Chemical Kinetics Flashcards
What is rate defined as?
A change in some quantity per unit of time
Ex. Delta X/delta t
What is delta equal? Is it positive or negative?
Change in
Final value - the initial value
It can be both
If the initial value is larger than the final value delta will be?
Negative
How can you write a rate of a reactant A with a coefficient of 3?
-1/3(delta[A]/delta t)
Minus sign, 1/coefficient and delta concentration divided by delta time
How can you write the rate of a product B with coefficient 1?
Delta[B]/delta t
Positive, 1/1, delta B over delta t
Is reaction rate positive or negative?
Always positive
If one reaction rate has a coefficient of -1/3 and the rate is 4.40x10^-3 then what is the rate of a reactant with rate with a coeffienct -1/2?
-2(4.40x10^-3)
=-8.80x10^-3
What is the problem with average reaction rate? What is the solution?
Depends on the width of the time interval, rate changes as reaction progresses
Calculate slope of tangent to get instantaneous reaction rate
What is a rate law? What do the variables represent?
Mathematical relationship between the rate of reaction and the concentrations of reactants
Ex. Rate=k[A]^n
K is the rate constant
N is the reaction order with respect to A
A is reactant
What is a zero order reaction mean? What happens when you double the concentration of A?
Rate of reaction is always the same
Has no affect on reaction rate
What does a first order reaction mean? What affect does doubling the concentration of A have?
Rate is directly proportional to the reactant concentration
Will double the rate of reaction
What does a second order reaction mean? What does doubling the concentration of A do?
Rate is directly proportional to the square of the reactant concentration
Will quadruple the rate of reaction
What would graphs of 0,1,2 order reaction look like for concentration vs time? What about rate vs concentration?
0: decreases the steepest in straight line
1: decreases exponentially
2: decreases exponentially but shallower than 1
0: straight horizontal line
1: straight increasing line
2: exponentially increasing line
How can you find the rate constant of a first order reaction?
K=rate/[A]
How can you find the order of reaction from chart when the concentrations weren’t doubled?
Rate 2/Rate 1=([A2]/[A1])^n
How can you find the overall reaction order?
Add up the different orders
How can you find the rate constant of a second order reaction?
k=rate/[A]^2
What is the units for rate constant for 0,1,2?
0:Ms^-1
1: s^-1
2: M^-1s^-1
What is the difference between the differential form and integrated form? What do they look like?
Relationship between the rate and reactant concentration, relationship between the concentration and time
Rate=k[A]^n
[A]=f(t) (function of time)
What is the integrated rate law of a first order reaction?
ln(concentration at time t)=-kt + ln(initial concentration)
How can you plot the first order integrated rate law in a straight line? What is the slope equal to?
ln[A] vs time
x vs y
-k
What is the integrated rate law of a second order reaction?
1/(concentration at time t)=kt + 1/(initial concentration)
How can you plot the second order integrated rate law in a straight line? What is the slope equal to?
1/[A] vs time
Slope is k
What is the integrated rate law of a zero order reaction?
(Concentration at time t)= -kt +(initial concentration)
How can you plot the zero order integrated rate law in a straight line? What is the slope equal to?
[A] vs time
-k
What is half life? What is lifetime?
t1/2 is the time it takes for a 1/2 of a reactant to be consumed
Lifetime is the time it takes for the concentration to decrease by 1/e, a measure of avg life expectancy of chemical species in reaction
What is half life for first order? What about lifetime? What about when the concentration is n times lower than initial?
Half-life=ln(2)/k
Lifetime=1/k
n-time= ln(2)/k
What half life does NOT depend on concentration?
First orders
What is half life for second order?
Half-life=1/k(initial concentration)
What is half life of zero order reaction? What is the decay time equation for n?
Half-life=(initial concentration)/2k
N-time= (n-1)(initial concentration)/nk
Why does the half-life value change over course of second and zero order reaction?
Intitial cocnentration changes
What is Arrhenius equation?
Rate constant is temperature dependent
k=Ae^(-Ea/RT)
K is rate constant
A is frequency factor
Ea is activation energy
R is gas constant
T is temperature in Kelvin
Is the frequency factor positive or negative? What about the activation energy?
Positive
Positive
When the value of the rate constant decreases what happens to Ea?
It increases
What is activation energy?
Amount of energy needed for reaction to happen
What is activated complex?
Transition state, inbetween reactants and products, not stable and cannot be isolated
What is the difference between the frequency factor and the exponential factor in Arrhenius equation?
A is the number of times the molecule tries to react
Exponential is the fraction of atoms that equal or have greater energy then Ea
What is the range of values for exponential factor?
0-1
Fractional so just be less then one
How can you plot Arrhenius plot? How can you relate this to equation of a line?
ln(k)=(-Ea/R)(1/T) +ln (A)
ln k is y
-Ea/R is m (slope)
1/T is x
ln A is b (y intercept)
How to find the activation energy given the slope of Arrhenius plot? What about frequency factor?
Ea = -(R)(slope)
Set y intercept equal to ln A, do e^y-intercept and then plug into calculator
What is the orientation factor?
p
Probability of molecules colliding in correct orientation
If p=0.16 what is the percent of successful collisions?
16%
Percent that collide in right orientation
For A(g) + B(g) —> AB(g) what is orientation factor?
1 (100%)
What is an example of a reaction with p>1? Why?
Production of KBR and Br
Harpoon mechanism (electron is transferred without collision and the resulting ions move together via coulumbic force)
If you are given two temperatures and two rate constants how can you find the activation energy?
Make two Arrhenius equations and divide them, A cancels out, rearrange to get the Ea/R x 1/T1 - 1/T2, solve for Ea
What is a reaction mechanism?
Series of individual chemical steps by which a chemical reaction occurs
What is each step in a reaction mechanism called? What are species that are in mechanism but not final reaction called?
Elementary step
Intermediates
What is molecularity? What are the types?
Number of reactant particles that collide in an elementary step
Unimolecular- one reactant
Bimolecular- 2 reactants
Termolecular- 3 reactants
If elementary step is A->products what is rate law? What about A+A -> products? A+B-> products?
Rate=k[A]
Rate=k[A]^2
Rate=k[A][B]
What is the rate determining step?
Limits overall rate of reaction
What is the rate of the rate determining elementary step generally proportional to?
Rate of overall reaction
How do you choose which step to make rate law for elementary step?
Rate determining or the one with the product formation
When do we use steady state approx?
When the rate law contains an intermediate
What are the general steps for doing a steady state approx?
Set d[intermediate]/dt =0
List all reactions where intermediate is involved, + for where it’s produced and - for where it’s consumed ex. K1[reactant]
Move negative terms over and isolate the concentration of intermediate
Plug value back into original rate law
What is a catalyst?
Substance that increases reaction rate by lowering activation energy
What is the difference between homo and heterogenous catalysts?
Homo is when they are in same phase as reactants
Herero is when they are in different phase than reactants
What is michealis menten equation? How to use?
k2[E]0[S] / [S] +Km
S is substrate cocnentration and E is enzyme concentration
Decide how to simplify denominator (decide if Km is greater or less than S)
If [S] is much greater than Km what is the rate equal to? What about us much less?
Rate = k2[E]0
Zero order
Rate= k2[E]0[S]/Km
First order
How can you find out how long it takes for 35% to decay?
1-0.35=0.65
ln(0.65)=-kt solve for t
What formula can be used to find the amount left after decaying?
Nt=N0e^-kt
Nt is amount after time decaying
N0 is initial amount
K is rate constant
T is time decaying
