Question

In an elementary reaction, $\mathrm{A}+2\mathrm{B}\to 2\mathrm{C}+\mathrm{D}$. If the concentration of A is increased four times and B is decreased to half of its initial concentration, then the rate becomes?

• A. Half
• B. Twice
• C. Unchanged
• D. None of these

Answer 1

The correct option is C

Unchanged

Explanation for the correct answer:-

Option (C) Unchanged

Step 1: Finding the rate for the reaction.

Reaction given in the question:-

$\mathrm{A}+2\mathrm{B}\to 2\mathrm{C}+\mathrm{D}$

Rate law for this reaction:-

Rate= $\left[\mathrm{A}\right]{\left[\mathrm{B}\right]}^2-\left[1\right]$

So, now if concentration of A is increased 4 times then the concentration of B is increased $\frac{1}{2}$ of the initial concentration .

Step 2:- Finding the new rate for the reation.

${\left(\mathrm{Rate}\right)}_{\mathrm{new}}=\left[4\mathrm{A}\right]{\left[\frac{\mathrm{B}}{2}\right]}^2$

= $4\left[\mathrm{A}\right]\frac{{\mathrm{B}}^2}{4}$

So,${\left(\mathrm{Rate}\right)}_{\mathrm{new}}=\left[\mathrm{A}\right]{\left[\mathrm{B}\right]}^2$$-\left[2\right]$

Now, from equation $\left[1\right]\mathrm{and}\left[2\right]$, There is no change is the rate.

Therefore, the correct answer is option (C) Unchanged.