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

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CH 104 CHEMICAL KINETICS

• Chemical kinetics is the study of the rates of
  reactions. The rate of a reaction is the change
  in concentration per unit of time.
• Some reactions are very fast. For example,
  H3O(aq) OH(aq) ? 2H2O(l) is completed in
  about 0.0000001 seconds.
• Some reactions are very slow. For example,
  2H2(g) O2(g) ? 2H2O(l) is completed in about
  1,000,000,000 years.
• Which is these reactions is faster, (a) Na(s) and
  Br2(l), or (b) the rusting of Fe(s)?
• (a) Na(s) and Br2(l)

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CHEMICAL KINETICS

• Factors affecting the rates of chemical
  reactions
• Nature of reactants
• Presence or absence of catalysts
• Solvent
• Concentration of reactants
• Temperature
• In Part A of todays experiment you will measure
  the affect of the concentration of reactants on
  rate.
• In Part B of todays experiment you will measure
  the affect of temperature on rate.

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CHEMICAL KINETICS

• Given the following general reaction
• aA bB cC ? dD eE fF
• The rate equation equals
• This rate has been arbitrarily defined as the
  disappearance of A
• (?A/?t). However, it could have been defined
  as the disappearance of any reactant, or the
  appearance of any product.
• m need not equal a, n need not equal b, etc.
• m is the order in A, n is the order in B,
  etc.
• m n p is the overall order
• m, n, p, etc. usually equals 0, 1, or 2 however,
  they may also equal 1/2, 3/2, etc.
• k is the specific rate constant. It is a
  constant for any given reaction in a specific
  solvent and at a specific temperature.
• What does k equal when all the concentrations are
  1 M?
• Rate k1m1n1p
• Rate k

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CHEMICAL KINETICS AND CONCENTRATION

• S2O82(aq) 3I(aq) ? 2SO42(aq) I3(aq)
• The method of initial rates is used to measure
  the orders of a reaction. For example, the order
  in S2O82(aq) is measured as follows.
• Step 1 To find the order in S2O82(aq), select
  the experiments with different initial
  concentrations of S2O82(aq) and equal
  concentrations of I(aq). What are these
  experiments?
• Experiments 1 and 2. In Part A of todays
  experiment you must assign the initial
  concentrations 3 different of reactants
  (CH3COCH3, I2, and H). How will 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 S2O82(aq).
• 2 2m
• m 1
• Therefore, the order in S2O82(aq) is 1.

Experiment Initial Concentrations, M Initial Concentrations, M Initial Rate, mol L1 s1
Experiment S2O82(aq) I(aq) Initial Rate, mol L1 s1
1 2 3 0.038 0.076 0.076 0.060 0.060 0.030 R1 1.4 x 105 R2 2.8 x 105 R3 1.4 x 105
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CHEMICAL KINETICS AND CONCENTRATION

• S2O82(aq) 3I(aq) ? 2SO42(aq) I3(aq)
• What is the order in I(aq)?
• Step 1 To find the order in I(aq), select the
  experiments with different initial concentrations
  of I(aq) and equal concentrations of S2O82(aq).
  What are these experiments?
• Experiments 2 and 3.
• Step 2 Use the ratio of these rate equations to
  solve for the order in I(aq).
• 2 2n
• n 1
• Therefore, the order in I(aq) is also 1.

Experiment Initial Concentrations, M Initial Concentrations, M Initial Rate, mol L1 s1
Experiment S2O82(aq) I(aq) Initial Rate, mol L1 s1
1 2 3 0.038 0.076 0.076 0.060 0.060 0.030 R1 1.4 x 105 R2 2.8 x 105 R3 1.4 x 105
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CHEMICAL KINETICS AND CONCENTRATION

• S2O82(aq) 3I(aq) ? 2SO42(aq) I3(aq)
• What is the overall order?
• The Order in S2O82(aq) The Order in I(aq) 1
  1 2
• Therefore, the overall order is 2.

Experiment Initial Concentrations, M Initial Concentrations, M Initial Rate, mol L1 s1
Experiment S2O82(aq) I(aq) Initial Rate, mol L1 s1
1 2 3 0.038 0.076 0.076 0.060 0.060 0.030 R1 1.4 x 105 R2 2.8 x 105 R3 1.4 x 105
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CHEMICAL KINETICS AND CONCENTRATION

• S2O82(aq) 3I(aq) ? 2SO42(aq) I3(aq)
• What is the rate constant (k) for this reaction?
• Rate kS2O821I1
• 1.4 x 105 k0.0380.060

Experiment Initial Concentrations, M Initial Concentrations, M Initial Rate, mol L1 s1
Experiment S2O82(aq) I(aq) Initial Rate, mol L1 s1
1 2 3 0.038 0.076 0.076 0.060 0.060 0.030 R1 1.4 x 105 R2 2.8 x 105 R3 1.4 x 105
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CHEMICAL KINETICS AND CONCENTRATION

• S2O82(aq) 3I(aq) ? 2SO42(aq) I3(aq)
• What is the rate of this reaction when S2O82
  0.050 M and I 0.025 M?
• Rate (6.1 x 103 L mol1 s1)S2O821I1
• Rate (6.1 x 103 L mol1 s1)0.0500.025
• Rate 7.7 x 106 mol L1 s1

Experiment Initial Concentrations, M Initial Concentrations, M Initial Rate, mol L1 s1
Experiment S2O82(aq) I(aq) Initial Rate, mol L1 s1
1 2 3 0.038 0.076 0.076 0.060 0.060 0.030 R1 1.4 x 105 R2 2.8 x 105 R3 1.4 x 105
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CHEMICAL KINETICS AND TEMPERATURE

• In Part B of todays experiment you will measure
  the affect of temperature on rate.
• Experience tells us that the rates of reactions
  increase with temperature.

• For example, fuels such as gasoline, oil, and
  coal are relatively inert at room temperature
  however, they rapidly burn at elevated
  temperatures.
• In addition, many foods last almost indefinitely
  in a freezer however, they spoil quickly at room
  temperature.

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CHEMICAL KINETICS AND TEMPERATURE

• The activation energy (Ea) is the minimum energy
  that is needed for molecules to react.
• In other words, Ea is the height of the energy
  barrier between reactants and products.

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CHEMICAL KINETICS AND TEMPERATURE

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

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CHEMICAL KINETICS AND TEMPERATURE

• The Arrhenius equation describes the relationship
  between the specific rate constant (k), the
  activation energy (Ea), and the absolute
  temperature (T). A graph of ln k versus 1/T is
  called an Arrhenius plot. It is a straight line
  with slope of m Ea/R and a y-intercept of b
  ln A.
• k is the specific rate constant.
• Ea is the activation energy.
• R is the gas constant, 8.314 J mol1 K1.
• T is the temperature in Kelvin.
• A is a constant for a given reaction.

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CHEMICAL KINETICS AND TEMPERATURE

• Calculate the Ea for this reaction.
• 2HI(g) ? H2(g) I2(g)
• Step 1 Complete this table.

k (M1 s1) ln k t (C) T (K) 1/T (K1)
3.52 x 107 3.02 x 105 2.19 x 104 1.16 x 103 3.95 x 102 283 356 393 427 508
14.860
556
0.00180
10.408
629
0.00159
8.426
666
0.00150
6.759
700
0.00143
3.231
781
0.00128
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CHEMICAL KINETICS AND TEMPERATURE

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

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CHEMICAL KINETICS AND TEMPERATURE

• Step 3 Calculate Ea.
• Slope (Ea/R)
• Ea (Slope)R
• Ea (Slope)R
• Ea (2.24 x 104 K)( 8.314 J mol1 K1)
• Ea 1.86 x 105 J mol1
• Ea 186 kJ mol1

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SAFETY

• Give at least 1 safety concern for the following
  procedure.
• Using acetone (CH3COCH3), hydrochloric acid
  (HCl), and iodine (I2).
• These are irritants. Wear your goggles at all
  times. Immediately clean all spills. If you do
  get either of these in your eye, immediately
  flush with water.
• Acetone is extremely flammable. Never use it
  near a flame or spark.
• Your laboratory manual has an extensive list of
  safety procedures. Read and understand this
  section.
• Ask your instructor if you ever have any
  questions about safety.

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SOURCES

• Barnes, D.S., J.A. Chandler. 1982. Chemistry
  111-112 Workbook and Laboratory Manual. Amherst,
  MA University of Massachusetts.
• McMurry, J., R.C. Fay. 2004. Chemistry, 4th ed.
  Upper Saddle River, NJ Prentice Hall.
• Petrucci, R.H. 1985. General Chemistry Principles
  and Modern Applications, 4th ed. New York, NY
  Macmillan Publishing Company.