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| Difference between rate law and the law of mass action |
You see, in chemical kinetics which is a study on measurement of how fast a chemical reaction occurs you will deal with two such laws, rate law expression and the law of mass action.
| Difference between rate law expression and the law of mass action | |||
|---|---|---|---|
| S.no | On the basis of | Rate law expression | Law of mass action |
| 1 | Definition | It expresses the rate of chemical reaction | Equation which represents the ratio between the mass of products and reactant raised to the power of the stoichiometric coefficient. |
| 2 | Equation components | The rate law is composed of rate constant and concentration of the reactants with the order of overall reaction | The law of mass action equation consists of a concentration of reactants and products raised to the power of the stoichiometric coefficient. |
| 3 | Product | The product is not included in the rate law | The product of the reaction is included in the equation of the law of mass action. |
| 4 | Proportionality constant | Rate law has proportionality constant ‘K’ | The law of mass action does not have a proportionality constant. |
| 5 | Stoichiometric coefficient | Concentrations of reactants are raised to some power which may or may not be equal to the stoichiometric coefficient of the reaction. | Concentrations of reactants and products are raised to the power of their stoichiometric coefficient. |
So to clear your doubt completely, let’s see the meaning of both the laws and what they both state.
Before going straight into the topic of rate law or law of mass action, we need to learn some basics.
So in any chemical reaction, the rate of a chemical reaction depends on the concentration of reactants or in simpler terms, “amount of reactant in a chemical equation decides the rate of reaction.”
Both the rate law and law of mass action convey the same meaning in some way or the other but with a slight difference and therefore people get confused between the two.
So first we will look at what is rate law and law of mass action and then will differentiate between both of them on various factors such as product, equation, proportionality constant just how we did it in the table above but in more depth.
Table of Contents
What is rate law ?
So as I mentioned earlier, both the laws signify that the rate of reaction depends on the concentration of reactant but in different ways.
Rate law or rate equations is a mathematical expression that describes the relationship between the rate law and concentration of reactants in the equation.
So, here is the equation of rate law:
Here [A] is the concentration of reactant A, [B] is the concentration of reactant B and [C] is the concentration of reactant C with raise to the power of ‘m’, ‘n’ and ‘p’ which may or may not be equal to the stoichiometric coefficient of reactant respectively and this goes on depending upon the number of reactant in a chemical equation.
Well, I know you may be wondering that what does stoichiometric coefficient mean, so let me tell you for a general chemical equation: “aA+bB ￫ cC+dD“
In the above equation, the stoichiometric coefficient of reactant ‘A’ and ‘B’ will be ‘a’ and ‘b’ respectively.
What is the law of mass action ?
The law of mass action states that the rate of chemical equation is directly proportional to the product of the active mass of the reactants, with each active mass term raised to the power equal to its respective stoichiometric coefficient in a balanced equation with a constant temperature.
Why in rate law expression the concentration of reactant, power is not always the stoichiometric coefficient ?
Conclusion:
So I hope, now that the difference between both the rate law expression and the law of mass action is clear to you, as now you know both the laws convey the same message that the rate of chemical reactions depends on the concentration of reactants.
But law of mass action only considers the concentration of reactant as the factor for the rate of reaction but in reality rate of a chemical reaction depends on many other factors as well and the reason why rate law and law of mass action are almost the same has a slight difference.
