CBSE Class 11 Chemistry Chapter 9 Hydrogen NCERT Solutions

Hydrogen, with the electronic configuration $1s^1$, holds a unique position in the periodic table. Its single valence electron allows it to exhibit characteristics similar to both alkali metals (Group 1) and halogens (Group 17), leading to a dual behaviour.

Resemblance with Alkali Metals (Group 1):

1. Like alkali metals, hydrogen has one electron in its outermost shell ($1s^1$). It can lose this electron to form a unipositive ion, $H^+$, similar to $Li^+$ ([He] 2s⁰) or $Na^+$ ([Ne] 3s⁰). For example: $2H\; \backslash rightarrow\; 2H^+\; +\; 2e^-$.
2. Hydrogen combines with electronegative elements to form compounds analogous to those formed by alkali metals. For instance, it forms oxides (like $H\_2O$, similar to $Na\_2O$), halides (like $HCl$, similar to $NaCl$), and sulphides (like $H\_2S$, similar to $Na\_2S$).

Resemblance with Halogens (Group 17):

1. Both hydrogen and halogens need one electron to achieve a stable noble gas configuration. Hydrogen requires one electron to become like Helium ($1s^2$), while halogens require one electron to complete their octet (e.g., Fluorine $1s^2\; 2s^2\; 2p^5$ needs one electron to become $1s^2\; 2s^2\; 2p^6$). Consequently, hydrogen can gain one electron to form a uninegative ion, $H^-$ (hydride ion), similar to halide ions like $F^-$ or $Cl^-$. For example: $H\_2\; +\; 2e^-\; \backslash rightarrow\; 2H^-$.
2. Hydrogen forms diatomic molecules ($H\_2$) and shares electrons to form covalent compounds with non-metals, much like halogens do (e.g., $HCl$, $HBr$).

Differences and Unique Position:

Despite these similarities, hydrogen differs significantly. It lacks metallic characteristics, has a much higher ionization enthalpy compared to alkali metals, and is less reactive than halogens. Furthermore, the $H^+$ ion is extremely small and cannot exist freely; it is always associated with other molecules. Due to this unique combination of properties and its inability to be definitively placed with either group, hydrogen is often placed separately at the top of the periodic table.

Hydrogen has three naturally occurring isotopes, distinguished by the number of neutrons in their nucleus:

1. Protium ($^1H$): This is the most common isotope, consisting of one proton and one electron. Its nucleus has no neutrons.
2. Deuterium ($^2H$ or D): This isotope consists of one proton and one neutron in its nucleus, along with one electron.
3. Tritium ($^3H$ or T): This isotope consists of one proton and two neutrons in its nucleus, along with one electron. Tritium is radioactive.

The mass of these isotopes is primarily determined by the number of protons and neutrons. Since Protium has a mass number of 1, Deuterium has a mass number of 2, and Tritium has a mass number of 3, their mass ratio is:

Mass Ratio of Protium : Deuterium : Tritium = 1 : 2 : 3

Under normal conditions, hydrogen exists predominantly as a diatomic molecule ($H\_2$) rather than as individual atoms (H) primarily due to its high ionization enthalpy and the stability gained by forming a covalent bond.

The ionization enthalpy of a hydrogen atom is very high ($1312\; \backslash text\{\; kJ\; mol\}^\{-1\}$). This means a significant amount of energy is required to remove its single electron to form a $H^+$ ion. Consequently, hydrogen atoms do not readily exist in the monoatomic form as $H^+$ ions.

Instead, two hydrogen atoms can achieve a more stable state by sharing their electrons to form a strong covalent bond. Each hydrogen atom contributes its single electron to form a shared pair, resulting in the $H\_2$ molecule where each atom effectively achieves the stable electronic configuration of Helium ($1s^2$).

Therefore, the formation of the stable $H-H$ covalent bond makes the diatomic form ($H\_2$) energetically more favorable than the monoatomic form under normal conditions.

Coal gasification is a process where coal reacts with steam at high temperatures to produce a mixture of gases known as water gas, which contains carbon monoxide ($CO$) and dihydrogen ($H\_2$). The primary reaction is:

$$C\_\{(s)\}\; +\; H\_2O\_\{(g)\}\; \backslash xrightarrow\{1270\; \backslash text\{\; K\}\}\; CO\_\{(g)\}\; +\; H\_\{2(g)\}$$

(Coal) (Steam) (Carbon Monoxide) (Dihydrogen)

To increase the yield of dihydrogen ($H\_2$) from this process, the carbon monoxide ($CO$) produced can be further reacted with more steam. This is achieved through the water-gas shift reaction. This reaction is typically carried out in the presence of a catalyst, such as iron chromate, at elevated temperatures.

The water-gas shift reaction is represented as:

$$CO\_\{(g)\}\; +\; H\_2O\_\{(g)\}\; \backslash xrightarrow\{\backslash text\{Catalyst,\; Heat\}\}\; CO\_2\_\{(g)\}\; +\; H\_\{2(g)\}$$

(Carbon Monoxide) (Steam) (Carbon Dioxide) (Dihydrogen)

In this reaction, the carbon monoxide reacts with steam to produce additional dihydrogen and carbon dioxide. By effectively converting the $CO$ into more $H\_2$, the overall production of dihydrogen from the initial coal gasification process is significantly increased.