Question:
Rate law for the reaction \( A + B \rightarrow C + D \) is \( r = k[A][B] \). Which from the following conditions does NOT affect the rate of reaction?
Rate law for the reaction \( A + B \rightarrow C + D \) is \( r = k[A][B] \). Which from the following conditions does NOT affect the rate of reaction?
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In rate laws, the rate depends on the concentrations of reactants as shown by the exponents in the rate law expression. If one of the concentrations is kept constant, it does not affect the rate of reaction.
Updated On: Jun 30, 2026
- \( \text{Concentration of A is doubled and concentration of B is kept constant.} \)
- \( \text{Concentration of B is doubled and concentration of A is kept constant.} \)
- \( \text{Concentration of B is doubled and concentration of A is halved.} \)
- \( \text{Concentration of A is kept constant and concentration of B is halved.} \)
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The Correct Option is D
Solution and Explanation
Step 1: Analyze the rate law.
The rate law for the reaction is given by: \[ r = k[A][B] \] This means that the rate of the reaction is directly proportional to the concentration of both A and B.
Step 2: Consider the options.
- Option (1): If the concentration of A is doubled, the rate will double since the rate depends on the concentration of A.
- Option (2): If the concentration of B is doubled, the rate will also double as the rate depends on the concentration of B.
- Option (3): If the concentration of B is doubled and the concentration of A is halved, the overall rate will change depending on the multiplication of both terms.
- Option (4): If the concentration of A is kept constant and the concentration of B is halved, the rate will decrease because the rate depends on the concentration of B.
Step 3: Identify the condition that does NOT affect the rate.
In Option (4), the concentration of A is kept constant, so it will not affect the rate of the reaction. The rate depends on both A and B, but halving the concentration of B reduces the rate. However, the concentration of A being constant doesn't change the proportionality factor for the rate.
Step 4: Final conclusion.
Thus, the condition that does NOT affect the rate is: \[ \boxed{(4)\ \text{Concentration of A is kept constant and concentration of B is halved.}} \]
The rate law for the reaction is given by: \[ r = k[A][B] \] This means that the rate of the reaction is directly proportional to the concentration of both A and B.
Step 2: Consider the options.
- Option (1): If the concentration of A is doubled, the rate will double since the rate depends on the concentration of A.
- Option (2): If the concentration of B is doubled, the rate will also double as the rate depends on the concentration of B.
- Option (3): If the concentration of B is doubled and the concentration of A is halved, the overall rate will change depending on the multiplication of both terms.
- Option (4): If the concentration of A is kept constant and the concentration of B is halved, the rate will decrease because the rate depends on the concentration of B.
Step 3: Identify the condition that does NOT affect the rate.
In Option (4), the concentration of A is kept constant, so it will not affect the rate of the reaction. The rate depends on both A and B, but halving the concentration of B reduces the rate. However, the concentration of A being constant doesn't change the proportionality factor for the rate.
Step 4: Final conclusion.
Thus, the condition that does NOT affect the rate is: \[ \boxed{(4)\ \text{Concentration of A is kept constant and concentration of B is halved.}} \]
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