Rate Expresssssions
Rate Expression Vs. Rate Law
Rate Expression: How fast concentrations change over time, uses balanced equation
Rate Law: an expression relating the rate of a reaction to the rate constant and the concentrations of the reactants.
Only reactants, determined experimentally
can I determine the rate law from the overall balanced equation?
No.
What is the difference between an elementary step and intermediates?
Elementary steps add up to the overall equation for the mechanism of reaction.
Intermediates: Are created in one elementary step and used in the next.
What are the 3 Integrated Rate Laws equations?
First: ln[A]t=-kt+ln[A]0
Second: 1/[A]t = kt + 1/[A]0
zero: [A]t=- kt + [A]0
What is meant by half-lives?
The time required for a concentration to reach 1/2 of its OG concentration
The rate of disappearance will be a ___ number
NEGATIVE!!
What is the overall reaction order for the following equations:
a.Rate = k [A][B]
b. Rate = k[A][B]^2
c. Rate = k[B]
d. Rate = k[A]
e. Rate = k[A]^5[B]
a. Overall order of 2
b. Overall order of 3
c. Overall order of 1
d. Overall order of 1
e. Overall order of 6
What does it mean when a reaction Unimolecular, Bimolecular, or Termolecular?
Unimolecular: 1 reactant rxn
Bimolecular:2 reactant rxn
Termolecular:3 reactant rxn
What are ways to determine whether a question is asking for a zero, first, or second order rxn?
- order of reaction (rate law)
- k constant units
- graphically
-Verbally
What effect does a large activation energy have on the speed of the reaction? What about a small Ea?
Large Ea= slower reaction
Small Ea= Faster reaction
What is the rate expression given:
C6H12O6 + 6O2 -> 6CO2 + 6H2O
Rate rxn= - ∆[C6H12O6]/∆t = - 1/6 ∆[O2]/∆t = 1/6 ∆[CO2 ]/∆t = 1/6 ∆[H2O ]/∆t
Determine the units of k rate constant given the following information.
a. Rate = k[A][B]3
b. Rate = k[A][B]
c. Rate = k[A]3 [B][C]
d. Rate = k
a. 4th overall -> M-3 S-1
b. 2nd overall -> M-1 s-1
c. 5th overall -> M-4 s-1
d. zero order -> M s-1
What 2 conditions must be true for a mechanism to be consistent with an observed rate law?
1. The slow step is consistent with the rate law.
2. AND the steps must explain the overall rxn!!! (the steps must add up tot overall rate law)
What is the difference between the graph of a zero order and a first order integrated rate law?
ln[A]t is on the y-axis of First Order
[A]t is on the y-axis of zero Order
"Calculate the half-life of the decomposition of gaseous N2O5 at 55 celsius if the rate constant is 1.5 x 10 s-1 ." What equation do I start with??
t1/2= .693/k
If the rate of decomposition of CH3OH(l) at some time is 5.00 x 10^-7
Given: 2CH3OH + 3O2 -> 2CO2 + 4H2O
What is the rate of appearance of CO2??
5x 10^-7 M/s
Determine the Rate Law for the reaction:
CH4 + 2O2 → CO2 + 2H2O (g)
(Table on PowerPoint Slide:)
Rate=k [CH4]^2[O2]^3
Is the following mechanism consistent with the observed rate law?
Given the reaction: H2 + 2ICl > 2HCl + I2
The Rate Law: Rate=k[H2][ICl]
Step 1: H2 + ICl > Hl + HCI SLOW
Step 2: HI+ ICl > HCl + I2 FAST
YES
The slow step is consistent with the rate law.
AND the steps explain the overall rxn!!! (the steps must add up tot overall rate law)
The Rate constant for a reaction is .50 M-1 s-1
2NO2(g) → 2NO(g) + O3 (g)
How long (in sec) would it take for the concentration of NO2 to decrease from 0.77 M to 0.35 M?
3.12 s-1
If the activation energy of a reaction is 155 kJ/ mol. What is the rate constant at 130o C if the rate constant is equal to 4.12 x 10 -5 S-1 at 190o C?
1.03 x 10^-7 s^-1
O2 disappears at a rare of O2=-3.22x10^-3 M/s. Find the rate of dissappearance of CH2?
CH2 +4O2 -> 2CO2 + 4H2O
-1.61 x 10^-3 M/s
Using the information, determine the value the rate constant for the rxn:
2A(g) + B(g) -> C(g) + D(g)
[A]o (M). [B]o(M). Rate (M/s)
.100 2.50 0.460
.200 2.50 0.920
.300 1.25 1.38
[A]^1[B]^y = 1.38/0.920
1.5^1[1/2]^y = 1.5
1/2^y = 1
y = 0
Rate=k[A]
In the follwoing rxn, can we also use (b) rxn mechanism to explain the observed rate law??
Given the reaction: H2 + 2ICl > 2HCl + I2
The Rate Law: Rate=k[H2][ICl]
(GO TO PPT)
No. When two rxn mechanisms meet the criteria, we choose the one with the simplest pathway...
KIS: KEEP IT SIMPLE
Calculate the concentration of O3 remaining 500 seconds after the start of its decomposition at 50oC when its concentration was 0.040 mol/L. The reaction and its rate are:
2NO2(g) → 2NO(g) + O3 (g), k=4.0 x 10-3 s-1
[O3]t = .00541
What units?
At 62.5o C, the first rate constant was 0.254 M-1sec-1. At 126.3o C, the second rate constant was 0.0453 M-1sec-1. What was the activation energy (kJ/mol)?
-30.12 kJ/mol