CHE 112 - MODULE 3 CHAPTER 15 LECTURE NOTES. Chemical Kinetics Chemical kinetics - study of the...

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CHE 112 - MODULE 3 CHAPTER 15 LECTURE NOTES
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Transcript of CHE 112 - MODULE 3 CHAPTER 15 LECTURE NOTES. Chemical Kinetics Chemical kinetics - study of the...

Page 1: CHE 112 - MODULE 3 CHAPTER 15 LECTURE NOTES. Chemical Kinetics  Chemical kinetics - study of the rates of chemical reactions and is dependent on the.

CHE 112 - MODULE 3

CHAPTER 15

LECTURE NOTES

Page 2: CHE 112 - MODULE 3 CHAPTER 15 LECTURE NOTES. Chemical Kinetics  Chemical kinetics - study of the rates of chemical reactions and is dependent on the.

Chemical Kinetics Chemical kinetics - study of the rates of

chemical reactions and is dependent on the characteristics of the reactants

Reaction mechanisms - detailed pathway that atoms and molecules take as a chemical reaction proceeds

CD-ROM Screen 6.2 & 15.2

Page 3: CHE 112 - MODULE 3 CHAPTER 15 LECTURE NOTES. Chemical Kinetics  Chemical kinetics - study of the rates of chemical reactions and is dependent on the.

Effects on Chemical Kinetics

Concentration of reactants – usually the rate of a reaction increases with increased concentration of reactants

Concentration of a catalyst - catalyst speeds up the reaction

Temperature – rate will increase with an increase in temperature (increased KE)

Surface area – as the surface area increases, the reaction will proceed at a faster rate

Page 4: CHE 112 - MODULE 3 CHAPTER 15 LECTURE NOTES. Chemical Kinetics  Chemical kinetics - study of the rates of chemical reactions and is dependent on the.

Rates of Chemical Reactions

Rates - the change in concentration of a chemical per unit of time ex. (M/sec)– much like the speed of a car ex. (miles/hr)– much like interest rates ex. (5.4%/year)– much like sales taxes ex. (7cents/$)

Rates are just a ratio or fraction of one thing changing with respect to another

Page 5: CHE 112 - MODULE 3 CHAPTER 15 LECTURE NOTES. Chemical Kinetics  Chemical kinetics - study of the rates of chemical reactions and is dependent on the.

Decomposition of N2O5

2 N2O5 4 NO2 + O2

We look at the disappearance of N2O5

Rate of Reaction = change in [N2O5] change in time

Rate of Reaction = [N2O5] t

Page 6: CHE 112 - MODULE 3 CHAPTER 15 LECTURE NOTES. Chemical Kinetics  Chemical kinetics - study of the rates of chemical reactions and is dependent on the.

Stoichiometry

2 N2O5 4 NO2 + O2

2moles : 4moles : 1mole

Therefore, for every 2 moles of N2O5 decomposed, you have 1 mole of O2

formed. The rate of formation of O2 is equal to ½ the rate of decomposition of N2O5

[O2]/t = -½ [N2O5]/ t

Page 7: CHE 112 - MODULE 3 CHAPTER 15 LECTURE NOTES. Chemical Kinetics  Chemical kinetics - study of the rates of chemical reactions and is dependent on the.

Decomposition of N2O5

Therefore we can determine the rate of decomposition by the rate of formation of either NO2 or the O2 with consideration to stoichiometry.

2 N2O5 4 NO2 + O2

Rf = [NO2] = -2Rd Rf = [O2] = -½ Rd

t t

Page 8: CHE 112 - MODULE 3 CHAPTER 15 LECTURE NOTES. Chemical Kinetics  Chemical kinetics - study of the rates of chemical reactions and is dependent on the.

Calculations

2 N2O5 4 NO2 + O2

Where ti = 600s and tf = 1200s and the concentration of N2O5 decomposed from 1.24 x 10-2 M to 0.93 x 10-2 M

Rate of decomposition = [N2O5 ] /t= (0.93 x 10-2 M -1.24 x 10-2 M)

(1200s – 600s) = -5.2 x 10-5 M/s

Rate of formation O2 = -½ [-5.2 x 10-5 M/s]= 2.6 x 10-5 M/s

Page 9: CHE 112 - MODULE 3 CHAPTER 15 LECTURE NOTES. Chemical Kinetics  Chemical kinetics - study of the rates of chemical reactions and is dependent on the.

Plot of Concentration vs. Time

Figure 15.2 on pg 702 shows the graph of the disappearance of N2O5.

Average rate = change in concentration over an interval in time (c/t)

Instantaneous rate = concentration at an instant in time; tangent to the curve at a particular point in time (dc/dt)

Page 10: CHE 112 - MODULE 3 CHAPTER 15 LECTURE NOTES. Chemical Kinetics  Chemical kinetics - study of the rates of chemical reactions and is dependent on the.

Rate Law

Rate Law – equation that relates the rate of reaction to the concentration of reactants raised to various powers

Rate = k [N2O5]x

– where k is the rate constant and x is the order of the reaction

Page 11: CHE 112 - MODULE 3 CHAPTER 15 LECTURE NOTES. Chemical Kinetics  Chemical kinetics - study of the rates of chemical reactions and is dependent on the.

Determining Rate Law

We can perform an experiment to decompose N2O5. If we doubled the concentration of the reactant we can observe the change in rate as follows:

Initial Conc. Rate of decompositionExp.1 1x10-2 mol/l 4.8x10-6 mol/l•secExp.2 2x10-2 mol/l 9.6x10-6 mol/l•sec

Observation: The rate of reaction doubled.

Page 12: CHE 112 - MODULE 3 CHAPTER 15 LECTURE NOTES. Chemical Kinetics  Chemical kinetics - study of the rates of chemical reactions and is dependent on the.

Determining Order

When a reaction has this observable result, it is said to be first order.

As we double the concentration, the rate is 2x, where x is the classification of the order of that particular reaction.

Or R2/R1 = 2x, where x=1, then we can see that it is clearly first order.

Page 13: CHE 112 - MODULE 3 CHAPTER 15 LECTURE NOTES. Chemical Kinetics  Chemical kinetics - study of the rates of chemical reactions and is dependent on the.

Overall Rate Law for N2O5

We previously stated that:

Rate = k [N2O5]x

Where x = 1 determined experimentally, therefore this reaction is a first order reaction and follows the rules of first order kinetics.

Page 14: CHE 112 - MODULE 3 CHAPTER 15 LECTURE NOTES. Chemical Kinetics  Chemical kinetics - study of the rates of chemical reactions and is dependent on the.

Order of Reactions

Considering other reactions where we double the initial reactant concentration, we can observe the rate and order as follows:

Quadruple rate = 4 (2x=4, x=2) second order

Double rate = 2 (2x=2, x=1) first order

No change = 1 (2x=1, x=0) zero order

Half rate = 1/2 (2x=1/2, x=-1) -1 order

1.4 the rate = 1.4 (2x=1.4, x=1/2) half order

Page 15: CHE 112 - MODULE 3 CHAPTER 15 LECTURE NOTES. Chemical Kinetics  Chemical kinetics - study of the rates of chemical reactions and is dependent on the.

Review of Logarithms

Logs are the inverse function of an exponential function (y = 3x)

Algebraically it is written as x = 3y or y = log3x where y is the exponent on 3 that results in a value of x– ex. 52 = 25 or log5 25 = 2

Written as a function logs are expressed as f(x) = logbx

If b is not designated, b=10 is assumed. If b = e it is called the natural log (ln).

Page 16: CHE 112 - MODULE 3 CHAPTER 15 LECTURE NOTES. Chemical Kinetics  Chemical kinetics - study of the rates of chemical reactions and is dependent on the.

First Order Kinetics

2 N2O5 4 NO2 + O2

Rate of decomposition = - [N2O5]

t Rate = kt

Therefore - [N2O5] = -kt (by substitution)

t ln [N2O5]t = -kt + ln [N2O5]0 (derive by integration)

Page 17: CHE 112 - MODULE 3 CHAPTER 15 LECTURE NOTES. Chemical Kinetics  Chemical kinetics - study of the rates of chemical reactions and is dependent on the.

Integrated Rate Equations

For a zero order reaction:

[A]t = -kt + [A]0

For a first order reaction:

ln [A]t = -kt + ln [A]0

For a second order reaction:

1/[A]t = kt + 1/[A]0

Page 18: CHE 112 - MODULE 3 CHAPTER 15 LECTURE NOTES. Chemical Kinetics  Chemical kinetics - study of the rates of chemical reactions and is dependent on the.

Iodine Clock Reaction Prep

Each solution is prepared in a 100ml volumetric flask and qs with distilled water. Accurately weigh the following:

2.998g of NaI = 0.2M 1.169g of NaCl = 0.2M 0.2482g of sodium thiosulfate = 0.01M 3.485g potassium sulfate = 0.2M 5.404g potassium persulfate = 0.2M

Page 19: CHE 112 - MODULE 3 CHAPTER 15 LECTURE NOTES. Chemical Kinetics  Chemical kinetics - study of the rates of chemical reactions and is dependent on the.

Half-life of Reaction

Half-life (t½) = the time it takes for the initial concentration of a reaction to disintegrate to half its original concentration

First order reaction: t½ = 0.693/k

Page 20: CHE 112 - MODULE 3 CHAPTER 15 LECTURE NOTES. Chemical Kinetics  Chemical kinetics - study of the rates of chemical reactions and is dependent on the.

Collision Theory

Three conditions must be met for a reaction to take place:– The reacting molecules must collide with

one another– Molecules must collide with sufficient

energy– Molecules must collide with the proper

orientation

Page 21: CHE 112 - MODULE 3 CHAPTER 15 LECTURE NOTES. Chemical Kinetics  Chemical kinetics - study of the rates of chemical reactions and is dependent on the.

Activation Energy

Ea = the minimum amount of energy that must be absorbed by a system to cause it to react where k = Ae -Ea/RT

Activation energy can be determined from the Arrhenius Equation:

ln k = ln A - Ea/R (1/T)

ln (k 2/k1) = - Ea/R [1/T2 - 1/T1]

Page 22: CHE 112 - MODULE 3 CHAPTER 15 LECTURE NOTES. Chemical Kinetics  Chemical kinetics - study of the rates of chemical reactions and is dependent on the.

Conditions Increasing Rates

Temperature - increases the KE and enough energy to overcome Ea

Presence of a catalyst - provides different pathways with lower Ea

Surface area - increases the probability of a collision with proper orientation

Page 23: CHE 112 - MODULE 3 CHAPTER 15 LECTURE NOTES. Chemical Kinetics  Chemical kinetics - study of the rates of chemical reactions and is dependent on the.

Reaction Mechanisms

Reaction mechanisms = sequence of steps that show the intermediates formed ( bonds broken and bonds formed) between the reactant side and product side of any reaction

See CD-ROM Screens 15.12 – 15.13

Page 24: CHE 112 - MODULE 3 CHAPTER 15 LECTURE NOTES. Chemical Kinetics  Chemical kinetics - study of the rates of chemical reactions and is dependent on the.

Rate Equations for Elementary Steps

Elementary Step - a singular molecular event classified by the # of reactant molecules involved (molecularity)– unimolecular: AP where R = k[A]– bimolecular: A + BP where R = k[A][B]

or A + AP where R = k[A]2

– termolecular: 2A +B P where R = k[A]2[B]

See Table at the bottom of page 734

Page 25: CHE 112 - MODULE 3 CHAPTER 15 LECTURE NOTES. Chemical Kinetics  Chemical kinetics - study of the rates of chemical reactions and is dependent on the.

Rate-determining Step

Step 1: A + B X + M; k1 is slow

Step 2: M + A Y; k2 is fast

Overall Rxn. 2A + B X + Y

Because step 2 is fast, it does not contribute to the overall rate, therefore:

R = k1[A][B]

Page 26: CHE 112 - MODULE 3 CHAPTER 15 LECTURE NOTES. Chemical Kinetics  Chemical kinetics - study of the rates of chemical reactions and is dependent on the.