Question

The rate of the substitution reaction of \([Co(CN)_5Cl]^3\)\(^-\) with OH\(^-\) to give \([Co(CN)_5(OH)]^3\)\(^-\) follows: 
 

• A. depends on the concentrations of both [Co(CN)\(_5\)Cl]\(^3\)\(^-\) and OH\(^-\)
• B. depends on the concentration of [Co(CN)\(_5\)Cl]\(^3\)\(^-\) only
• C. is directly proportional to the concentration of OH\(^-\) only
• D. is inversely proportional to the concentration of OH\(^-\)

Hint:

In substitution reactions, the rate is often determined by the concentration of the complex being attacked by the nucleophile.

Answer 1

The Correct Option is B

The rate of substitution reactions in coordination chemistry often depends on the concentration of the complex undergoing substitution and the nucleophile involved. In this case, the reaction involves the substitution of chloride (Cl\(^-\)) by hydroxide (OH\(^-\)) in the complex \([Co(CN)_5Cl]^{3-}\) to form \([Co(CN)_5(OH)]^{3-}\). To understand how the rate of this reaction depends on the concentrations, we need to consider the following:

Step 1: Identifying the nucleophile and the complex.
- The complex \([Co(CN)_5Cl]^{3-}\) contains a chloride ion (Cl\(^-\)) that is substituted by OH\(^-\), which is the nucleophile in the reaction.
- The rate of the reaction will primarily depend on the availability of the complex \([Co(CN)_5Cl]^{3-}\) and the nucleophile OH\(^-\). The more of the complex present, the more collisions there will be between the complex and OH\(^-\), increasing the rate of the substitution.

Step 2: Understanding the mechanism.
In most substitution reactions in coordination compounds, the rate is governed by the concentration of the complex undergoing substitution. For this reaction, OH\(^-\) acts as a nucleophile, but the reaction rate is primarily dependent on the concentration of the complex \([Co(CN)_5Cl]^{3-}\). This is because the substitution occurs when OH\(^-\) attacks the metal center, displacing the chloride ion. The rate law for such a substitution would be first order with respect to \([Co(CN)_5Cl]^{3-}\), meaning the reaction rate increases with the concentration of the complex.

Step 3: Conclusion.
Since the rate of the substitution reaction is dependent on the concentration of the complex \([Co(CN)_5Cl]^{3-}\), the correct answer is (B).