Characteristics of Chemical Reactions

Chemical reactions are often accompanied by some features which can be observed easily. These important characteristics of chemical reactions are:

• Evolution of gas- The reaction between sodium carbonate and dilute hydrochloric acid is characterised by the evolution of carbon dioxide gas.
• Formation of a precipitate- The reaction between sulphuric acid and barium chloride is accompanied by the formation of a white precipitate of barium sulphate.
• Change in colour- When sulphur dioxide gas is passed through an acidified solution of potassium dichromate, the colour changes from orange to green.
• Change in temperature- The reaction between quicklime (calcium oxide) and water to form slaked lime (calcium hydroxide) is accompanied by a rise in temperature. Such heat producing reactions are called exothermic reactions.  A lot of heat is released when carbon bums in air to form carbon dioxide. The reaction between barium hydroxide and ammonium chloride to give barium chloride, water and ammonia is accompanied by a fall in temperature. Such heat absorbing reactions are called endothermic reactions.
• Change in state- When wax (solid) is burnt in the form of a candle, water (liquid) and carbon dioxide (gas) are produced.

Some chemical reactions may show two or more characteristics.

Chemical Equations

The representation of a chemical reaction with the help of symbols and formulae of the substances involved in it is known as a chemical equation. The chemical equation is a short-hand method of representing a chemical reaction in which the reactants are written on the left hand side, separated by a plus sign (+), and the products are written on the right hand side, separated by a plus sign (+). The arrow sign (→), pointing towards the right, is put between the reactants and products.

Matter can neither be created nor destroyed in a chemical reaction. Thus, the number of atoms of different elements in reactants must be equal to the number of same type of atoms in products. The process of making the number of atoms equal on both sides of an equation is called Balancing of equation.

The reaction of hydrogen with oxygen to form water can be written in an equation form as:

H₂ + O → H₂O

But in the above reaction, there are 2 atoms of H and 2 atoms of O on the left hand side and 2 atoms of H and 1 atom of  O on the right hand side. Thus the number of atoms of each element is not the same on both the sides. The balanced equation can be written as -

2H₂ + O₂ → 2H₂O

Now the number of atoms of each element is the same on both the sides (4 atoms of H and 2 atoms of O)

The chemical equations can be made more informative by indicating the physical states of the reactants and products, heat changes taking place in the reaction, and the conditions under which the reaction takes place.

Example: 1. Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l)

(s) stands for solid, l for liquid, (g) for gas, (aq) for aqueous solution, and (ppt) for precipitate.

2. The equation for photosynthesis can be written as

6CO₂(g) + 6H₂O(l) → sunlight, chlorophyll → C₆H₁₂O₆(aq) + 6O₂(g)

3. All combustion reactions are exothermic, like burning of wood, coal, kerosene, petrol, diesel, and natural gas (consisting mainly of methane, CH₄).

CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(g) + Heat energy

The combustion of food (like glucose) in our body during respiration is also an exothermic reaction.

C₆H₁₂O₆(aq) + 6O₂(g) → 6CO₂(g) + 6H₂O(l) + energy

The combination or nitrogen and oxygen to form nitrogen monoxide, which takes place inside the engines of motor vehicles is an endothermic reaction.

N₂(g) + O₂(g) + Heat → 2NO(g)

4. All decomposition reactions are endothermic in nature. An example is:

CaCO₃(s) + Heat → CaO(s) + CO₂(g)

Photosynthesis is an endothermic reaction because sunlight is absorbed during the process. The electrolysis of water to form hydrogen and oxygen is also endothermic because electrical energy is absorbed.