Chemical Reactions and Stoichiometry Fundamentals

Chemical Reactions and Stoichiometry

A chemical reaction is a process by which substances are transformed into others with the release or absorption of energy. Initial substances are called reactants, and the final substances are called products.

Balancing Chemical Equations

Stoichiometry indicates the proportion of molecules (microscopic scale) or moles (macroscopic scale) involved in a reaction to ensure the conservation of atoms. Common balancing methods include:

• Mathematical Method: A matching method consisting of solving a system of equations arising from the equality of atoms on either side of the equation.
• Trial and Error: A method where coefficients are adjusted starting with elements involved in the fewest substances and ending with those in the most compounds.

Types of Chemical Reactions

• Decomposition (Analysis): AB → A + B
• Synthesis: A + B → AB
• Displacement (Substitution): AB + C → AC + B
• Double Substitution: AB + CD → AC + BD
• Reverse Reaction: Products → Reactants
• Chained Reactions: A + B → C + D; D + E → F + G

Acids, Bases, and Neutralization

An acid is a substance that dissociates to release H⁺ ions (HA → H⁺ + A^-). A base is a substance that dissociates to release OH^- ions (BOH → B⁺ + OH^-).

Strength and Weakness: Strong acids and bases dissociate completely, while weak ones dissociate only partially.

Neutralization Reaction: A reaction between an acid and a base to produce a salt and water:
HA (acid) + BOH (base) → BA (salt) + H₂O (water)

Combustion and Energy Changes

Combustion Reactions: In the combustion of any organic substance, carbon dioxide and water are always produced:
C_xH_y + O₂ → xCO₂ + H₂O

• Exothermic Reaction: A chemical reaction in which energy is released.
• Endothermic Reaction: A chemical reaction which absorbs energy.

Thermochemical Equations and Enthalpy

A thermochemical equation includes the chemical energy absorbed or evolved, the physical state of reactants and products, and the conditions of pressure and temperature. Standard conditions are defined as P = 1 atm and T = 25 °C.

• Variation of Internal Energy (ΔU): Reaction heat measured at constant volume (q_v).
• Variation of Enthalpy (ΔH): Reaction heat measured at constant pressure (q_p).

The enthalpy of chemical elements in standard conditions and in their most stable state is considered zero. The enthalpy of reaction can be expressed as:

• ΔH_reaction = ΣH_products - ΣH_reactants
• ΔH_reaction = ΣH_{bonds broken} - ΣH_{bonds formed}

Stoichiometric Calculations and Yield

Stoichiometric calculations are always performed in moles using conversion factors.

• Limiting Reagent: The reactant present in the smallest stoichiometric amount; it is the one completely consumed in the reaction.
• Excess Reactants: All reagents remaining beyond the stoichiometric ratio.
• Performance (Yield): The ratio of the actual product yield to the theoretically expected amount of product.