Rate of reaction, Rate Law and Rate Constant Chemistry Notes
Reaction Rate, Rate Law and Rate Constant :
According to law of mass action, the rate of reaction is directly proportional to the concentration of reactants at a certain temperature. For following reaction, A → B
If concentration of A at any time is CA, then from law of mass action,
Here, k is a proportionality constant which depends on nature of substance, temperature etc. It is called rate constant or rate coefficient or specific rate of reaction.
Hence, “specific reaction rate or rate constant or rate coefficient is the rate of a reaction where the concentrations of all reactants are unity.”
Characteristics of Rate Constant :
The main characteristics of rate constant (k)are as follow:
- The rate of a reaction depends on rate constant and increases with increase in rate constant.
- The rate constant of each reaction at a specific temperature is definite.
- The value of rate constant for a reaction changes with change in temperature.
- The value of rate constant is completely independent from concentration of species taking part in a reaction.
- The unit of rate constant depends on nature of reaction or order of reaction.
Differences between Reaction Rate and Rate Constant :
|The rate of reaction is change in molar concentration of reactants per unit time.
|It is a constant and its value is similar to rate of reaction when molar concentrations of each reactants are same.
|Its value at any time : depends on molar concentration of reactants.
|Its value depends on temperature. It does not depend on molar concentration.
|Its unit is always mol L-1s-1
|Its unit depends on nature of reaction.
|Its value decreases with time on initiating the reaction.
|Its value is always constant. It does not depend on progress of reaction.