Question

Select the correct statements: (1) \(\mathrm{H}_{2}\) is thermally stable (2) Ionisation of \(\mathrm{CH}_{3} \mathrm{COOH}\) is slower than that of \(\mathrm{CH}_{3} \mathrm{COOD}\) (3) Kinetic isotopic effect is observed when there is retardation in the rate when \(\mathrm{H}_{2} \mathrm{O}\) is replaced by \(\mathrm{D}_{2} \mathrm{O}\) (4) \(\mathrm{H}^{+}\) can exist as \(\mathrm{H}_{4} \mathrm{O}_{4}^{+}\) in water (a) 1,2 and 3 (b) 1,2 and 4 (c) 1,3 and 4 (d) 2,3 and 4

Short answer

Correct statements are (1) and (3), so option (c) 1, 3 and 4 is partially correct.

Step-by-step solution

Analyzing Statement 1

The hydrogen molecule \(\mathrm{H}_2\) is known for its stability under normal conditions due to the strong covalent bond between the two hydrogen atoms. Therefore, \(\mathrm{H}_2\) is indeed thermally stable.

Analyzing Statement 2

The ionization of acetic acid \(\mathrm{CH}_3\mathrm{COOH}\) compared to its deuterated version \(\mathrm{CH}_3\mathrm{COOD}\) often involves similar mechanisms, but deuterium has a lower zero-point energy compared to hydrogen. Typically, the ionization of the deuterated compound is slightly slower due to the kinetic isotope effect.

Analyzing Statement 3

The kinetic isotopic effect is observed when a chemical reaction involving isotopic substitution results in a change in reaction rate. Replacing \(\mathrm{H}_2\mathrm{O}\) with \(\mathrm{D}_2\mathrm{O}\), where deuterium replaces hydrogen, often slows down the reaction, confirming the statement.

Analyzing Statement 4

In aqueous solutions, \(\mathrm{H}^+\) ions do not exist as free ions but often associate with water to form complex ions. However, the configuration \(\mathrm{H}_4\mathrm{O}_4^+\) is not a commonly accepted or standard species; hence, the statement is incorrect.

Key concepts

Kinetic Isotope Effect

The Kinetic Isotope Effect (KIE) is a phenomenon where the rate of a chemical reaction is affected by the presence of different isotopes. This occurs because isotopes have different masses. Since reactions involve breaking and forming chemical bonds, the precise mass of the atoms involved can influence how those reactions occur. When you replace hydrogen (\(\mathrm{H}\)) with its isotope deuterium (\(\mathrm{D}\)), you see the most noticeable effects. Deuterium has one additional neutron compared to hydrogen, which increases its mass noticeably. This mass difference influences the chemical properties and how reactions proceed.

• Heavier atoms move more slowly and form weaker bonds than lighter ones.
• This results in a change in zero-point energy, which in turn affects reaction rate.

For example, when \(\mathrm{H}_2\mathrm{O}\) is replaced by \(\mathrm{D}_2\mathrm{O}\) in a reaction, the rate often slows down. This is what is observed in statement 3 of the original exercise. The slower reaction is due to the increased mass of deuterium, illustrating the KIE. Understanding KIE is crucial in fields such as biochemistry and pharmacology, where reaction rates can significantly influence biological processes.

Deuteration

Deuteration is the process of replacing hydrogen atoms in compounds with deuterium. Deuterium is a stable isotope of hydrogen that contains one neutron in addition to the single proton found in hydrogen. This subtle change can significantly affect the physical and chemical properties of a molecule.

• Deuteration often reduces the zero-point energy of bonds.
• This change leads to a decrease in bond vibrational energy, which can consequentially slow down reaction rates.

For example, in the ionization of acetic acid (\(\mathrm{CH}_3\mathrm{COOH}\)) compared to its deuterated version (\(\mathrm{CH}_3\mathrm{COOD}\)), the introduction of deuterium results in a phenomena akin to the Kinetic Isotope Effect. The result is a slower reaction rate for the deuterated compound, as seen in statement 2. This knowledge is valuable in chemical synthesis and pharmaceuticals for controlling reaction rates or investigating reaction mechanisms.

Hydrogen Ion Association

Hydrogen ions \((\mathrm{H}^+)\) in aqueous solutions are not present as naked protons. Instead, they associate with water molecules to form complexes due to the high charge density and small size of the proton. This interaction results in the formation of hydronium ions \((\mathrm{H}_3\mathrm{O}^+)\), which are commonly found in acidic solutions.While \(\mathrm{H}^+\) ions can associate with water to form more complex ions, certain proposed configurations like \(\mathrm{H}_4\mathrm{O}_4^+\) don't typically exist. Such forms are not standard and aren't recognized in usual chemical contexts or literature.

• Commonly accepted configurations include \(\mathrm{H}_3\mathrm{O}^+\) (hydronium ion) and solvated forms like \((\mathrm{H}_2\mathrm{O})\) \(\mathrm{H}_3\mathrm{O}^+\).
• These ions play a crucial role in acid-base chemistry, influencing pH and reactivity in solutions.

Understanding how \(\mathrm{H}^+\) associates with water can help in grasping broader concepts in chemistry such as acidity and electrolyte reactions, as shown in statement 4 of the exercise, which is deemed incorrect.