CHEM2311: Kinetics; Nucleophiles and Electrophiles Lecture 12

These flashcards cover key terms and concepts related to kinetics, nucleophiles and electrophiles discussed in the lecture.

Kinetics

The study of reaction rates, often measured as a change in concentration over time.

What factors can increase the rate of a chemical reaction?

1. Increasing reactant concentration. 2. Increasing temperature. 3. Increasing surface area (for heterogeneous reactions). 4. Adding a catalyst.

Why does raising the temperature increase the rate of a chemical reaction?

Increasing temperature increases the average kinetic energy of molecules, leading to more frequent collisions and a higher proportion of collisions with energy greater than the activation energy (Ea).

Activation Energy (Ea)

The minimum amount of kinetic energy required for molecules to collide and react.

Transition State

A high-energy structure formed during a collision where bonds break and form, existing fleetingly.

What is the relationship between the activation energy barrier and the rate of reaction?

A higher activation energy barrier generally leads to a slower reaction rate, as fewer molecules possess the necessary kinetic energy to overcome it.

Catalyst

A substance that speeds up a reaction by lowering the activation energy without being consumed in the reaction.

How do catalysts enhance the rate of reaction?

Catalysts provide an alternative reaction pathway with a lower activation energy, thus allowing a larger fraction of reactant molecules to overcome the energy barrier and react, without being consumed in the overall reaction.

Rate Law

An expression that relates the rate of a reaction to the concentration of reactants, typically involving stoichiometric coefficients.

How is the rate law for an elementary step determined?

For an elementary step, the rate law can be written directly from its stoichiometry, where the rate is proportional to the product of the reactant concentrations, each raised to the power of its stoichiometric coefficient (e.g., if A + B \rightarrow Products, Rate = k[A][B]).

Reaction Coordinate Diagram

A graphical representation that traces how the energy of a system changes as a reaction progresses.

What key elements should be labeled on a Reaction Coordinate Diagram?

Reactants, products, transition state(s), intermediates (if present), activation energy (Ea), and the overall enthalpy change (\Delta E or \Delta H).

How are Activation Energy (Ea) and the Transition State identified on a Reaction Coordinate Diagram?

The transition state is the peak of an energy hill. Activation energy (Ea) is the energy difference between the reactants (or intermediates in a multi-step reaction) and the transition state that immediately follows it.

Multistep Mechanism

A reaction pathway that occurs through a series of elementary steps, each with its own transition state.

Intermediates

Species that are formed and consumed during the course of a reaction, not appearing in the overall reaction.

How are intermediates identified in a multi-step reaction mechanism?

Intermediates are species formed in one elementary step and consumed in a subsequent elementary step; they do not appear in the overall balanced chemical equation and are usually local minima between transition states on an energy diagram.

What is the rate-determining step in a multi-step mechanism, and how is it identified on a Reaction Coordinate Diagram?

The rate-determining step (or rate-limiting step) is the slowest elementary step in a multi-step reaction mechanism. On a Reaction Coordinate Diagram, it corresponds to the elementary step with the highest activation energy barrier.

How is the overall reaction for a multi-step mechanism determined?

By summing all the elementary steps and canceling out any intermediates and catalysts that appear on both the reactant and product sides of the steps.

Nucleophile

An electron donor in a chemical reaction, often a negatively charged species or atom rich in electrons.

Electrophile

An electron acceptor in a chemical reaction, often a