Which of the following does not affect the rate of reaction?

Options

A. Amount of the reactant taken
B. Physical state of the reactant
C. `DeltaH` of reaction
D. Size of vessel

Answer

To determine which factor does not affect the rate of a reaction, we can analyze each option provided in the question:

1. **Amount of the Reactant Taken**:
- The rate of a reaction is directly influenced by the concentration of the reactants. More reactants mean more molecules available to collide, leading to an increased rate of reaction.
- **Conclusion**: This factor affects the rate of reaction.

2. **Physical State of the Reactant**:
- The physical state (solid, liquid, gas) of the reactants plays a significant role in the reaction rate. For example, gases react faster than solids due to greater mobility and collision frequency.
- **Conclusion**: This factor affects the rate of reaction.

3. **Enthalpy of Reaction**:
- Enthalpy change (ΔH) refers to the heat content change during a reaction. While it indicates whether a reaction is exothermic or endothermic, it does not influence how fast the reaction occurs. The rate of reaction is not determined by the energy change but rather by the frequency of collisions and the energy of those collisions.
- **Conclusion**: This factor does not affect the rate of reaction.

4. **Size of Vessel**:
- The size of the vessel can affect the concentration of reactants, especially in gas reactions. A larger vessel may dilute the reactants, leading to fewer collisions and a slower reaction rate.
- **Conclusion**: This factor affects the rate of reaction.

Based on the analysis above, the correct answer is that the **enthalpy of reaction** does not affect the rate of reaction.

### Final Answer:
The factor that does not affect the rate of reaction is **Enthalpy of Reaction**.

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