Chemical Reactions, Equilibria, and Industrial Processes

Posted on Dec 3, 2024 in Chemistry

Hess’s Law

If a reaction can be performed in several steps, whether real or theoretical, the total enthalpy change is equal to the sum of the enthalpy changes of these intermediate reactions.

Standard Enthalpy of Reaction

From standard enthalpies of formation.

From bond enthalpies.

Standard Entropy of Reaction

Free Energy

Spontaneous

Theories of Chemical Reactions

Collision Theory

For a chemical reaction to occur, reactants (atoms, molecules, or ions) must collide. Effective collisions require:

• Sufficient kinetic energy for bond rearrangement and new substance formation.
• Proper orientation during collision.

Activated Complex Theory

As reactant molecules approach, they distort, forming a high-energy, short-lived activated complex during collision. Activation energy is the extra energy reactants absorb to form this complex.

Rate Equation or Rate Law

Factors Influencing Reaction Rate

Temperature

Generally, increasing temperature increases reaction rate.

The Arrhenius equation relates the rate constant (k) with temperature:

k = Ae^-Ea/RT

Where:

• A: Collision frequency factor.
• e: Base of natural logarithms.
• Ea: Activation energy (kJ/mol).
• R: Gas constant.
• T: Absolute temperature (K).

Reactant Concentration

Higher reactant concentration increases reaction rate.

Chemical Nature of Substances

Physical State

Degree of Solid Separation

Greater contact surface area increases reaction probability.

Catalyst Use

A catalyst alters reaction speed without being consumed. It can be positive (increases rate) or negative/inhibitory (decreases rate).

Le Chatelier’s Principle

External changes (temperature, pressure, concentration) to a system at equilibrium cause a shift to counteract the change and establish a new equilibrium.

• Increased concentration: System shifts to consume the substance.
• Decreased concentration: System shifts to produce the substance.
• Increased pressure: System shifts to reduce gas moles.
• Decreased pressure: System shifts to increase gas moles.
• Increased temperature: System favors the endothermic reaction.
• Decreased temperature: System favors the exothermic reaction.
• Catalysts do not alter the equilibrium state.

Arrhenius Theory of Ionic Dissociation

Acids and bases exist only in aqueous solution.

• Acid: Dissociates to form hydrogen ions (H⁺).
• Base: Dissociates to form hydroxide ions (OH^–).

Neutralization: Complete reaction of acid and base to form salt and water.

HA + BOH → AB + H₂O

Brønsted-Lowry Theory

• Acid: Donates a hydrogen ion (proton).
• Base: Accepts a hydrogen ion (proton).

Neutralization: Proton transfer from acid to base, forming conjugate acid and base.

HA + B → A^– + BH⁺

HA/A^– and B/BH⁺ are conjugate acid-base pairs.

Buffer Solutions

Maintain near-constant pH despite dilution or small acid/base additions.

Acid-Base Indicators

Weak acids or bases with different colors at different pHs.

• Methyl Orange: Orange (lower pH), transition (3.1-4.4), blue (higher pH).
• Bromothymol Blue: Yellow (lower pH), transition (6.0-7.6), blue (higher pH).
• Phenolphthalein: Colorless (lower pH), transition (8.3-10.0), red (higher pH).

Industrial Ammonia Synthesis

Ammonia (NH₃) is a colorless, pungent gas used in fertilizers, fibers, plastics, and more. The Haber-Bosch process catalytically synthesizes ammonia from nitrogen and hydrogen:

N₂(g) + 3H₂(g) ↔ 2NH₃(g) ΔH° = -92.6 kJ

High pressure and low temperature favor ammonia formation. Industrially, purified nitrogen and hydrogen (200-1000 atm) react at ~450°C with an iron/metal oxide catalyst. Cooling liquefies ammonia (-33.4°C boiling point) for separation, and unreacted gases are recycled.

Voltaic Cell

Chemical reaction produces electricity, two electrolytes, spontaneous redox, anode negative, cathode positive.

Electrolytic Cell

Electricity produces chemical reaction, single electrolyte, non-spontaneous redox, anode positive, cathode negative.

Aluminum Electrolysis (Hall-Héroult Process)

Bauxite (Al₂O₃·nH₂O) is purified to alumina (Al₂O₃) and electrolyzed with cryolite (Na₃AlF₆) at ~950°C. Aluminum deposits at the graphite cathode, oxygen evolves at the graphite anode.

Cathode: 4[Al³⁺ + 3e^– → Al(l)]

Anode: 3[2O^2- → O₂(g) + 4e^–]

Overall: 2Al₂O₃ → 4Al(l) + 3O₂(g)

Sulfuric Acid (H₂SO₄)

Sulfuric acid is a colorless, viscous, strong acid and dehydrating agent. Dilute solutions react with active metals to release hydrogen:

Mg(s) + H₂SO₄(aq) → MgSO₄(aq) + H₂(g)

Concentrated, hot acid oxidizes non-metals:

C(s) + 2H₂SO₄(aq) → CO₂(g) + 2SO₂(g) + 2H₂O(l)

Industrial Production (Contact Process)

1. SO₂ collection (from sulfide ore roasting or sulfur combustion):
   4FeS₂(s) + 11O₂(g) → 2Fe₂O₃(s) + 8SO₂(g)
2. Catalytic SO₂ oxidation:
   2SO₂(g) + O₂(g) → 2SO₃(g)
3. SO₃ absorption and reaction with water:
   SO₃(g) + H₂SO₄(aq) → H₂S₂O₇(l) (oleum)
   H₂S₂O₇(l) + H₂O(l) → 2H₂SO₄(l)