How fast is that reaction AND how is it happening?

Kinetics: Chemistry's Demolition Derby - Crash Course Chemistry #32

Factors that Affect Reaction Rate

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Expressing Reaction Rates

Reaction Rates

Rates of reactions can be determined by monitoring the change in concentration
of either reactants or products as a function
of time.

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General Rate Expressions and Stoichiometry

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Rate Laws: The dependence of rate on concentration

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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.

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

Kinetics: Initial Rate Method
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Integrated Rate Laws expresses how concentration changes with time

from the figure to the right, where

[A]0 is the initial concentration of A, and

[A]t is the concentration of A at some time, t, during the course of the reaction.

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

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

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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].

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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].

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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].

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

Half lives as it pertains to chemical reactions can be thought of as a situation where [A]t = 0.5 as compared to [A]o.

The graph for a first order equation on the left shows that the half-life does not depend on concentration as in

dicated by the equation in the table above.

Second order reactions do depend on concentration but it is 1/[A]...

If the graph above is confusing, as in half-life is changing, look at the equation for half-life of a second order reaction in the table to the right.


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Temperature and rate

  • Generally, as temperature increases, so does the reaction rate.

  • This is because k is temperature-dependent.

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The Collision Model

  • In a chemical reaction, bonds are broken and new bonds are formed.

  • Molecules can only react if they collide with each other.

  • Furthermore, molecules must collide with the correct orientation and with enough energy to cause bond breakage and formation.

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

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Reaction Energy Diagrams

  • The diagram shows the energy of the reactants and products (and, therefore, ΔE, or ΔH 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

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

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Catalysts speed up chemical reactions by lowering the energy of activation.

It is important to keep reminding yourself that this happens because catalysts provide alternative paths to reactions that have lower energies of activation.
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Catalysts lower energies of activation by providing an alternative mechanism for a reaction

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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

The Arrhenius equation

Svante Arrhenius developed a mathematical relationship between k and Ea where A is the frequency factor, a number that represents the likelihood that collisions would occur with the proper orientation for reaction.

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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

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Reaction Mechanisms

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

There are some rules however! For example, intermediates cannot be present in a rate law. In the example to the left, N2O2, is an intermediate and, therefore, must cancel in the reaction mechanism to give the overall reaction. We do not know which step in this mechanism is the slow step but both steps involve N2O2. 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

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Rate Limiting Step and the Requirements for a Valid Mechanism
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Rate Law for a Mechanism with a Fast Initial Step
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Video on additional problems.

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

Solutions to practice problems on this smore

additional problems from the notes also worked out

What can you do to help your understanding?

Chemistry is a subject that must be practiced everyday if possible. Work through the lecture examples stopping the video clips and then restarting to check yourself. Take advantage of the practice in Mastering Chemistry to give you the practice you need to be successful. DO NOT PROCRASTINATE! Check announcements on Blackboard everyday. Keep a printed copy of the most recent course calendar (included in syllabus) next to your work area. Email me with questions!!