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In a reaction, if the concentration of reactant A is halved, and the rate of rea

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Question: In a reaction, if the concentration of reactant A is halved, and the rate of reaction decreases to one-fourth, what is the order of the reaction? (2020)

Options:

  1. Zero
  2. First
  3. Second
  4. Third

Correct Answer: Second

Solution: 

If the rate decreases to one-fourth when concentration is halved, the reaction is second-order.

In a reaction, if the concentration of reactant A is halved, and the rate of rea

Practice Questions

Q1
In a reaction, if the concentration of reactant A is halved, and the rate of reaction decreases to one-fourth, what is the order of the reaction? (2020)
  1. Zero
  2. First
  3. Second
  4. Third

Questions & Step-by-Step Solutions

In a reaction, if the concentration of reactant A is halved, and the rate of reaction decreases to one-fourth, what is the order of the reaction? (2020)
  • Step 1: Understand that the rate of a reaction depends on the concentration of the reactants.
  • Step 2: Identify that halving the concentration of reactant A means we change it from [A] to [A]/2.
  • Step 3: Note that the rate of reaction decreases to one-fourth when the concentration of A is halved.
  • Step 4: Use the formula for reaction rate: Rate = k[A]^n, where k is the rate constant and n is the order of the reaction.
  • Step 5: If the concentration is halved, we can express the new rate as Rate' = k([A]/2)^n.
  • Step 6: Simplify the new rate: Rate' = k[A]^n / 2^n.
  • Step 7: Since we know Rate' is one-fourth of the original rate, we can set up the equation: k[A]^n / 2^n = (1/4) * k[A]^n.
  • Step 8: Cancel k[A]^n from both sides of the equation, leading to 1/2^n = 1/4.
  • Step 9: Recognize that 1/4 can be rewritten as 1/2^2, so we have 1/2^n = 1/2^2.
  • Step 10: Conclude that n must equal 2, indicating that the reaction is second-order.
  • Rate Law – The relationship between the concentration of reactants and the rate of a chemical reaction.
  • Order of Reaction – The exponent to which the concentration of a reactant is raised in the rate law, indicating how the rate is affected by changes in concentration.
  • Second-Order Reactions – Reactions where the rate is proportional to the square of the concentration of one reactant or the product of the concentrations of two reactants.
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