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Chemical Kinetics — Chemistry STD 12 Science — Question

Gujarat BoardEnglish MediumSTD 12 ScienceChemistryChemical Kinetics4 Marks
Question
Decrease in concentration of reactant or increase in concentration of product per unit time is called rate of reaction. lt is of two types:
  1. Instantaneous rate of reaction: Rate of change of concentration of reactant or product at a particular time is called instantaneous rate of reaction.
$\text{r}_\text{inst.}=\frac{\text{dC}}{\text{dt}}$
where, dC = infinitely small change in concentration
dt = infinitely small change in time.
  1. Average rate of reaction: Ratio of change in concentration and time required for the change is average rate of reaction.
$\text{r}_\text{av}=\frac{\triangle\text{x}}{\triangle\text{t}}=\frac{\text{Change in concentration}}{\text{Time required for the change}}$
For a reaction of the type, $m_1A + m_2B → n_1C + n_2D$
Rate of reaction is given as
$\frac{1}{\text{m}_1}\frac{\text{d[A]}}{\text{dt}}=-\frac{1}{\text{m}_2}\frac{\text{d[B]}}{\text{dt}}=+\frac{1}{\text{n}_1}\frac{\text{d[C]}}{\text{dt}}=+\frac{1}{\text{n}_2}\frac{\text{d[D]}}{\text{dt}}$
In these questions (Q. No. i-iv), a statement of assertion followed by a statement ofreason is given. Choose the correct answer out of the following choices.
  1. Assertion and reason both are correct statements and reason is correct explanation for assertion.
  2. Assertion and reason both are correct statements but reason is not correct explanation for assertion.
  3. Assertion is correct statement but reason is wrong statement.
  4. Assertion is wrong statement but reason is correct statement.
  1. Assertion: The kinetics of the reaction, $\text{mA}+\text{nB}+\text{pC}\rightarrow\text{m}'\text{ X}+\text{n}'\text{ Y}+\text{p}'\text{ Z}$ obey the rate expression as $\frac{\text{dx}}{\text{dt}}=\text{k}[\text{A}]^\text{m}[\text{B}]^\text{n}.$
Reason: The rate of the reaction does not depend upon the concentration of C.
  1. Assertion: Instantaneous rate of reaction is equal to $\frac{\text{dx}}{\text{dt}}.$
Reason: lt is the rate of reaction at any particular instant of time.
  1. Assertion: For the reaction, $\text{RCl}+\text{NaOH}\rightarrow\text{ROH}+\text{NaCl},$ the rate of reaction is reduced to half on reducing the concentration of RCl to half.
Reason: The rate of reaction is represented by k[RCl].
  1. Assertion: ln rate law, unlike in the expression for equilibrium constants, the exponents for concentrations do not necessarily match the stoichiometric coefficients.
Reason: It is the mechanism and not the balanced chemical equation for the overall change that governs the reaction rate.
  1. Assertion: ln a reaction, $2A + B → A_2B$, the reactant B will disappear at twice the rate as A will decrease.
Reason: The rate of disappearance of reactant will be $-\frac{1}{2}\frac{\text{d[A]}}{\text{dt}}=-\frac{\text{d[B]}}{\text{dt}}$
✓

Answer

  1. (a) Assertion and reason both are correct statements and reason is correct explanation for assertion.
Explanation:
Rate expression $\frac{\text{dx}}{\text{dt}}=\text{k}[\text{A}]^\text{m}[\text{B}]^\text{n}$
shows that the total order of reactions is m + n + 0 = m + n, as the rate of reaction is independent of concentration of C, i.e., the order with respect to C is zero. This is the reason that C does not figure in the rate expression.
  1. (b) Assertion and reason both are correct statements but reason is not correct explanation for assertion.
Explanation:
Instantaneous rate of a reaction is equal to small change in concentration (dx) during a small interval of time (dt) at that particular instant of time divided by the time interval.
  1. (a) Assertion and reason both are correct statements and reason is correct explanation for assertion.
Explanation:
For the given reaction, rate of reaction (r) = k[RCl] (where k is rate constant). Therefore if the concentration of [RCl] is reduced to half, then new rate $(\text{r}')=\frac{\text{k}}{2}[\text{RCl}].$
  1. (a) Assertion and reason both are correct statements and reason is correct explanation for assertion.
  1. (d) Assertion is wrong statement but reason is correct statement.
Explanation:
$2A + B → A_2B$
The rate of disappearance of reactant will be
$-\frac{1}{2}\frac{\text{d[A]}}{\text{dt}}=\frac{\text{d[B]}}{\text{dt}}=\frac{\text{d}[\text{A}_2\text{B}]}{\text{dt}}$
Therefore, the reactant B will disappear at half the rate as A will decrease.

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