Kinetics

Chapter 6, Topic 16, 7+6 IB hours

Graph-o-Rama!

Useful reference website

SL Notes

Overview

Chem:

Collision Theory- 3 tenets

Factors which affect rate- temp/concentration/particle size/catalyst

Graphs of Δ____/time

Experiments to find rate w/ varying temp/concentration/particle size

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SL:�Maxwell-Boltzmann distribution

Enthalpy level diagram w/ and w/o catalyst

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HL:�Rate Laws

expression

calculating k (w/ units)

Rate determining steps- finding/graphing

Arrhenius equation for calculating activation energy

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Mr. Massey’s Review Quizzes

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Flip through the SL Kinetics Paper one

Graphs- Review

Rates

Measure factors like vol. of gas, pressure, pH, temp… over time

Maxwell-Boltzmann

E_A

Can you explain this?

Why inverse?

Why not linear?

Inverse b/c:

• As reactants get used up, products form @ same rate

Not linear b/c:

• Rate decreases (slope lower) as products used up
• Lower collision frequency

Rate Laws- what are they? (16.1)

Rate constant k:

Rate (∆con/∆t) is ∝ to the concentrations of the reactants

Rate can be expressed as rate=k[reactants]

where k= the rate constant, specific to each reaction (only changes with temperature)

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Rate Law/Rate Expression:

Each reaction has an equation: rate=k[A]^x[B]^y where A and B are reactants and x and y can be determined from experimental data

← M/sec or mol dm^-3 sec^-1

Rate Laws- what are they? (16.1)

Reaction Order:

The “order” with respect to each reactant is the reactant’s exponent in the rate law

ex: rate=k[A]²[B]² is 2nd order w/ respect to A

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Overall Order:

The overall order is the sum of the orders of each reactant

ex: rate=k[A]²[B]² is 4th order overall

Little math moment- Exponents...

2⁰ =

2¹ =

2² =

2³ =

1

2

4

8

4 x 2⁰ =

4 x 2¹ =

4 x 2² =

4 x 2³ =

4

8

16

32

3

1

0

2

5.1x10^-3 x 2^? = 4.1x10^-2

1.2x10² x 2^? = 2.4x10²

6.9x10⁹ x 2^? = 6.9x10⁹

3.1x10^-2 x 2^? = 1.2x10^-1

Finding the rate law (16.1.2)

rate= k

[A]¹

[B]²

When Ax2, rate x2, so A impacts the rate by a factor of 1

When Bx2, rate x4, so B impacts the rate by a factor of 2

Rate Expression Problems (16.1)

Click here (or scan the QR) for some good Kinetics Notes

and scroll to the bottom resources section for Solving the Rate Equation - test yourself

Another MC Check-in Quiz

Unit for k

rate is ∆con/∆time (M/s or mol dm^-3s^-1)

• rate = k [A] [B]
• M/sec = ? x M x M
• k= 1/Msec or M^-1s^-1 or dm³mol^-1s^-1

(a/e) M^-1s^-1 or dm³mol^-1s^-1 (b) s^-1 (c) Ms^-1 or mol dm^-3s^-1

(d) M^-2s^-1

Mechanisms Intro….

Imagine you’re making Mac-n-Cheese

• List out at least 4 steps in the process
• This is the “mechanism”
• Which is the “rate-limiting step”?

Mechanism (16.2)

What does "mechanism" mean?

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Own analogies

Crash Course Video (skip the equilibrium part)

Rate Determining Step= slowest step in a mechanism. Rate Law is actually based on the RDS

ex: A + B → C slow

C + D → E fast

overall: A + B + D → E

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rate= k[A][B]

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Mechanism (16.2.)

This gets harder if the RDS is not the first step in the mechanism

ex: A + B → C fast

C + D → E slow

overall: A + B + D → E

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rate = k[C][D]...but, C depends on A and B, so

rate = k[A][B][D]

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This explains why the overall rate law is not linked to the coefficients of the overall reaction, and why reactants can be zero order

Try p.300 #22- 25

Try this:

What is the overall reaction? And rate law?

Overall: 2NO + O₂ → 2NO₂

rate= k[NO]²[O₂]

Why? RDS has rate=k[N₂O₂][O₂], BUT N₂O₂ is not in overall reaction, so back up to step 1. [N₂O₂] depends on [NO]², so replace [N₂O₂] with [NO]²

Try this:

Write the Rate Law for each

S_N1: rate= k[C_mess]

S_N2: rate= k[C_mess][OH^-]

Brace yourself…. The S_N1 is first order, and S_N2 is second order! That’s what the 1 and 2 mean! And….

Slow

Molecularity (part of 16.2)

Check out this description

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= # of particles involved in the rate determining step

• “unimolecular”,
• “bimolecular”
• “termolecular”

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3. Which of the following elementary reactions is a termolecular reaction?

a. A+2B+C→D

b. A+B+B→C

c. a and b

d. 2A+2B+2C→2D

e. b and d

f. none of the above

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4. Which rate law corresponds to a bimolecular reaction?

a. rate=k[A][A]²

b. rate=k[A][B]

c. all of the above

d. rate=k[A]²

e. b and d

f. none of the above

Really unlikely to be 1 step

(b) The slowest step is the rate-determining step. The rate expression for the rate-determining step is: rate = k[AB₂]²

Because the rate-determining step is the first step this is also the overall rate expression for the reaction.

Challenge!

On Page 289…

Pick one of the reactions at the bottom

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Write a mechanism which matches

• The overall reaction
• The rate law

Rate order from graphs (16.1)

Rate=k, Rate=k[A], Rate=k[A]²

Think about it…

What does a graph look like for y=m, y=mx, y=mx²?

Create a graph of [A] vs rate for a zero, first, and second order reaction

(assume k=1)

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Then, try a second one of [A] vs. time

Summary...

A good site: from Purdue U.

Practice Question #1 from text

rate= k[A]

so rate vs. [A] is linear

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(A would be 2nd order, but backwards, C is an inverse relationship, Why not D?)

Probably since D would be a curve?

Half-life

the time it takes for half of a reactant to be used up

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First order reactions always have a constant half-life,

so a constant half-life can be used to ID a reaction as first order with respect to that reactant

Activation Energy (16.2)

A is found by measuring rate at different temperatures

16.3.1

16.3.2

(kJ/mol)

8.31 J/molK

Temperature

(K)

Frequency Factor (L/mol sec)

Rearranged Arrhenius Equation

Slope = -Activation/8.31

Yes! It’s in the data book

lnk