Question

Consider the following two-step reaction: $$ \begin{aligned} \mathrm{Cu}^{2+}+\mathrm{H}_{2} & \longrightarrow \mathrm{CuH}^{+}+\mathrm{H}^{+} \\ \mathrm{CuH}^{+}+\mathrm{H}^{+}+\mathrm{H}_{2} \mathrm{C}=\mathrm{CH}_{2} & \longrightarrow \mathrm{Cu}^{2+}+\mathrm{H}_{3} \mathrm{C}-\mathrm{CH}_{3} \end{aligned} $$ (a) Identify any catalysts or intermediates. (b) Write the net reaction.

Short answer

(a) The catalyst is the Cu2+ ion and the intermediate is CuH+. (b) The net reaction is H2 + H+ + H2C=CH2 ➝ H3C-CH3.

Step-by-step solution

Identify catalysts and intermediates

Looking at the two reactions, there seems to be one possible catalyst and one possible intermediate:The Cu2+ ion starts as a reactant in the first step, becomes a product in the second step, and reappears as a reactant. This meets our definition for a catalyst.Meanwhile, CuH+ is formed as a product in the first step, then is used up as a reactant in the second step, thereby fitting the definition of an intermediate.

Write out the net reaction

To find the net reaction, add the two given reactions together:1. Cu2+ + H2 ➝ CuH+ + H+2. CuH+ + H+ + H2C=CH2 ➝ Cu2+ + H3C-CH3When these reactions are combined,Cu2+ + H2 + CuH+ + H+ + H2C=CH2 ➝ CuH+ + H+ + Cu2+ + H3C-CH3Then, cancel out any substances that appear on both sides of the reaction to leave the net reaction:H2 + H+ + H2C=CH2 ➝ H3C-CH3

Key concepts

Catalysts

In chemical reactions, catalysts are substances that speed up the rate of a reaction without being consumed in the process. They achieve this by providing an alternative reaction pathway that requires less energy. Catalysts make reactions occur more easily or more rapidly. They are crucial in many industrial and chemical processes because they can be reused.
In the given reaction, the role of a catalyst is played by the \( \mathrm{Cu}^{2+} \) ion. It is initially present as a reactant, and it reappears as a product at the end of the series of reactions. This cycling feature of being regenerated at the end of the reactions is a hallmark of catalytic behavior. By facilitating the conversion of reactants to products and returning in its original form, \( \mathrm{Cu}^{2+} \) fulfills the role of a catalyst in this sequence of reactions.

Intermediates

Intermediates are temporary species that form during the course of a reaction mechanism. They are typically produced in one step and consumed in another, making them crucial for the overall progress of a reaction but are not seen in the final net reaction. They can provide insight into the pathway a reaction takes and are often unstable or short-lived.
In the reaction presented, the \( \mathrm{CuH}^{+} \) acts as an intermediate. It is produced as a product in the first step and is consumed as a reactant in the second step. This transient nature of \( \mathrm{CuH}^{+} \) demonstrates its role as an intermediate, bridging the steps and facilitating the transformation from reactants to final products. Understanding the role of intermediates helps chemists map out detailed mechanisms that describe how starting substances are converted to final products.

Net Reaction

A net reaction simplifies the overall process by removing all intermediates and catalysts to showcase the path from reactants to products, essentially summarizing the entire reaction sequence without the detailed steps. It represents the transformation that occurs from the initial reactants to the final products while highlighting the actual chemical change.
To deduce the net reaction, we combine given reaction steps and cancel substances present on both equal sides of the reaction, such as intermediates and recyclable catalysts. For this exercise, once we combine the reactions and remove \( \mathrm{CuH}^{+} \) and \( \mathrm{Cu}^{2+} \), we get: \[ \mathrm{H}_{2} + \mathrm{H}^{+} + \mathrm{H}_{2}\mathrm{C}=\mathrm{CH}_{2} \longrightarrow \mathrm{H}_{3}\mathrm{C}-\mathrm{CH}_{3} \] This equation provides the net change and showcases the essential conversion of \( \mathrm{H}_{2} \) and \( \mathrm{H}_{2}\mathrm{C}=\mathrm{CH}_{2} \) into \( \mathrm{H}_{3}\mathrm{C}-\mathrm{CH}_{3} \), illustrating the overall chemical process.