Chem 12 - Chapter 1: Reaction Kinetics

let's all do well on this one!

Chemical kinetics

the investigation of the rate at which these reactions occur and the factors that affect them

Reaction rates may be determined by…

observing the disappearance of a reactant OR the appearance of product

What to consider when determining the reaction rate?

• measurable property associated with a change in quantity of a reactant or product?

• how to measure this quantity in the lab?

• what units will be associated with the measurement and what will the units of the rxn. rate be?

Equation for avg. reaction rate

(change in a measurable quantity of a chemical species) / (change of time)

When is the reaction rate the greatest?

At the beginning of the reaction, as it decreases as the reaction continues.

How do we express rate?

As an absolute or positive value

What are methods to measure a reaction and what do they measure?

• titration: concentration of a reactant

• manometer: change in pressure of a gas

• eudiometer: change in volume of a gas

• scale: change in mass of a reactant (usually a gas leaving an open system)

• pH meter: pH of a solution

• spectrophotometer: intensity of a color

What are the FIVE FACTORS that influence reaction rate?

• surface area (affects heterogenous reactions)

• concentration (affects rxns. with solutions and gases)

• nature of the reactants

• temperature

• presence of a catalyst

How does increasing the temperature affect the rate of a reaction?

It results in more frequent and more forceful collisions

Which reactants react at the fastest rate, from fastest to slowest?

monatomic ions > aq solutions > gases > liquids > solids

How is the concentration of reactants related to the reaction rate?

The rate of a reaction is directly proportional to the product of the concentrations of the reactants

What is the rate law equation?

reaction rate = k[A]^x[B]^y

What are “x” and “y” in the rate law equation?

Reactant orders. The higher the order of a particular reactant, the more a change in the concentration of that reactant affects the reaction rate

What part of the rate law is affected by temp?

The rate/proportionality constant

Collision theory states that the reaction rates depend upon what two considerations?

1. the number of collisions per unit time

2. the fraction of these collisions that succeed in producing products

What are the requirements for effective (reacting) collisions?

• must attain activation energy

• the geometric shape AND the collision geometry must be favorable

What do the five factors affecting reaction rate actually do in terms of collision theory?

Increased temperature, concentration, and surface area all increase the frequency of collisions. The presence of a catalyst and increased temperature increase the fraction of collisions that are successful.

What is enthalpy?

Enthalpy is potential energy that may be evolved (given off) or absorbed as heat.

What is bond energy?

The chemical potential energy able to break a bond (the amount of energy required to break one mole of the stated bond)

What is an activated complex?

A high-energy, unstable, transitory intermediate state or configuration of atoms that occurs while reactants are transforming into products.

How do we find Ea in terms of potential energy diagrams?

Ea = PE of activated complex - PE of reactants

What is the enthalpy change in terms of potential energy diagrams?

ΔH = PE of products - PE of reactants

What is always true for a reversible potential energy diagram?

• ΔH for the forward and reverse reactions have the same magnitude and opposite signs

• Ea is always smaller for the exothermic reaction

• Altering temp, pressure, concentration, or surface area will have no effect on a potential energy diagram. Only catalysts will alter a diagram.

What do catalysts do in a chemical reaction?

Catalysts provide an additional reaction mechanism with a lower activation energy, which results in an increased reaction rate. Catalysts must be consumed in one step of the reaction and regenerated in a later step of the reaction.

What do potential energy profiles look like with catalyzed reactions?

Potential energy profiles for catalyzed reactions will usually involve the formation of more than one activated complex and more than one peak in the diagram.

What is a reaction mechanism?

A series of steps that may be added together to give an overall chemical reaction.

How do chemists determine reaction mechanisms?

They alter the concentrations of the reactants involved and determine the reaction mechanism using the results produced by the alterations.

What are elementary processes?

The individual steps in a series that make up a reaction mechanism.

What is the molecularity of an elementary process?

the number of reactant species that must collide to produce the reaction indicated by that step

What is a reaction intermediate?

A species that is formed in one step and consumed in a subsequent step and so does not appear in the overall reaction.

What is the rate-determining step?

A step that limits the rate of the overall reaction. This may be due to the elementary process having:

• complex collision geometry

• a high activation energy

• low concentrations of reactants

• a termolecular collision

How do we speed up the rate of a reaction after knowing the slowest elementary process?

We must increase the concentration of the reactants present in the rate-determining step.

How do we determine Ea and rate-determining step when looking at a potential energy diagram of a catalyzed reaction?

Highest peak = overall Ea of reaction

Biggest energy difference from plateau to peak in a single elementary process = rate-determining step

How can catalysts be classified?

Heterogenous or homogenous

What are heterogenous catalysts?

A form of catalysis in which the phase of the catalyst differs from that of the reactants. These are limited to reactions on their surface area only, and undergo adsorption of reactants onto their surface, allowing them easy interaction between reacting species.

What are homogenous catalysts?

Catalysts that exist in the same phase as the rest of the reaction system. This is often seen through acid catalysis, where the hydrogen ions of an acid make a reactant more susceptible to reacting with the other reagent.