Chemical Reactions and Equations

A chemical equation is a shorthand description of a reaction. It symbolically represents the reactants, products and their physical states. In a balanced chemical equation, number of atoms of each type involved in the chemical reaction is equal on the reactants and products sides of the equation.

If charged species are involved, the sum of the charges on reactants should be equal to sum of charges on the products. During balancing of a chemical equation, no change in the formula of reactants and products is allowed. A balanced chemical equation obeys the law of conservation of mass and the law of constant proportions.

Reactants → Products

In a combination reaction two or more substances combine to form a new single substance.

C (s) + O2 (g) → CO2 (g)

In a decomposition reaction, a single substance decomposes to give two or more substances. Thus, decomposition reactions are opposite to combination reactions.

CaCO3 (s) + heat → CaO(s) + CO2 (g)

Reactions in which heat is given out during product formation are called exothermic reactions and reactions in which heat is absorbed during product formation are called endothermic reactions.

A displacement reaction is one in which an element displaces another element from its compound. 

Zn (s) + CuSO4 (aq) → ZnSO4 (aq) + Cu (s)

When two different ions are exchanged between two reactants double displacement reaction occurs.

Na2SO4 (aq) + BaCl2 (aq) → BaSO4 (s) ↓ + 2NaCl (aq)

Precipitation reactions are the result of ion exchange between two substances, producing insoluble salts.

Oxidation is the gain of oxygen or loss of hydrogen and reduction is loss of oxygen or gain of hydrogen. Oxidation and reduction reactions occur simultaneously and are jointly called redox reactions. Redox reactions can also be defined in terms of loss and gain of electrons. Gain of electrons is reduction and loss of electrons is oxidation.

Oxidizing agent is the substance which oxidizes another substance. It itself gets reduced in the reaction. Reducing agent is the substance which reduces another substance. It itself gets oxidized in the reaction.

Effects of Redox Reactions in Everyday Life: Two commonly observed effects are corrosion and rancidity.

Corrosion: It is a destructive chemical process in which metals are oxidized in presence of air and moisture. For example, Rusting of iron, tarnishing of silver, formation of green coating on copper, brass and bronze items.

Rancidity is the process of oxidation of fats and oils resulting in the formation of acids. This process changes the smell and taste of stale fats and oil.