Chemical Reactions and Equations - CHEMIC1.1 CHEMIC1.1 CHEMIC1.1 CHEMIC1.1 CHEMIC AL EQUA AL EQUAAL EQUA AL EQUAAL EQUA TIONS TIONSTIONSTIONSTIONS

A chemical reaction is a process where the original substances lose their nature and identity and form new chemical substances with different properties. Indicators include change in state, color, temperature, or evolution of a gas.

A chemical equation is a symbolic representation of a chemical reaction using symbols and formulae of the substances involved. For example, the burning of magnesium in air: $2Mg(s) + O_2(g) \rightarrow 2MgO(s)$.

Balanced Chemical Equation: According to the Law of Conservation of Mass, mass can neither be created nor destroyed in a chemical reaction. Therefore, the number of atoms of each element on the reactant side must equal the number of atoms on the product side.

Combination Reaction: A reaction in which a single product is formed from two or more reactants. Example: $CaO(s) + H_2O(l) \rightarrow Ca(OH)_2(aq) + \text{Heat}$.

Decomposition Reaction: A reaction in which a single reactant breaks down to give simpler products. This can be Thermal ($CaCO_3 \xrightarrow{\Delta} CaO + CO_2$), Electrolytic ($2H_2O \xrightarrow{\text{electricity}} 2H_2 + O_2$), or Photolytic ($2AgCl \xrightarrow{h\nu} 2Ag + Cl_2$).

Displacement Reaction: A reaction in which a more reactive element displaces a less reactive element from its compound. Example: $Fe(s) + CuSO_4(aq) \rightarrow FeSO_4(aq) + Cu(s)$.

Double Displacement Reaction: A reaction in which there is an exchange of ions between the reactants to form new compounds. Often produces a precipitate. Example: $Na_2SO_4(aq) + BaCl_2(aq) \rightarrow BaSO_4(s) + 2NaCl(aq)$.

Redox Reactions (Oxidation and Reduction): Oxidation is the gain of oxygen or loss of hydrogen. Reduction is the loss of oxygen or gain of hydrogen. When both occur simultaneously, it is a Redox reaction. Example: $CuO + H_2 \rightarrow Cu + H_2O$.

Exothermic reactions release energy to the surroundings (e.g., respiration), while Endothermic reactions absorb energy (e.g., photosynthesis).

Corrosion: The slow eating up of metals by the action of air, moisture, or chemicals. Example: Rusting of Iron ($Fe_2O_3 \cdot xH_2O$).

Rancidity: The oxidation of fats and oils in food resulting in a change in smell and taste. Prevented by using antioxidants or flushing bags with nitrogen gas ($N_2$).