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Chemistry · 2025 · 3 marks
CBSE 2025 · Region 1 · Set 1 · Q24
A certain reaction is 50% complete in 20 minutes at 300 K and the same reaction is 50% complete in 5 minutes at 350 K. Calculate the activation…
A certain reaction is $\displaystyle 50$% complete in $\displaystyle 20$ minutes at $\displaystyle 300$ K and the same reaction is $\displaystyle 50$% complete in $\displaystyle 5$ minutes at $\displaystyle 350$ K. Calculate the activation energy if it is a first order reaction. $\displaystyle 3$ $\displaystyle \left[\mathrm{R}=8.314 \mathrm{~J} \mathrm{~K}^{-1} \mathrm{~mol}^{-1} ; \log 4=0.602\right]$
Marking-scheme solution
t1/$\displaystyle 2$=
𝑘
k1=
$\displaystyle 20$ = $\displaystyle 0.03465$ / $\displaystyle 3.465$ × $\displaystyle 10$-$\displaystyle 2$ min-$\displaystyle 1$
k2=
$\displaystyle 5$ = $\displaystyle 0.1386$ / $\displaystyle 1.386$ × $\displaystyle 10$-$\displaystyle 1$ min-$\displaystyle 1$
log
𝐸𝑎
$\displaystyle 2.303$ ×$\displaystyle 8.314$
[$\displaystyle 350$−$\displaystyle 300$]
[$\displaystyle 350$ ×$\displaystyle 300$]
log $\displaystyle 4$ =
𝐸𝑎
[$\displaystyle 50$]
[$\displaystyle 350$ ×$\displaystyle 300$]
Ea = $\displaystyle 24209$ J mol-$\displaystyle 1$ or $\displaystyle 24.209$ kJ mol-$\displaystyle 1$
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CBSE Class 12 Chemistry past-paper question from the 2025 board exam, with the answer as CBSE’s own marking scheme gives it. Where our answers come from.