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Question #44799

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The Question

A kinetic investigation of the oxidation of benzaldehyde ( $\ce{C6H5CHO}$ ) by Tollens' reagent ( $\ce{[Ag(NH3)2]+}$ ) in an aqueous alkaline medium at 298 K reveals that the empirical rate of deposition of the silver mirror is given by:

$\frac{d[\ce{Ag}]}{dt} = k_{obs}[\ce{C6H5CHO}][\ce{[Ag(NH3)2]+}]$

In a parallel experiment, the oxidation of benzaldehyde is attempted using Fehling's solution under identical thermal and solvent conditions, with the rate of formation of the red precipitate ( $\ce{Cu2O}$ ) denoted as $R_{\ce{Cu}}$ .

Accounting for the exact stoichiometric electron transfer balance and specific reagent reactivity, which of the following correctly represents the rate of disappearance of hydroxide ions ( $-\frac{d[\ce{OH-}]}{dt}$ ) in the Tollens' reaction, and the value of $R_{\ce{Cu}}$ ?

Options

A.
$1.5 k_{obs}[\ce{C6H5CHO}][\ce{[Ag(NH3)2]+}]$ and $R_{\ce{Cu}} = 0$
B.
$1.0 k_{obs}[\ce{C6H5CHO}][\ce{[Ag(NH3)2]+}]$ and $R_{\ce{Cu}} = 0$
C.
$1.5 k_{obs}[\ce{C6H5CHO}][\ce{[Ag(NH3)2]+}]$ and $R_{\ce{Cu}} = k'_{obs}[\ce{C6H5CHO}][\ce{Cu^2+}]$
D.
$0.67 k_{obs}[\ce{C6H5CHO}][\ce{[Ag(NH3)2]+}]$ and $R_{\ce{Cu}} = 0$

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Correct Answer

Step-by-Step Solution

Explanation

How the Examiner Thinks

Numerical_Precision

Improve calculation stamina and avoid rounding errors in high-stakes competitive examinations.

Concepts Behind This Question

Stoichiometric Coefficient Division|Tollens vs. Fehling Specificity

Why This Question Is Tricky

Common Mistakes

The problem tests two major traps.

First, the 'Tollens vs Fehling Specificity' trap: students often assume all aldehydes react with Fehling's, forgetting that aromatic aldehydes (like benzaldehyde) are exceptions due to the lack of alpha-hydrogens and the lower oxidation potential of Fehling's reagent.

Second, the 'Stoichiometric Coefficient Division' trap in kinetics: students frequently fail to balance the complex Tollens' redox reaction, specifically missing the 3:2 ratio between $\ce{OH-}$ consumed and $\ce{Ag}$ produced.

They might incorrectly use the n-factor (2 for aldehyde, 1 for Ag) directly as stoichiometric coefficients or assume a 1:1 ratio, leading to incorrect differential rate expressions.

Quick Facts About This Question

Question Information

Subject	Chemistry
Chapter	Chemical Kinetics - Class 12
Concept	Stoichiometric Coefficient Division
Difficulty	8
Cognitive Load	7
Target Solve Time	160 sec
Access	Free