Q22.29P - Chemistry: Molecular Nature Of Matter And Change

Question

(a) What is a kinetic isotope effect? 

(b) Do compounds of hydrogen exhibit a relatively large or small kinetic isotope effect? Explain. 

(c) Carbon compounds also exhibit a kinetic isotope effect. How do you expect it to compare in magnitude with that for hydrogen compounds? Why?

Step-by-Step Solution

Answer

1. Deuterium is isolated using the isotope effect.
2. The mass ratio of the isotopes is larger for H than for any other atom, hydrogen compounds exhibit a relatively large kinetic isotope effect.
3. It is stronger and harder to break. So, carbon compound exhibit relatively low kinetic isotope effect.

Step 1: Definition of Concept

Oxidation is the process by which a molecule, atom, or ion loses an electron in a chemical reaction.

Reduction is the process by which a molecule, atom, or ion gains electrons from one of the atoms in a chemical reaction.

Step 2: Subpart (a)

Different bond energies result from the mass difference between two atoms, which affects reactivity. As a result, H-atom bonded to a given atom vibrates at a higher frequency than a D atom, resulting in a higher energy bond to H. As a result, it is weaker and easier to defeat break. 

Therefore, any reaction that includes breaking a bond to hydrogen in the rate-determining step occurs faster with H than with D. This phenomenon is called a kinetic isotope effect; No other element displays a kinetic isotope effect nearly as large.

Step 3: Subpart (b)

${}^{\text{2}}\text{H}$  has two times the mass of  ${}^{\text{1}}\text{H}$ , whereas ${}^{\text{13}}\text{C}$  has only  $\text{1.08}$ times the mass of  ${}^{\text{12}}\text{C}$. Since the mass ratio of the isotopes is larger for H than for any other atom, hydrogen compounds exhibit a relatively large kinetic isotope effect.

Step 4: Subpart (c)

${}^{\text{13}}\text{C}$  has only  $\text{1.08}$ times the mass of ${}^{\text{12}}\text{C}$ . The mass difference leads to different bond energies, which affect reactivity. As a result,  ${\text{a}}^{\text{12}}\text{C}$  atom bonded to a given atom vibrates at a lower frequency than ${}^{\text{13}}\text{C}$  atom does, so the bond to ${}^{\text{12}}\text{C}$  is lower in energy. Thus, it is stronger and harder to break. Hence, carbon compounds exhibit a relatively low kinetic isotope effect.