CBSE Class 11 Chemistry Chapter 9: Hydrogen NCERT Solutions

Hydrogen, with the electronic configuration $1s^1$, is the first element in the periodic table. Its single valence electron allows it to exhibit dual behavior, showing similarities to both alkali metals (Group 1) and halogens (Group 17).

Similarities with Alkali Metals:

1. Like alkali metals, hydrogen has one electron in its outermost shell ($1s^1$), similar to Lithium ($[He]\; 2s^1$) and Sodium ($[Ne]\; 3s^1$). It can lose this electron to form a unipositive ion ($H^+$).
2. Hydrogen combines with electronegative elements to form compounds analogous to those formed by alkali metals, such as oxides (e.g., $H\_2O$), halides (e.g., $HCl$), and sulphides (e.g., $H\_2S$).

Similarities with Halogens:

1. Both hydrogen and halogens need one electron to complete their outermost shell. Hydrogen requires one electron to achieve the stable helium configuration ($1s^2$), while halogens need one electron to complete their octet.
2. Hydrogen can gain one electron to form a uninegative ion ($H^-$), similar to halide ions (e.g., $F^-$, $Cl^-$).
3. Like halogens, hydrogen exists as a diatomic molecule ($H\_2$) and forms numerous covalent compounds.

Differences and Justification for Separate Placement:

Despite these similarities, hydrogen differs significantly. It lacks metallic characteristics unlike alkali metals. Its ionization enthalpy ($1312\; \backslash text\{\; kJ\; mol\}^\{-1\}$) is very high compared to alkali metals. Also, it is less reactive than halogens. Furthermore, the $H^+$ ion is extremely small and cannot exist freely; it is always associated with other atoms or molecules. Due to these distinct properties and the instability of the free $H^+$ ion, hydrogen is best placed separately in the periodic table, often above Group 1.

Hydrogen has three naturally occurring isotopes, each differing in the number of neutrons:

1. Protium ($^1H$): This is the most common isotope, consisting of one proton and no neutrons. Its nucleus is just a single proton.
2. Deuterium ($^2H$ or $D$): This isotope consists of one proton and one neutron. It is also known as 'heavy hydrogen'.
3. Tritium ($^3H$ or $T$): This isotope consists of one proton and two neutrons. It is radioactive.

The mass of an atom is primarily determined by the number of protons and neutrons in its nucleus. 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:

Protium : Deuterium : Tritium = 1 : 2 : 3

Under normal conditions, hydrogen exists predominantly as a diatomic molecule ($H\_2$) rather than as individual monoatomic hydrogen atoms (H) due to its high ionization enthalpy. The ionization enthalpy of hydrogen is $1312\; \backslash text\{\; kJ\; mol\}^\{-1\}$, which is a very large amount of energy required to remove its single electron. Consequently, forming a stable $H^+$ ion is energetically unfavorable.

Instead of existing as separate atoms, two hydrogen atoms achieve a more stable electronic configuration by sharing their electrons. Each hydrogen atom contributes its single electron to form a shared pair, creating a strong covalent bond ($H-H$). This results in the formation of the $H\_2$ molecule, where each hydrogen atom effectively has a stable $1s^2$ configuration, similar to helium.

Coal gasification is a process where coal reacts with steam at high temperatures to produce a mixture of carbon monoxide and dihydrogen, known as water gas.

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

To increase the yield of dihydrogen from this process, the water-gas shift reaction is employed. In this reaction, the carbon monoxide (CO) produced is reacted with more steam at a moderate temperature, typically in the presence of an iron(III) oxide-chromium(III) oxide ($Fe\_2O\_3/Cr\_2O\_3$) catalyst.

$$CO\_\{(g)\}\; +\; H\_2O\_\{(g)\}\; \backslash xrightarrow\{\backslash text\{Catalyst,\; ~673\; K\}\}\; CO\_2\_\{(g)\}\; +\; H\_\{2(g)\}$$

This reaction converts the carbon monoxide into carbon dioxide, producing an additional mole of dihydrogen for every mole of CO consumed. This significantly enhances the overall production of dihydrogen from the initial coal gasification process.