Hydrogen (Position & Properties)

Position Of Hydrogen In The Periodic Table

Hydrogen: Hydrogen (H) is the first and simplest element in the periodic table, with atomic number 1. Its position in the periodic table is unique due to its properties, which allow it to be placed in multiple groups.

Common Placement: Hydrogen is typically placed at the top of Group 1 (alkali metals) because:

It has one valence electron ($1s^1$), similar to alkali metals.
It readily loses its electron to form a cation ($H^+$), similar to alkali metals forming $M^+$ ions.
Its electronic configuration ($1s^1$) resembles that of alkali metals.

Similarities with Group 1 Elements (Alkali Metals):

Loses one electron to form $H^+$.
Reacts with halogens to form halides (e.g., $HCl$, $HBr$).
Reacts with oxygen to form oxides (e.g., $H_2O$).

Similarities with Group 17 Elements (Halogens):

Needs one electron to achieve a stable noble gas configuration ($1s^2$), similar to halogens needing one electron to achieve $ns^2np^5$ configuration.
Can gain an electron to form an anion ($H^-$), called a hydride ion, similar to halogens forming halide ions ($X^-$).
Forms diatomic molecules ($H_2$), similar to halogens ($Cl_2$, $Br_2$).
Reacts with alkali metals to form ionic hydrides (e.g., $NaH$).
Reacts with halogens to form covalent halides (e.g., $HCl$).

Placement Above Group 17: Due to its ability to gain an electron, hydrogen is sometimes shown above Group 17 (halogens) as well.

Other Possible Placements:

Above Group 14: Its electronic configuration ($1s^1$) has one electron in the s-orbital, which could be loosely related to the elements in Group 14 having electrons in the s-orbital.

Conclusion on Position: Hydrogen's unique electronic structure and properties make it an anomaly. While usually placed in Group 1, it shares characteristics with both Group 1 and Group 17 elements, justifying its unique position.

Dihydrogen, H2

Dihydrogen ($H_2$) is the most abundant element in the universe and a fundamental molecule in chemistry.

Occurrence

Universe: Dihydrogen is the most abundant element in the universe, constituting about 75% of the total elemental mass. It exists predominantly in stars like the Sun, where it undergoes nuclear fusion to form helium, releasing vast amounts of energy.

Earth's Atmosphere: Dihydrogen is present in the Earth's atmosphere in very small amounts (about 0.00005% by volume) because its low molecular weight causes it to escape the Earth's gravitational pull into space.

In Combination: Dihydrogen is rarely found in its elemental form on Earth. It is abundant in combined form as:

Water ($H_2O$): The most common compound of hydrogen on Earth, found in oceans, rivers, lakes, and as ice and water vapor.
Organic Compounds: Hydrogen is a constituent of all organic compounds, including carbohydrates, fats, proteins, and nucleic acids. It is also present in fossil fuels like coal, petroleum, and natural gas.
Biomass: Hydrogen is found in all living organisms in various organic molecules.
Hydrocarbons: Such as methane ($CH_4$) and other alkanes.
Hydrides: Compounds of hydrogen with metals and non-metals.

Isotopes Of Hydrogen

Hydrogen has three isotopes, which differ in the number of neutrons in their nucleus:

Protium ($^1H$ or H):

Most abundant isotope ($\approx$ 99.985%).
Nucleus consists of 1 proton only.
Atomic mass $\approx$ 1.0078 u.

Deuterium ($^2H$ or D):

Less abundant isotope ($\approx$ 0.015%).
Nucleus consists of 1 proton and 1 neutron (called a deuteron).
Atomic mass $\approx$ 2.0141 u.
Deuterium compounds are called "heavy" compounds (e.g., $D_2O$ or $HDO$ is heavy water).

Tritium ($^3H$ or T):

Extremely rare and radioactive isotope.
Nucleus consists of 1 proton and 2 neutrons.
Atomic mass $\approx$ 3.0160 u.
Radioactive decay: Tritium decays by beta emission with a half-life of about 12.3 years.
Used in tracers and luminous paints.

Chemical Properties of Isotopes: The chemical properties of isotopes of an element are very similar because they have the same number of electrons and similar electronic configurations. However, there can be slight differences in reaction rates due to the difference in mass (this is known as the kinetic isotope effect). For example, the rate of reaction of $D_2O$ is generally slower than that of $H_2O$ in many chemical reactions.

Preparation Of Dihydrogen, H2

Dihydrogen can be prepared in the laboratory and on a commercial scale using various methods.

Laboratory Preparation Of Dihydrogen

Reaction of Active Metals with Dilute Acids: This is the most common laboratory method for preparing small quantities of hydrogen gas.

General Reaction:

$$\text{Metal} + \text{Dilute Acid} \rightarrow \text{Salt} + \text{Hydrogen gas}$$

Examples:

Zinc with dilute sulfuric acid: $Zn(s) + H_2SO_4(aq) \rightarrow ZnSO_4(aq) + H_2(g)$
Magnesium with dilute hydrochloric acid: $Mg(s) + 2HCl(aq) \rightarrow MgCl_2(aq) + H_2(g)$
Iron with dilute sulfuric acid: $Fe(s) + H_2SO_4(aq) \rightarrow FeSO_4(aq) + H_2(g)$

Reaction of Active Metals with Strong Alkalis: Amphoteric metals like zinc, aluminum, and silicon react with strong bases to produce hydrogen gas.

Examples:

Zinc with sodium hydroxide: $Zn(s) + 2NaOH(aq) + 2H_2O(l) \rightarrow Na_2[Zn(OH)_4](aq) + H_2(g)$
Aluminum with sodium hydroxide: $2Al(s) + 2NaOH(aq) + 6H_2O(l) \rightarrow 2Na[Al(OH)_4](aq) + 3H_2(g)$

Other Methods:

Reaction of water with reactive metals: Sodium or Calcium with cold or hot water (vigorous reaction).
Electrolysis of water: $2H_2O(l) \xrightarrow{electrolysis} 2H_2(g) + O_2(g)$. This method produces very pure hydrogen.

Commercial Production Of Dihydrogen

Dihydrogen is produced on a large scale primarily through methods that are economically viable for industrial applications.

1. From Fossil Fuels (Steam Reforming of Hydrocarbons): This is the most common industrial method.

Process: Natural gas (mainly methane, $CH_4$) or other hydrocarbons are reacted with steam at high temperatures (700-1000°C) in the presence of a catalyst (like Nickel or Rhodium). This process yields synthesis gas (syngas), a mixture of carbon monoxide ($CO$) and hydrogen ($H_2$).

Reaction:

$$CH_4(g) + H_2O(g) \xrightarrow{Ni \text{ catalyst}, 700-1000^\circ C} CO(g) + 3H_2(g)$$

Water-Gas Shift Reaction: The carbon monoxide produced in the steam reforming process can be further reacted with steam to produce more hydrogen. This is called the water-gas shift reaction, typically carried out at around 400°C with iron(III) oxide as a catalyst.

$$CO(g) + H_2O(g) \xrightarrow{Fe_2O_3 \text{ catalyst}, 400^\circ C} CO_2(g) + H_2(g)$$

The overall process from methane yields a mixture rich in hydrogen. Carbon dioxide is then removed by passing the mixture through solutions of potassium carbonate ($K_2CO_3$) or monoethanolamine.

2. Electrolysis of Water:

While effective for producing pure hydrogen, the direct electrolysis of pure water is inefficient due to its low conductivity. Electrolysis is usually performed on water with a small amount of acid or alkali added to increase conductivity.

$$2H_2O(l) \xrightarrow{electrolysis} 2H_2(g) + O_2(g)$$

This method is energy-intensive and generally more expensive than steam reforming, making it less common for large-scale production unless very pure hydrogen is needed or electricity is cheap.

3. From Coal (Coal Gasification):

Coal can be reacted with steam at high temperatures to produce synthesis gas, similar to the process with hydrocarbons.

$$C(s) + H_2O(g) \xrightarrow{high \ T} CO(g) + H_2(g)$$

The $CO$ is then converted to $H_2$ via the water-gas shift reaction.

4. Other Methods (Less Common):

Reaction of steam with coke (carbon) at very high temperatures ($1000^\circ C$).
Electrolysis of brine solutions (though this primarily produces $H_2$ as a byproduct, along with $Cl_2$ and $NaOH$).

Properties Of Dihydrogen

Dihydrogen ($H_2$) is a simple molecule with distinct physical and chemical properties.

Physical Properties

Appearance: Dihydrogen is a colorless gas.

Odor and Taste: It is odorless and tasteless.

Molecular Weight: It is the lightest element, with a molecular weight of approximately 2.016 g/mol.

Density: It is the lightest gas known. Its density is about 1/14th that of air.

Flammability: Dihydrogen is highly flammable and burns with a pale blue flame. It forms explosive mixtures with air or oxygen over a wide range of concentrations (from 4% to 75% by volume).

Solubility: It is insoluble in water and most common solvents.

Liquefaction: Dihydrogen liquefies at a very low temperature (-239.9°C or 33.15 K) and solidifies at -259.2°C (14.15 K).

Atomic vs. Molecular Hydrogen: Atomic hydrogen (H), produced by passing an electric discharge through hydrogen gas at low pressure, is highly reactive and exists at very high temperatures.

Chemical Properties

Dihydrogen exhibits a wide range of chemical reactivity, acting as both a reducing agent and, less commonly, an oxidizing agent.

1. Action as a Reducing Agent: Dihydrogen readily loses its electron to form $H^+$ or acts as a source of hydrogen atoms, making it a strong reducing agent.

Reactions with Non-metals:

With Halogens: Reacts directly with halogens (except iodine) in the presence of a catalyst (like platinum) or upon heating to form hydrogen halides.

$H_2(g) + Cl_2(g) \xrightarrow{Pt \text{ or } UV \ light} 2HCl(g)$

With Oxygen: Burns in air or oxygen with a pale blue flame, forming water. The reaction is highly exothermic and can be explosive in the presence of oxygen. It can be catalyzed by platinum or nickel.

$2H_2(g) + O_2(g) \xrightarrow{Pt \text{ catalyst}} 2H_2O(l) \quad (\Delta H = -285.8 \text{ kJ/mol})$

With Nitrogen: Reacts with nitrogen at high temperature (400-750°C) and pressure in the presence of a catalyst (iron) to form ammonia (Haber process).

$N_2(g) + 3H_2(g) \xrightarrow{Fe \text{ catalyst}, 400-750^\circ C, \text{ high pressure}} 2NH_3(g)$

With Sulfur: Reacts with sulfur vapor at around 450°C to form hydrogen sulfide.

$H_2(g) + S(s) \rightarrow H_2S(g)$

With Phosphorus: Does not react directly with phosphorus under normal conditions.

2. Action as an Oxidizing Agent: Dihydrogen can also gain electrons to form hydride ions ($H^-$) when it reacts with very electropositive metals (metals with low ionization energies), acting as an oxidizing agent.

With Alkali Metals: Reacts with alkali metals upon heating to form ionic hydrides (salt-like hydrides).

$2Li(s) + H_2(g) \xrightarrow{heat} 2LiH(s)$

With Alkaline Earth Metals: Reacts with alkaline earth metals (except Be) upon heating to form ionic hydrides.

$Ca(s) + H_2(g) \xrightarrow{heat} CaH_2(s)$

3. Reducing Agent in Metallurgy: Hydrogen is used as a reducing agent to reduce metal oxides to metals, especially for less reactive metals where other reducing agents might be too harsh.

Example:

Reduction of copper(II) oxide: $CuO(s) + H_2(g) \rightarrow Cu(s) + H_2O(g)$
Reduction of tungsten(VI) oxide: $WO_3(s) + 3H_2(g) \rightarrow W(s) + 3H_2O(g)$

4. Catalytic Hydrogenation: Dihydrogen is used in catalytic hydrogenation to reduce unsaturated organic compounds (like alkenes and alkynes) to saturated compounds (alkanes).

Example: Hydrogenation of ethene to ethane:

$$CH_2=CH_2(g) + H_2(g) \xrightarrow{Ni \text{ catalyst}} CH_3-CH_3(g)$$

5. Formation of Hydrides: Hydrogen combines with many elements to form hydrides. These can be classified based on the nature of bonding:

Ionic (Salt-like) Hydrides: Formed with alkali and alkaline earth metals (except Be). They are crystalline solids, non-volatile, and non-conducting in solid state but conduct electricity when molten or dissolved in water, producing $H_2$ at the anode.
Covalent (Molecular) Hydrides: Formed with non-metals (e.g., $CH_4$, $NH_3$, $H_2O$, $HF$). These can be further classified as electron-deficient, electron-precise, or electron-rich depending on the bonding.
Metallic (Interstitial) Hydrides: Formed with many transition metals. These are typically non-stoichiometric compounds where hydrogen atoms occupy interstitial positions in the metal lattice. They retain metallic properties.