Problem 15 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following gives a precipitate with \(\mathrm{Pb}\left(\mathrm{NO}_{3}\right)_{2}\) but not with \(\mathrm{Ba}\left(\mathrm{NO}_{3}\right)_{2} ?\) (a) sodium acetate (b) sodium nitrate (c) sodium chloride (d) sodium hydrogen phosphate

Step-by-Step Solution

Answer

(c) sodium chloride

Step 1: Review Reactants

We need to identify which compound forms a precipitate with \(\mathrm{Pb}\left(\mathrm{NO}_{3}\right)_{2}\) but not with \(\mathrm{Ba}\left(\mathrm{NO}_{3}\right)_{2}\). This means that the cation from \(\mathrm{Pb}^{2+}\) should form an insoluble compound, while \(\mathrm{Ba}^{2+}\) should not.

Step 2: Analyze Sodium Acetate (Option a)

Sodium acetate \((\mathrm{NaC_2H_3O_2})\) generally forms soluble acetates with all metals.Therefore, it does not form precipitates with either \(\mathrm{Pb}\) or \(\mathrm{Ba}\) nitrates.

Step 3: Analyze Sodium Nitrate (Option b)

Sodium nitrate \((\mathrm{NaNO_3})\) is highly soluble, and its salts remain soluble with both \(\mathrm{Pb}\) and \(\mathrm{Ba}\).Thus, it does not form precipitates.

Step 4: Analyze Sodium Chloride (Option c)

Sodium chloride \((\mathrm{NaCl})\) reacts with \(\mathrm{Pb}\left(\mathrm{NO}_{3}\right)_{2}\) to form \(\mathrm{PbCl_2}\), which is not very soluble, resulting in a precipitate. However, \(\mathrm{BaCl_2}\) is soluble and does not form a precipitate with \(\mathrm{Ba}\left(\mathrm{NO}_{3}\right)_{2}\).

Step 5: Check Sodium Hydrogen Phosphate (Option d)

Sodium hydrogen phosphate \((\mathrm{Na_2HPO_4})\) can react with both \(\mathrm{Pb}\) and \(\mathrm{Ba}\) to form precipitates such as \(\mathrm{Pb_3(PO_4)_2}\) and \(\mathrm{Ba_3(PO_4)_2}\). Therefore, it does not satisfy the condition.

Step 6: Conclusion

Only \(\mathrm{NaCl}\) (sodium chloride) forms a precipitate with \(\mathrm{Pb}\left(\mathrm{NO}_{3}\right)_{2}\) but not with \(\mathrm{Ba}\left(\mathrm{NO}_{3}\right)_{2}\).This is because \(\mathrm{PbCl_2}\) is poorly soluble, whereas \(\mathrm{BaCl_2}\) remains soluble.

Key Concepts

Solubility Rules

When working with precipitation reactions, solubility rules are essential. They help predict whether a compound will dissolve in water or form a precipitate. A precipitate is a solid that forms out of a solution due to a chemical reaction. To determine if a precipitate will form, we use solubility rules, which guide us by indicating the solubility of different ionic compounds.

Here are some of the key solubility rules to remember:

• Nitrates (\( ext{NO}_3^{-}\)) are always soluble. That means compounds like sodium nitrate will dissolve in water without forming a precipitate.
• Chlorides (\( ext{Cl}^-\)) are generally soluble except for those formed with \( ext{Pb}^{2+}\), \( ext{Ag}^+\), and \( ext{Hg}_2^{2+}\). This explains why sodium chloride forms a precipitate with \( ext{Pb(NO}_3 ext{)}_2\) but not with \( ext{Ba(NO}_3 ext{)}_2\).
• Acetates (\( ext{C}_2 ext{H}_3 ext{O}_2^-\)) are typically soluble.
• Phosphates (\( ext{PO}_4^{3-}\)) and carbonates (\( ext{CO}_3^{2-}\)) are usually insoluble, unless paired with alkali metals like sodium or potassium, or ammonium.

This knowledge helps in predicting and understanding chemical reactivity in various reactions.

Insoluble Salts

Insoluble salts play a significant role in precipitation reactions as they are the compounds that actually form the precipitate. When certain ions in a solution combine to form a compound that is not soluble in water, an insoluble salt is formed, leading to precipitation. Let's explore this concept further.

Consider the compound lead(II) chloride, \( ext{PbCl}_2\). It forms when \( ext{Pb}^{2+}\) ions meet \( ext{Cl}^-\) ions. Although both ions are separately soluble with other ions in many cases, their combination here results in a compound that is not very soluble in water. This reduced solubility causes the formation of a solid precipitate.

• The insolubility of \( ext{PbCl}_2\) is a key reason why a precipitate forms when lead(II) nitrate and sodium chloride solutions mix.
• By contrast, when the same \( ext{Cl}^-\) ions meet with \( ext{Ba}^{2+}\) ions, the resulting compound \( ext{BaCl}_2\) is soluble in water due to different chemical properties, and no precipitate forms.

Understanding which salts are insoluble helps predict the outcome of chemical reactions and is beneficial for identifying unknown substances in qualitative analysis.

Chemical Reactivity

Chemical reactivity refers to how substances chemically interact to form new substances. In precipitation reactions, chemical reactivity is driven by the formation of insoluble products. Let's dive deeper into this concept.

In the reaction between \( ext{Pb(NO}_3 ext{)}_2\) and \( ext{NaCl}\), the strong ionic bonds between \( ext{Pb}^{2+}\) and \( ext{Cl}^-\) result in a more stable, albeit less soluble product: \( ext{PbCl}_2\). This formation drives the reaction and results in a visible change—a solid precipitate appearing in the solution. This is a direct consequence of the chemical reactivity between lead(II) ions and chloride ions.

• Reactivity is influenced by several factors, including the concentration of reactants, temperature, and the nature of the ions involved.
• The difference in reactivity between \( ext{Pb}^{2+}\) with \( ext{Cl}^-\) compared to \( ext{Ba}^{2+}\) with \( ext{Cl}^-\) highlights the unique chemical behavior and interactions of different substances.

This understanding helps chemists predict reactions and design experiments to synthesize new compounds or break down existing ones.