The following reactions are carried out with HCl(aq)...

The following reactions are carried out with HCl(aq) containing some tritium ( $_{1}^{3} \mathrm{H}$ ) as a tracer. Would you expect any of the tritium radioactivity to appear in the $\mathrm{NH}_{3}(\mathrm{g}) ?$ In the $\mathrm{H}_{2} \mathrm{O} ?$ Explain. $$\begin{array}{c} \mathrm{NH}_{3}(\mathrm{aq})+\mathrm{HCl}(\mathrm{aq}) \longrightarrow \mathrm{NH}_{4} \mathrm{Cl}(\mathrm{aq}) \\ \mathrm{NH}_{4} \mathrm{Cl}(\mathrm{aq})+\mathrm{NaOH}(\mathrm{aq}) \longrightarrow\\\mathrm{NaCl}(\mathrm{aq})+\mathrm{H}_{2} \mathrm{O}(1)+\mathrm{NH}_{3}(\mathrm{g})\end{array}$$

The following reactions are carried out with HCl(aq) containing some tritium ( $_{1}^{3} \mathrm{H}$ ) as a tracer. Would you expect any of the tritium radioactivity to appear in the $\mathrm{NH}_{3}(\mathrm{g}) ?$ In the $\mathrm{H}_{2} \mathrm{O} ?$ Explain. $$\begin{array}{c}
\mathrm{NH}_{3}(\mathrm{aq})+\mathrm{HCl}(\mathrm{aq}) \longrightarrow \mathrm{NH}_{4} \mathrm{Cl}(\mathrm{aq}) \\
\mathrm{NH}_{4} \mathrm{Cl}(\mathrm{aq})+\mathrm{NaOH}(\mathrm{aq}) \longrightarrow\\\mathrm{NaCl}(\mathrm{aq})+\mathrm{H}_{2} \mathrm{O}(1)+\mathrm{NH}_{3}(\mathrm{g})\end{array}$$

Key Concepts

Isotopic Labeling

Isotopic labeling involves incorporating a distinctive isotope into a molecule so that its pathway through a chemical reaction can be traced. In a reaction, if a particular atom (for example, a hydrogen atom) is replaced by its radioactive isotope, one can follow which products incorporate that isotope, thereby providing insight into the mechanism of the reaction.

Acid?Base Reactions

Acid–base reactions are processes in which a proton (H+) is transferred from an acid to a base. In these reactions, the acid donates a proton to the basic species, forming a conjugate base and a conjugate acid. The movement of the labeled proton through the reaction pathway is central to understanding the fate of the tracer.

Proton Exchange in Aqueous Media

In aqueous solutions, protons are highly mobile and can undergo rapid exchange among different species, including water molecules and ions such as ammonium. This dynamic proton exchange means that any originally labeled (tritiated) proton can quickly become randomly distributed within the solvent, diminishing its retention on a specific molecule.

Chemical Equilibration and Isotopic Scrambling

Chemical equilibration in systems with rapid proton exchange leads to isotopic scrambling, where the labeled protons do not remain confined to the initially targeted species. In reactions where the intermediate undergoes such exchange with water, the final products, particularly those returning to solution, will have the label dispersed, often resulting in negligible retention of the isotope in species removed from the solution phase.

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