Order & Molecularity of a Reaction — NEET Chemistry MCQs with Solutions
Free NEET Chemistry Order & Molecularity of a Reaction MCQs with step-by-step solutions (30 questions). Part of Chemical Kinetics. Practise online on Prepizo — no login needed.
▶ Practise Order & Molecularity of a Reaction online (free)
Questions with solutions
Q1 — Order & Molecularity of a Reaction · easy · numerical
For a reaction the rate law is Rate $= k[A][B]^2$. The overall order of the reaction is:
A. 2
B. 3 ✓ Correct
C. 4
D. 1
Solution: Overall order is the sum of the exponents in the rate law: $1 + 2 = 3$.
Q2 — Order & Molecularity of a Reaction · easy · numerical
For a reaction, Rate $= k[A]^2[B]$. The order with respect to $B$ is:
A. 1 ✓ Correct
B. 0
C. 3
D. 2
Solution: The exponent of $[B]$ in the rate law is 1, so the order with respect to $B$ is 1.
Q3 — Order & Molecularity of a Reaction · easy · numerical
The rate of a reaction is found to be independent of the concentration of $A$, i.e. Rate $= k[A]^0$. The order with respect to $A$ is:
A. fractional
B. 2
C. 0 ✓ Correct
D. 1
Solution: Since the exponent of $[A]$ is 0, the reaction is zero order with respect to $A$; the rate does not change with $[A]$.
Q4 — Order & Molecularity of a Reaction · medium · numerical
For a reaction, Rate $= k[A]^{1/2}[B]^2$. The overall order of the reaction is:
A. 2.5 ✓ Correct
B. 2
C. 1.5
D. 3
Solution: Overall order $= \dfrac{1}{2} + 2 = \dfrac{5}{2} = 2.5$.
Q5 — Order & Molecularity of a Reaction · easy · numerical
The molecularity of the elementary reaction $2HI \rightarrow H_2 + I_2$ is:
A. 2 (bimolecular) ✓ Correct
B. 1 (unimolecular)
C. 3 (termolecular)
D. 0
Solution: Two $HI$ molecules take part in the single elementary step, so the molecularity is 2 (bimolecular).
Q6 — Order & Molecularity of a Reaction · medium · numerical
When the concentration of $A$ is doubled, the rate of a reaction increases by a factor of 8 (other concentrations unchanged). The order with respect to $A$ is:
A. 1
B. 8
C. 3 ✓ Correct
D. 2
Solution: If Rate $\propto [A]^n$, then $2^n = 8 = 2^3$, so $n = 3$.
Q7 — Order & Molecularity of a Reaction · medium · numerical
On tripling the concentration of $A$, the rate of a reaction becomes 9 times the original (other concentrations constant). The order with respect to $A$ is:
A. 9
B. 3
C. 2 ✓ Correct
D. 1
Solution: $3^n = 9 = 3^2$, so the order with respect to $A$ is 2.
Q8 — Order & Molecularity of a Reaction · medium · numerical
For a reaction, Rate $= k[A][B]^{-1}$. Which statement is correct?
A. The order with respect to $B$ is $-1$; increasing $[B]$ decreases the rate. ✓ Correct
B. The order with respect to $B$ is 0; the rate is unaffected by $[B]$.
C. The order with respect to $B$ is $-2$.
D. The order with respect to $B$ is $+1$; increasing $[B]$ increases the rate.
Solution: The exponent of $[B]$ is $-1$ (a negative order), so raising $[B]$ lowers the rate.
Q9 — Order & Molecularity of a Reaction · medium · numerical
Which of the following statements about order and molecularity is INCORRECT?
A. Order is determined experimentally, whereas molecularity is a theoretical concept.
B. Order of a reaction can be zero, fractional or negative.
C. Molecularity of a reaction can be zero or a fraction. ✓ Correct
D. Molecularity is defined only for elementary reactions (or elementary steps).
Solution: Molecularity is always a whole number (1, 2 or 3) and can never be zero or fractional; the other statements are correct.
Q10 — Order & Molecularity of a Reaction · easy · numerical
Which of the following can never be zero or a fractional value?
A. Rate of a reaction
B. Order of a reaction
C. Rate constant of a reaction
D. Molecularity of a reaction ✓ Correct
Solution: Molecularity counts the species colliding in an elementary step, so it is always a whole number (1, 2 or 3). Order is experimental and may be zero or fractional.
Q11 — Order & Molecularity of a Reaction · medium · numerical
The units of the rate constant of a second-order reaction are:
A. $L\,mol^{-1}s^{-1}$ ✓ Correct
B. $mol\,L^{-1}s^{-1}$
C. $s^{-1}$
D. $L^2\,mol^{-2}s^{-1}$
Solution: Units $= (mol\,L^{-1})^{1-2}s^{-1} = mol^{-1}L\,s^{-1} = L\,mol^{-1}s^{-1}$.
Q12 — Order & Molecularity of a Reaction · medium · numerical
The units of the rate constant of a third-order reaction are:
A. $s^{-1}$
B. $mol\,L^{-1}s^{-1}$
C. $L^2\,mol^{-2}s^{-1}$ ✓ Correct
D. $L\,mol^{-1}s^{-1}$
Solution: Units $= (mol\,L^{-1})^{1-3}s^{-1} = mol^{-2}L^2 s^{-1} = L^2\,mol^{-2}s^{-1}$.
Q13 — Order & Molecularity of a Reaction · easy · numerical
The rate constant of a reaction has the units $mol\,L^{-1}s^{-1}$. The order of the reaction is:
A. 0 ✓ Correct
B. 2
C. 1
D. 3
Solution: Units $mol\,L^{-1}s^{-1}$ correspond to $(mol\,L^{-1})^{1-n}s^{-1}$ with $n = 0$; a zero-order rate constant has these units.
Q14 — Order & Molecularity of a Reaction · medium · numerical
The rate constant of a reaction is expressed in $L\,mol^{-1}s^{-1}$. The overall order of the reaction is:
A. 3
B. 1
C. 0
D. 2 ✓ Correct
Solution: These units correspond to $(mol\,L^{-1})^{1-n}s^{-1}$ with $n = 2$, so the reaction is second order.
Q15 — Order & Molecularity of a Reaction · medium · numerical
A reaction proceeds by the mechanism: (slow) $NO_2 + NO_2 \rightarrow NO_3 + NO$; (fast) $NO_3 + CO \rightarrow NO_2 + CO_2$. The rate law is:
A. Rate $= k[NO_2]^2$ ✓ Correct
B. Rate $= k[NO_2][CO]$
C. Rate $= k[NO_2]^2[CO]$
D. Rate $= k[CO]$
Solution: The slow (rate-determining) step is bimolecular in $NO_2$, so Rate $= k[NO_2]^2$; $CO$ appears only in the fast step.
Q16 — Order & Molecularity of a Reaction · medium · numerical
For a reaction, Rate $= k[A][B]^{1/2}$. The overall order of the reaction is:
A. 1
B. 1.5 ✓ Correct
C. 0.5
D. 2
Solution: Overall order $= 1 + \dfrac{1}{2} = \dfrac{3}{2} = 1.5$.
Q17 — Order & Molecularity of a Reaction · medium · numerical
Which of the following statements is correct?
A. The order of a reaction is always equal to its molecularity.
B. A reaction with a fractional order is always an elementary reaction.
C. A fractional order is impossible for any reaction.
D. A reaction showing a fractional order must proceed through more than one step (a complex mechanism). ✓ Correct
Solution: A fractional order cannot arise from a single elementary step; it signals a multi-step (complex) mechanism.
Q18 — Order & Molecularity of a Reaction · easy · numerical
The molecularity of the elementary reaction $PCl_5 \rightarrow PCl_3 + Cl_2$ is:
A. 2 (bimolecular)
B. 3 (termolecular)
C. 0
D. 1 (unimolecular) ✓ Correct
Solution: A single $PCl_5$ molecule decomposes in the elementary step, so the molecularity is 1 (unimolecular).
Q19 — Order & Molecularity of a Reaction · medium · numerical
For a reaction, Rate $= k[A]^{3/2}[B]^{-1/2}$. The overall order of the reaction is:
A. $-1$
B. 0
C. 2
D. 1 ✓ Correct
Solution: Overall order $= \dfrac{3}{2} - \dfrac{1}{2} = 1$.
Q20 — Order & Molecularity of a Reaction · medium · numerical
Which statement correctly distinguishes order from molecularity?
A. Molecularity can be fractional but order is always an integer.
B. Order is always a whole number while molecularity can be fractional.
C. Both order and molecularity are always equal for every reaction.
D. Order is found experimentally and may be zero, fractional or negative, whereas molecularity is a whole number (1, 2 or 3). ✓ Correct
Solution: Order is an experimental quantity (can be 0, fractional or negative); molecularity is theoretical and always a small whole number.
Q21 — Order & Molecularity of a Reaction · easy · numerical
For a reaction, Rate $= k[A]^2$. If the concentration of $A$ is doubled, the rate becomes:
A. 8 times the original
B. 4 times the original ✓ Correct
C. 2 times the original
D. unchanged
Solution: Rate $\propto [A]^2$, so doubling $[A]$ multiplies the rate by $2^2 = 4$.
Q22 — Order & Molecularity of a Reaction · medium · numerical
For a reaction $A \rightarrow$ products, the rate is $2\times10^{-3}$ mol $L^{-1}s^{-1}$ when $[A] = 0.1$ M and $8\times10^{-3}$ mol $L^{-1}s^{-1}$ when $[A] = 0.2$ M. The order of the reaction is:
A. 0.5
B. 3
C. 1
D. 2 ✓ Correct
Solution: Doubling $[A]$ (0.1 to 0.2 M) makes the rate 4 times larger; $2^n = 4$ gives $n = 2$.
Q23 — Order & Molecularity of a Reaction · medium · numerical
For $A + B \rightarrow$ products, doubling $[A]$ (with $[B]$ fixed) doubles the rate, while doubling $[B]$ (with $[A]$ fixed) makes the rate 4 times larger. The overall order is:
A. 3 ✓ Correct
B. 2
C. 1
D. 4
Solution: Order in $A = 1$ (rate $\times 2$) and order in $B = 2$ (rate $\times 4 = 2^2$); overall order $= 1 + 2 = 3$.
Q24 — Order & Molecularity of a Reaction · medium · numerical
Why does the molecularity of a reaction rarely exceed 3?
A. Reactions involving more than three molecules are forbidden by thermodynamics.
B. The order of such reactions is always zero.
C. Rate constants become negative when the molecularity exceeds three.
D. The simultaneous collision of more than three molecules with the correct orientation and energy is highly improbable. ✓ Correct
Solution: Molecularity counts the species colliding in a single step; the chance of four or more colliding at once is negligibly small.
Q25 — Order & Molecularity of a Reaction · medium · numerical
Which of the following is true for an elementary reaction?
A. Its order is always zero.
B. It always has a fractional order.
C. Its molecularity is determined experimentally.
D. Its order is equal to its molecularity. ✓ Correct
Solution: For a single-step (elementary) reaction the rate law follows the stoichiometry, so order equals molecularity.
Q26 — Order & Molecularity of a Reaction · medium · numerical
For a reaction, Rate $= k[A]^0[B]^2$. The order with respect to $A$ is:
A. 3
B. 2
C. 1
D. 0 ✓ Correct
Solution: The exponent of $[A]$ is 0, so the reaction is zero order in $A$ (the overall order is $0 + 2 = 2$).
Q27 — Order & Molecularity of a Reaction · medium · numerical
For a reaction, doubling $[B]$ leaves the rate unchanged, while doubling $[A]$ quadruples the rate. The overall order of the reaction is:
A. 3
B. 1
C. 2 ✓ Correct
D. 4
Solution: Order in $B = 0$ (no change) and order in $A = 2$ (rate $\times 4 = 2^2$); overall order $= 2 + 0 = 2$.
Q28 — Order & Molecularity of a Reaction · medium · numerical
The molecularity of the elementary reaction $2NO + O_2 \rightarrow 2NO_2$ is:
A. 5
B. 1 (unimolecular)
C. 2 (bimolecular)
D. 3 (termolecular) ✓ Correct
Solution: Three molecules (two $NO$ and one $O_2$) collide in the single step, so the molecularity is 3 (termolecular).
Q29 — Order & Molecularity of a Reaction · easy · numerical
Which statement about molecularity is correct?
A. Molecularity is always a whole number and can never be zero or a fraction. ✓ Correct
B. Molecularity is determined experimentally from rate data.
C. Molecularity can be a fraction for complex reactions.
D. Molecularity can be zero for photochemical reactions.
Solution: Molecularity is the number of species in an elementary step, so it must be a small whole number (1, 2 or 3).
Q30 — Order & Molecularity of a Reaction · medium · numerical
For $A + B \rightarrow$ products, the rate doubles when $[A]$ is doubled (with $[B]$ fixed) and also doubles when $[B]$ is doubled (with $[A]$ fixed). The overall order of the reaction is:
A. 2 ✓ Correct
B. 1
C. 0
D. 3
Solution: Order in $A = 1$ and order in $B = 1$; overall order $= 1 + 1 = 2$.