Introduction
We observe various chemical processes in our daily life. It is a common observation that the rates of these chemical reactions vary and depend on certain factors. The knowledge of the rate of a reaction is of importance in industries to ensure optimum productivity.
Rate of Chemical Reactions
Everyday experience tells us that chemical reactions take place in widely varying rates. Infact rates of various reactions vary from very slow to very fast. Some reactions are so rapid that they occur as soon as the reactants are mixed. For example, the reactions involving ionic species (known as ionic solutions) are very fast.
Reversible Reactions, Irreversible Reactions and Equilibrium
It is a common observation that most of the reactions when carried out in closed vessels do not go to completion, under a given set of conditions of temperature and pressure. Infact in all such cases, in the initial state, only the reactants are present but as the reaction proceeds, the concentration of reactants decreases and that of products increases. Finally a stage is reached when no further change in concentration of the reactants and products is observed.
Changing the Position of Equilibrium
A reversible reaction attains equilibrium when the opposing reactions occur at an equal rate. Therefore, if a reaction at equilibrium is subjected to some change it changes the rate of one reaction more than the other. By changing the concentration, temperature or pressure of any of the reactants or products, the position of equilibrium can be changed. The effect of these changes can be summarised by Le-Chatelier's Principle.
Dynamic Nature of Chemical Equilibrium
The state of equilibrium is a state in which the measurable properties of the system do not undergo any noticeable change under a particular set of conditions.
Acids and Bases - pH Scale
Arrehenius' Concept of Acids and Bases
An acid is a substance which can furnish hydrogen ions in its solution.
Examples:
Conjugate Acid - Base Pair
An acid after losing a proton becomes a base whereas a base after accepting the proton becomes an acid.
For example, let us consider the reaction between water and ammonia as represented by the following equilibrium:
Lewis Concept of Acids and Bases
Although Bronsted-Lowry theory was more general than Arrhenius' theory of acids and bases, it failed to explain the acid-base reactions which do not involve transfer of proton. For e.g., it fails to explain how acidic oxide such as anhydrous CO2, SO2, SO3 etc can neutralise basic oxides such as CaO, BaO, etc. even in the absence of a solvent.
Heat Changes During Chemical Equilibrium
There are many reactions in which energy is evolved while in others energy is absorbed. Depending upon evolution or absorption of energy, the chemical reactions can be classified into two types: (i) Exothermic reactions and (ii) Endothermic reactions.
Summary
Rate of a Reaction
Rate of a reaction is defined as the speed at which the reactants are converted into products.
