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Rate of a Reaction

Chemistry, by its very nature, is .concerned with change. Substances with well defined properties are converted by chemical reactions into other substances with different  properties.

For any chemical reaction, chemists try to find out

(a) the feasibility of a chemical reaction

(b) extent to which a reaction will proeeed

(c) speed of a reaction i.e., time taken by a reaction to reach equilibrium.

It is important to know the rate and the factors controlling the rate of a chemical reaction for its complete understanding. For example,

(i) Which parameters determine as to how rapidly food gets spoiled?

(ii) How to design a rapidly setting material for dental filling?

(iii) What controls the rate at which fuel burns in an auto engine?

During the course of our studies we come across a wide variety of chemical reactions having different rates. In general, various reactions can be categorised into three types depending upon the reaction rates.

1. Very Fast Reactions (Type 1). This type consists of reactions which occur almost instantaneously. For example, the precipitation of AgCI occurs instantaneously by addition of aqueous solution of sodium chloride to aqueous solution of silver nitrate.

AgNO3(aq) + NaCl(aq) à AgCl(s) + NaNOiaq)

The rates of such reactions (ionic reactions) cannot be determined easily.

2. Very Slow Reactions (Type 2). This type comprises reactions which occur at a

very slow rate. These reactions may require months or even years together for their completion.

For example, rusting of iron.

The rates of such reactions are hardly of any physical importance.

 3. Moderately Slow Reactions. This type refers to reactions in between the very fast

(type 1) and very slow (type 2) reactions. These reactions proceed at moderate speed which can be easily measured. Inversion of cane sugar and hydrolysis of starch are common examples of this type of reactions.

RATE OF A REACTION

The rate of a chemical reaction is the speed or velocity with which a reaction takes place. It can be expressed qualitatively as well as quantitatively.

(a) Qualitative Rate. The qualitative rate is based on certain visual parameters like disappearance of reactants, colour change, effervescence, etc. The rate expressed as such provides arbitrary conclusions about the rate of a reaction.

(b) Quantitative Rate. The quantitative rate of a chemical reaction provides us with much better information about the rate data and the accurate rate of a reaction. Quantitatively, the rate of chemical reaction may be expressed in any of the following ways :

(i) The rate of decrease in concentration of any one of the reactant, or

(ii) The rate of increase in concentration of any one of the product.

For example, for a hypothetical reaction, assuming that the volume of the system remains constant.

R

RATE OF A REACTION

 

The rate of a chemical reaction is the speed or velocity with which a reaction takes place. It can be expressed qualitatively as well as quantitatively.

 

(a) Qualitative Rate. The qualitative rate is based on certain visual parameters like disappearance of reactants, colour change, effervescence, etc. The rate expressed as such provides arbitrary conclusions about the rate of a reaction.

 

(b) Quantitative Rate. The quantitative rate of a chemical reaction provides us with much better information about the rate data and the accurate rate of a reaction. Quantitatively, the rate of chemical reaction may be expressed in any of the following ways :

 

(i) The rate of decrease in concentration of any one of the reactant, or

 

(ii) The rate of increase in concentration of any one of the product.

For example, for a hypothetical reaction, assuming that the volume of the system remains constant.

P

One mole of the reactant R produces one mole of the product P. If [R]1  and [P]1  are the concentrations of R and P respectively at time t 1 and [R]2 and [P]2 are their concentrations at time t2 then,

One mole of the reactant R produces one mole of the product P. If [R]1  and [P]1  are the concentrations of R and P respectively at time t 1 and [R]2 and [P]2 are their concentrations at time t2 then,

The square brackets in the above expressions are used to express molar concentration.

Rate of disappearance of R

= Decrease in concentration of R / Time taken = Δ [R] / Δt

Rate of appearance of p

= Increase in concentration of P / Time taken = + Δ[p] / Δ t

While writing the expressions for rate we use a negative sign in the expression employed for expressing rate in terms of reactants.

 

PHYSICAL CHANGES WHICH INDICATE PROGRESS OF REACTION

Experimental determination of reaction rates depends upon the nature of reaction under investigation. Rates, can be conveniently studied by measuring the suitable physical property of the system as a whole or of one of the constituent of the reaction. Some common examples are given below:

1. Disappearance of Reactants and Appearance of Products. The visual disappearance of reactants or appearance of products indicates progress of reaction. For example, in reaction of zinc granules with HCl, the granules disappear after sometime and the reaction stops.

2. Colorimetry or Colour Change. Here increase or decrease in the intensity of colour is observed at regular intervals

3. Rotation of the Plane of Polarised Light. It is observed that some molecules rotate the plane of polarized light. The amount of rotation can be measured using a polarimeter. The reaction of sucrose with water is an example of this category. In the reaction, surcose changes into a mixture of glucose and fructose. This brings about a change in the amount of rotation. The rotation can be measured at various time intervals. A graph of angle of rotation plotted against time allows the rate of the reaction to be determined.

4. Volume Changes. We have already seen how to measure volume changes when gases are given off in a reaction. Some reactions involving liquids also show volume changes. An example is the reaction between 2-methylpropene and water:

(CH3)2 C = CH2 + H2O à (CH3)3 COH

In the reaction, two molecules are replaced by one and there is a small decrease in volume. The reaction can be followed using a dilatometer.

5. pH Measurement. In hydrolysis of ethyl acetate, the gradual increase in concentration. of acetic acid is measured by measuring the pH of solution

6. Pressure Changes. Reactions between gases are often followed by measuring changes in pressure rather than volume. (It is easier to keep the volume of the apparatus constant and measure pressure changes.) Examples of reactions that have been followed using pressure changes are:

2NO(g) + O2(g) à 2NO (g)

2N2O(g) à 2N2(g) + O2

Notice that in both cases there is a change in the number of moles of gas. Gradual increase or decrease of pressure is noticed in gaseous system

7. Change in Mass. The rate of a reaction can also be determined by change in mass during the reaction. For example, when a reaction is carried out between CaCO3 and HCl, carbon dioxide gas is evolved causing a loss in mass. This indicates progress of reaction.

8. Change in Temperature. Changing temperature can also help predicting progress of reaction. For-example, the colour of KMnO4 in its reaction with oxalic acid discharges very slowly at room temperature but when the solution is warmed i.e., temperature is increased, it decolourises faster.