# Kinetics

## How fast is that reaction AND how is it happening?

## Factors that Affect Reaction Rate

## Physical State of the Reactants Which state of matter do you think would result in more reactions: solid, liquid, gas, solution? The more homogeneous a mixture of reactants is, the faster the molecules can react. The picture above shows this in terms of surface area. | ## Temperature If the goal is for reactants to collide with enough energy for a reaction to proceed, does this happen at lower or higher temperatures in general? The picture above is our second introduction to the big idea that not all particles in a system have the same energy. At the higher temperature, T2, a greater proportion of particles have the | ## Concentration Many reactions occur in solution. WIll more collisions occur in dilute or in concentrated solutions? In the picture above, steel wool burns in air on the left but in 100% oxygen gas on the right. |

## Physical State of the Reactants

The more homogeneous a mixture of reactants is, the faster the molecules can react. The picture above shows this in terms of surface area.

## Temperature

The picture above is our second introduction to the big idea that not all particles in a system have the same energy. At the higher temperature, T2, a greater proportion of particles have the *threshold energy* necessary for the reaction to occur as compared to the lower temperature, T1.

## Expressing Reaction Rates

## General rate expressions This is the simplest form of a rate expression. As seen in the diagram above, it is an "average rate" for one reactant (disappearance) or product (appearance) over a unit of time. | ## Rate Laws Rate laws are specific for a given reaction where the dependence of rate on concentration is expressed mathematically. Some known rate laws have taken years of careful research to obtain. See the larger picture below which dissects the information contained within a rate law. | ## Instantaneous rates These must be determined graphically. It is the rate of a reaction at a given point in time. |

## General rate expressions

## Rate Laws

## Practice

## General Rate Expressions and Stoichiometry

## Example using stoichiometry to equate rates | ## Example 2 using stoichiometry to equate rates | ## Example 3For the following gas-phase reactions, write the rate expression in terms of the appearance of each product and disappearance of each reactant. (a) 2H2O(g) ---> 2H2(g) + O2(g) (b) 2SO2(g) + O2(g) ---> 2SO3(g) |

## Rate Laws: The dependence of rate on concentration

## What Rate Laws Tell Us

A rate law shows the relationship between the reaction rate and the concentrations of reactants.

The exponents tell the

**order**of the reaction with respect to each reactant.The overall reaction order can be found by adding the exponents on the reactants in the rate law.

## We get rate laws from EXPERIMENTS only! The method of initial rates shows you how.

## Using calculus, we can manipulate the equation above to get a graphable form of the equation, y = mx + b

## Since we have a graphable equation, it can be used to determine the order of a reaction by graphing variations of concentration vs time.

## A first order reaction would produce the graphs seen above. Since the one that produces a straight line is found by taking the natural log of [A], we know this is a first order reaction with respect to [A].

## A second order reaction would produce the graphs seen above. Since the one that produces a straight line is found by taking 1/[A], we know this is a second order reaction with respect to [A].

## A zero order reaction would produce the graphs seen above. Since the one that produces a straight line is found by graphing [A] vs time, we know this is a zero order reaction with respect to [A].

## A summary of integrated rate law equations and half-life equations.

## The Collision Model

## there is a minimum amount of energy required for reaction: the activation energy, Ea

## Reaction Energy Diagrams

The diagram shows the energy of the reactants and products (and, therefore, Δ

*E, or*Δ*H*if pressure is constant).The high point on the diagram is the

**transition state**.The species present at the transition state is called the

**activated complex**.The energy gap between the reactants and the activated complex is the

**activation-energy barrier**.

## temperature affects the fraction of molecules that have sufficient energy to attain Ea

## adding a catalyst also affects the number of molecules that can attain Ea

## Catalysts lower energies of activation by providing an alternative mechanism for a reaction

## Question for diagram above

(a) How many elementary reactions are in the reaction mechanism? (b) How many intermediates are formed in the reaction? (c) Which step is rate-limiting? (d) Is the overall reaction endothermic or exothermic?

## There is a mathematical realtionship between k and Ea

## Our final topic in kinetics: using rate laws to determine how reactions really occur

## Reaction Mechanisms

The sequence of events that describes the actual process by which reactants become products is called the **reaction mechanism**.

Reactions may occur all at once or through several discrete steps.

Each of these processes is known as an

**elementary reaction**or**elementary process**.

## If we look at each elementary step, we can make some assumptions about the rate laws for those steps

## We can write rate laws from the slow step for a reaction mechanism.

**Note:**rate laws are usually written from the disappearance of reactants.

## Example reaction mechanism: the slow step is the most important as it determines the rate for a reaction

In this video I go over Arrhenius equation examples, half-life examples, integrated rate law examples, and reaction mechanism examples.

additional problems from the notes also worked out