Principles of Kinetics

What is chemical kinetics?

Chemical kinetics is the study of how quickly reactions occur and the factors that affect their rates, including concentration, temperature, catalysts, and molecular structure.

What is reaction rate?

Reaction rate is the change in concentration of a reactant or product per unit time, showing how fast a reaction proceeds.

What are the typical units of reaction rate?

Moles per liter per second (mol·L⁻¹·s⁻¹), often abbreviated as molarity per second (M/s).

How is the rate of disappearance of a reactant related to the rate of appearance of a product?

The rates are related by stoichiometric coefficients. For example, in the reaction aA → bB, the rate equals negative one over a times the change in [A] over time, which equals positive one over b times the change in [B] over time.

What does the rate constant (k) represent?

The rate constant is a proportionality constant that relates the rate of reaction to the concentrations of reactants, depending on temperature and the reaction mechanism.

What is a rate law?

A rate law expresses the rate of a reaction as a function of reactant concentrations, each raised to an experimentally determined power: rate equals k times [A]^x times [B]^y.

What is the order of reaction?

The order of reaction is the sum of the exponents in the rate law, showing how the rate depends on concentration.

What is the difference between the rate law and the balanced chemical equation?

The rate law cannot be predicted from the stoichiometric equation because it depends on the mechanism, not the overall balanced reaction.

What does collision theory state?

Reactions occur when molecules collide with sufficient energy and correct orientation to break and form bonds.

What is activation energy?

The minimum energy that reacting particles must possess for a collision to result in a reaction.

How does temperature affect reaction rate?

Higher temperatures increase molecular motion, causing more frequent and energetic collisions, which increases the rate of reaction.

What is the transition state?

A high-energy, unstable arrangement of atoms at the point of maximum potential energy along the reaction pathway, also known as the activated complex.

What is the relationship between activation energy and the rate constant?

The rate constant increases exponentially as activation energy decreases, described by the Arrhenius equation.

What is the Arrhenius equation?

k equals A times e to the power of negative Ea over RT, where A is the frequency factor, Ea is activation energy, R is the gas constant, and T is temperature in kelvins.

What is the frequency factor (A) in the Arrhenius equation?

A represents the number of collisions with correct orientation per unit time. It reflects how often particles collide effectively.

How is activation energy determined experimentally?

By measuring the rate constant at different temperatures and plotting the natural log of k versus one over T. The slope equals negative Ea over R.

What does an energy profile diagram show?

It plots potential energy versus reaction progress, illustrating the energy of reactants, products, and the activation barrier. The peak corresponds to the transition state.

What is a reaction mechanism?

A step-by-step description of the sequence of elementary reactions by which an overall reaction occurs.

What are elementary steps?

Individual molecular events that occur during a reaction mechanism, each with its own rate law derived directly from its molecularity.

What is the rate-determining step?

The slowest step in a reaction mechanism, which limits the overall rate of the reaction.

How does a catalyst affect reaction rate?

A catalyst provides an alternative pathway with a lower activation energy, increasing the rate without being consumed.