Why does the rate of any reaction generally decreases during the course of the reaction?

Explanation

### Step-by-Step Solution

1. **Understanding Reaction Rate**:
The rate of a chemical reaction is defined as the change in concentration of reactants or products over time. For a reaction where A is converted into B, the rate can be expressed mathematically as:
\[ \text{Rate} = -\frac{d[A]}{dt} \]
where \([A]\) is the concentration of the reactant A.

2. **Change in Concentration**:
As the reaction proceeds, the concentration of A (the reactant) decreases while the concentration of B (the product) increases. This can be represented as:
\[ [A] \text{ decreases} \quad \text{and} \quad [B] \text{ increases} \]

3. **Effect on Rate**:
Since the rate of reaction is dependent on the concentration of the reactants, a decrease in the concentration of A leads to a decrease in the rate of reaction. This is because fewer molecules of A are available to collide and react with each other. Therefore, we can conclude:
\[ \text{As } [A] \text{ decreases, Rate decreases.} \]

4. **Collision Theory**:
According to collision theory, the rate of a reaction is proportional to the number of effective collisions between reactant molecules. As the concentration of A decreases, the number of molecules available for collisions also decreases, leading to fewer effective collisions over time. Thus:
\[ \text{Fewer collisions} \Rightarrow \text{Decreased rate of reaction.} \]

5. **Conclusion**:
Therefore, the overall reason why the rate of any reaction generally decreases during the course of the reaction is that the concentration of reactants decreases, leading to fewer collisions and a lower rate of reaction.