Which one of the following statements about dihydrogen (H 2) is not correct?
A mixture of NO 2and H 2is known as Syngas.
Analyzing Dihydrogen (H<sub>2</sub>) Properties
The question asks us to identify the statement that is NOT correct about dihydrogen (H<sub>2</sub>). Let's examine each statement.
Statement 1: H<sub>2</sub> is lighter than air and insoluble in water.
Dihydrogen (H<sub>2</sub>) has a molar mass of approximately $2.016 \text{ g/mol}$. The average molar mass of air is about $28.97 \text{ g/mol}$. Since the molar mass of H<sub>2</sub> is significantly less than that of air, H<sub>2</sub> is indeed lighter than air.
Hydrogen is a nonpolar molecule. Water is a polar solvent. Nonpolar substances generally have very low solubility in polar solvents like water. Therefore, H<sub>2</sub> is largely insoluble in water.
This statement is correct regarding the properties of dihydrogen.
Statement 2: H<sub>2</sub> is inert at room temperature due to high H-H bond dissociation enthalpy.
The bond dissociation enthalpy of the H-H bond in dihydrogen is very high, approximately $435 \text{ kJ/mol}$ at $298 \text{ K}$. This high energy required to break the bond makes dihydrogen relatively unreactive at room temperature. High temperatures or the presence of a catalyst are usually needed for H<sub>2</sub> to react significantly.
This statement is correct about the inertness of dihydrogen at room temperature.
Statement 3: H<sub>2</sub> reacts with alkali metals at high temperature to yield metal hydrides.
Dihydrogen reacts with highly electropositive metals, such as alkali metals (Group 1) and alkaline earth metals (Group 2), usually at high temperatures, to form ionic hydrides. In these hydrides, hydrogen exists as the hydride ion (H<sup>-</sup>).
For example, the reaction with sodium (an alkali metal):
$\text{2Na (s) } + \text{ H}_2\text{ (g) } \xrightarrow{\text{heat}} \text{ 2NaH (s)}$
Sodium hydride (NaH) is a metal hydride.
This statement is correct about the reaction of dihydrogen with alkali metals.
Statement 4: A mixture of NO<sub>2</sub> and H<sub>2</sub> is known as Syngas.
Syngas, or synthesis gas, is a fuel gas mixture consisting primarily of carbon monoxide (CO) and hydrogen (H<sub>2</sub>). It is typically produced from coal, natural gas, or biomass. The composition can vary, but the main components are CO and H<sub>2</sub>. Sometimes, it also contains carbon dioxide (CO<sub>2</sub>) and other trace gases.
Nitrogen dioxide (NO<sub>2</sub>) is a toxic reddish-brown gas and is not a component of Syngas. A mixture of NO<sub>2</sub> and H<sub>2</sub> is not known as Syngas.
This statement is incorrect.
Based on the analysis of each statement, the incorrect statement is the one claiming that a mixture of NO<sub>2</sub> and H<sub>2</sub> is known as Syngas.
Revision Table: Dihydrogen Properties Summary
| Property/Reaction | Description | Correct? |
|---|---|---|
| Density vs Air | Lighter than air | Yes |
| Solubility in water | Insoluble | Yes |
| Reactivity at room temperature | Inert (due to high bond enthalpy) | Yes |
| Reaction with alkali metals | Forms metal hydrides at high temperature | Yes |
| Syngas composition | Mixture of CO and H<sub>2</sub> (not NO<sub>2</sub> and H<sub>2</sub>) | No |
Additional Information: Dihydrogen and Syngas
Dihydrogen (H<sub>2</sub>) is the simplest and lightest element. It exists as a diatomic molecule. It is colorless, odorless, and tasteless.
- Physical Properties: Besides being lighter than air and insoluble in water, H<sub>2</sub> has a very low melting point ($\text{259.1}$ °C) and boiling point ($\text{252.8}$ °C).
- Chemical Properties: While inert at room temperature, H<sub>2</sub> reacts under specific conditions. It reacts with halogens, oxygen (combustion to form water), nitrogen (Haber process to form ammonia), and many metal oxides (reduction).
- Syngas Production: Syngas can be produced via steam reforming of natural gas ($ \text{CH}_4\text{ (g)} + \text{H}_2\text{O (g)} \xrightarrow{\text{heat}} \text{CO (g)} + \text{3H}_2\text{ (g)} $), partial oxidation of hydrocarbons, or gasification of coal or biomass.
- Uses of Syngas: Syngas is an important intermediate in the production of various chemicals, including methanol, ammonia, and synthetic fuels (Fischer-Tropsch process). It can also be used directly as a fuel.
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