2. Reaction Kinetics Flashcards
What is the basic way of working out the rate of a reaction?
Change in concentration/time for the change to occur
Delta[X]
————
Delta t
What is the sign for rate of reaction?
Rate of reaction is always a positive number
What is the integrated rate law for zeroeth order?
[A]=[A]o -it
What is the integrated rate law for first order?
[A]=[A]o e^-kt
Or
Ln[A]= -kt + ln[A]o
What is the integrated rate law for second order?
1/[A] = 1/[A]o + kt
What do you need to plot for a linear relationship for zero order?
[A] vs t
What do you need to plot for a linear relationship in first order?
Ln[A] vs t
What do you need to plot for a linear relationship in second order?
1/[A] vs t
What does the slope indicate for zero order?
-k
What does the slope indicate for first order?
-k
What does the slope indicate for second order?
K
What are the units for k in a zero order?
Moles per litre per second
What are the units for k in first order?
Per second
What are the units for k in second order?
Per moles L per second
What is the formula for half life in a zero order reaction?
t(1/2) = [A]o
——-
2k
What is the formula for half life for first order?
t(1/2)=ln2/k
What is the formula for half life in a second order reaction?
t(1/2) = 1
——-
K[A]o
What is collision theory?
- The rate of a reaction is proportional to the number of effective collisions per second among the reactant molecules
- An effective collision is one that actually gives product molecules, only a small fraction of collisions lead to a net change
- Concentration and temperature can influence the number of effective collisions per second
What is activation energy?
The threshold energy which must be overcome for a reaction to occur
What does the size of the activation energy barrier suggest about the speed of the reaction?
High barrier gives a slow reaction
Low barrier gives a fast reaction
How does temperature affect the rate of reaction?
- The rate constants (rates) of most reactions increase with temperature
- Increase in T increases velocity of the molecules and hence increases the collision frequency
What is the Arrhenius equation?
K = Ae^(Ea/RT)
What does A represent in the Arrhenius equation?
Z (collision frequency or number of collisions per second) x p (steric factor, fraction of collisions with correct orientation)
What does the e in the Arrhenius equation represent?
The fraction of collisions of energy Ea or greater at temperature T
What is the graphical way of evaluating activation energy?
Plot of ln(k) vs 1/T has the slope -Ea/R where T is in kelvin
What is the second way of evaluating activation energy?
If you have measured k at 2 temperatures, you can obtain the equation:
Ln(k2/k1) = -Ea/R (1/T1 - 1/T2)
Ea = -R ln(k1/k2)
——————
(1/T1 - 1/T2)
What is the mechanism of a reaction?
The sequence of actual events that take place as reactant molecules are converted into products
In what two ways do elementary reactions differ from ordinary net chemical reactions?
- The rate law of an elementary reaction can be written by inspection of the stoichiometry
- Elementary steps often involve unstable or reactive species that do not appear in the net reaction equation
What is the steady state approximation?
In a reaction mechanism involving an intermediate species, the concentration of the intermediate remains constant during the course of the reaction
Rate of change of the intermediate concentration = 0
What is a catalyst?
A catalyst speeds up a reaction by providing a route with lower Ea, without being consumed itself in the reaction
How do you work out how many times faster a reaction is with a catalyst than without?
Work out k with and without catalyst using the Arrhenius equation and then divide k (cat) by k (no cat)
