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Number of molecules which must collide simultaneously to give product is called molecularity. It is equal to sum of coefficients of reactants present in stoichiometric chemical equation.
For reaction, $m_1A + m_2B \rightarrow$ Product
Molecularity $= [m_1 + m_2]$
ln complex reaction each step has its own molecularity which is equal to the sum of coefficients of reactants present in a particular step. Molecularity is a theoretical property. Its value is any whole number. Number of concentration terms on which rate of reaction depends is called order of reaction or sum of powers of concentration terms present in the rate equation is called order of reaction.
If rate equation ofreaction is: Rate $=\text{k}\cdot\text{C}^{\text{m}_1}_\text{A}\cdot\text{C}^{\text{m}_2}_\text{B}$
Then order of reaction $= m_1 + m_2.$
ln simple reaction, order and molecularity are same.
ln complex reaction, order of slowest step is the order ofover all reaction. This step is known as rate determining step. Order is an experimental property. Its value may be zero, fractional or negative.
The following questions are multiple choice questions. Choose the most appropriate answer:

1. Higher order$(> 3)$ reactions are rare due to:

1. Shifting of equilibrium towards reactants due to elastic collisions.
2. Loss of active species on collision.
3. Low probability of simultaneous collision of all the reacting species.
4. Increase in entropy and activation energy as more molecules are involved.

2. The molecularity of the reaction:

$6\text{FeSO}_4+3\text{H}_2\text{SO}_4+\text{KClO}_3\rightarrow\text{KCl}+3\text{Fe}_2(\text{SO}_4)_3+3\text{H}_2\text{O}$ is:

1. $6$
2. $10$
3. $3$
4. $7$

3. Which of the following statements is false in the following?

1. Order of a reaction may be even zero.
2. Molecularity of a reaction is always a whole number.
3. Molecularity and order always have same values for a reaction.
4. Order of a reaction depends upon the mechanism of the reaction.

4. The rate of reaction, $A + 2B →$ products, is given by the following equation:

$-\frac{\text{d}[\text{A}]}{\text{dt}}=\text{k}[\text{A}][\text{B}]^2$

If $B$ is present in large excess, the order of the reaction is:

1. Zero
2. First
3. Second
4. Third

5. The rate of the reaction, $A + B + C →$ products, is given by $\text{r}=\frac{\text{d}[\text{A}]}{\text{dt}}=\text{k}[\text{A}]^\frac{1}{2}[\text{B}]^\frac{1}{3}[\text{C}]^\frac{1}{4}.$ The order of the reaction is:

1. $\frac{1}{3}$
2. $\frac{1}{4}$
3. $\frac{1}{2}$
4. $\frac{13}{12}$