15 extra multiple-choice questions for Chemical Kinetics (12th Standard Chemistry, Samacheer Kalvi), beyond the ones printed in the textbook — each with the correct option highlighted and a clear, worked explanation. Free to read in English and Tamil.
Q1
Which of the following units is used to express the rate of a gas phase reaction when the concentration of species is expressed in terms of partial pressure?
- A. mol L\(^{-1}\) s\(^{-1}\)
- B. atm s\(^{-1}\)Correct
- C. mol s\(^{-1}\)
- D. atm mol\(^{-1}\) s\(^{-1}\)
Explanation. For reactions occurring in the gas phase, the concentration is typically measured using partial pressure units like atmospheres, resulting in the rate unit atm s\(^{-1}\).
Q2
For a general reaction \(x A + y B \to l C + m D\), what is the correct relationship between the rates of change of concentration for the different species?
- A. Rate = \(-x \frac{d[A]}{dt} = -y \frac{d[B]}{dt}\)
- B. Rate = \(-\frac{1}{x} \frac{d[A]}{dt} = \frac{1}{l} \frac{d[C]}{dt}\)Correct
- C. Rate = \(\frac{1}{y} \frac{d[B]}{dt} = \frac{1}{m} \frac{d[D]}{dt}\)
- D. Rate = \(-x \frac{d[A]}{dt} = l \frac{d[C]}{dt}\)
Explanation. The overall reaction rate is equal to the rate of consumption of a reactant or formation of a product divided by its corresponding stoichiometric coefficient.
Q3
How is the instantaneous rate of a chemical reaction at a particular time 't' determined from a experimental concentration vs. time curve?
- A. By calculating the total change in concentration over the total time.
- B. By finding the area under the curve between zero and time t.
- C. By calculating the slope of the tangent drawn to the curve at time t.Correct
- D. By taking the reciprocal of the reactant concentration at time t.
Explanation. The instantaneous rate represents the rate at a specific moment and is calculated as the slope of the tangent to the concentration-time graph at that point.
Q4
Which of the following characteristics correctly describes the rate constant 'k' of a reaction?
- A. It depends on the initial concentration of the reactants.
- B. It is equal to the rate of reaction when the concentration of each reactant is unity.Correct
- C. Its value remains constant even if the temperature changes.
- D. It represents the speed at a specific instant during the reaction.
Explanation. The rate constant is a proportionality constant that equals the reaction rate specifically when all reactant concentrations are exactly 1 M.
Q5
Which property distinguishes molecularity from the order of a reaction?
- A. Molecularity can be zero or fractional.
- B. Order is always a whole number and cannot be zero.
- C. Molecularity is always a whole number and cannot be zero or fractional.Correct
- D. Order is assigned for each individual elementary step.
Explanation. Molecularity counts the number of species colliding in an elementary step and must be a non-zero integer, unlike order, which is determined experimentally.
Q6
What is the order of the reaction for the decomposition of dinitrogen pentoxide (\(N_2O_5\))?
- A. Zero order
- B. First orderCorrect
- C. Second order
- D. Third order
Explanation. The decomposition of \(N_2O_5\) is a classic example of a first-order reaction where the rate depends only on the concentration of \(N_2O_5\).
Q7
Which condition allows a higher-order reaction to be studied as a pseudo-first order reaction?
- A. Carrying out the reaction at a very high pressure.
- B. Taking one of the reactants in large excess.Correct
- C. Using a heterogeneous catalyst.
- D. Decreasing the temperature to near absolute zero.
Explanation. When one reactant is in large excess, its concentration remains essentially constant, simplifying the rate law to depend only on the other reactant.
Q8
For a zero-order reaction, a plot of reactant concentration \([A]\) versus time yields a straight line. What does the slope of this line represent?
- A. The rate constant \(k\)
- B. The negative rate constant \(-k\)Correct
- C. The initial concentration \([A]_0\)
- D. The natural logarithm of the rate constant \(\\ln k\)
Explanation. The integrated rate law for a zero-order reaction is \([A] = -kt + [A]_0\). Comparing this to \(y = mx + c\), the slope \(m\) is \(-k\).
Q9
How does the half-life (\(t_{1/2}\)) of a first-order reaction change if the initial concentration of the reactant is doubled?
- A. The half-life also doubles.
- B. The half-life is reduced by half.
- C. The half-life remains unchanged.Correct
- D. The half-life increases by a factor of four.
Explanation. For a first-order reaction, \(t_{1/2} = 0.693/k\). This expression shows that the half-life is constant and does not depend on the initial concentration.
Q10
Which of the following is true for the half-life of a zero-order reaction?
- A. It is independent of the initial concentration.
- B. It is inversely proportional to the initial concentration.
- C. It is directly proportional to the initial concentration.Correct
- D. It is inversely proportional to the square of the initial concentration.
Explanation. In a zero-order reaction, \(t_{1/2} = [A]_0 / 2k\). This formula indicates that the half-life is directly proportional to the initial concentration of the reactant.
Q11
According to collision theory, even if molecules collide with sufficient energy, they may not react. What additional factor is necessary for an effective collision?
- A. High pressure
- B. Presence of an inert gas
- C. Proper orientation of colliding moleculesCorrect
- D. Small molecular size
Explanation. Effective collisions require both sufficient activation energy and a correct spatial orientation to allow for the formation of the transition state.
Q12
In the Arrhenius equation \(k = A e^{-E_a/RT}\), what does the term 'A' represent?
- A. Activation energy
- B. Frequency factorCorrect
- C. Gas constant
- D. Reaction rate
Explanation. The pre-exponential factor 'A' is known as the frequency factor, which relates to the total number of collisions between reactant molecules per second.
Q13
When plotting \(\\ln k\) versus \(1/T\) based on the Arrhenius equation, a straight line is obtained. What is the slope of this line?
- A. \(\\ln A\)
- B. \(-E_a / R\)Correct
- C. \(E_a / 2.303 R\)
- D. \(R / E_a\)
Explanation. The logarithmic form is \(\\ln k = \\ln A - (E_a/R)(1/T)\). Comparing this to the equation of a straight line, the slope is \(-E_a / R\).
Q14
How does a catalyst affect the rate of a chemical reaction?
- A. It increases the rate by increasing the enthalpy of the reaction.
- B. It decreases the rate by increasing the activation energy.
- C. It increases the rate by providing an alternative path with lower activation energy.Correct
- D. It has no effect on the reaction rate.
Explanation. A catalyst provides a new reaction mechanism that has a lower energy barrier, thus increasing the speed of the reaction without being consumed.
Q15
In the reaction between marble chips (\(CaCO_3\)) and hydrochloric acid, why does powdered marble react faster than larger chips of the same mass?
- A. The powder has a higher temperature.
- B. The powder has a larger surface area available for collisions.Correct
- C. The powder acts as a catalyst for the reaction.
- D. The powder decreases the activation energy of the reaction.
Explanation. A greater surface area in powdered reactants allows for more frequent collisions with the other reactant species, thereby increasing the reaction rate.
Frequently asked questions
How many MCQs are there in Chemical Kinetics?
This chapter has 15 book-back multiple-choice questions, each with the correct answer and a step-by-step explanation.
Are these 12th Standard Chemistry MCQs free to practise online?
Yes. Every question, answer and explanation here is free, and you can also take them as a timed practice test.
Where can I find the Chemical Kinetics book-back answers?
The correct option for each question is highlighted on this page with a worked explanation, plus a quick answer-key summary at the top.
