Kinetics:Part I: Rates of Reaction

Dr. C. YauSpring 2014

Jespersen Chapter 14 Sec 1 & 2

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Chemical Kinetics

"Chemical Kinetics" refer to how fast a reaction runs.

Gen Chem I – studied stoichiometry

• We predicted how much product will be made.

• We assumed reaction has finished.

• Actually some reactions are very slow and this can be a reason why sometimes % yield is low.

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Chemical Kinetics

In this chapter,we study the rates of reaction.

It's like going on a trip. Stoichiometry is like noting at the end of the trip how many miles we have traveled.

Kinetics is like noting how fast, in miles per hour, we are traveling during the trip. The speed may fluctuate during the trip.

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Chemical Kinetics

We measure how fast a reaction runs by measuring…

• the rate of formation of a product

or

• the rate of disappearance of a reactant

reactant product.

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Factors Affecting Reaction Rate

1. Chemical nature of the reactants• Bond strengths• General reactivity (instability of reactants)

e.g. Reactivity of Group IA metals increases as you go down the column (Li, Na, K, Rb, Cs).

Na in water K in water Rb & Cs in waterWhy is there such a trend?What is the reaction of these metals with water?

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Factors Affecting Reaction Rate (contd.)2. Ability of the reactants to come in contact with each

other• Physical state (consider solid + solid; solid +liquid etc.)• Surface area for liquids, solids, and heterogeneous mixtures • Amount of Mixing• Particle shape/size

3. Concentration of reactants– Molarity for solutions (higher conc means reactants are closer

to each other.)– Pressure effects for gases– Volume effects for gases

4. Temperature5. Catalysts

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Which of the following would speed a reaction?

A. stirring it

B. dissolving the reactants in water, if ionic

C. adding a catalyst

D. grinding any solids

E. boil off some of the solvent

F. all of these

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Measuring Rates

• Can be measured using any substance in the reaction. We choose one that can be easily detected.

• Units: M/s or

• Measured in three ways:

– instantaneous rate

– initial rate (most often used)

– average rate

1-1/ molmol L

L

ss

s

moles L

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Fig. 14.5 p.643: Instantaneous Rate2HI(g) H2(g) + I2 (s)

At time 100s, the rate is determined by the slope of the tangent at 100s.

The graph is for disappearance of HI, thus slope is negative.

BUT rate is always given as a positive number:

Rate = (.027M)/110s= 2.5x10-4 M/s 9

Why is the graph not linear?

What happens to the rate as time progresses?

What happens to the

slope as time

progresses?

Why?

As HI is consumed,

there are less HI to

collide with each other

to form the products….

& reaction slows down. 10

Slope= -0.01M/100s

rate=1x 10-4 M/s

Instantaneous Rates Changes With Time

At 100s, rate =2.5x10-4M/sAt 250s, rate = 1 x10-4M/sAs time progresses,reaction slows down.

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Initial Rate of Reaction"Initial Rate of Reaction" is the instantaneous rate at time

zero.A tangent is drawn at time zero & the slope is determined.

See Example 14.2 p.643

Do Practice Exercises 3 & 4 p.644 12

NO2 appearance

0

0.005

0.01

0.015

0.02

0.025

0.03

0.035

0 200 400 600 800

Time (s)

[NO

2]

Instantaneous Reaction Rates• Instantaneous rate: the slope of the

tangent to the curve at any specific time

• Initial rate determined at the initial time

Why does this graph look so different from the previous graphs? 13

Average Reaction Rates

rateΔtime

]Δ[Reactant

Average rate of reaction: the slope of the line connecting the starting and ending coordinates for a specified time frame.

NO2 appearance

0

0.005

0.01

0.015

0.02

0.025

0.03

0.035

0 200 400 600 800

Time (s)

[NO

2]

e.g. Average rate for time period of 200-600 seconds is measured thus...

-5

(0.027-0.012)M

(600-200)s

0.015 M=

400 s

= 3.8x10 M/s 14

(600s, 0.027 M)

(200 s, 0.012 M)

What is the average rate of B between 10 and 40 s?

A. -0.006 M/s

B. +0.006 M/s

C. -0.002 M/s

D. +0.002 M/s

E. can’t tell from the information

10 20 30 40

Time Elapsed in Reaction Progress (s)

Concentration of B (M)

0.0

0.1

0.2

0.3

0.4

X (10s, 0.20M)

X

(40s, 0.03M)0.20M - 0.03M

slope = 10s - 40 s

0.17M = = - 0.0057 = - 0.006 M/s

- 30s

Is B a reactant or product? 15What happened to the negative sign?

Rates And Stoichiometry Example 14.1 p. 640

Butane, C4H10, the fuel in cigarette lighters, burns in oxygen to give carbon dioxide and water.

If, in a certain experiment, the butane concentration is decreasing at a rate of 0.20 mol L-1 s-1, what is the rate at which the oxygen concentration is decreasing, and what are the rates at which the product concentrations are increasing?

16Do Practice Exercise 1 & 2 on p. 641.

• In the reaction: 2A + 3B →5D we measured the rate of disappearance of substance A to be 3.5×10-5M/s. What is the rate of appearance of D?

• In the reaction 3A + 2B →C, we measured the rate of B. How does the rate of C relate?

Rate of appearance of D = RD = 8.8×10-5 M/s

RC=1/2 RB

Rate of formation of C should be slower than rate of consumption of B (by ½). 17

In the reaction 2CO(g) + O2(g) →2CO2(g), the rate of the reaction of CO is measured to be 2.0 M/s. What would be the rate of the production of CO2?

A. the same

B. twice as great

C. half as large

D. you cannot tell from the given information

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