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So far, we have discussed different types of chemical reactions and the characteristic of the reactants and products. Let us consider the following reactions:
  • Rusting of iron
  • Digestion of food
  • Burning of petrol
  • Weathering of rock
What is the speed of each reaction? Sort them in order of slowest to fastest. How will you know which one is the fastest and which is the slowest?
 
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One method of determining the speed of a reaction is as follows:
 
Determine the amount of reactants or products present before and after a specified time period. For example, let us assume that \(100\) \(g\) of a substance ‘\(A\)’ undergoes a reaction and after an hour, \(50\) \(g\) of ‘\(A\)’ is left.
 
\(A → Product\)
 
In another case, \(100\) \(g\) of substance '\(C\)' undergoes a reaction and after an hour, \(20\) \(g\) of '\(C\)' is left.
 
\(C → Product\)
 
Which reaction do you think is faster?

In the first reaction, \(50\) \(g\) of the reactant (\(A\)) is converted into products, whereas in the second reaction, \(80\) \(g\) of the reactant is converted into products in one hour. So, the second reaction is faster than the first reaction. This measurement is called the 'Reaction rate'.
Rate of a reaction is the change in the amount or concentration of any one of the reactants or products per unit time.
Consider the reaction below:

\(A → B\)

The rate of reaction is given by
 
Rate=dAdt=+dBdt
 
Where,
 
\([A]\) is the concentration of \(A\).
 
\([B]\) is the concentration of \(B\).
 
Note: ‘[ ]’ represents the concentration, ‘\(d\)’ represents the infinitesimal change in the concentration.
 
The negative sign implies that the concentration of \(A\) is decreasing over time.

The positive sign implies that the concentration of \(B\) is increasing over time.
 
Why is reaction rate important?

The faster the reaction, the more will be the amount of the product in a specified time. So, the reaction rate is important for a chemist for designing a process to ensure a high yield of a product.
 
The reaction rate is also important for a food processor who hopes to slow down the reactions that cause food to spoil.