Question

The isotopes of hydrogen have different physical properties due to difference in mass. They have almost same chemical properties with a difference in their rates of reactions which is mainly due to (a) their different enthalpy of bond dissociation (b) different electronic configurations (c) different atomic masses (d) different physical properties.

Short answer

The rates of reactions vary in isotopes of hydrogen mainly due to (c) different atomic masses.

Step-by-step solution

Understanding Isotopes and Chemical Properties

Isotopes of an element have the same number of protons and electrons but different numbers of neutrons, leading to different atomic masses. This does not affect the electronic configuration, which is responsible for chemical properties. Therefore, isotopes generally have the same chemical properties.

Considering the Factors that Affect Reaction Rates

The mass of an isotope can affect the rate of chemical reactions due to the kinetic isotope effect. This effect happens because isotopes have different masses, which can influence the vibration frequency of bonds involving these isotopes, affecting the bond dissociation energy and thus the reaction rate.

Eliminating Incorrect Options

Since the isotopes have the same electronic configuration (option b) and varying rates of reactions are not primarily due to different enthalpy of bond dissociation (option a) or different physical properties (option d), we can deduce that the correct option directly related to the mass is different atomic masses.

Key concepts

Kinetic Isotope Effect

When studying isotopes of hydrogen, one crucial concept to grasp is the kinetic isotope effect (KIE). This phenomenon arises due to the differences in mass between isotopes. When a chemical reaction involves the breaking or forming of bonds with isotopic atoms, these mass differences can influence reaction rates. For example, bonds involving heavier isotopes, like deuterium (an isotope of hydrogen), vibrate at lower frequencies compared to those involving lighter isotopes, such as protium (the most common isotope of hydrogen).

As a result, since heavier isotopes form stronger bonds, more energy is needed to break them, which often makes chemical reactions involving heavier isotopes slower. This is particularly noticeable in hydrogen because the mass difference between its isotopes is proportionally large compared to other elements. In essence, the kinetic isotope effect helps explain why isotopically substituted compounds can react at significantly different rates.

Chemical Properties of Isotopes

Isotopes, despite having different masses, usually exhibit nearly identical chemical properties. This is because the electronic configuration, which dictates an atom's chemical behavior, remains unchanged among isotopes. They still have the same number of protons and electrons, which are the primary particles involved in chemical reactions.

To understand the exercise from the textbook, it's important to recognize that while isotopes of hydrogen—protium, deuterium, and tritium—differ in their number of neutons, hence their atomic mass, they all inhabit the same position in the periodic table and have one electron forming chemical bonds. Consequently, these isotopes tend to form the same types of bonds and participate in the same chemical reactions despite their mass differences. Nevertheless, mass can be a limiting factor for reaction rates, reinforcing the connection with the kinetic isotope effect.

Isotope Reaction Rates

Exploring isotope reaction rates further, it is evident that differences in reaction rates among isotopes are primarily due to their varying atomic masses. This aligns with the correct answer in the textbook exercise. Since the mass of an isotope can affect how fast or slow a reaction takes place, heavier isotopes generally participate in slower reactions.

For instance, if we compare the rates of reaction for protium and deuterium when they are part of a chemical reaction, we would notice that due to the greater mass of deuterium (twice the mass of protium), its reactions are typically slower. This phenomenon is explained by the fact that molecules with heavier isotopes have reduced zero-point energies leading to a higher activation energy barrier that needs to be overcome for the reaction to proceed. Consequently, when considering isotope reaction rates, it is pivotal to take into account the mass-dependent kinetic isotope effect that predominately drives the differences in reaction kinetics for isotopes.