Chemical Kinetics Chemistry - Chemical Kinetics

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.

2. 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, m₁A + m₂B → n₁C + n₂D

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.

2. 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.

3. 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].

4. 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.

5. Assertion: ln a reaction, 2A + B → A₂B, the reactant B will disappear at twice the rate as A will decrease.

Reason: The rate of disappearance of reactant will be $-\frac{1}{2}\frac{\text{d[A]}}{\text{dt}}=-\frac{\text{d[B]}}{\text{dt}}$

1. The rate constant for the reaction is:

1. 0.4s^-1
2. 0.02s^-1
3. 0.01s^-1
4. 0.3s^-1

2. The pressure of the gas A after 230 s will be:

1. 0.04 atm
2. 0.36 atm
3. 0.4 atm
4. 0.036 atm

3. The total pressure of the system after 230 swill be:

1. 2.15 atm
2. 1.12 atm
3. 0.4 atm
4. 3.08 atm

4. The plot ofln[A] vs twill be:

1. Linear with slope = k
2. Linear with intercept = In[A]₀
3. Linear with slope = In[A]₀
4. Linear with intercept = [A]₀

5. Which of the following is not an example of first order reaction?

1. $\text{C}_2\text{H}_{4(\text{g})}+\text{H}_{2(\text{g})}\rightarrow\text{C}_2\text{H}_{6(\text{g})}$
   
2. $2\text{N}_2\text{O}_{5(\text{g})}\rightarrow4\text{NO}_{2(\text{g})}+\text{O}_{2(\text{g})}$
   
3. $2\text{N}\text{H}_{3(\text{g})}\xrightarrow[\triangle]{\text{pt}}\text{N}_{2(\text{g})}+3\text{H}_{2(\text{g})}$
   
4. $2\text{N}_2\text{O}_{(\text{g})}\xrightarrow{\ \ \triangle\ \ }2\text{N}_{2(\text{g})}+\text{O}_{2(\text{g})}$
   

1. Higher order(> 3) reactions are rare due to:

1. Shifting of equilibrium towards reactants due to elastic collisions.
2. Loss of active species on collision.
3. Low probability of simultaneous collision of all the reacting species.
4. Increase in entropy and activation energy as more molecules are involved.

2. The molecularity of the reaction:

$6\text{FeSO}_4+3\text{H}_2\text{SO}_4+\text{KClO}_3\rightarrow\text{KCl}+3\text{Fe}_2(\text{SO}_4)_3+3\text{H}_2\text{O}$ is:

1. 6
2. 10
3. 3
4. 7

3. Which of the following statements is false in the following?

1. Order of a reaction may be even zero.
2. Molecularity of a reaction is always a whole number.
3. Molecularity and order always have same values for a reaction.
4. Order of a reaction depends upon the mechanism of the reaction.

4. The rate of reaction, A + 2B → products, is given by the following equation:

$-\frac{\text{d}[\text{A}]}{\text{dt}}=\text{k}[\text{A}][\text{B}]^2$

If B is present in large excess, the order of the reaction is:

1. Zero
2. First
3. Second
4. Third

5. The rate of the reaction, A + B + C → products, is given by $\text{r}=\frac{\text{d}[\text{A}]}{\text{dt}}=\text{k}[\text{A}]^\frac{1}{2}[\text{B}]^\frac{1}{3}[\text{C}]^\frac{1}{4}.$ The order of the reaction is:

1. $\frac{1}{3}$​​​​​​​

2. $\frac{1}{4}$

3. $\frac{1}{2}$

4. $\frac{13}{12}$

1. The unit of rate constant for a first order reaction is:

1. S^-1
2. mol L^-1 s^-1
3. L mol^-1 s^-1
4. L² mol^-2 s^-1

2. Half-life period of a first order reaction is 10min. Starting with initial concentration 12M, the rate after 20min is:

1. 0.693 × 3M min^-1
2. 0.0693 × 4M min^-1
3. 0.0693 × M min^-1
4. 0.0693 × 3M min^-1

3. 50% of a first order reaction is complete in 23 minutes. Calculate the ti me required to complete 90% of the reaction.

1. 70.4 minutes.
2. 76.4 minutes.
3. 38.7 minutes.
4. 35.2 minutes.

4. For a first order reaction, (A) → products, the concentration of A changes from 0.1M to 0.025M in 40 minutes. The rate of reaction when the concentration of A is 0.01M, is:

1. 3.47 × 10^-4 M/ min
2. 3.47 × 10^-5 M/ min
3. 1.73 × 10^-4 M/ min
4. 1.73 × 10^-5 M/ min

5. The half-life period ofa 1st order reaction is 60 minutes. What percentage will be left over after 240 minutes?

1. 6.25%
2. 4.25%
3. 5%
4. 6%

1. What is the value of n?

1. 1
2. 2
3. 3
4. 4

2. Find the value of the equilibrium constant.

1. 0.6M
2. 1.2M
3. 0.3M
4. 2.4M

3. The initial rate of conversion of A will be:

1. 0.1 mol L^-1hr^-1
2. 0.2 mol L^-1hr^-1
3. 0.4 mol L^-1hr^-1
4. 0.8 mol L^-1hr^-1

4. For the reaction, if $\frac{\text{d}[\text{B}]}{\text{dt}}=2\times10^{-4},$ value of $-\frac{\text{d}[\text{A}]}{\text{dt}}$ will be:
   

1. 2 × 10^-4
2. 10^-4
3. 4 × 10^-4
4. 0.5 × 10^-4

5. Which factor has no effect on rate of reaction?

1. Temperature.
2. Nature of reactant.
3. Concentration of reactant.
4. Molecularity.