In inorganic salt analysis, for the indicator test of Cl-, conc. H2SO4 is used. This is because:
Conc. H2SO4 has lower volatility than HCl.
Understanding the Chloride Test and Sulfuric Acid's Role
The indicator test for chloride ions ($\text{Cl}^-$) in inorganic salt analysis is a common qualitative analysis procedure. This test typically involves heating a small amount of the solid salt with concentrated sulfuric acid ($\text{H}_2\text{SO}_4$). A pungent-smelling gas, hydrogen chloride ($\text{HCl}$), is evolved, which forms dense white fumes when a glass rod dipped in aqueous ammonia is brought near the mouth of the test tube.
The Chemical Reaction
When a chloride salt (like $\text{NaCl}$) reacts with concentrated sulfuric acid, a double displacement reaction occurs:
$\text{NaCl (s)} + \text{H}_2\text{SO}_4 \text{(conc.)} \rightarrow \text{NaHSO}_4 \text{(s)} + \text{HCl (g)}$
The gaseous $\text{HCl}$ produced is then identified.
Why Concentrated $\text{H}_2\text{SO}_4$ is Used for Chloride Test
The choice of concentrated sulfuric acid for this test is crucial. Several properties of concentrated $\text{H}_2\text{SO}_4$ are important, but the primary reason it's used to displace $\text{HCl}$ from a chloride salt is related to its volatility.
- Concentrated sulfuric acid is a strong acid.
- It is also a strong dehydrating agent.
- It is a relatively non-volatile acid compared to $\text{HCl}$.
Acids with lower volatility can displace more volatile acids from their salts. In this case, concentrated $\text{H}_2\text{SO}_4$ has a significantly lower volatility than $\text{HCl}$. Therefore, it can react with a solid chloride salt to liberate gaseous $\text{HCl}$, which is more volatile and escapes, driving the reaction forward.
Analyzing the Given Options
Let's look at why the other options are not the primary reason for using concentrated $\text{H}_2\text{SO}_4$ in this specific test:
- Option 1: Conc. $\text{H}_2\text{SO}_4$ is a strong oxidising agent and $\text{Cl}_2$ is produced. While concentrated $\text{H}_2\text{SO}_4$ is an oxidizing agent, the primary reaction in the chloride test is not oxidation to produce $\text{Cl}_2$ gas under typical conditions. Oxidation of $\text{HCl}$ to $\text{Cl}_2$ usually requires stronger oxidizing agents or heating with concentrated $\text{H}_2\text{SO}_4$, which is not the main principle behind the standard chloride test.
- Option 2: Conc. $\text{H}_2\text{SO}_4$ is a strong reducing agent and $\text{HCl}$ is produced. Concentrated $\text{H}_2\text{SO}_4$ is an oxidizing agent, not a strong reducing agent. This statement is incorrect.
- Option 3: Conc. $\text{H}_2\text{SO}_4$ is a strong dehydrating agent and $\text{Cl}_2$ is produced. Concentrated $\text{H}_2\text{SO}_4$ is indeed a strong dehydrating agent, but this is not the primary reason it's used to liberate $\text{HCl}$. The liberation of $\text{HCl}$ is a displacement reaction based on volatility. Also, $\text{Cl}_2$ is not the primary product in the standard test conditions.
- Option 4: Conc. $\text{H}_2\text{SO}_4$ has lower volatility than $\text{HCl}$. This is the correct reason. The lower volatility of concentrated $\text{H}_2\text{SO}_4$ allows it to displace the more volatile $\text{HCl}$ from the chloride salt, causing $\text{HCl}$ gas to evolve and be detected.
Conclusion
The use of concentrated sulfuric acid in the indicator test for chloride ions is primarily due to its low volatility relative to $\text{HCl}$. This property enables it to effectively displace $\text{HCl}$ from chloride salts, facilitating the detection of the chloride ion.
Revision Table: Key Properties
| Property | Concentrated $\text{H}_2\text{SO}_4$ | $\text{HCl}$ | Relevance to Chloride Test |
|---|---|---|---|
| Volatility | Low | High (as gas) | Enables displacement of $\text{HCl}$ gas from salt. |
| Acidity | Strong | Strong | Required to react with salt. |
| Oxidizing Power | Strong (especially hot) | Weak (can be oxidized) | Not the primary principle for $\text{HCl}$ liberation. |
| Dehydrating Power | Strong | - | Important in other contexts, but not main reason for $\text{HCl}$ liberation. |
Additional Information: Qualitative Analysis Concepts
Qualitative inorganic analysis involves identifying the ions present in an unknown sample. Various tests are performed based on the unique chemical properties of different ions.
- Displacement Reactions: Acids can displace weaker or more volatile acids from their salts. This principle is widely used in qualitative analysis. For example, concentrated $\text{H}_2\text{SO}_4$ is used to test for chlorides, bromides, iodides, nitrates, and acetates because it is less volatile than the corresponding acids ($\text{HCl}$, $\text{HBr}$, $\text{HI}$, $\text{HNO}_3$, $\text{CH}_3\text{COOH}$) or can cause decomposition leading to characteristic products ($\text{HNO}_3$, $\text{CH}_3\text{COOH}$).
- Indicator Tests: These tests use specific reagents or conditions that produce a visible change (like color, precipitate, or gas evolution) indicating the presence of a particular ion. The chloride test using concentrated $\text{H}_2\text{SO}_4$ and subsequent identification of $\text{HCl}$ gas is an example.
- Volatility: The tendency of a substance to vaporize. Acids with high volatility (like $\text{HCl}$) exist as gases at relatively low temperatures, while non-volatile acids (like $\text{H}_2\text{SO}_4$) have high boiling points and low vapor pressure.
Important Questions from p-block Elements
Second most abundant element in alloy misch metal is:
Match List-I with List-II:
List-I List-II (A) Gel (I) Hair cream (B) Foam (II) Dust (C) Emulsion (III) Cheese (D) Aerosol (IV) Whipped cream Choose the correct answer from the options given below:
Rate of a reaction changes from 2.48 × 10⁻³ mol⁻¹ sec⁻¹ to 4.96 × 10⁻³ mol⁻¹ sec⁻¹ when concentration of reactant is changed from 0.6 M to 2.4 M respectively, the order of reaction is:
Degree of dissociation, when molar conductivity of X at its concentration C is 24.14 and its limiting molar conductivity is 48.28 will be:
A divalent ion of 'V' (Atomic no. 23) in aqueous solution is:
