Chemical Reactions and Equations

A chemical reaction involves the breaking of bonds in reactants and forming of new bonds in products. The total mass of reactants equals the total mass of products (Law of Conservation of Mass). Reactions are classified as: combination (A + B → AB), decomposition (AB → A + B), displacement (A + BC → AC + B), double decomposition/precipitation, and redox (involving electron transfer). Reaction rates — how fast a reaction proceeds — depend on: concentration (more particles → more collisions), temperature (higher temperature → particles move faster → more energetic collisions), surface area (more contact → faster), and catalysts (lower activation energy without being consumed). Endothermic reactions absorb heat (cold packs); exothermic reactions release heat (burning, neutralisation). The activation energy is the minimum energy needed to start a reaction. Collision theory explains why these factors matter.