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Unit 14 - Chemical Kinetics - Coggle Diagram
Unit 14 - Chemical Kinetics
14.1 - 14.2 Factors that Affect Reaction Rates
Chemical kinetics studies the rate at which chemical reactions occur.
Kinetics describes how a reaction occurs - the reaction mechanism.
In order to react, molecules must come into contact and “collide”
activation energy is what's needed to break reactant chemical bonds and form new product bonds
The collision must take place with the proper orientation at the correct place on the molecule, the reaction site.
Reaction Rates
rate is generally defined as the change in a given quantity per time
The rate of a reaction, therefore, is calculated by monitoring the change in both the reactant disappearance and the product formation as a function of time.
Molarity per second = (ΔM/Δs) or (M/s)
rate of appearance
change in concentration of B/change in time= Δ[B]/Δt
rate of disappearance
-change in concentration of A/change in time= - Δ[A]/Δt
14.3 The Rate Law: The Effect of Concentration on Rate
This equation is called the rate law, and k is the rate constant.
A rate law shows the relationship between the reaction rate and the concentrations of reactants.
Rate = k[A]^m*[B]^n
The exponents of each reactant concentration tell the order of the reaction with respect to each reactant.
Reaction Order
Zero order (exponent = 0, and therefore no effect on rate)
First order (exponent = 1, as concentration doubles, rate doubles)
Second order (exponent = 2), as concentration doubles, rate quadruples (22)
The overall reaction order can be found by adding the exponents on the reactants in the rate law.
14.4 The Change of Concentration with Time
First Order Reactions
Rate = - Δ[A]Δt= k[A]
Integrated Rate = ln[A]t = -kt + ln[A]0
Second Order Reactions
Rate = - Δ[A]Δt= k[A]2
Integrated Rate = 1[A]t= kt + 1[A]0
*
If a reaction is second order, a plot of 1/[A] vs. t will yield a straight line with a slope of k.
Half-life
[A]t1/2 = ½ [A]0
First Order
t1/2 = - ln(1/2)/k= 0.693/k
Second Order
t1/2 = 1/k[A]0
14.5 Temperature and Rate
Generally, as temperature increases, reaction rate increases due to the fact that k is temperature dependent.
The collision model accounts for these changes. To react:
There must be a collision between molecules.
This collision must occur with a sufficient amount of energy (activation energy (Ea).
This collision must occur at the correct orientation to cause bond breakage and formation.
The activation energy is the minimum amount of energy needed to be present in a collision for a reaction to occur and break the initial bonds.
14.6 Reaction Mechanisms
reaction intermediates, chemical species that are produced and consumed during the reaction but do not appear in the overall reaction.
Classification of Molecularity of a Process
A unimolecular reaction involves only one molecule of a substance decomposing into smaller atoms or molecules.
A bimolecular reaction has two molecules colliding (which could be two of the same substance)
A termolecular reaction has three molecules colling (at least could involve the same substance colliding)
most reactions involving multiple elementary steps, there is generally one step that is slower than the others, called the rate determining step.
Each individual reaction in the mechanism is called an elementary step or elementary reaction
Each elementary step has its own rate of reaction
One of the steps is slower than the rest and is called the rate limiting step which determines how fast the overall reaction can occur.
