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Factors Influencing Rate of a Reaction

The rate of reactions is influenced by the following factors:

(i) Concentration of the reactants,

(ii) Temperature of the reactants,

(iii) Particle size and nature of the reacting substances,

(iv) Presence of catalyst, and

(v) Exposure to radiations.

 

1. Dependence of Rate on Concentration

When a chemical reaction occurs, the reactants change over to products. It is observed that with the passage of time the concentrations of reactants decrease while those of products increase. It is graphically shown in Fig. 20.8.

Fig. 20.8. Time dependence of the concentrations of reactants and products in a reaction.

 

Now, if we assume that other factors are constant then the rate of a chemical reaction decreases with the decrease in concentration of the reactants. For example, we find that a piece of’ wood bums at a much faster rate in oxygen than in air. It is because of higher concentration of O2 in the fanner.

Cato Guldberg and Peter Waage proposed a qualitative relationship between the rates of reactions and the concentration of the reacting species. This generalisation is known as ”Law of Mass Action”. It states that:

At a given temperature, the rate of a chemical reaction is directly proportional to the product of molar concentrations of reacting species with each concentration term raised to the power equal to numerical coefficient of  that species in the chemical equation.

Thus, for a hypothetical reaction;

A + B à Produces

The rate according to law of mass action is given as Rate of reaction

where [A] and [B) are the molar concentrations of the reactants A and B respectively and k is a constant of proportionality.

 

Particle Size and Nature of Reactants

It is commonly observed that the nature of the reacting substances has a marked influence on the reaction rates. The effect of the nature of reactants can be described in terms of the following factors:

(a) Physical state of reactants

(b) Surface area and particle size of reactants

(c) Chemical nature of reactants.

 

(a) Physical State of the Reactants

We know that the chemical reaction takes place as a result of the collisions between the reacting particles. In order to collide with one another, the reacting particles must come close and intermix. Now intermixing of reacting particles cannot take place easily if they are in solid state. This is due to restriction in their molecular motion. However, in liquid state as well as in gaseous state the intermixing is possible and chances of their collision become larger. Thus, the reaction will be faster if the reactants are mixed in liquid phase or solution phase.

 

This can be illustrated with the help of following reaction between lead nitrate and potassium iodide.

Pb (NO3)2 + KI à PbI2 + KNO3

 

(i) In one of the experiment, solid Pb(NO3)2 is mixed with solid KI in a china dish with the help of spatula.

(ii) In another experiment, aqueous solution of Pb(NO3)2 is mixed with aqueous solution of KI in a beaker.

It is observed that rate of formation of ( yellow coloured Pbl2 is much faster in the second experiment because probability of encoumers between the- reacting particles is much larger in liquid phase than that in solid phase.

 

(b) Surface Area and Particle Size of Reactants

Particle size and surface area play an important role in rates of heterogeneous reactions particularly when one of reacting species is in .solid phase. Smaller the size of the reacting particles present in the solid phase larger will be their surface area. As a result, more will be the probability of the reacting species coming in contact with each other and therefore, more will be the .number of encounters between them. For example,

 

  • combustion of a piece of coal in air is relatively slower, whereas combustion of coal dust in air is much faster. dissolution of sugar crystals in water is slower but
  • dissolution of powdered sugar is rapid. Thus, we find that the rate of the process is increased, if the particle size in the solid state is decreased. In fact, decrease in the particle size increases the surface area.

 

(c) Chemical Nature of the Reactants

The chemical nature of reacting substances also affects the reaction rates significantly. Depending upon the nature of reacting species, the rates of different reactions may differ very widely from one another. For example, the oxidation of ferrous ions (Fe+2) by potassium permanganate in acidic medium is practically instantaneous. Whereas oxidation of oxalate ions (C2O 4- 2) by potassium permanganate in acidic solution is comparatively much slower:

In these two reactions, everything is identical except the nature of reducing agents.

 

3. The Effect of Pressure

The change in pressure also affects the rate of a reaction.

  • If one or more .of the reactant§ is a gas then increasing pressure will effectively increase the concentration of the reactant molecules and speed up the reaction.
  • Changing the pressure i.e., lower or higher, results in lesser or greater concentration and so slower or faster reaction, all because of the increased chance of a ‘fruitful’ collision.

 

4. The Effect of Stirring

In doing rate experiments with a solid and solution reactant e.g., marble chips-acid solution or a solid catalyst like manganese(IV) oxide catalysing the decomposition of hydrogen peroxide solution, it is sometimes forgotten that stirring the mixture is ail important rate factor.

  • If the reacting mixture is not stirred ‘evenly’, the reactant concentration in solution becomes much less near the solid, which tends to settle out at the bottom of the flask.
  • Therefore, at the bottom of the flask the reaction prematurely slows down distorting the overall rate measurement and making the results uneven and therefore inaccurate. The ‘unevenness’ of the results is even more evident by giving the reaction mixture the ‘odd stir’ ! You get jumps in the graph!! !

 

5. The Effect of Radiations

Reactions carried out in the presence of light radiations are known as photochemical reactions. The presence/absence of radiations greatly affects the rate of a reaction. Some examples are given below to illustrate the idea.

  • Halogenation of methane can readily occur in the presence of uv radiations. The reaction does not occur in dark. The radiations supply sufficient energy to break the bonds of Cl2 molecule giving free radicals and hence the reaction.

Photosynthesis in green plants occurs in the presence of sunlight. The green chlorophyll molecules absorb the photon energy packets of sunlight and initiate the chemical changes summarised below:

Water  + carbon dioxide  à glucose + oxygen

6H2O(l) + 6CO2(g)  à C6H12O6(aq) + 6O2(g)

 

6. Effect of Temperature

The rates of almost all reactions increase with the increase in temperature. For example,

• In decomposition of N2O 5, the time taken for half of original amount of material to decompose is 12 min at 50°C, 5 hours at 25°C and 10 days at 0°C. ·

• In a mixture of KMnO 4 and Oxalic acid (H2Cp 4), potassium permanganate gets decolourised faster at higher temperature that at lower temperature.

In most of the cases it has been found that the rate of the reaction becomes almost double for every 10° rise of temperature. This is also expressed in the terms of Temperature coefficient which is the ratio of rate constants of the reaction at two temperatures differing by 10°. The two temperatures generally selected are 298 K and 308 K.

Thus,

Temperature coefficient = Rate constant of 308 K / Rate constant at 298 K

 

The rate constants for the decomposition of N2O5 at different temperatures is given in Table 20.1. From the table, it is seen that rate constant at 273 K is 7.87 x w-1 and at 298 K is 3.56 X w-5 This shows that for 25° rise of temperature of the rate constant increases by 43 times.

 

Table 20.1. Rate Constants for the Decomposition of N2O 5 at Different Temperatures