All first order reactions are unimolecular.
T or F

Answer

To determine whether the statement "All first order reactions are unimolecular" is true or false, we can analyze the definitions of order and molecularity in the context of chemical kinetics.

### Step-by-Step Solution:

1. **Understand the Definitions**:
- **Order of Reaction**: The order of a reaction is the sum of the powers of the concentration terms in the rate law. For a first-order reaction, the rate is directly proportional to the concentration of one reactant raised to the power of one.
- **Molecularity of Reaction**: Molecularity refers to the number of reactant molecules involved in an elementary reaction step. A unimolecular reaction involves one molecule, while a bimolecular reaction involves two.

2. **Identify the Relationship**:
- A first-order reaction implies that the rate depends on the concentration of one reactant. However, this does not necessarily mean that the reaction is unimolecular.
- A reaction can be first-order even if it involves two reactants, provided that one of the reactants is in excess. In such cases, the concentration of the excess reactant remains relatively constant, allowing the reaction to behave as if it were first-order with respect to the limiting reactant.

3. **Example of a Bimolecular Reaction**:
- Consider a reaction where A and B react to form C, and B is in excess:
\[ A + B \rightarrow C \]
- If B is in large excess, the rate can be expressed as:
\[ \text{Rate} = k[A][B] \approx k'[A] \]
- Here, \(k' = k[B]\) is a new rate constant that incorporates the constant concentration of B. The reaction appears first-order with respect to A, even though the molecularity is bimolecular (2).

4. **Conclusion**:
- Since there are first-order reactions that can arise from bimolecular reactions where one reactant is in excess, the statement "All first order reactions are unimolecular" is **False**.

### Final Answer:
**False**