KCET · Chemistry · p Block Elements (Group 15, 16, 17 & 18)

For a reaction \( \frac{1}{2} \mathrm{~A} \rightarrow 2 \mathrm{~B} \) rate of disappearance of \( \mathrm{A} \) is related to rate of appearance of \( \mathrm{B} \) by the
expression

A \( \frac{-d[A]}{d t}=4 \frac{d[B]}{d t} \)
B \( \frac{-\mathrm{d}[\mathrm{A}]}{\mathrm{d} \mathrm{t}}=\frac{1}{4} \frac{\mathrm{d}[\mathrm{B}]}{\mathrm{dt}} \)
C \( \frac{-\mathrm{d}[\mathrm{A}]}{\mathrm{d} \mathrm{t}}=\frac{1}{2} \frac{\mathrm{d}[\mathrm{B}]}{\mathrm{dt}} \)
D \( \frac{-d[A]}{d t}=\frac{d[B]}{d t} \)

Verified Solution

Answer & Solution

Correct Answer

(B) \( \frac{-\mathrm{d}[\mathrm{A}]}{\mathrm{d} \mathrm{t}}=\frac{1}{4} \frac{\mathrm{d}[\mathrm{B}]}{\mathrm{dt}} \)

Step-by-step Solution

Detailed explanation

For the given reaction,
\( \frac{1}{2} A \rightarrow 2 B \) or \( A \rightarrow 4 B \)
Rate of disappearance of \( A=-\frac{\mathrm{d}[\mathrm{A}]}{\mathrm{dt}} \)
Rate of appearance of \( B=-\frac{\mathrm{d}[\mathrm{B}]}{\mathrm{dt}} \)
Rate of disappearance of \( A= \) Rate of appearance of \( B \).
Therefore,
\( -\frac{d[A]}{d t}=\frac{1}{4} \frac{d[B]}{d t} \)

Apply the relevant identity from the chapter and substitute the values established in the previous step, keeping the original constraint in view.

Use the simplified expression to isolate the unknown, then plug the result back into the question setup to confirm the working is internally consistent.

Cross-check against a boundary or limiting case. Both the dimensional balance and the qualitative behaviour at the extreme should agree with the candidate answer.

Combine the steps above to identify the correct option. The remaining choices each fail at least one of the consistency, dimensional, or boundary checks.

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For a reaction \( \frac{1}{2} \mathrm{~A} \rightarrow 2 \mathrm{~B} \) rate of disappearance of \( \mathrm{A} \) is related to rate of appearance of \( \mathrm{B} \) by theexpression

Step-by-step Solution

For a reaction \( \frac{1}{2} \mathrm{~A} \rightarrow 2 \mathrm{~B} \) rate of disappearance of \( \mathrm{A} \) is related to rate of appearance of \( \mathrm{B} \) by the
expression