Flashcards in Lecture 18: Kinetics I Deck (43)
What is kinetics?
The study of the rate at which chemical reactions occur
Gives us info about the rate at which change takes place and the mechanism by which a reactant is converted into products
What are reaction rates?
Rates of change in concentrations or amounts if either reactants or products
In chemistry we would be interested in the yield, what are we interested in terms of kinetics in pharmaceutical science?
We want to monitor how much the product or drug changes over time until it becomes unsafe and unable to give to patients.
As well as knowing the speed of reaction we can also get an idea of the reaction mechanisms involved
What are reaction rates?
What we measure, the speed of the reaction
What are rate laws.
The relationship between the reaction and the product
What are integrated rate laws?
The integrated form of the rate law, which we use to calculate the amount of reactants and products
What is half life?
The time taken for the drug or reactant or product to reduce to half of its original amount
How are rate of reactions determined?
We monitor the change in concentration of either reactants or products as a function of time.
What is the formula for rate?
Δ[A] vs. Δt
What is the law of mass theory?
The chemical reaction is proportional to the product of the molar concentration of the reactants
Or in other words, the rate is proportional to the concentration of reactant A to the exponent of its amount
How can the rate law be expressed generally, given there action aA + bB + .... = products
Rate = [A]^a
Rate = [B]^b
So rate law:
Rate = k[A]^a[B]^b
Where k is the rate constant
What is the reaction order?
This is the exponents of the concentration terms
Consider the reaction of ethyl acetate reacting with sodium hydroxide.
If the rate of reactants is:
ethyl acetate = first order,
and sodium hydroxide = first order, what is the rate order of the overall reaction?
Add the exponents together, = second order reaction
Describe the reaction of ethyl acetate with sodium hydroxide in a rate law term
The Δ(ethyl acetate) over time = Δ(sodium hydroxide over time.
This would give a constant term x the concentration of each reactant.
So rate of reaction is 1st order with respect to ethyl acetate
1st order with respect to sodium hydroxide
And 2nd order with respect to the overall reaction
What is a pseudo order reaction?
The reaction is dependent on the first order reaction.
Consider the same reaction of ethyl acetate and sodium hydroxide.
Instead of having equal molar amounts, we will have NaOH presented in excess
As the reaction progresses, ethyl acetate depletes to a greater extent (because NaOH is excess)
So the reaction is dependent on the first order reaction of ethyl acetate, once it has all depleted the reaction obviously cannot continue.
So it is termed pseudo first order
What is molecularity?
This describes the number of molecules, atoms or ions reacting in an elementary process
E.g. Unimolecular, bimolecular, and complex reactions
What is unimolecular?
A reaction involving a single molecule e.g.
Br2 --> 2Br
What is bimolecular?
A reaction involving two molecules
E.g. H2 + I2 --> 2HI
What are complex reactions?
Reactions which require more than one step
2NO + O2 --> 2NO2 (overall reaction)
1. 2NO --> N2O2
2. N2O2 + O2 --> 2NO2
What is the rate constant?
Value K, a constant for each reaction
It is a specific value usually determined experimentally
It is affected by factors such as temperature, solvent, reactants, etc
Can be determined either graphically or via a substitution method
How is K determined graphically?
K is the slope of a straight line (LINEAR)
Determined by the equation y=mx+c
What kind of graph must be plotted to determine k?
It has to be some form of a concentration vs. Time graph
Zero order: concentration vs. Time
First order : log concentration vs. Time
Second order: 1/log concentration vs. Time
How is k determined via the substitution method?
Data accumulated in a kinetic study is substituted in the integrated form of the equations
When the equation is found in which the k values remain constant within the limits of experimentation, the reaction is considered to be of that order.
What is a zero order reaction.
Reaction is dependent on time
Independent of concentration
Expressed as Δ[A]/time
Depends on the rate constant, and nothing else
(rate constant can be anything like light intensity, temperature, etc
What can we determine from a graph of a zero order reaction?
We know that the Δ[A]/time is dynamic
So we need to integrate,
When this is plotted, we get a straight slope.
The half life reaction is exponential
How can we determine the half life of a zero order reaction?
We can easily modify the integrated zero order reaction rate to determine the half life.
We write it as the time taken to drop the concentration to half.
In this case the half life is exponential
What is the equation for half life of a zero order reaction?
t1/2 = [A]o/2k
What is the differential equation for zero order?
ko = dA/dt
Or rate = -k
What is the integrated rate equation for zero order?
[A]t = [A]o - kot
What is a first order reaction?
Dependent on concentration
Reaction rate is affected by temperature or use of solvent
We get an exponential slope in the initial curve but this must be integrated in order to obtain the k value
So we log both sides of the equation
What is the rate law equation for first order?
Rate = -k[A]
What is the integrated rate law equation for first order?
lnAt = lnAo - kt
Or log At = log Ao - kt/2.303
What is the first order half life equation?
t1/2 = 0.693/k
What is the first order shelf life equation?
t90 = 0.1054/ k
What is apparent zero order reaction?
This reaction often occurs when only part of a drug is soluble e.g. In a vessel of a suspension, ideally particles suspend forever, but in reality some of the particles dissolve over time
There is a reaction between the undissolved particles and the suspension medium - this follows first order reaction
As the particles dissolve and react with the suspension medium they are gone, but more suspension particles dissolve and react so it no longer follows a first order reaction. As it dissolves, more particles follow the system.
So it is not technically a zero order reaction (independent of concentration) but it appears to be one
What is a second order reaction?
Involves 2 elements in order to form products i.e. A bimolecular reaction.
The integrated graph has 1/concentration /time
What is the differential equation for second order reaction?
Rate = k[A]^2
Or kA^2 = -dA/dt
What is the integrated rate law for second order reaction?
1/At = 1/Ao + kt
What is the halflife equation for second order?
t1/2 = 1/[A]o
What are complex reactions?
Equilibrium / reversible reactions
E.g. As A depletes to form B, at some point it will reverse and form A again
Reaction is dynamic, there is no net change
Expressing into rate terms, it is the Δ[A]/Δt and is dependent on the rate constant of A and rate constant of B
What is the unit for k for a zero order reaction?
What is the unit for k for a first order reaction?