In a chemical reaction, the substances that combine are called reactants and the new substances formed are called products. For example, when and (reactants) combine they form (product).
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Most of the reactions, in which the products do not recombine to form reactants, are called irreversible reactions. They are supposed to complete and are represented by putting a single arrow ( between the reactants and products.
On the other hand, reactions in which the products can recombine to form reactants are called reversible reactions. These reactions never go to completion. They are represented by a double arrow ( between reactants and products. These reactions proceed in both ways, i.e., they consist of two reactions; forward and reverse. So, a reversible reaction is one which can be made to proceed in either direction depending upon the conditions.
Let us discuss a reaction between hydrogen and iodine. Because one of the reactants, iodine is purple, while the product hydrogen iodide is colourless, proceedings of the reaction are easily observable. On heating, hydrogen and iodine vapours in a closed flask, hydrogen iodide is formed. As a result, purple colour of iodine fades as it reacts to form colourless hydrogen iodide, as shown in figure 9.1. This reaction is called as forward reaction.
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On the other hand, when only hydrogen iodide is heated in a closed flask, purple colour appears because of formation of iodine vapours. Such as In this case, hydrogen iodide acts as reactant and produces hydrogen and iodine vapours. This reaction is reverse of the above. Therefore, it is called as reverse reaction. When both of these reactions are written together as a reversible reaction, they are represented as:
Let us have another example, when calcium oxide and carbon dioxide react, they produce calcium carbonate:
On the other hand, when is heated in an open flask, it decomposes to form calcium oxide and carbon dioxide. escapes out and reaction goes to completion:
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In these two reactions, decomposition is reverse to combination or vice versa. When calcium carbonate is heated in a closed flask, so that can't escape out as shown in figure 9.2. Initially only decomposition take place on (forward reaction), but after a while starts combining with CaO to form (reverse reaction). In the beginning, forward reaction is fast and reverse reaction is slow. But eventually, the reverse reaction speeds up and both reactions go on at the same rate. At this stage, decomposition and combination take place at the same rate but in opposite directions, as a result amounts of , CaO and do not change. It is written as