Question

In a pseudo first order reaction, the following results were obtained. {|c|c|c|c|c|c|c|c|} Time s & 0 & 10 & 20 & 30 & 40 & 50 & 60
$[A]$/mol lit$^{-1}$ & 0.65 & 0.55 & 0.46 & 0.38 & 0.26 & 0.20 & 0.13
Average rate of the reaction between 20 and 40 seconds is

• A. $0.01\ \text{mol lit}^{-1}\text{ s}^{-1}$
• B. $0.02\ \text{mol lit}^{-1}\text{ s}^{-1}$
• C. $0.001\ \text{mol lit}^{-1}\text{ s}^{-1}$
• D. $0.1\ \text{mol lit}^{-1}\text{ s}^{-1}$
• E. $0.04\ \text{mol lit}^{-1}\text{ s}^{-1}$

Hint:

Average rate: - Always use $\frac{\text{change in concentration}}{\text{time interval}}$ - Use minus sign for reactants

Answer 1

The Correct Option is A

Concept: Average rate of reaction: \[ \text{Rate} = -\frac{\Delta [A]}{\Delta t} \]

Step 1: Take concentration values.
\[ [A]_{20} = 0.46,\quad [A]_{40} = 0.26 \]

Step 2: Find change in concentration.
\[ \Delta [A] = 0.26 - 0.46 = -0.20 \]

Step 3: Find time interval.
\[ \Delta t = 40 - 20 = 20\ \text{s} \]

Step 4: Calculate rate.
\[ \text{Rate} = -\frac{-0.20}{20} = \frac{0.20}{20} = 0.01\ \text{mol lit}^{-1}\text{ s}^{-1} \]