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.
Common examples of such type are:
i) Acid-Base Neutralisation
ii) Precipitation from aqueous solution of NaCl and AgNO3
iii) Ionic Reactions
These reactions occur in about 10-14 to 10-16 seconds and are very fast. These reactions involve only the ions and no bonds are broken. Therefore their rates are very fast.
On the other hand there are certain reactions which occur at an extremely slow speed.
Examples of very slow reactions are:
- Rusting of iron
- Conversion of monoclinic sulphur to rhombic sulphur etc.
In between very slow and very fast reactions, we have reactions which proceed at a moderate speed which can easily be measured.
Examples of moderate speed reactions are:
i) The reaction between nitrogen peroxide and carbon monoxide
ii) Decomposition of hydrogen peroxide
iii) Decomposition of dinitrogen pentoxide
Rate of a Reaction
The rate of a reaction is broadly defined as the speed with which the reactants are converted into products. This can be determined both quantitatively and qualitatively.
Qualitatively, (concerned with or depending on constitiuents) an idea about the rate of a reaction can be obtained by observing either the speed of disappearance of the reactants or the speed of appearance of the products. For example when a piece of magnesium is put into a beaker containing dilute hydrochloric acid, hydrogen gas is rapidly evolved and magnesium disappears at once. On the other hand, when a piece of iron is placed in the same acid, hydrogen gas evolves at a slower rate and iron also disappears very slowly. This means that the reaction of magnesium and hydrochloric acid is faster than reaction of iron and hydrochloric acid.
Units of rate of reaction
Units of concentration (in aqueous solutions) is moles per litre
Unit of time is seconds, minutes etc.
For e.g., Consider a hypothetical reaction
The rate of a reaction may be expressed in either of the following two ways:
1) The rate of disappearance or decrease in concentration of A (reactants)
2) The rate of appearance or increase in concentration of B (products)
Equation A 
B shows that one mole of A produces one mole of B and thus the rate at which the concentration of A decreases will be the same as the rate at which the concentration of B increases.
The decrease or increase in the concentration of the reactants or products may also be expressed in terms of change in their concentration during time interval Dt as
Where D[A] gives the decrease in concentration of A and D[B] represents the increase in concentration of B. The square brackets around the substances are used to express the concentration (in terms of mol/litre). It may be noted that in the case of concentration of reactants minus sign is used. This implies that the concentration of the reactants is decreasing with time.
For e.g., consider the reaction:
The rate of the reaction can be expressed either by decrease of concentration of any of the reactants (NO or O3) or by increase in concentration of any of the products (NO2 or O2). Thus,
However for the reaction
We observe from the stoichiometry of the reaction, that one mole of H2 reacts with one mole of I2 to form two moles by hydrogen iodide. This means that the rate of disappearance of H2 and I2 is the same but the rate of appearance of HI must be twice the rate of disappearance of H2 and I2.
Thus we may write,
But for having unique value of the reaction rate (independent of the concentration term chosen), we divide the rate of the reaction, defined with respect to any of the reactants or products by the number of moles involved in the reaction.
Thus, for the above reaction, rate may be expressed as (by dividing by 2).
Consider another example,
When acidified hydrogen peroxide (H2O2) is added to a solution of potassium iodide (KI), iodine is liberated.
Concentration of iodine is zero initially. With the passage of time, the concentration of iodine increases and the reaction solution becomes brownish.
Concentration of iodine can be measured at different intervals of time by titrating against sodium thiosulphate solution.
If the concentration of iodine rises from 0 to 10-5 mol L-1 in seconds,
[Here symbol D represents a change and I2 represents the molar concentration of iodine].
This change in concentration of the product (iodine) takes place in ten seconds. Thus the rate can be called the average reaction rate.
The change in concentration over a shorter time interval will give a more accurate estimate of reaction at any moment.
Rate of reaction depends on the concentration of the reactants. With lapse of time, concentration of reactant/(s) decreases and therefore, rate of reaction also decreases.
In order to determine the rate of a particular instant, one should make the time interval almost zero.
d[I2] denotes a very small change in concentration of I2, and dt denotes very small change in time.
This rate is instantaneous rate.
The concentration of H2O2 decreases with time and thus rate in terms of H2O2 and KI is expressed as
The negative sign indicates that the concentration of H2O2 and KI decreases with time.
In this above reaction, rate of consumption of KI is twice the rate of consumption of H2O2.
Thus to make the two rates equal we divide by two and write
Consider another example,
Cisplatin, a compound with a molecular formula of Pt (NH3)2 Cl2 is used in chemotherapy of cancer. It reacts with water and releases chloride ion bound to central platinum metal.
The reaction is represented as:
The rate at which this reaction can occur is found by measuring the amounts of Pt (NH3)2 Cl2 consumed or the amount of Cl- released per unit volume of the reaction mixture.
The amount of cisplatin per unit volume at different intervals of time is obtained and provided in the given table.
| Time/minutes | Concentration of Cisplatin /mol L - 1 |
|---|---|
| 0 | 0.0100 |
| 20 | 0.00970 |
| 40 | 0.00940 |
| 80 | 0.00887 |
If we consider concentrations at t1 = 0 min and t2 = 40 min, then the rate of change of concentration of Cisplatin is calculated as follows:
Rate of change of concentration of Cisplatin
The reaction rate is defined as the rate of decrease of concentration of a reactant, or the rate of increase of concentration of a product and is always given as a positive number.
When rate is expressed in terms of 
which is a negative quantity, a minus sign is always used to make the rate positive.
Therefore the average rate of reaction between t1 = 0 minutes and t2 = 40 minutes will be
Factors Affecting the Rate of a Reaction
The rate of reaction depends on the following factors:- Concentration of the reactants
- Surface area of the reactants
- Temperature
- Catalyst
- Light
Effect of Concentration
The rate of a reaction generally increases with increase in concentration of the reactants. For a reaction to take place between two molecules, they must collide or come into contact. The probability of such collisions increase with the increase in the concentration of the reacting molecules.
Different reactants can affect the rate of a particular reaction to different extent.For example, consider the reaction
The rate of this reaction doubles when concentration of oxygen is doubled. On the other hand, when the concentration of nitric oxide is doubled, the rate of reaction increases four times.
In systems where the reactants and the products are in different states, the area of contact between the reacting substances influences the rate considerably. It is found that magnesium powder reacts much more rapidly than magnesium ribbon with dilute H2SO4.Activity to Study the Effect of Concentration
Requirements: 6g of granular zinc10 mL of 1 M HCl, 10 mL of 2 M HCl, A graduated syringe and a boiling tube.
The syringe is attached to the boiling tube and the reaction is carried out with 3g of zinc and 5 mL of 1 M HCl and next with 3g of zinc and 5 mL of 2 M HCl.The volume of the gas at time intervals of 20s, 40s and 60s is recorded and the results are plotted.
Data Plot Demonstrating Concentration Effects at 1 M and 2 M HCl, at constant Temperature
Effect of Surface Area of the Reactants
The rate of the reaction increases with increase in surface area of the reactants.
It is a common observation that small pieces of wood burn more rapidly than large logs of wood.As mentioned earlier, magnesium powder reacts with dilute sulphuric acid more rapidly than magnesium ribbons. Thus smaller the size of reacting particles, the greater is the total surface area exposed for reaction and consequently greater is the rate of the reaction.
Effect of Temperature
Generally, the rates of chemical reactions increase with increase in temperature. It is a common observation that perishable foods like milk, cooked vegetables decay more quickly in summers than in winter.
Consider the chemical reaction:
At 273 K, the reaction is extremely slow and the amount of CO2 collected is very small. When the temperature is raised to 323 K (50o C) the evolution of CO2 is more rapid.
Showing Effect of Temperature on the Rate of Reaction
The reaction between zinc and hydrochloric acid can be performed at two different temperatures: 293 K and 308 K.
It will be found that the volume of hydrogen formed during the same time interval will be more at 308K then at 298 K.
Data Plot Demonstrating Temperature Effects at 293 K and 308 K for 1M HCl
Effect of Catalyst
Catalysts are substances, which alter the rate of chemical reactions without undergoing any overall chemical change themselves.
In the manufacture of ammonia, iron is used as a catalyst and it enhances the rate of this reaction.
as a catalyst.
Sometimes catalysts are used to retard or slow down the rates of reaction. For example., glycerol is used to slow down the decomposition of hydrogen peroxide. Such catalysts are known as negative catalysts.
Many industrial processes like the manufacture of polythene and polystyrene require catalysts.In the living bodies, a large number of complex chemical reactions occur which are catalysed by complex organic molecules called enzymes. For example., enzyme amylase present in saliva catalyses the decomposition of starch into maltose.
Effect of Light
There are many reactions, which are influenced by light. For example., photosynthesis and photography. Such reactions are known as photochemical reactions.
Other examples are:
Light supplies the necessary energy for the reaction to take place. Photographic films which are normally coated with silver bromide (and a very small amount of silver iodide) undergo chemical reaction when exposed to sunlight. This basic reaction is used in photography.
