Types of Reaction The
chemical reactions are classified into three types on the basis of reaction rule: 1. Fast Reaction They are ionic in nature and takes place instantaneously. Such reactions have very low energy of activation. Since these reactions involve ions, hence are also called ionic reaction. It is possible to determine rates of such reaction. Ag+NO3-+ Na+Cl- ® AgCl + Na+NO3- 2. Slow Reaction These are the reactions, which proceed at extremely slow speed and take very long time for completion. Such reactions have very high energy of activation. It is difficult to determine experimentally velocity of such reactions for example rusting of Iron, radioactive decay of elements. 3. Moderate Reaction These are the reactions, which proceed at experimentally measurable rate. Generally reactions of
organic and covalent compounds are such type. CH3COOH + C2H5OH ® CH3COOC2H5 + H2O Determination of
rate of Reaction Two methods are employed for the determination of rate of chemical reaction. 1. Physical Method In this method, rate of a reaction is determined by observing the change in some physical property of either reactant or product. Physical properties involved may be: (i) Change in colour of reactant or product. (ii) Evolution of gas its volume is measure. (iii) Change in pH (iv) Change in Optical rotation of plane polarized light. 2. Chemical Method In this method, samples are drawn from the reacting vessel at regular intervals of time. Reaction is stopped at that particular moment by suddenly chilling the sample adding it to a suitable chemical. The amount of a reactant or product present at that time is generally found by titrating the sample against a proper reagent. For example, acidic hydrolysis of ethyl acetate is followed by determining the amount of CH3COOH formed at different time by titration against standard NaOH. H+ CH3COOC2H5 + H2O Û CH3COOH + C2H5OH Rate is determined from the graph.
Rate = Slop= Factors Affecting Rate of Reaction The rate of a reaction involving collisions of molecules is influenced by many factors. 1. Concentration of
reactants 2. Surface area of reactants 3. Nature of reactants 4.
Temperature 5. Catalyst 6. Radiation
1. Concentration of Reactants Concentration is defined as, “Number of molecules of a substance per unit volume.” Rate of reaction varies with the concentration of reactants. According to law of mass action, the rate of a chemical reaction is directly proportional to the concentration of reactants. By increasing the concentration of reactants, more crowding the molecules so frequency of collisions between them increases, resulting in the increase in rate of formation of products. Consider a general reaction: XA + YB ® Product Rate of Reaction µ
XY
In this rate of expression the sum of exponents of concentration (X + Y) is called Order of Reaction. The order of reaction is defined as, “Sum of all the exponents of the concentration in terms of the reactants involved in the rate equation.” Thus, (i) A + 2B ® Product
Order of reaction = 1 + 2 = 3 (Third order reaction) (ii) 2A ® Product (Second order reaction) (iii) A ® Product (First order reaction) 2. Nature of Reactant The nature of the reactants also affects the rate because activation energy for effective collision is different for different reacting substance. 2NO + O2 ® 2NO2 (Fast) 2CO + O2 ® 2CO2 (Slow) Reaction between NO and O2 is fast while that for CO and O2 is slow. This is because activation energy for CO is higher than that of NO. 4. Temperature The rate of a chemical reaction generally increases with the rise in temperature. As a general rate of thumb, the rate of reaction doubles for every 10°K rise in temperature. The reasons are: (i) As temperature increases the velocity of molecules also increases, and increases the frequency of collisions. (ii) The rise in temperature raises the kinetic energy of each molecule. It has been found by rai
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