Chemical Reactions

Introduction to Chemical Reactions

• Definition: A chemical reaction is a change in which matter transforms into one or more new substances.
• External Indications: Reactions are often identified by characteristics such as a change in color, evolution of a gas, evolution or absorption of heat, and the formation of an insoluble precipitate.

Main Types of Chemical Reactions

• Direct Combination (Synthesis): A reaction where two or more elements or compounds combine to form a single new compound.
• Decomposition Reaction: A process where a single chemical compound breaks down into two or more simpler substances (elements or compounds).
• Displacement Reaction (Substitution): A reaction in which a more reactive element replaces another element from its compound.
• Double Decomposition Reaction: A reaction where two reactants exchange their radicals to form two new compounds. This includes Neutralization (acid + base → salt + water) and Precipitation (formation of an insoluble solid).

Specific Decomposition Processes

• Thermal Decomposition: Decomposition brought about by the application of heat.
• Thermal Dissociation: A reversible decomposition reaction where a substance breaks down upon heating but recombines upon cooling.
• Electrolytic Decomposition: A reaction triggered by the passage of an electric current through a substance.
• Photochemical Decomposition: A reaction that occurs specifically in the presence of light, such as the decomposition of silver salts.

The Activity Series of Metals

• The activity series arranges metals in order of their reactivity, with the most active metals at the top and the least active at the bottom.
• Metals higher in the series displace those lower in the series from their salt solutions.
• The series determines how metals react with water, acids, and air; for example, metals above hydrogen displace it from dilute acids.

Energy Changes in Reactions

• Exothermic Reactions: Chemical reactions that proceed with the evolution of heat energy.
• Endothermic Reactions: Chemical reactions that proceed with the absorption of heat energy.

Catalytic and Reversible Reactions

• Catalytic Reaction: A reaction where a catalyst is used to alter the rate of the reaction without being consumed itself.
• Promoters: Substances added to a catalyst to increase its efficiency.
• Enzymes: Biological catalysts found in the human body that accelerate biochemical reactions.
• Reversible Reaction: A reaction where the products can react together to reform the original reactants under specific conditions.

Classification of Oxides