Question

Which of the following properties of hydrogen is incorrect? (a) Like halogens, hydrogen exists as a diatomic gas. (b) Like halogens, hydrogen exhibits \(-1\) oxidation state in its compounds with metals. (c) Like halogens, hydrogen is liberated at cathode. (d) The ionisation energy of hydrogen is quite close to halogens.

Short answer

The incorrect property of hydrogen is that hydrogen is liberated at the cathode (Option (c)).

Step-by-step solution

Evaluate Statement (a)

Determine if hydrogen indeed exists as a diatomic gas, similar to halogens. Most of the halogens (group 17 elements) exist in diatomic form (e.g., F2, Cl2), and hydrogen also exists as H2, which is diatomic.

Evaluate Statement (b)

Analyze if hydrogen exhibits a -1ewline) oxidation state in its compounds with metals. Hydrogen can indeed exhibit -1ewline) oxidation state, typically when it forms hydrides with metals, similar to halides formed by halogens.

Evaluate Statement (c)

Investigate the electrolysis process to find out where hydrogen is liberated. During electrolysis, hydrogen is not liberated at the cathode when its salts are electrolyzed; it is halogens that are liberated at the anode. Therefore, this statement is incorrect.

Evaluate Statement (d)

Compare the ionization energies of hydrogen and halogens. While the ionization energy of hydrogen is significant, it is not as close to halogens since halogens have higher electron affinities and lower ionization energies due to their desire to gain an electron to achieve a noble gas configuration.

Key concepts

Diatomic Gas

Hydrogen, often compared with the halogens in group 17 of the periodic table, exists naturally as a diatomic gas. This means that two hydrogen atoms bond together to form the hydrogen molecule, represented as H₂. Diatomic molecules consist of two atoms, either of the same or different chemical elements. The hydrogen molecule is the simplest and most abundant form of a diatomic gas in the universe. It's important to note that this diatomic nature contributes to the stability of the hydrogen molecule through a shared pair of electrons, completing the valence shell to mimic the noble gas configuration.

Understanding this property is crucial for grasping why hydrogen exhibits certain chemical behaviors. In the context of our exercise, statement (a) correctly identifies that hydrogen, like halogens, exists as a diatomic gas.

Oxidation State of Hydrogen

The oxidation state, or oxidation number, is a concept in chemistry that allows us to keep track of electron movement in chemical reactions. Hydrogen typically has an oxidation state of +1 in most of its compounds. However, it can also exhibit an oxidation state of -1 when it forms compounds known as hydrides with metals. This is similar to halogens, which typically have an oxidation state of -1 in their compounds.

For example, in lithium hydride (LiH), hydrogen has an oxidation state of -1. This is analogous to how chlorine would have an oxidation state of -1 in sodium chloride (NaCl). The -1 oxidation state reveals that hydrogen, in this instance, is gaining an electron, which mirrors the behavior of halogens in their respective compounds. Statement (b) is accurate in asserting that hydrogen can exhibit a -1 oxidation state in compounds with metals.

Ionisation Energy of Hydrogen

Ionisation energy is the amount of energy required to remove the most loosely bound electron from an atom to form a cation. It is a critical property that gives insight into an element's reactivity and chemical behavior. Hydrogen has a notable ionisation energy because it has only one electron in its outer shell. Although hydrogen's ionisation energy is high, it remarkably deviates from those of halogen gases.

The halogens have lower ionisation energies and higher electron affinities, as they tend to gain an electron to achieve a stable octet structure akin to that of noble gases. Hydrogen, on the other hand, lacks this tendency to gain an electron as it is positioned in group 1 of the periodic table. Therefore, statement (d) is misleading because, while hydrogen has a significant ionisation energy, it is not 'quite close' to that of halogens.

Electrolysis

Electrolysis is a process that uses electric current to drive a non-spontaneous chemical reaction. This technique is often employed to decompose chemical compounds, in particular, to extract elements from their naturally occurring sources. For instance, electrolysis of water leads to the liberation of hydrogen gas at the cathode (negative electrode) and oxygen gas at the anode (positive electrode).

Contrary to halogens, which are liberated at the anode during the electrolysis of their salts, hydrogen is not liberated at the cathode under analogous conditions for halogen salts. This understanding is vital when considering experimental setups and predicting the outcome of electrolysis processes. Therefore, statement (c) is incorrect as it erroneously states that hydrogen, like halogens, is liberated at the cathode during electrolysis.