Chemical Kinetics

Question 1

What is rate defined as?

Answer 1

A change in some quantity per unit of time

Ex. Delta X/delta t

Question 2

What is delta equal? Is it positive or negative?

Answer 2

Change in

Final value - the initial value

It can be both

Question 3

If the initial value is larger than the final value delta will be?

Answer 3

Negative

Question 4

How can you write a rate of a reactant A with a coefficient of 3?

Answer 4

-1/3(delta[A]/delta t)

Minus sign, 1/coefficient and delta concentration divided by delta time

Question 5

How can you write the rate of a product B with coefficient 1?

Answer 5

Delta[B]/delta t

Positive, 1/1, delta B over delta t

Question 6

Is reaction rate positive or negative?

Answer 6

Always positive

Question 7

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?

Answer 7

-2(4.40x10^-3)
=-8.80x10^-3

Question 8

What is the problem with average reaction rate? What is the solution?

Answer 8

Depends on the width of the time interval, rate changes as reaction progresses

Calculate slope of tangent to get instantaneous reaction rate

Question 9

What is a rate law? What do the variables represent?

Answer 9

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

Question 10

What is a zero order reaction mean? What happens when you double the concentration of A?

Answer 10

Rate of reaction is always the same

Has no affect on reaction rate

Question 11

What does a first order reaction mean? What affect does doubling the concentration of A have?

Answer 11

Rate is directly proportional to the reactant concentration

Will double the rate of reaction

Question 12

What does a second order reaction mean? What does doubling the concentration of A do?

Answer 12

Rate is directly proportional to the square of the reactant concentration

Will quadruple the rate of reaction

Question 13

What would graphs of 0,1,2 order reaction look like for concentration vs time? What about rate vs concentration?

Answer 13

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

Question 14

How can you find the rate constant of a first order reaction?

Answer 14

K=rate/[A]

Question 15

How can you find the order of reaction from chart when the concentrations weren’t doubled?

Answer 15

Rate 2/Rate 1=([A2]/[A1])^n

Question 16

How can you find the overall reaction order?

Answer 16

Add up the different orders

Question 17

How can you find the rate constant of a second order reaction?

Answer 17

k=rate/[A]^2

Question 18

What is the units for rate constant for 0,1,2?

Answer 18

0:Ms^-1

1: s^-1

2: M^-1s^-1

Question 19

What is the difference between the differential form and integrated form? What do they look like?

Answer 19

Relationship between the rate and reactant concentration, relationship between the concentration and time

Rate=k[A]^n

[A]=f(t) (function of time)

Question 20

What is the integrated rate law of a first order reaction?

Answer 20

ln(concentration at time t)=-kt + ln(initial concentration)

Question 21

How can you plot the first order integrated rate law in a straight line? What is the slope equal to?

Answer 21

ln[A] vs time

x vs y

-k

Question 22

What is the integrated rate law of a second order reaction?

Answer 22

1/(concentration at time t)=kt + 1/(initial concentration)

Question 23

How can you plot the second order integrated rate law in a straight line? What is the slope equal to?

Answer 23

1/[A] vs time

Slope is k

Question 24

What is the integrated rate law of a zero order reaction?

Answer 24

(Concentration at time t)= -kt +(initial concentration)

Question 25

How can you plot the zero order integrated rate law in a straight line? What is the slope equal to?

Answer 25

[A] vs time -k

Question 26

What is half life? What is lifetime?

Answer 26

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

Question 27

What is half life for first order? What about lifetime? What about when the concentration is n times lower than initial?

Answer 27

Half-life=ln(2)/k Lifetime=1/k n-time= ln(2)/k

Question 28

What half life does NOT depend on concentration?

Answer 28

First orders

Question 29

What is half life for second order?

Answer 29

Half-life=1/k(initial concentration)

Question 30

What is half life of zero order reaction? What is the decay time equation for n?

Answer 30

Half-life=(initial concentration)/2k N-time= (n-1)(initial concentration)/nk

Question 31

Why does the half-life value change over course of second and zero order reaction?

Answer 31

Intitial cocnentration changes

Question 32

What is Arrhenius equation?

Answer 32

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

Question 33

Is the frequency factor positive or negative? What about the activation energy?

Answer 33

Positive Positive

Question 34

When the value of the rate constant decreases what happens to Ea?

Answer 34

It increases

Question 35

What is activation energy?

Answer 35

Amount of energy needed for reaction to happen

Question 36

What is activated complex?

Answer 36

Transition state, inbetween reactants and products, not stable and cannot be isolated

Question 37

What is the difference between the frequency factor and the exponential factor in Arrhenius equation?

Answer 37

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

Question 38

What is the range of values for exponential factor?

Answer 38

0-1 Fractional so just be less then one

Question 39

How can you plot Arrhenius plot? How can you relate this to equation of a line?

Answer 39

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)

Question 40

How to find the activation energy given the slope of Arrhenius plot? What about frequency factor?

Answer 40

Ea = -(R)(slope) Set y intercept equal to ln A, do e^y-intercept and then plug into calculator

Question 41

What is the orientation factor?

Answer 41

p Probability of molecules colliding in correct orientation

Question 42

If p=0.16 what is the percent of successful collisions?

Answer 42

16% Percent that collide in right orientation

Question 43

For A(g) + B(g) —> AB(g) what is orientation factor?

Answer 43

1 (100%)

Question 44

What is an example of a reaction with p>1? Why?

Answer 44

Production of KBR and Br Harpoon mechanism (electron is transferred without collision and the resulting ions move together via coulumbic force)

Question 45

If you are given two temperatures and two rate constants how can you find the activation energy?

Answer 45

Make two Arrhenius equations and divide them, A cancels out, rearrange to get the Ea/R x 1/T1 - 1/T2, solve for Ea

Question 46

What is a reaction mechanism?

Answer 46

Series of individual chemical steps by which a chemical reaction occurs

Question 47

What is each step in a reaction mechanism called? What are species that are in mechanism but not final reaction called?

Answer 47

Elementary step Intermediates

Question 48

What is molecularity? What are the types?

Answer 48

Number of reactant particles that collide in an elementary step Unimolecular- one reactant Bimolecular- 2 reactants Termolecular- 3 reactants

Question 49

If elementary step is A->products what is rate law? What about A+A -> products? A+B-> products?

Answer 49

Rate=k[A] Rate=k[A]^2 Rate=k[A][B]

Question 50

What is the rate determining step?

Answer 50

Limits overall rate of reaction

Question 51

What is the rate of the rate determining elementary step generally proportional to?

Answer 51

Rate of overall reaction

Question 52

How do you choose which step to make rate law for elementary step?

Answer 52

Rate determining or the one with the product formation

Question 53

When do we use steady state approx?

Answer 53

When the rate law contains an intermediate

Question 54

What are the general steps for doing a steady state approx?

Answer 54

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

Question 55

What is a catalyst?

Answer 55

Substance that increases reaction rate by lowering activation energy

Question 56

What is the difference between homo and heterogenous catalysts?

Answer 56

Homo is when they are in same phase as reactants Herero is when they are in different phase than reactants

Question 57

What is michealis menten equation? How to use?

Answer 57

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)

Question 58

If [S] is much greater than Km what is the rate equal to? What about us much less?

Answer 58

Rate = k2[E]0 Zero order Rate= k2[E]0[S]/Km First order

Question 59

How can you find out how long it takes for 35% to decay?

Answer 59

1-0.35=0.65 ln(0.65)=-kt solve for t

Question 60

What formula can be used to find the amount left after decaying?

Answer 60

Nt=N0e^-kt Nt is amount after time decaying N0 is initial amount K is rate constant T is time decaying