The rate of a chemical reaction:

Options

A. Increases as the reaction proceeds
B. Decreases as the reaction proceeds
C. May increase or decrease during the reaction
D. Remains constant as the reaction proceeds

Answer

To solve the question regarding the rate of a chemical reaction, we will analyze the options given and understand the relationship between the concentration of reactants and the rate of reaction.

### Step-by-Step Solution:

1. **Understanding the Rate of Reaction**:
The rate of a chemical reaction is defined by how quickly reactants are converted into products. It can be expressed mathematically as:
\[ \text{Rate} = k \cdot [A]^m \cdot [B]^n \]
where \( k \) is the rate constant, \([A]\) and \([B]\) are the concentrations of the reactants, and \( m \) and \( n \) are the orders of the reaction with respect to each reactant.

2. **Observation of Reactants and Products**:
Consider a reaction where reactants A and B produce products C and D:
\[ A + B \rightarrow C + D \]
Initially, at \( t = 0 \), the concentrations of A and B are at their maximum (100%), and the concentrations of C and D are at 0%.

3. **Progress of the Reaction**:
As the reaction proceeds to time \( t = T_1 \), some of the reactants A and B are converted into products C and D. For example, if 80% of the reactants have reacted, then:
- Concentration of A and B decreases (let's say to 20% each).
- Concentration of C and D increases (to 80% each).

4. **Effect on Rate of Reaction**:
The rate of reaction depends on the concentration of the reactants. As the concentrations of A and B decrease, the rate of reaction will also decrease. This is because the rate is directly proportional to the concentration of the reactants.

5. **Evaluating the Options**:
Now, let's evaluate the options provided in the question:
- **Option A**: Increases as the reaction proceeds - **Incorrect** (the rate decreases).
- **Option B**: Decreases as the reaction proceeds - **Correct** (as explained, the concentration of reactants decreases).
- **Option C**: May increase or decrease during the reaction - **Incorrect** (the rate consistently decreases).
- **Option D**: Remains constant as the reaction proceeds - **Incorrect** (the rate cannot remain constant if reactants are being consumed).

6. **Conclusion**:
The correct answer is that the rate of the chemical reaction **decreases as the reaction proceeds**.

### Final Answer:
**The rate of a chemical reaction decreases as the reaction proceeds.**