Both sides are balanced. Equation 2 is correctly formulated.

Step 5: Analyze Equation 3

Check the conservation of atoms in the reaction: \(3 \mathrm{Cu} + 8 \mathrm{HNO}_{3} \rightarrow 3 \mathrm{Cu(NO}_{3})_{2} + 2 \mathrm{NO} + 4 \mathrm{H}_{2} \mathrm{O}\). Balance the copper, nitrogen, hydrogen, and oxygen atoms.

Step 6: Verify Atom Counts in Equation 3

Equation 3: Left has 8H and 24O while Right has 8H and 24O. Both sides are balanced. Equation 3 is correctly formulated.

Step 7: Analyze Equation 4

Check the conservation of atoms in the reaction: \( \mathrm{As} + 3 \mathrm{HNO}_{3} \rightarrow \mathrm{H}_{3}\mathrm{AsO}_{3} + 3 \mathrm{NO}_{2} \). Balance arsenic, nitrogen, hydrogen, and oxygen atoms.

Step 8: Verify Atom Counts in Equation 4

Equation 4: Left has 3H and 9O while Right has 3H and 9O. Both sides are balanced. Equation 4 is correctly formulated.

Key Concepts

Balancing Equations

Balancing chemical equations is like making sure a recipe uses the exact ingredients needed for the perfect dish. Chemical equations show how substances interact in a reaction. A balanced equation means that the number of each type of atom on the reactant side (starting materials) equals the number on the product side (substances formed). This is important in chemistry because matter is neither created nor destroyed.
For example, when checking equation (1) from the exercise, which describes the reaction of tin with nitric acid, we start by looking at every element involved. We balance tin, nitrogen, hydrogen, and oxygen. In this specific equation, there are equal numbers of each atom on both sides. Simplifying the equation keeps reactions understandable and predictable.
Use these tips to balance equations:

• List all the elements involved in the reactants and products.
• Add coefficients (numbers in front of compounds) to balance each kind of atom.
• Double-check that all atoms balance and simplify.

Balancing equations supports a foundational chemical law – conservation of mass, which we'll explore further.

Redox Reactions

Redox (reduction-oxidation) reactions are fascinating processes where electrons are transferred between substances. In any redox reaction, one substance loses electrons (is oxidized) while another gains them (is reduced). Identifying which substances undergo oxidation and reduction can be essential in balancing equations, especially with reactions involving metals and acid.
In our exercise, when examining the reactions, notice how metal atoms like zinc or copper interact with nitric acid. Metals often give up electrons (oxidized) and transform into their corresponding ions, while the ions in nitric acid typically gain electrons (reduced), forming various products.
Recognizing redox processes in these equations can involve:

• Identifying changes in oxidation states – observe if the metal's state increases or decreases.
• Calculating losing/gaining of electrons to ensure electron transfer is balanced.
• Applying half-reactions – think of them as mini-reactions showing either oxidation or reduction.

This concept helps ensure that reactions maintain charge balance in the equations.

Conservation of Mass

The principle of Conservation of Mass is one of the cornerstones of chemistry. According to this law, mass cannot be created or destroyed in a closed system during a chemical reaction. This concept is crucial when balancing chemical equations.
For instance, consider our examined equations. Each equation represents a reaction where all reactants and products must weigh the same before and after the reaction, affirming our Conservation of Mass principle.
How does this affect equations?

• The sum of the masses of all reactions equals the sum of the masses of all products.
• Ensures that when balancing equations, you're lining up equal numbers of each kind of atom on either side of the equation.
• Helps predict amounts of substances needed or produced in a reaction.

Knowing that mass remains constant aids in maintaining precision in chemical reactions and aligns with consistent scientific observations.