Kinetics
How fast is that reaction AND how is it happening?
Factors that Affect Reaction Rate
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.
Concentration
Expressing Reaction Rates
General rate expressions
Rate Laws
Instantaneous 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.
Practice
General Rate Expressions and Stoichiometry
Example using stoichiometry to equate rates
Example 2 using stoichiometry to equate rates
Example 3
For 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.
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
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.
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.
dicated by the equation in the table above.
Second order reactions do depend on concentration but it is 1/[A]...
.
Temperature and rate
Generally, as temperature increases, so does the reaction rate.
This is because k is temperature-dependent.
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
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
adding a catalyst also affects the number of molecules that can attain Ea
Catalysts speed up chemical reactions by lowering the energy of activation.
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
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.
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.