Quiz: Chemical Kinetics — 28 questions

Question 1

1. Which field of chemistry examines reaction rates, the factors that control them, and the mechanisms by which reactions occur?

Quantum chemistry

Analytical chemistry

Chemical kinetics

Chemical thermodynamics

Explanation

Chemical kinetics focuses on how fast reactions occur, what affects their rates, and the pathways they follow. Chemical thermodynamics instead addresses energetic feasibility and related equilibrium questions.

Answer

Chemical kinetics

Question 2

2. A reaction has a negative Gibbs free-energy change at constant temperature and pressure but proceeds imperceptibly slowly. What does this illustrate?

A slow reaction must have a positive Gibbs free-energy change

Thermodynamic feasibility and reaction speed measure the same property

Thermodynamic feasibility does not determine reaction speed

Reaction speed determines whether Gibbs free energy is negative

Explanation

A negative $\Delta G$ indicates that the reaction is feasible under the stated conditions, while kinetics determines how rapidly it proceeds. The diamond-to-graphite conversion exemplifies a feasible reaction with an extremely low rate.

Answer

Thermodynamic feasibility does not determine reaction speed

Question 3

3. How is the rate of disappearance of a reactant related to the rate of appearance of a product in a reaction such as $R\rightarrow P$ ?

Both changes are represented as positive concentration changes because concentrations define rate directly

Both changes are represented as negative concentration changes because reaction concentrations decrease

Reactant disappearance is represented by a negative concentration change, while product appearance is positive

Reactant disappearance is positive, while product appearance is negative because products are consumed

Explanation

A reactant concentration decreases, so its change is negative, whereas a product concentration increases, so its change is positive. The minus sign before the reactant change makes the reaction rate positive.

Answer

Reactant disappearance is represented by a negative concentration change, while product appearance is positive

Question 4

4. For $R\rightarrow P$ at constant volume, the concentrations change from $[R]_1$ and $[P]_1$ to $[R]_2$ and $[P]_2$ over a finite time interval $\Delta t$ . Which expression gives the average reaction rate?

r_{av}=-\frac{d[R]}{dt}=\frac{d[P]}{dt}

r_{av}=-\frac{\Delta[R]}{\Delta t}=\frac{\Delta[P]}{\Delta t}

r_{av}=\frac{\Delta[R]}{\Delta t}=-\frac{\Delta[P]}{\Delta t}

r_{av}=-\Delta[R]\times\Delta t=\Delta[P]\times\Delta t

Explanation

Average rate uses concentration changes measured over a finite interval, with a negative sign for reactant disappearance. The derivative expression describes instantaneous rate rather than average rate.

Answer

$$r_{av}=-\frac{\Delta[R]}{\Delta t}=\frac{\Delta[P]}{\Delta t}$$

Question 5

5. What does the instantaneous reaction rate represent on a concentration–time graph?

The concentration value at the point where the reaction begins

The slope of a secant spanning the entire experiment

The total concentration change divided by the initial concentration

The slope of the tangent at a particular time

Explanation

Instantaneous rate is defined by $r_{inst}=-\frac{d[R]}{dt}=\frac{d[P]}{dt}$ and corresponds to the tangent slope at a particular time. A secant slope instead represents an average rate over a finite interval.

Answer

The slope of the tangent at a particular time

Question 6

6. For $aA+bB\rightarrow cC+dD$ , why are concentration changes divided by their stoichiometric coefficients when defining a single reaction rate?

To ensure reactant concentration changes have positive signs like product changes

To make the reaction rate independent of whether time is measured in seconds or minutes

To convert every concentration into a pressure before calculating the rate

To account for the different amounts of each species consumed or produced per reaction event

Explanation

Dividing by each coefficient normalizes the species-specific concentration changes to the same reaction progress. Without this adjustment, species with larger coefficients would appear to have different reaction rates for the same reaction.

Answer

To account for the different amounts of each species consumed or produced per reaction event

Question 7

7. For $2HI\rightarrow H_2+I_2$ , which expression correctly represents the reaction rate?

r=-\frac{1}{2}\frac{d[HI]}{dt}=\frac{d[H_2]}{dt}=\frac{d[I_2]}{dt}

r=-\frac{d[HI]}{dt}=\frac{1}{2}\frac{d[H_2]}{dt}=\frac{d[I_2]}{dt}

r=\frac{1}{2}\frac{d[HI]}{dt}=-\frac{d[H_2]}{dt}=-\frac{d[I_2]}{dt}

r=-2\frac{d[HI]}{dt}=\frac{d[H_2]}{dt}=\frac{d[I_2]}{dt}

Explanation

The coefficient of HI is 2, so its concentration change is divided by 2 and given a negative sign because HI is consumed. The products have coefficients of 1, so their positive derivatives directly equal the reaction rate.

Answer

$$r=-\frac{1}{2}\frac{d[HI]}{dt}=\frac{d[H_2]}{dt}=\frac{d[I_2]}{dt}$$

Question 8

8. What does an experimentally determined rate law relate?

The reaction enthalpy to the stoichiometric coefficients

The product yield to the volume of the reaction vessel

The equilibrium constant to the temperature of the reaction

The reaction rate to molar concentrations of reacting species

Explanation

A rate law expresses how reaction rate depends on the molar concentrations of reacting species. A balanced equation supplies stoichiometric information but does not by itself establish the rate law.

Answer

The reaction rate to molar concentrations of reacting species

Question 9

9. For the reaction $aA+bB\rightarrow cC+dD$ , which expression represents the general rate law?

r=(a+b)k[A][B]

r=k[C]^c[D]^d

r=\frac{[A]^x[B]^y}{k}

r=k[A]^x[B]^y

Explanation

The general rate law uses the rate constant multiplied by reactant concentrations raised to experimentally determined powers. The stoichiometric coefficients and product concentrations do not automatically define this expression.

Answer

$$r=k[A]^x[B]^y$$

Question 10

10. What is the role of the rate constant $k$ in a rate law at a specified temperature?

It is the concentration of products at chemical equilibrium

It is the stoichiometric coefficient of the fastest reacting species

It is the proportionality constant relating rate to concentration terms

It is the exponent assigned to every reactant concentration

Explanation

At a specified temperature, $k$ is the proportionality constant connecting the rate to the concentration expression. It is not an exponent or a stoichiometric coefficient.

Answer

It is the proportionality constant relating rate to concentration terms

Question 11

11. How is the overall order of a reaction determined?

By counting the number of products formed in the balanced equation

By summing the powers of reactant concentrations in the experimental rate law

By adding the stoichiometric coefficients of reactants in the balanced equation

By comparing the initial and final temperatures of the reaction

Explanation

Reaction order is the sum of the concentration exponents in the experimentally determined rate law. Stoichiometric coefficients belong to the balanced equation and may not match those exponents.

Answer

By summing the powers of reactant concentrations in the experimental rate law

Question 12

12. What happens to the rate of a zero-order reaction when a reactant concentration changes?

The rate remains independent of that reactant concentration

The rate changes with the square of that concentration

The rate becomes inversely proportional to that concentration

The rate changes in direct proportion to that concentration

Explanation

A zero-order rate law contains a zero power for the relevant reactant, so changing its concentration does not affect the rate. Direct proportionality describes first-order dependence instead.

Answer

The rate remains independent of that reactant concentration

Question 13

13. What does molecularity describe for an elementary reaction?

The number of products formed after the complete reaction sequence

The number of elementary steps included in a complex reaction

The sum of concentration powers in the overall experimental rate law

The number of reacting species colliding simultaneously in one elementary step

Explanation

Molecularity counts the reacting species involved in a single elementary collision or step. The concentration powers describe reaction order, while the number of steps characterizes a mechanism.

Answer

The number of reacting species colliding simultaneously in one elementary step

Question 14

14. Which statement correctly distinguishes molecularity from reaction order?

Molecularity is measured for overall reactions, whereas order applies to single collisions

Molecularity is a positive integer from one to three, whereas order may be zero or fractional

Molecularity comes from concentration data, whereas order comes from collision counting

Molecularity may be fractional, whereas reaction order must be a positive integer

Explanation

Molecularity refers to the integer number of species in an elementary step and ranges from one to three. Reaction order is obtained from a rate law and may be zero, fractional, or negative.

Answer

Molecularity is a positive integer from one to three, whereas order may be zero or fractional

Question 15

15. In a complex reaction mechanism, which step controls the overall reaction rate?

The step with the greatest number of reacting species

The final step that releases the largest amount of heat

The first product-forming step in the sequence

The slowest elementary step in the sequence

Explanation

A complex reaction occurs through multiple elementary steps, and its slowest step limits the overall rate. The position, molecularity, or heat release of another step does not by itself determine rate control.

Answer

The slowest elementary step in the sequence

Question 16

16. For a zero-order reaction, how can the rate constant be calculated from the initial and current reactant concentrations?

k=\frac{[R]_0-[R]}{t}

k=\frac{[R]_0+[R]}{t}

k=\frac{\ln([R]_0/[R])}{t}

k=\frac{[R]}{[R]_0t}

Explanation

The zero-order integrated law gives $[R]=[R]_0-kt$ , which rearranges to the stated expression for $k$ . The logarithmic expression belongs to the first-order integrated law rather than the zero-order law.

Answer

$$k=\frac{[R]_0-[R]}{t}$$

Question 17

17. Which concentration-time relationship describes a first-order reaction?

[R]=[R]_0e^{-kt}

[R]=[R]_0-kt

[R]=[R]_0e^{kt}

[R]=[R]_0+kt

Explanation

A first-order reaction follows exponential decay, expressed as $[R]=[R]_0e^{-kt}$ or equivalently $\ln([R]_0/[R])=kt$ . The linear expression with a subtraction describes a zero-order reaction.

Answer

$$[R]=[R]_0e^{-kt}$$

Question 18

18. What does the half-life of a reaction represent?

The time required for the reactant concentration to reach half its initial value

The time required for the rate constant to decrease to half its initial value

The time required for the reactant concentration to become equal to the product concentration

The time required for the product concentration to reach its maximum value

Explanation

Half-life is defined as the time needed for a reactant concentration to decrease to half of its initial value. It is not defined by the product concentration or by a change in the rate constant.

Answer

The time required for the reactant concentration to reach half its initial value

Question 19

19. A zero-order reaction has its initial reactant concentration doubled while its rate constant remains unchanged; what happens to its half-life?

It is reduced by half because the reaction consumes more reactant

It remains unchanged because the rate constant is unchanged

It increases fourfold because half-life depends quadratically on concentration

It doubles because

t_{1/2}=\frac{[R]_0}{2k}

Explanation

For a zero-order reaction, $t_{1/2}=\frac{[R]_0}{2k}$ , so the half-life is directly proportional to the initial concentration. The first-order result, in which half-life is independent of initial concentration, does not apply here.

Answer

It doubles because $$t_{1/2}=\frac{[R]_0}{2k}$$

Question 20

20. Which expression gives the half-life of a first-order reaction?

t_{1/2}=\frac{0.693}{k}

t_{1/2}=0.693k

t_{1/2}=\frac{2k}{[R]_0}

t_{1/2}=\frac{[R]_0}{2k}

Explanation

For a first-order reaction, the half-life is $t_{1/2}=\frac{0.693}{k}$ and does not depend on the initial concentration. The expression involving $[R]_0$ is the zero-order half-life formula.

Answer

$$t_{1/2}=\frac{0.693}{k}$$

Question 21

21. Why can a reaction involving several reactants exhibit pseudo-first-order behavior?

The reaction has only one reactant and therefore has first-order stoichiometry

The rate constant becomes independent of temperature during the reaction

One reactant is in large excess, so its concentration remains nearly constant

All reactant concentrations decrease at the same constant rate

Explanation

A higher-order reaction can behave as first order when one reactant is present in large excess and its concentration remains nearly constant. Treating that concentration as effectively constant reduces the observed concentration dependence.

Answer

One reactant is in large excess, so its concentration remains nearly constant

Question 22

22. In the Arrhenius equation $k=Ae^{-E_a/(RT)}$ , what does $A$ represent?

The frequency factor associated with the reaction

The activation energy required to form the activated complex

The gas constant used to relate energy and temperature

The absolute temperature at which the reaction occurs

Explanation

In the Arrhenius equation, $A$ is the pre-exponential frequency factor, while $E_a$ is the activation energy. The activation energy appears in the exponential term rather than representing the frequency factor.

Answer

The frequency factor associated with the reaction

Question 23

23. For a plot of $\ln k$ against $1/T$ , what are the slope and intercept predicted by the logarithmic Arrhenius equation?

Slope

-R/E_a

and intercept

A

Slope

-E_a/R

and intercept

\ln A

Slope

\ln A

and intercept

-E_a/R

Slope

E_a/R

and intercept

\ln A

Explanation

Rearranging the Arrhenius equation gives $\ln k=-\frac{E_a}{RT}+\ln A$ , so the slope versus $1/T$ is $-E_a/R$ and the intercept is $\ln A$ . The intercept and slope are therefore not interchangeable.

Answer

Slope $$-E_a/R$$ and intercept $$\ln A$$

Question 24

24. What is activation energy in the context of a chemical reaction?

The energy stored in the frequency factor before collisions occur

The total energy released when products form from the reactants

The energy required to form the activated complex from the reacting molecules

The average kinetic energy of all molecules in the reaction mixture

Explanation

Activation energy is the energy required for reacting molecules to form the activated complex. It is not the overall energy change of the reaction or the average kinetic energy of the mixture.

Answer

The energy required to form the activated complex from the reacting molecules

Question 25

25. What does a catalyst do to increase the rate of a chemical reaction?

It increases the Gibbs energy difference between reactants and products

It provides an alternative pathway with lower activation energy

It raises the equilibrium constant by stabilizing the products

It permanently changes into a new chemical species during reaction

Explanation

A catalyst speeds a reaction by offering an alternative pathway whose activation energy is lower, while remaining chemically unchanged overall. Increasing the equilibrium constant would alter the equilibrium state, which catalysis does not do.

Answer

It provides an alternative pathway with lower activation energy

Question 26

26. How does a catalyst affect a reversible reaction approaching equilibrium?

It accelerates the reverse direction and shifts equilibrium toward reactants

It slows both directions while leaving the equilibrium composition unchanged

It accelerates the forward direction and shifts equilibrium toward products

It accelerates both directions similarly, so equilibrium is reached sooner

Explanation

A catalyst accelerates the forward and reverse reactions to the same extent, reducing the time required to reach equilibrium. It does not favor one direction or change the equilibrium composition.

Answer

It accelerates both directions similarly, so equilibrium is reached sooner

Question 27

27. In the collision-theory rate relation $r=Z_{AB}e^{-E_a/(RT)}P$ , what does the factor $P$ represent?

The proportion of collisions whose energy exceeds the activation threshold

The pressure exerted by reactant molecules on the container walls

The product concentration accumulated after the reaction begins

The probability that colliding molecules have a suitable orientation

Explanation

The factor $P$ is the steric factor, which accounts for whether molecules meet in an orientation suitable for reaction. The energy requirement is represented by the exponential factor involving $E_a$ , not by $P$ .

Answer

The probability that colliding molecules have a suitable orientation

Question 28

28. Which collision between reactant molecules is most likely to produce products according to collision theory?

A collision with sufficient energy but an unsuitable molecular orientation

A collision with sufficient energy and a suitable molecular orientation

A collision involving molecules that remain far enough apart to avoid contact

A collision with suitable orientation but energy below the activation threshold

Explanation

An effective collision requires both energy at least equal to the activation threshold and a suitable orientation. A collision that lacks either condition does not form products, even if it satisfies the other condition.

Answer

A collision with sufficient energy and a suitable molecular orientation

Quiz: Chemical Kinetics — 28 questions

Detailed questions and answers

1. Which field of chemistry examines reaction rates, the factors that control them, and the mechanisms by which reactions occur?

2. A reaction has a negative Gibbs free-energy change at constant temperature and pressure but proceeds imperceptibly slowly. What does this illustrate?

3. How is the rate of disappearance of a reactant related to the rate of appearance of a product in a reaction such as $R\rightarrow P$ ?

4. For $R\rightarrow P$ at constant volume, the concentrations change from $[R]_1$ and $[P]_1$ to $[R]_2$ and $[P]_2$ over a finite time interval $\Delta t$ . Which expression gives the average reaction rate?

5. What does the instantaneous reaction rate represent on a concentration–time graph?

6. For $aA+bB\rightarrow cC+dD$ , why are concentration changes divided by their stoichiometric coefficients when defining a single reaction rate?

7. For $2HI\rightarrow H_2+I_2$ , which expression correctly represents the reaction rate?

8. What does an experimentally determined rate law relate?

9. For the reaction $aA+bB\rightarrow cC+dD$ , which expression represents the general rate law?

10. What is the role of the rate constant $k$ in a rate law at a specified temperature?

11. How is the overall order of a reaction determined?

12. What happens to the rate of a zero-order reaction when a reactant concentration changes?

13. What does molecularity describe for an elementary reaction?

14. Which statement correctly distinguishes molecularity from reaction order?

15. In a complex reaction mechanism, which step controls the overall reaction rate?

16. For a zero-order reaction, how can the rate constant be calculated from the initial and current reactant concentrations?

17. Which concentration-time relationship describes a first-order reaction?

18. What does the half-life of a reaction represent?

19. A zero-order reaction has its initial reactant concentration doubled while its rate constant remains unchanged; what happens to its half-life?

20. Which expression gives the half-life of a first-order reaction?

21. Why can a reaction involving several reactants exhibit pseudo-first-order behavior?

22. In the Arrhenius equation $k=Ae^{-E_a/(RT)}$ , what does $A$ represent?

23. For a plot of $\ln k$ against $1/T$ , what are the slope and intercept predicted by the logarithmic Arrhenius equation?

24. What is activation energy in the context of a chemical reaction?

25. What does a catalyst do to increase the rate of a chemical reaction?

26. How does a catalyst affect a reversible reaction approaching equilibrium?

27. In the collision-theory rate relation $r=Z_{AB}e^{-E_a/(RT)}P$ , what does the factor $P$ represent?

28. Which collision between reactant molecules is most likely to produce products according to collision theory?

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Quiz: Chemical Kinetics — 28 questions

Detailed questions and answers

1. Which field of chemistry examines reaction rates, the factors that control them, and the mechanisms by which reactions occur?

2. A reaction has a negative Gibbs free-energy change at constant temperature and pressure but proceeds imperceptibly slowly. What does this illustrate?

3. How is the rate of disappearance of a reactant related to the rate of appearance of a product in a reaction such as R→PR\rightarrow PR→P?

4. For R→PR\rightarrow PR→P at constant volume, the concentrations change from [R]1[R]_1[R]1​ and [P]1[P]_1[P]1​ to [R]2[R]_2[R]2​ and [P]2[P]_2[P]2​ over a finite time interval Δt\Delta tΔt. Which expression gives the average reaction rate?

5. What does the instantaneous reaction rate represent on a concentration–time graph?

6. For aA+bB→cC+dDaA+bB\rightarrow cC+dDaA+bB→cC+dD, why are concentration changes divided by their stoichiometric coefficients when defining a single reaction rate?

7. For 2HI→H2+I22HI\rightarrow H_2+I_22HI→H2​+I2​, which expression correctly represents the reaction rate?

8. What does an experimentally determined rate law relate?

9. For the reaction aA+bB→cC+dDaA+bB\rightarrow cC+dDaA+bB→cC+dD, which expression represents the general rate law?

10. What is the role of the rate constant kkk in a rate law at a specified temperature?

11. How is the overall order of a reaction determined?

12. What happens to the rate of a zero-order reaction when a reactant concentration changes?

13. What does molecularity describe for an elementary reaction?

14. Which statement correctly distinguishes molecularity from reaction order?

15. In a complex reaction mechanism, which step controls the overall reaction rate?

16. For a zero-order reaction, how can the rate constant be calculated from the initial and current reactant concentrations?

17. Which concentration-time relationship describes a first-order reaction?

18. What does the half-life of a reaction represent?

19. A zero-order reaction has its initial reactant concentration doubled while its rate constant remains unchanged; what happens to its half-life?

20. Which expression gives the half-life of a first-order reaction?

21. Why can a reaction involving several reactants exhibit pseudo-first-order behavior?

22. In the Arrhenius equation k=Ae−Ea/(RT)k=Ae^{-E_a/(RT)}k=Ae−Ea​/(RT), what does AAA represent?

23. For a plot of ln⁡k\ln klnk against 1/T1/T1/T, what are the slope and intercept predicted by the logarithmic Arrhenius equation?

24. What is activation energy in the context of a chemical reaction?

25. What does a catalyst do to increase the rate of a chemical reaction?

26. How does a catalyst affect a reversible reaction approaching equilibrium?

27. In the collision-theory rate relation r=ZABe−Ea/(RT)Pr=Z_{AB}e^{-E_a/(RT)}Pr=ZAB​e−Ea​/(RT)P, what does the factor PPP represent?

28. Which collision between reactant molecules is most likely to produce products according to collision theory?

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3. How is the rate of disappearance of a reactant related to the rate of appearance of a product in a reaction such as $R\rightarrow P$ ?

4. For $R\rightarrow P$ at constant volume, the concentrations change from $[R]_1$ and $[P]_1$ to $[R]_2$ and $[P]_2$ over a finite time interval $\Delta t$ . Which expression gives the average reaction rate?

6. For $aA+bB\rightarrow cC+dD$ , why are concentration changes divided by their stoichiometric coefficients when defining a single reaction rate?

7. For $2HI\rightarrow H_2+I_2$ , which expression correctly represents the reaction rate?

9. For the reaction $aA+bB\rightarrow cC+dD$ , which expression represents the general rate law?

10. What is the role of the rate constant $k$ in a rate law at a specified temperature?

22. In the Arrhenius equation $k=Ae^{-E_a/(RT)}$ , what does $A$ represent?

23. For a plot of $\ln k$ against $1/T$ , what are the slope and intercept predicted by the logarithmic Arrhenius equation?

27. In the collision-theory rate relation $r=Z_{AB}e^{-E_a/(RT)}P$ , what does the factor $P$ represent?