Chemical Reactions

Reactants, products, conservation of mass, and energy changes

A chemical reaction transforms reactants (starting materials) into products (ending materials) by breaking and forming chemical bonds. The atoms themselves are never created or destroyed — they just rearrange. This is the Law of Conservation of Mass: mass of reactants = mass of products. Reactions are classified by type: synthesis (A+B→AB), decomposition (AB→A+B), single displacement (A+BC→AC+B), double displacement (AB+CD→AD+CB), and combustion (fuel+O₂→CO₂+H₂O+heat). Energy changes: exothermic reactions release heat (ΔH<0), endothermic reactions absorb heat (ΔH>0). Reaction rate increases with temperature, concentration, surface area, and catalysts. A catalyst speeds up a reaction without being consumed.