Reaction Rate and Stecheiometry

Let us consider a general reaction between hydrogen and iodide.

$$\ce{H2 + O2->2HI}……(i)$$

The rate of this reaction can be determined by the rate of disappearance of hydrogen or rate of formation of hydrogen iodide.

For each molecule of Hydrogen, two moles of hydrogen iodide is formed.


$$\text{Rate of disappearance of H}_2 = -\frac{d{[H]}_2}{{dt}}$$

$$\text{Rate of formation of HI} = -\frac{d{[HI]}}{{dt}}$$

The rate of reaction which are equivalent to each other for the above reaction is given by:

$$\frac{d{[H]}_2}{{dt}} = -\frac{d{[HI]}}{{2dt}}$$

Rate law

We know that at given temperature, the rate of reaction depends upon the concentration of reaction. So, the expression which shows how the reaction is related with the concentration of reactant is called rate law or rate equation.

Let us consider a general reaction.

$${\text{A} \rightarrow \text{Product}}$$

$$\text{Rate} {\propto} \text{[A}^C]$$

$$\text{Rate} = \text{K[A}^C……….(i)]$$

Equation (i) is the rate equation.

For a reaction,

$${\text{2A + B} \rightarrow \text{Product}}$$

$$\text{Rate} = \text{K[A]}^C.[B]^C$$

Some other specific examples:

Reaction Rate Law
2N2O5  → 4NO2 + O2 K[N­2O5]
H2 +I2 → 2HI K[H2]· [I2]
2NO  → N2 + 2H2O K[NO]2 · [H2]

From the above examples, it is found that the power of concentration expressed in rate law may or may not be equal to its stechiometric  coefficient.

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