Chemical Reactions, Rates, Energy & Environmental Impact

Activation Energy

Activation energy: When bonds are broken, other bonds are formed → products.

Reaction energy: E_products - E_reactants

Reaction rate: The rate at which a reaction proceeds.

Factors Influencing Reaction Rate

Factors influencing the reaction rate:

• Nature of the reactants

  The nature of the substances: substances with covalent bonds generally react more slowly; reactivity depends on bond type and molecular structure.

• Temperature

  Reaction rate increases with temperature because particle motion intensifies, increasing the number and energy of collisions.

• Degree of division (surface area)

  Liquids and gases typically react faster than solids. Smaller particle size (greater surface area) accelerates reactions.

• Concentration

  Higher reactant concentration usually increases the reaction rate by increasing collision frequency.

• Catalysts, enzymes and inhibitors

  Catalyst: A substance that lowers the activation energy and modifies the reaction rate. In living systems, organic catalysts called enzymes accelerate reactions. Substances that decrease or stop reaction rate are called inhibitors.

Lavoisier's Law

Lavoisier's law: The sum of the mass of the reactants must be equal to the sum of the mass of the products (law of conservation of mass).

Types of Chemical Reactions

• Synthesis (combination)

  Synthesis reaction: A + B → AB

• Decomposition

  Decomposition reaction: AB → A + B

• Single displacement (substitution)

  Displacement or substitution reaction: AB + X → AX + B

• Double displacement (exchange)

  Double displacement or exchange: AB + XY → AX + YB

• Oxidation

  Oxidation: A reaction of a substance with oxygen (classical definition). In modern terms, oxidation is the loss of electrons.

Reduction and Redox Processes

Reduction: Classically, the loss of oxygen by part of a substance; modern definition: gain of electrons.

Oxidation process: An element that is oxidized loses electrons and is called the reducing agent (reducer).

Reduction process: An element that is reduced gains electrons and the substance that causes reduction is called the oxidizing agent (oxidizer).

Types of Reactions by Energy Transfer

• Exothermic reactions

  Exothermic: The internal energy of the reactants is greater than that of the products; energy is released as heat, light, etc.

• Endothermic reactions

  Endothermic: Energy is absorbed from the surroundings (heat, light, etc.).

Pollution and Environmental Impact

Pollution: Presence of any agent in the environment that causes damage or contamination.

Environmental impact: A set of negative effects on the environment resulting from various actions or agents.

Types of Environmental Impact

• Atmospheric impact

  Destruction of the ozone layer reduces its ability to absorb ultraviolet (UV) radiation from the sun, increasing UV levels at the surface.

• Hydrosphere impact

  Water can be contaminated in different ways: naturally or through human intervention and activities.

• Greenhouse effect

  Excessive warming of the atmosphere and land surface occurs because many atmospheric gases absorb and trap heat.

• Acid rain

  Pollutants reach clouds; when precipitation occurs, it may fall as acid rain containing dissolved chemical compounds harmful to ecosystems.