Chemical Kinetics Flashcards

Flashcards about Chemical Kinetics

Chemical Kinetics

The study of reaction mechanisms and rates of chemical reactions under various conditions.

What controls the thermodynamics of a reaction?

Enthalpy change, entropy change, and temperature.

What influences the kinetics of a reaction?

Mechanism, activation barrier, concentration of species, and temperature.

What three conditions must be met for a reaction to occur?

Reaction species collide, collision has enough energy to overcome the activation barrier, reacting species collide in an orientation that allows the necessary bond breaking and bond forming needed to transform reactants to products to occur.

How does the number of collisions affect reaction speed?

More collisions between reacting species occur, the reaction will proceed faster.

For a reaction to occur as a result of a specific collision, what must the collision have?

The collision must have enough energy to overcome the activation barrier.

How does temperature affect the number of molecules with enough energy to react?

As the temperature increases, the number of molecules with enough energy to react increases.

How does the magnitude of the energy barrier affect the number of reactant molecules that undergo a reaction?

More reactant molecules undergo reaction when three is a smaller activation barrier

How is reaction rate calculated?

Ratio of change in concentration over change in time.

Reaction stoichiometry is directly related to the relative rate at which what happens?

Reactants are consumed and products are formed.

Instantaneous rate is equal to what?

Slope of the tangent to the concentration–time curve at a given point in time.

How is the initial rate found?

Tangent to the concentration–time curve at t = 0.

What are the main components of rate laws?

Rate constant (k), concentrations of reacting species, order of reaction with respect to reacting species.

Rate constant(k)

Proportionality constant that relates rate and concentration; Function of temperature.

How to find the units of the rate constant

Divide the rate units by the molarity raised to the power of the overall reaction order

How is reaction order indicated and determined?

Exponents x, y, and z; Determined from experiment, not stoichiometric coefficients.

Overall reaction order

Sum of individual reaction orders.

Method of Initial Rates

Reaction is repeated multiple times with different initial concentrations; Initial rate is measured from each experiment; Result is used to determine order of reaction.

Integrated Rate Laws

Mathematical equations that show the relationship between the concentration of a reactant and time for reactions that involve a single reactant.

What are the three different ways to plot concentration–time data?

Concentration versus time, natural log of concentration versus time, 1/(concentration) versus time.

Reaction Half-life (t1/2)

Amount of time required for the concentration of a reactant to fall to one half of its initial value; Independent of the initial concentration of reactant for first order reactions

For zero- and second-order reactions, half-life depends on what?

Rate constant and initial concentration of reactant.

Activation Barrier

Energy that must be overcome for a reaction to proceed to form products; Changing the temperature affects ability to overcome this barrier.

Reaction Coordinate Diagram

Plot that displays the energy (y-axis) as a function of the progress of a chemical reaction from reactants to products (x-axis).

Activation Energy (Ea)

Energy required to reach transition state; Always a positive quantity.

When temperature is increased, what happens to the rate of reaction and Ea?

Rate of reaction increases, Ea remains unchanged.

The Arrhenius Equation

k = rate constant; A = frequency factor; Ea = activation energy; R = gas constant (8.3.145 J/K . mol); T = temperature (K).

Frequency factor (A)

Measure of the number of collisions that take place with the correct orientation during a reaction.

Reaction Mechanisms

Detailed molecular level description of steps by which products are converted to reactants; Series of individual steps that happen in sequence, with each step being an elementary step whose sum gives the overall equation.

Molecularity

Provides number of species that react in the step; Unimolecular: One reacting species; Bimolecular: Two species collide to react; Termolecular: Three species collide simultaneously.

Intermediate

Chemical species formed in one step of a reaction mechanism and consumed in a later step.

Reaction Mechanism and Rate Law

Rate-determining step is the slowest step in a reaction; Rate of a reaction can be no faster than the slowest step.

Some elementary steps are reversible and have

ability to proceed in forward and reverse directions; Dynamic equilibrium is attained when both reverse and forward reactions occur at a fast and at the same rate

Catalyst

Chemical that changes the reaction mechanism of a reaction, thereby influencing its rate; Consumed in one step of a multistep reaction and produced in a subsequent step.

Homogeneous catalyst

Exist in the same phase as the species undergoing the reaction

Heterogeneous catalyst

Do not exist in the same phase as the species undergoing the reaction