Chemistry - Chemical Changes and Reactions (Types of Reactions, Energy Changes)

A chemical reaction is a process in which one or more substances, the reactants, are converted to one or more different substances, the products, involving the breaking and making of chemical bonds.

Direct Combination (Synthesis) Reaction: Two or more elements or compounds react to form a single product. Example: $2Mg + O_2 \rightarrow 2MgO$.

Decomposition Reaction: A single compound breaks down into two or more simpler substances, usually by heat (Thermal Decomposition), light (Photolysis), or electricity (Electrolysis). Example: $2KClO_3 \xrightarrow{MnO_2/\Delta} 2KCl + 3O_2 \uparrow$.

Displacement Reaction: A more reactive element displaces a less reactive element from its compound. Example: $Fe + CuSO_4 \rightarrow FeSO_4 + Cu$.

Double Displacement Reaction: Two compounds react by an exchange of ions to form two new compounds. This includes Precipitation (forming an insoluble solid) and Neutralization (Acid + Base). Example: $AgNO_3 + NaCl \rightarrow AgCl \downarrow + NaNO_3$.

Exothermic Reactions: Chemical reactions that release energy in the form of heat to the surroundings. The enthalpy change $\Delta H$ is negative. Example: $CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O + Heat$.

Endothermic Reactions: Chemical reactions that absorb energy from the surroundings. The enthalpy change $\Delta H$ is positive. Example: $N_2 + O_2 \xrightarrow{\Delta} 2NO$.

Catalysis: A catalyst is a substance that alters the rate of a chemical reaction without itself undergoing any permanent chemical change. Positive catalysts (like $MnO_2$) increase the rate, while negative catalysts (inhibitors) decrease it.