Chemical Reactions: Kinetics, Types, and Redox Principles

Introduction to Chemical Reactions

Chemical reactions are chemical phenomena that cause a modification of the internal molecular structure. Examples include the oxidation of iron and the degradation of food.

Energy in Chemical Processes

Synthesis reactions (manufacture) or anabolic processes are endothermic reactions; for example, photosynthesis and lipid synthesis. Conversely, exothermic reactions involve catabolism (not synthesis), such as cellular respiration and fermentation.

Chemical Equation for Cellular Respiration

The chemical equation for cellular respiration with oxygen is: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O.

Factors Affecting the Rate of a Reaction

• Temperature: With increasing temperature, chemical reactions occur with more numerous shocks (collisions), and this generates an increase in the speed of the reaction.
• Pressure: The higher the pressure, the closer the gaseous molecules approach each other, and therefore they collide and compress more frequently.
• Surface Contact: If instead of placing a single reagent, it is divided into pulverized pieces, there will be a larger surface area available, and therefore it accelerates the speed of the reaction.
• Nature of the Reagents: Some substances undergo reactions faster than others when facing the same reagent (e.g., different metals react differently with oxygen in the air).
• Catalysts: These are compounds that accelerate the rate of reactions. Positive catalysts increase the speed of reactions, while negative catalysts decrease the speed. The catalyst molecule does not undergo any transformation; it only increases or decreases the speed of the chemical reaction.

Manifestations of Chemical Reactions

Chemical reactions can manifest in several ways:

• Obtaining precipitates: In some reactions, products are insoluble solids in the water column where the reaction occurred (found as sediment at the bottom).
• Discoloration: This manifests as the appearance of a colored product, the disappearance of a color reagent, or any color change of a reagent when it becomes a product (e.g., the oxidation of iron).
• Detachment of gases: Many chemical reactions give off gases as a form of their products that are released into the atmosphere. Examples include the combustion of petrol, photosynthesis, fermentation, and respiration.
• Explosions, emission of light, or flames: Many chemical reactions occur with such violence that an explosion or flame occurs.

Types of Chemical Reactions

• Combination or Synthesis: Two products are formed from reactants (A + B = C). For example: H₂ + O₂ → H₂O.
• Displacement: Components are rearranged between two substances (A + B + CD = AC + BD). This includes precipitation types (indicated by a small downward arrow) and acid-base neutralization (forming salts).
• Decomposition: A substance is broken down into two or more products. For example: H₂O₂ → H₂O + O₂.

Oxidation-Reduction (Redox) Reactions

In oxide-reduction (redox) reactions, there is a change in the oxidation number of the reagents when they become products (e.g., Fe + 3Cl → FeCl₃). Whenever there is a reduction, there is an oxidation.

• Oxidation: Any process where a chemical species loses electrons (e-).
• Reduction: Any process in which a chemical species wins electrons.
• Reducing Agent: The reactive species that is oxidized.
• Oxidizing Agent: The reagent in the reduction process in which a chemical species wins electrons, while the oxidation process is one where a chemical species loses electrons.