AP Chemistry 5.4-5.5: Elementary Reactions and Collision Model

What is an elementary reaction?

A single molecular event represented by one step in a reaction mechanism. Its molecularity is determined by the number of reactant particles involved in that step.

What is molecularity?

The number of reactant particles that participate in a single elementary reaction step. Molecularity is defined only for elementary steps, not for overall reactions.

What is a unimolecular elementary reaction?

An elementary step involving one reactant particle, such as $\mathrm{A\rightarrow products}$. Its elementary-step rate law is typically $\mathrm{rate}=k[\mathrm{A}]$.

What is a bimolecular elementary reaction?

An elementary step involving two reactant particles, such as $\mathrm{A+B\rightarrow products}$ or $\mathrm{2A\rightarrow products}$. The corresponding rate laws are typically $\mathrm{rate}=k[\mathrm{A}][\mathrm{B}]$ and $\mathrm{rate}=k[\mathrm{A}]^2$, respectively.

What is a termolecular elementary reaction?

An elementary step involving three reactant particles, such as $\mathrm{A+B+C\rightarrow products}$. Its rate law is typically $\mathrm{rate}=k[\mathrm{A}][\mathrm{B}][\mathrm{C}]$. Termolecular steps are uncommon because simultaneous three-particle collisions are unlikely.

How is the rate law for an elementary reaction related to its molecularity?

For an elementary step, the reactant coefficients in the balanced step become the exponents in its rate law. For example, $\mathrm{2A+B\rightarrow products}$ has $\mathrm{rate}=k[\mathrm{A}]^2[\mathrm{B}]$.

Why can the molecularity of an overall reaction not usually be assigned?

An overall reaction may consist of multiple elementary steps, so its coefficients do not necessarily describe a single molecular event. Molecularity applies only to individual elementary steps.

What five factors commonly affect the rate of a chemical reaction?

The chemical identities of the reactants, their physical states and surface area, temperature, reactant concentrations, and the presence of a catalyst.

Chemical nature of reactants

The identities and bonding of the reacting substances influence reaction rate. Different substances can react at very different rates even under the same conditions.

Why does the physical state of a reactant affect reaction rate?

Reacting particles must come into contact. For reactants in different phases, reaction occurs at the interface, so the amount of available contact area influences the rate.

How does decreasing the particle size of a solid usually affect its reaction rate with a liquid or gas?

Smaller particles expose a greater total surface area, creating more opportunities for collisions at the interface and generally increasing the reaction rate.

How does increasing reactant concentration increase reaction rate according to collision theory?

A greater concentration means more particles occupy a given volume, increasing the frequency of collisions and therefore usually increasing the number of successful collisions per unit time.

Catalyst

A substance that increases the rate of a reaction by providing an alternative reaction pathway with a lower activation energy. A catalyst is not consumed overall in the reaction.

What does collision theory state about the relationship between collisions and reaction rate?

Reaction rate is proportional to the frequency of collisions that can lead to chemical change. However, only collisions with suitable orientation and sufficient energy are successful.

What three conditions must be met for a collision to be successful according to collision theory?

The reactant particles must collide, have an orientation that allows the necessary atoms to interact, and possess enough energy to overcome the activation energy.

Why do not all collisions between reactant particles produce products?

Some collisions have insufficient energy to overcome the activation barrier, while others occur with an orientation that does not allow the required bonds to break and form.

How does increasing temperature increase reaction rate according to collision theory?

Higher temperature increases the average kinetic energy and changes the energy distribution so that a greater fraction of particles can participate in collisions with energy at least equal to $E_a$.

What approximate temperature rule is often observed for reaction rates?

For many reactions, increasing the temperature by $10\ ^\circ\mathrm{C}$ approximately doubles the reaction rate. This is only a rough guideline and is not universal.

Activation energy, $E_a$

The minimum energy required for reactants to reach the transition state and form products during a reaction. It is commonly expressed in $\mathrm{J\ mol^{-1}}$ or $\mathrm{kJ\ mol^{-1}}$.

Transition state (activated complex)

A very short-lived, high-energy arrangement of atoms formed when reactants collide with suitable orientation and sufficient energy. It lies at the top of the activation-energy barrier.

On a reaction energy diagram, how is forward activation energy represented?

For a forward reaction, $E_a$ is the energy difference between the reactants and the transition state: $E_a=E_{\mathrm{transition\ state}}-E_{\mathrm{reactants}}$.

How does a catalyst affect activation energy and reaction rate?

A catalyst provides an alternative pathway with a lower activation energy. At the same temperature, more collisions have sufficient energy to react, so the reaction rate increases.

What does the steric factor represent in collision theory?

The fraction of collisions with an orientation suitable for reaction. It accounts for the fact that geometrically possible collisions are not necessarily productive.

What simplified collision-theory rate expression is given for a bimolecular gas reaction?

For $\mathrm{A+B\rightarrow products}$, the rate can be represented as $r(T)=Z\rho e^{-E_a/(RT)}$, where $Z$ is the collision frequency and $\rho$ is a steric factor accounting for orientation.