Chemistry Reaction Kinetics PMDC 2020
PMDC Verified Question 48 of 80
For which reaction, \( \text{A}_{(g)} \longrightarrow \text{product} \)
When the concentration of \( \text{A}_{(g)} \) doubles, the rate of reaction increased four folds, which means it is:
A
Negative order of reaction
B
Zero order of reaction
C
1st order reaction
D
2nd order reaction
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Propolis Cognitive Error Autopsy

Official Correct Choice:
Option D: 2nd order reaction
Concept:

The mathematical relationship between concentration change and rate change determines the reaction order.

Formula:

$$ \text{Rate}_1 = k[\text{A}]^n $$
$$ \text{Rate}_2 = k[2\text{A}]^n $$

Solution:

  • We are given that when concentration doubles (factor of 2), the rate increases by a factor of 4.
  • Set up the ratio: \( \frac{\text{Rate}_2}{\text{Rate}_1} = \frac{k[2\text{A}]^n}{k[\text{A}]^n} \)
  • $$ 4 = (2)^n $$
  • Since \( 2^2 = 4 \), the exponent \( n \) must be exactly 2.
  • Therefore, this is a second-order reaction.


Why other options are incorrect:

If it were 1st order, doubling concentration would only double the rate. If zero order, the rate wouldn't change at all.

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