CH 104: CHEMICAL KINETICS

• Chemical kineticsChemical kinetics is the study of the rates of reactions. The rate of a is the study of the rates of reactions. The rate of a reaction is the change in concentration per unit of time.reaction is the change in concentration per unit of time.

• Some reactions are very fast. For example, HSome reactions are very fast. For example, H33OO++(aq)(aq) + OH + OH––

(aq)(aq) → 2H → 2H22OO(l)(l) is is

completed in about 0.0000001 seconds.completed in about 0.0000001 seconds.

• Some reactions are very slow. For example, 2HSome reactions are very slow. For example, 2H2(g)2(g) + O + O2(g)2(g) → 2H → 2H22OO(l)(l) is is

completed in about 1,000,000,000 years.completed in about 1,000,000,000 years.

• Which is these reactions is faster, (a) NaWhich is these reactions is faster, (a) Na(s)(s) and Br and Br2(l)2(l), or (b) the rusting of , or (b) the rusting of

FeFe(s)(s)??• (a) Na(a) Na(s)(s) and Br and Br2(l)2(l)

CH 104: CHEMICAL KINETICSCH 104: CHEMICAL KINETICS

• Factors affecting the rates of chemical reactions:Factors affecting the rates of chemical reactions:1.1. Nature of reactantsNature of reactants2.2. Presence or absence of catalystsPresence or absence of catalysts3.3. SolventSolvent4.4. Concentration of reactantsConcentration of reactants5.5. TemperatureTemperature

• In Part A of today’s experiment you will measure the In Part A of today’s experiment you will measure the affect of the concentration of reactants on rate.affect of the concentration of reactants on rate.

• In Part B of today’s experiment you will measure the In Part B of today’s experiment you will measure the affect of temperature on rate.affect of temperature on rate.

CHEMICAL KINETICSCHEMICAL KINETICS

• Given the following general reaction:Given the following general reaction:aA + bB + cC + … → dD + eE + fF + …aA + bB + cC + … → dD + eE + fF + …

• The rate equation equals:The rate equation equals:

• This rate has been arbitrarily defined as the disappearance of A This rate has been arbitrarily defined as the disappearance of A (–Δ[A]/Δt). However, it could have been defined as the disappearance (–Δ[A]/Δt). However, it could have been defined as the disappearance of any reactant, or the appearance of any product.of any reactant, or the appearance of any product.

• m need not equal a, n need not equal b, etc.m need not equal a, n need not equal b, etc.• m is “m is “the order in Athe order in A”, n is “”, n is “the order in Bthe order in B”, etc.”, etc.• m + n + p + … is “m + n + p + … is “the overall orderthe overall order””• m, n, p, etc. usually equals 0, 1, or 2; however, they may also equal 1/2, m, n, p, etc. usually equals 0, 1, or 2; however, they may also equal 1/2,

3/2, etc.3/2, etc.• k is the k is the specific rate constantspecific rate constant. It is a constant for any given reaction in . It is a constant for any given reaction in

a specific solvent and at a specific temperature.a specific solvent and at a specific temperature.• What does k equal when all the concentrations are 1 M?What does k equal when all the concentrations are 1 M?• Rate = k[1]Rate = k[1]mm[1][1]nn[1][1]pp

• Rate = kRate = k

CHEMICAL KINETICSCHEMICAL KINETICS

SS22OO882–2–

(aq)(aq) + 3I + 3I––(aq)(aq) → 2SO → 2SO44

2–2–(aq)(aq) + I + I33

––(aq)(aq)

• The The method of initial ratesmethod of initial rates is used to measure the orders of a reaction. For is used to measure the orders of a reaction. For example, the order in Sexample, the order in S22OO88

2–2–(aq)(aq) is measured as follows. is measured as follows.

• Step #1: To find the order in SStep #1: To find the order in S22OO882–2–

(aq)(aq), select the experiments with different , select the experiments with different

initial concentrations of Sinitial concentrations of S22OO882–2–

(aq)(aq) and equal concentrations of I and equal concentrations of I––(aq)(aq). What are . What are

these experiments?these experiments?• Experiments 1 and 2. In Part A of today’s experiment you must assign the Experiments 1 and 2. In Part A of today’s experiment you must assign the

initial concentrations 3 different of reactants (CHinitial concentrations 3 different of reactants (CH33COCHCOCH33, I, I22, and H, and H++). How will ). How will

you do this so that you can measure the order of each reactant?you do this so that you can measure the order of each reactant?• Step #2: Use the ratio of these rate equations to solve for the order in SStep #2: Use the ratio of these rate equations to solve for the order in S22OO88

2–2–(aq)(aq)..

• 2 = 22 = 2mm

• m = 1m = 1• Therefore, the order in STherefore, the order in S22OO88

2–2–(aq)(aq) is 1. is 1.

CHEMICAL KINETICS AND CONCENTRATIONCHEMICAL KINETICS AND CONCENTRATION

ExperimentExperiment Initial Concentrations, MInitial Concentrations, M Initial Rate,Initial Rate,mol Lmol L–1–1 s s–1–1

SS22OO882–2–

(aq)(aq) II––(aq)(aq)

112233

0.0380.0380.0760.0760.0760.076

0.0600.0600.0600.0600.0300.030

RR11 = 1.4 x 10 = 1.4 x 10–5–5

RR22 = 2.8 x 10 = 2.8 x 10–5–5

RR33 = 1.4 x 10 = 1.4 x 10–5–5

SS22OO882–2–

(aq)(aq) + 3I + 3I––(aq)(aq) → 2SO → 2SO44

2–2–(aq)(aq) + I + I33

––(aq)(aq)

• What is the order in IWhat is the order in I––(aq)(aq)??

• Step #1: To find the order in IStep #1: To find the order in I––(aq)(aq), select the experiments with different initial , select the experiments with different initial

concentrations of Iconcentrations of I––(aq)(aq) and equal concentrations of S and equal concentrations of S22OO88

2–2–(aq)(aq). What are these . What are these

experiments?experiments?• Experiments 2 and 3.Experiments 2 and 3.• Step #2: Use the ratio of these rate equations to solve for the order in IStep #2: Use the ratio of these rate equations to solve for the order in I––

(aq)(aq)..

• 2 = 22 = 2nn

• n = 1n = 1• Therefore, the order in ITherefore, the order in I––

(aq)(aq) is also 1. is also 1.



SS22OO882–2–


112233

0.0380.0380.0760.0760.0760.076

0.0600.0600.0600.0600.0300.030

RR11 = 1.4 x 10 = 1.4 x 10–5–5

RR22 = 2.8 x 10 = 2.8 x 10–5–5

RR33 = 1.4 x 10 = 1.4 x 10–5–5

SS22OO882–2–

(aq)(aq) + 3I + 3I––(aq)(aq) → 2SO → 2SO44

2–2–(aq)(aq) + I + I33

––(aq)(aq)

• What is the overall order?What is the overall order?

• The Order in SThe Order in S22OO882–2–

(aq)(aq) + The Order in I + The Order in I––(aq)(aq) = 1 + 1 = 2 = 1 + 1 = 2

• Therefore, the overall order is 2.Therefore, the overall order is 2.



SS22OO882–2–


112233

0.0380.0380.0760.0760.0760.076

0.0600.0600.0600.0600.0300.030

RR11 = 1.4 x 10 = 1.4 x 10–5–5

RR22 = 2.8 x 10 = 2.8 x 10–5–5

RR33 = 1.4 x 10 = 1.4 x 10–5–5

SS22OO882–2–

(aq)(aq) + 3I + 3I––(aq)(aq) → 2SO → 2SO44

2–2–(aq)(aq) + I + I33

––(aq)(aq)

• What is the rate constant (k) for this reaction?What is the rate constant (k) for this reaction?

• Rate = k[SRate = k[S22OO882–2–]]11[I[I––]]11

• 1.4 x 101.4 x 10–5–5 = k[0.038][0.060] = k[0.038][0.060]



SS22OO882–2–


112233

0.0380.0380.0760.0760.0760.076

0.0600.0600.0600.0600.0300.030

RR11 = 1.4 x 10 = 1.4 x 10–5–5

RR22 = 2.8 x 10 = 2.8 x 10–5–5

RR33 = 1.4 x 10 = 1.4 x 10–5–5

SS22OO882–2–

(aq)(aq) + 3I + 3I––(aq)(aq) → 2SO → 2SO44

2–2–(aq)(aq) + I + I33

––(aq)(aq)

• What is the rate of this reaction when [SWhat is the rate of this reaction when [S22OO882–2–] = 0.050 M and [I] = 0.050 M and [I––] = 0.025 M?] = 0.025 M?

• Rate = (6.1 x 10Rate = (6.1 x 10–3–3 L mol L mol–1–1 s s–1–1)[S)[S22OO882–2–]]11[I[I––]]11

• Rate = (6.1 x 10Rate = (6.1 x 10–3–3 L mol L mol–1–1 s s–1–1)[0.050][0.025])[0.050][0.025]

• Rate = 7.7 x 10Rate = 7.7 x 10–6–6 mol L mol L–1–1 s s–1–1



SS22OO882–2–


112233

0.0380.0380.0760.0760.0760.076

0.0600.0600.0600.0600.0300.030

RR11 = 1.4 x 10 = 1.4 x 10–5–5

RR22 = 2.8 x 10 = 2.8 x 10–5–5

RR33 = 1.4 x 10 = 1.4 x 10–5–5

• In Part B of today’s experiment you will measure the affect In Part B of today’s experiment you will measure the affect of temperature on rate.of temperature on rate.

• Experience tells us that the rates of reactions increase with Experience tells us that the rates of reactions increase with temperature.temperature.

CHEMICAL KINETICS AND TEMPERATURECHEMICAL KINETICS AND TEMPERATURE

• For example, fuels such as gasoline, For example, fuels such as gasoline, oil, and coal are relatively inert at room oil, and coal are relatively inert at room temperature; however, they rapidly burn temperature; however, they rapidly burn at elevated temperatures.at elevated temperatures.

• In addition, many foods last almost In addition, many foods last almost indefinitely in a freezer; however, they indefinitely in a freezer; however, they spoil quickly at room temperature.spoil quickly at room temperature.

• The activation energy (EThe activation energy (Eaa) is the minimum energy that is needed for ) is the minimum energy that is needed for

molecules to react.molecules to react.• In other words, EIn other words, Eaa is the height of the energy barrier between reactants is the height of the energy barrier between reactants

and products.and products.


• Svante Arrhenius noted that the temperature dependence of the Svante Arrhenius noted that the temperature dependence of the specific rate constant is mathematically similar to the Boltzmann specific rate constant is mathematically similar to the Boltzmann distribution of energies.distribution of energies.


• The The Arrhenius equationArrhenius equation describes the relationship between describes the relationship between the specific rate constant (k), the activation energy (Ethe specific rate constant (k), the activation energy (Eaa), ),

and the absolute temperature (T). A graph of ln k versus and the absolute temperature (T). A graph of ln k versus 1/T is called an 1/T is called an Arrhenius plotArrhenius plot. It is a straight line with . It is a straight line with slope of m = –Eslope of m = –Eaa/R and a y-intercept of b = ln A./R and a y-intercept of b = ln A.

• k is the specific rate constant.k is the specific rate constant.• EEaa is the activation energy. is the activation energy.• R is the gas constant, 8.314 J molR is the gas constant, 8.314 J mol–1–1 K K–1–1..• T is the temperature in Kelvin.T is the temperature in Kelvin.• A is a constant for a given reaction.A is a constant for a given reaction.


• Calculate the ECalculate the Eaa for this reaction. for this reaction.

2HI2HI(g)(g) → H → H2(g)2(g) + I + I2(g)2(g)

• Step #1: Complete this table.Step #1: Complete this table.


kk(M(M–1–1 s s–1–1))

ln kln k tt(°C)(°C)

TT(K)(K)

1/T1/T(K(K–1–1))

3.52 x 103.52 x 10–7–7

3.02 x 103.02 x 10–5–5

2.19 x 102.19 x 10–4–4

1.16 x 101.16 x 10–3–3

3.95 x 103.95 x 10–2–2

283283356356393393427427508508

––14.86014.860––10.40810.408––8.4268.426––6.7596.759––3.2313.231

556556629629666666700700781781

0.001800.001800.001590.001590.001500.001500.001430.001430.001280.00128

• Step #2: Use Excel to plot ln k versus 1/T. Then calculate Step #2: Use Excel to plot ln k versus 1/T. Then calculate the slope (–Ethe slope (–Eaa/R) of this Arrhenius plot./R) of this Arrhenius plot.


Arrhenius Plot

-16

-14

-12

-10

-8

-6

-4

-2

0.00120 0.00140 0.00160 0.00180 0.00200

1/T (K-1)

ln k

• Step #3: Calculate EStep #3: Calculate Eaa..

• Slope = (–ESlope = (–Eaa/R)/R)

• ––EEaa = (Slope)R = (Slope)R

• EEaa = –(Slope)R = –(Slope)R

• EEaa = –(–2.24 x 10 = –(–2.24 x 1044 K)( 8.314 J mol K)( 8.314 J mol–1–1 K K–1–1))

• EEaa = 1.86 x 10 = 1.86 x 1055 J mol J mol–1–1

• EEaa = 186 kJ mol = 186 kJ mol–1–1


• Give at least 1 safety concern for the following procedure.Give at least 1 safety concern for the following procedure.

• Using acetone (CHUsing acetone (CH33COCHCOCH33), hydrochloric acid (HCl), and ), hydrochloric acid (HCl), and

iodine (Iiodine (I22).).

• These are irritants. Wear your goggles at all times. These are irritants. Wear your goggles at all times. Immediately clean all spills. If you do get either of these in Immediately clean all spills. If you do get either of these in your eye, immediately flush with water.your eye, immediately flush with water.

• Acetone is extremely flammable. Never use it near a flame Acetone is extremely flammable. Never use it near a flame or spark.or spark.

• Your laboratory manual has an extensive list of safety Your laboratory manual has an extensive list of safety procedures. Read and understand this section.procedures. Read and understand this section.

• Ask your instructor if you ever have any questions about Ask your instructor if you ever have any questions about safety.safety.

SAFETYSAFETY

• Barnes, D.S., J.A. Chandler. 1982. Chemistry 111-112 Barnes, D.S., J.A. Chandler. 1982. Chemistry 111-112 Workbook and Laboratory Manual. Amherst, MA: University Workbook and Laboratory Manual. Amherst, MA: University of Massachusetts.of Massachusetts.

• McMurry, J., R.C. Fay. 2004. Chemistry, 4th ed. Upper McMurry, J., R.C. Fay. 2004. Chemistry, 4th ed. Upper Saddle River, NJ: Prentice Hall.Saddle River, NJ: Prentice Hall.

• Petrucci, R.H. 1985. General Chemistry Principles and Petrucci, R.H. 1985. General Chemistry Principles and Modern Applications, 4th ed. New York, NY: Macmillan Modern Applications, 4th ed. New York, NY: Macmillan Publishing Company.Publishing Company.

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CH 104: CHEMICAL KINETICS