Define elementary reaction .

An elementary reaction is a single molecular event — one step in a reaction mechanism — that cannot be broken down further. The set of elementary reactions that combine to give the overall equation is called the reaction mechanism.

What is the molecularity of an elementary reaction, and what are the three types?

Molecularity is the number of reactant molecules in a single elementary step. Unimolecular: one molecule (rate = k[A]). Bimolecular : two molecules (rate = k[A][B] or k[A]²). Termolecular : three molecules — rare.

True or False? The rate law for an elementary reaction can be written directly from its stoichiometric equation.

True. For elementary reactions only, the reaction order for each reactant equals its stoichiometric coefficient. This is not valid for overall (non-elementary) balanced equations.

A chemical species that appears in elementary steps but not in the overall balanced equation is called a reaction .......... .

A chemical species that appears in elementary steps but not in the overall balanced equation is called a reaction intermediate .

Why can the rate law NOT be determined from the overall balanced equation of a multi-step reaction?

The overall equation is the sum of all elementary steps and does not reveal the mechanism. Rate laws depend on which elementary step is rate-limiting , so they must be determined experimentally — not read off the stoichiometry of the overall equation.

True or False? Termolecular elementary reactions are the most common type in chemical mechanisms.

False. Termolecular reactions are rare because the simultaneous collision of three molecules with the correct orientation and sufficient energy is highly unlikely. Unimolecular and bimolecular steps are far more common.

Define effective collision .

An effective collision is a collision between reactant molecules that results in a chemical reaction. It requires (1) kinetic energy equal to or greater than the activation energy, and (2) correct molecular orientation .

According to the collision model, what three factors determine the rate constant k?

k = p × Z × f, where:\n\nZ = collision frequency (number of collisions per unit time at unit concentration)\n\nf = fraction of collisions with energy ≥ activation energy \n\np = steric factor (fraction of collisions with correct molecular orientation)

True or False? Every collision between reactant molecules leads to a chemical reaction.

False. Only effective collisions — those with sufficient energy (≥ activation energy) AND correct molecular orientation — result in product formation. Most collisions are ineffective.

When temperature increases, the Maxwell-Boltzmann distribution curve peak shifts to the .......... and the curve .......... .

When temperature increases, the Maxwell-Boltzmann distribution curve peak shifts to the right and the curve flattens , because more molecules have higher kinetic energies — increasing the fraction exceeding the activation energy.

What does the area to the right of the activation energy line on a Maxwell-Boltzmann distribution curve represent?

It represents the fraction of molecules with kinetic energy equal to or greater than the activation energy — i.e., the fraction capable of undergoing an effective collision and reacting. Increasing temperature increases this area.

True or False? The total area under a Maxwell-Boltzmann distribution curve increases when temperature is raised.

False. The total area under the curve represents the total number of molecules, which is constant regardless of temperature. Only the shape changes — the curve flattens and shifts right.

Why does increasing the concentration of a reactant increase the rate of reaction at the molecular level?

Higher concentration means more molecules per unit volume, which increases the collision frequency (Z). More collisions per unit time means more effective collisions , so the rate increases.

Define activated complex .

The activated complex (transition state) is an unstable, high-energy species formed transiently when reactant bonds are partially broken and product bonds are partially formed. It sits at the peak of the energy profile. The energy required to reach it is the activation energy .

On an energy profile, how is the activation energy (E a ) of the forward reaction identified?

E a of the forward reaction is the energy difference between the peak of the energy barrier (the transition state ) and the energy level of the reactants. It is always a positive value.

True or False? A highly exothermic reaction must have a low activation energy.

False. The activation energy is independent of the enthalpy change. A highly exothermic reaction can still have a high activation energy and therefore proceed very slowly.

The activation energy for the reverse reaction equals E a (forward) .......... Δ H .

The activation energy for the reverse reaction equals E a (forward) minus Δ H . For example, if E a (forward) = +134 kJ and Δ H = −226 kJ, then E a (reverse) = 134 − (−226) = 360 kJ.

Why is the activation energy of a reaction much smaller than the energy needed to break all bonds in the reactants?

In the activated complex , bonds are only partially broken (and new bonds partially formed) — so only enough energy to weaken bonds is required, not the full bond dissociation energy . This is the key insight of transition state theory.

True or False? The rate of a reaction depends on the magnitude of its activation energy, not on its enthalpy change.

True. Slow reactions have high E a (few molecules exceed it); fast reactions have low E a . The enthalpy change (Δ H ) tells you whether products are higher or lower in energy than reactants, but does not determine speed.

Define frequency factor (A) in the Arrhenius equation.

The frequency factor (A) is a constant in the Arrhenius equation related to the collision frequency (Z) and the steric factor (p). It represents the maximum possible rate constant if every collision were effective.

Write the Arrhenius equation and state what each symbol represents.

k = Ae (−E a /RT)\n\nk = rate constant ; A = frequency factor ; E a = activation energy (J mol -1 ); R = gas constant (8.314 J mol -1 K -1^); T = absolute temperature (K).\n\nAs E a increases, k decreases because fewer molecules have sufficient energy to react.

Taking the natural log of the Arrhenius equation gives ln k = .......... − E a /RT, which means a plot of ln k against 1/T gives a straight line with a .......... slope.

Taking the natural log of the Arrhenius equation gives ln k = ln A − E a /RT, which means a plot of ln k against 1/T gives a straight line with a negative slope.

True or False? Increasing the activation energy of a reaction increases the rate constant k.

False. In k = Ae (−E a /RT), a larger E a makes the exponent more negative, so e (−E a /RT) decreases — meaning k decreases. Higher activation energy → smaller rate constant → slower reaction.

Why does temperature have a much greater effect on reaction rate than collision frequency alone would predict?

Collision frequency (Z) increases only modestly with temperature (less than 10% from 500 to 600 K), so collision frequency alone cannot explain the large rate increase. The dominant effect is the exponential term e^(−E a /RT): even small temperature rises dramatically increase the fraction of molecules with energy ≥ E a , causing a large increase in k.

True or False? The slope of a ln k vs 1/T (Arrhenius) plot can be used to determine the activation energy of a reaction.

True. The slope of the ln k vs 1/T graph equals −E a /R. Multiplying the slope by −R gives E a in J mol -1 .

AP®ChemistryCollege BoardFlashcardsUnit 5: KineticsElementary Reactions & Collisions

Elementary Reactions & Collisions (College Board AP® Chemistry): Flashcards

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Define elementary reaction.

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Cards in this collection (25)

Define elementary reaction.
An elementary reaction is a single molecular event — one step in a reaction mechanism — that cannot be broken down further. The set of elementary reactions that combine to give the overall equation is called the reaction mechanism.
What is the molecularity of an elementary reaction, and what are the three types?
Molecularity is the number of reactant molecules in a single elementary step.
Unimolecular: one molecule (rate = k[A]).
Bimolecular: two molecules (rate = k[A][B] or k[A]²).
Termolecular: three molecules — rare.
True or False?
The rate law for an elementary reaction can be written directly from its stoichiometric equation.
True.
For elementary reactions only, the reaction order for each reactant equals its stoichiometric coefficient. This is not valid for overall (non-elementary) balanced equations.
A chemical species that appears in elementary steps but not in the overall balanced equation is called a reaction ...........
A chemical species that appears in elementary steps but not in the overall balanced equation is called a reaction intermediate.
Why can the rate law NOT be determined from the overall balanced equation of a multi-step reaction?
The overall equation is the sum of all elementary steps and does not reveal the mechanism. Rate laws depend on which elementary step is rate-limiting, so they must be determined experimentally — not read off the stoichiometry of the overall equation.
True or False?
Termolecular elementary reactions are the most common type in chemical mechanisms.
False.
Termolecular reactions are rare because the simultaneous collision of three molecules with the correct orientation and sufficient energy is highly unlikely. Unimolecular and bimolecular steps are far more common.
Define effective collision.
An effective collision is a collision between reactant molecules that results in a chemical reaction. It requires (1) kinetic energy equal to or greater than the activation energy, and (2) correct molecular orientation.
According to the collision model, what three factors determine the rate constant k?
k = p × Z × f, where:\n\nZ = collision frequency (number of collisions per unit time at unit concentration)\n\nf = fraction of collisions with energy ≥ activation energy\n\np = steric factor (fraction of collisions with correct molecular orientation)
True or False?
Every collision between reactant molecules leads to a chemical reaction.
False.
Only effective collisions — those with sufficient energy (≥ activation energy) AND correct molecular orientation — result in product formation. Most collisions are ineffective.
When temperature increases, the Maxwell-Boltzmann distribution curve peak shifts to the .......... and the curve ...........
When temperature increases, the Maxwell-Boltzmann distribution curve peak shifts to the right and the curve flattens, because more molecules have higher kinetic energies — increasing the fraction exceeding the activation energy.
What does the area to the right of the activation energy line on a Maxwell-Boltzmann distribution curve represent?
It represents the fraction of molecules with kinetic energy equal to or greater than the activation energy — i.e., the fraction capable of undergoing an effective collision and reacting. Increasing temperature increases this area.
True or False?
The total area under a Maxwell-Boltzmann distribution curve increases when temperature is raised.
False.
The total area under the curve represents the total number of molecules, which is constant regardless of temperature. Only the shape changes — the curve flattens and shifts right.
Why does increasing the concentration of a reactant increase the rate of reaction at the molecular level?
Higher concentration means more molecules per unit volume, which increases the collision frequency (Z). More collisions per unit time means more effective collisions, so the rate increases.
Define activated complex.
The activated complex (transition state) is an unstable, high-energy species formed transiently when reactant bonds are partially broken and product bonds are partially formed. It sits at the peak of the energy profile. The energy required to reach it is the activation energy.
On an energy profile, how is the activation energy (E_a) of the forward reaction identified?
E_a of the forward reaction is the energy difference between the peak of the energy barrier (the transition state) and the energy level of the reactants. It is always a positive value.
True or False?
A highly exothermic reaction must have a low activation energy.
False.
The activation energy is independent of the enthalpy change. A highly exothermic reaction can still have a high activation energy and therefore proceed very slowly.
The activation energy for the reverse reaction equals E_a(forward) .......... ΔH.
The activation energy for the reverse reaction equals E_a(forward) minus ΔH.
For example, if E_a(forward) = +134 kJ and ΔH = −226 kJ, then E_a(reverse) = 134 − (−226) = 360 kJ.
Why is the activation energy of a reaction much smaller than the energy needed to break all bonds in the reactants?
In the activated complex, bonds are only partially broken (and new bonds partially formed) — so only enough energy to weaken bonds is required, not the full bond dissociation energy. This is the key insight of transition state theory.
True or False?
The rate of a reaction depends on the magnitude of its activation energy, not on its enthalpy change.
True.
Slow reactions have high E_a (few molecules exceed it); fast reactions have low E_a. The enthalpy change (ΔH) tells you whether products are higher or lower in energy than reactants, but does not determine speed.
Define frequency factor (A) in the Arrhenius equation.
The frequency factor (A) is a constant in the Arrhenius equation related to the collision frequency (Z) and the steric factor (p). It represents the maximum possible rate constant if every collision were effective.
Write the Arrhenius equation and state what each symbol represents.
k = Ae^(−E^_a^{/RT)\n\nk =}^{rate constant}^{; A =}^{frequency factor}^{; E}^_a⁼^{activation energy}^{(J mol}-1^{); R = gas constant (8.314 J mol}-1^K-1^); T = absolute temperature (K).\n\nAs E_a increases, k decreases because fewer molecules have sufficient energy to react.
Taking the natural log of the Arrhenius equation gives ln k = .......... − E_a/RT, which means a plot of ln k against 1/T gives a straight line with a .......... slope.
Taking the natural log of the Arrhenius equation gives ln k = ln A − E_a/RT, which means a plot of ln k against 1/T gives a straight line with a negative slope.
True or False?
Increasing the activation energy of a reaction increases the rate constant k.
False.
In k = Ae^(−E_^a^{/RT), a larger E}_^a^{makes the exponent more negative, so e}(−E_a/RT) decreases — meaning k decreases. Higher activation energy → smaller rate constant → slower reaction.
Why does temperature have a much greater effect on reaction rate than collision frequency alone would predict?
Collision frequency (Z) increases only modestly with temperature (less than 10% from 500 to 600 K), so collision frequency alone cannot explain the large rate increase. The dominant effect is the exponential term e^(−E_a/RT): even small temperature rises dramatically increase the fraction of molecules with energy ≥ E_a, causing a large increase in k.
True or False?
The slope of a ln k vs 1/T (Arrhenius) plot can be used to determine the activation energy of a reaction.
True.
The slope of the ln k vs 1/T graph equals −E_a/R. Multiplying the slope by −R gives E_a in J mol^-1.

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