Hydrogen and Compounds Study Notes

Chapter IX Hydrogen and Compounds

IX.1. Hydrogen

Definition and Characteristics
- Hydrogen is the simplest element known; its most common atomic form contains only one proton and one electron.
- The atomic form of hydrogen exists only at very high temperatures.
- Normally, elemental hydrogen exists as a diatomic molecule (H2) formed through an exothermic reaction:
 $ext{H} + ext{H} ightarrow ext{H}_2$
- The hydrogen molecule's chemical formula is H2.
- Hydrogen is a colorless and non-poisonous gas.
Physical Properties
- At 1 atm, liquid hydrogen has a boiling point of -259 °C.
- Average atomic mass: $A_r(H) = 1.008$ .
- Average molecular mass: $Mr(H2) = 2.016$ .
- Hydrogen is the most abundant element in the universe and the tenth most abundant in Earth's crust. It is found combined with other elements.
- Earth lacks a strong gravitational pull to retain lightweight H2 molecules; thus, hydrogen is not found in the atmosphere.
Industrial Preparation Methods of Hydrogen
1. From methane (natural gas):
- The reaction between methane and steam in the presence of a catalyst at 900 °C:
 $ext{CH}4 ( ext{g}) + ext{H}2 ext{O} ( ext{g}) ightarrow ext{CO} ( ext{g}) + 3 ext{H}_2 ( ext{g})$
1. From steam over red-hot coke:
- $ext{C}(s) + ext{H}2 ext{O} ( ext{g}) ightarrow ext{CO} ( ext{g}) + ext{H}2 ( ext{g})$
- Carbon monoxide produced is converted to carbon dioxide mixed with steam over heated $Fe2O3$ catalyst:
 $ext{CO} ( ext{g}) + ext{H}2 ext{O} ( ext{g}) ightarrow ext{CO}2 ( ext{g}) + ext{H}2 ( ext{g})$ Z=1, $Ar=1.008$
1. From active metals (replacement reaction):
- For example, zinc reacts with dilute hydrochloric acid:
 $ext{Zn} (s) + 2 ext{HCl} ightarrow ext{ZnCl}2 + ext{H}2 ( ext{g})$
1. Electrolysis of water (though it consumes too much energy for practical use):
- $2 ext{H}2 ext{O} (l) ightarrow 2 ext{H}2 ( ext{g}) + ext{O}_2 ( ext{g})$
- At the anode (oxidation): $2 ext{H}2 ext{O} (l) ightarrow ext{O}2 ( ext{g}) + 4 ext{H}^+ (aq) + 4e^-$
- At the cathode (reduction): $4 ext{H}^+ (aq) + 4e^- ightarrow 2 ext{H}_2 ( ext{g})$
Physical Properties of Hydrogen
- Hydrogen is the lightest of all gases, approximately 15 times lighter than air.
- It is colorless and odorless.
- It is nearly insoluble in water.
- Boiling point: -259 °C.
Chemical Properties of Hydrogen
- Hydrogen exhibits different oxidation numbers in various compounds.
- With nonmetals, hydrogen has a +1 oxidation number (e.g., in HCl).
- With metals, hydrogen has a -1 oxidation number (e.g., in LiH).
- Hydrogen compounds with another element are termed hydrides.
- Ionic hydrides form when hydrogen combines directly with alkali metals or alkaline earth metals such as calcium, strontium, or barium:
- $2 ext{Li} (s) + ext{H}_2 (g) ightarrow 2 ext{LiH} (s)$
- Due to their high reactivity with water, ionic hydrides are used to remove traces of water from organic solvents:
- $ext{LiH} + ext{H}2 ext{O} ightarrow ext{LiOH} + ext{H}2 (g)$
Common Reactions of Hydrogen
- 1. Formation of water by reaction with oxygen: $2 ext{H}2 + ext{O}2 ightarrow 2 ext{H}_2 ext{O} (l)$
 - The reaction releases a large amount of energy, used in fuel cells and space rockets. It can be explosive.
- 2. Formation of hydrogen sulfide: $ext{H}2 + ext{S} ightarrow ext{H}2 ext{S} (g)$
 - Hydrogen sulfide has the smell of rotten eggs.
- 3. Synthesis of ammonia from nitrogen: $ext{N}2 + 3 ext{H}2 ightleftharpoons 2 ext{NH}_3 ext{(g)}$
 - This reaction occurs at 450-500 °C, at high pressure (1000 atm), with iron catalyst.
- 4. Formation of methane (synthesis):
 $ext{C} + 2 ext{H}2 ightarrow ext{CH}4$
- 5. Acting as a reducing agent by removing oxygen; for example, copper(II) oxide is reduced to copper when heated in hydrogen: $ext{CuO} (s) + ext{H}2 ightarrow ext{Cu} + ext{H}2 ext{O} (l)$
 - Presence of hydrogen sulfide (H2S) provides a recognizable odor.
- 6. Hydrogenation reaction refers to the addition of hydrogen to multiple bonds (particularly C=C):
 - Example: Ethylene to Ethane:
 $ext{H}2 + ext{C}2 ext{H}4 ightarrow ext{C}2 ext{H}_6$
 - Catalysts like nickel or platinum speed up this reaction.
Uses of Hydrogen
- Hydrogenation in food technology to harden oils.
- Hydrogen serves as a fuel for space rockets due to its clean reaction with oxygen producing water, making it a potential renewable resource as well.

IX.2. Hydrogen Peroxide

Definition
- Hydrogen peroxide (H2O2) is a well-known peroxide, containing oxygen with an oxidation number of -1, symbolized by O2^2- ion.
- Structure: H-O-O-H; it is a colorless, syrupy liquid, and polar molecule.
Preparation
- Prepared in the lab via reaction with dilute sulfuric acid on barium peroxide:
 $ext{BaO}2 + ext{H}2 ext{SO}4 ightarrow ext{BaSO}4 + ext{H}2 ext{O}2$
Chemical Properties
- 1. Hydrogen peroxide decomposes when heated, exposed to sunlight, or certain metals (like iron and copper):
 $2 ext{H}2 ext{O}2 ightarrow 2 ext{H}2 ext{O} + ext{O}2 + Q$
- 2. Miscible with water in all proportions, forms hydrogen bonds.
- 3. Acts as a strong oxidizing agent, e.g.
 $ext{H}2 ext{O}2 + 2 ext{KI} ightarrow ext{I}_2 + 2 ext{KOH}$
- 4. Reduces stronger oxidizers in acidic solutions, such as permanganate:
 $2 ext{KMnO}4 + 5 ext{H}2 ext{O}2 + 3 ext{H}2 ext{SO}4 ightarrow ext{K}2 ext{SO}4 + 2 ext{MnSO}4 + 5 ext{O}2 + 8 ext{H}2 ext{O}$
Uses
- 1. 3% solutions as antiseptics available in drugstores.
- 2. Concentrated solutions used as bleaching agents.
- 3. Used in rocket fuels for its high heat of decomposition.

IX.3. Water

Chemical Composition
- Water is a compound of hydrogen and oxygen, with a chemical formula H2O.
- Produced by burning hydrogen in air:
 $2 ext{H}2 + ext{O}2 ightarrow 2 ext{H}_2 ext{O}$
Physical Properties
- Covers ¾ of Earth’s surface (referred to as World Ocean).
- Notable for existing in three states (solid, liquid, gas).
- Key points include:
- Solid (ice), liquid (water), and gas (water vapor or steam).
- Colorless, tasteless, and odorless.
- High capacity to dissolve substances, hence called a universal solvent.
- Testing Water: Two common tests include:
- Using white anhydrous copper(II) sulfate turns blue in water presence.
- Cobalt(II) chloride paper turns pink in pure water presence.
Chemical Properties
- Water reacts as both an acid and a base (amphoteric), e.g. it can hydrolyze salts.
- Water undergoes reaction with metals to form hydroxides, e.g. :
 $ext{Ca} + 2 ext{H}2 ext{O} ightarrow ext{Ca(OH)}2 + ext{H}_2$
Water Pollution
- Waste from industries, farms, and urban areas can overwhelm water purification systems, leading to polluted water harming aquatic life and humans.

IX.4. Applications of Water in Everyday Life

Water is crucial for various human activities, including drinking, cooking, sanitation, and as a raw material in industry.
Treated potable water derives from rivers and reservoirs, its purification includes multiple stages such as screening, sedimentation, filtration, and chlorination.