Kinetics: Part I: Rates of Reaction

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KineticsPart I Rates of Reaction

• Dr. C. Yau
• Fall 2009

Chapter 13 of Brady Senese 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.

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Factors Affecting Reaction Rate

• 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 water
• Why 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
• Surface area for liquids, solids, and
  heterogeneous mixtures
• Amount of Mixing
• Particle shape/size
• Concentration of reactants
• Molarity for solutions
• Pressure effects for gases
• Volume effects for gases
• Temperature
• Catalysts

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• Which of the following would speed a reaction?
• stirring it
• dissolving the reactants in water, if ionic
• adding a catalyst
• grinding any solids
• all of these

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Measuring Rates

• Can be measured using any substance in the
  reaction
• Units M/s or
• Measured in three ways
• instantaneous rate (use this unless specified)
• average rate
• initial rate

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Fig. 13.5 p.524 Instanteous Rate
2HI(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. Rate is always given as a
positive number.
Do Practice Exercises 1 2 p.523
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• 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.

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Instantaneous Rates Changes With Time

• At 100s, rate 2.5x10-4M/s
• At 250s, rate 1 x10-4M/s
• As time progresses,
• reaction slows down.

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Initial Rate of Reaction

• "Initial Rate of Reaction" is the instanteous
  rate at time zero.
• A tangent is drawn at time zero the slope is
  determined.

See Example 13.2 p.525 Do Practice Exercises 3
4 p.526
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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?
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Average Reaction Rates

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

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• What is the average rate of B between 10 and 40
  s?
• -0.006 M/s
• 0.006 M/s
• -0.002 M/s
• 0.002 M/s
• cant tell from the information

X (10s, 0.20M)
(40s, 0.03M)
X
Is B a reactant or product?
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Rates And Stoichiometry

• Example 13.1 p. 523
• 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.

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• In the reaction 2A 3B ?5D we measured the
  rate of disappearance of substance A to be
  3.510-5M/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 8.810-5 M/s
RC1/2 RB Rate of formation of C should be slower
than rate of consumption of B (by ½).
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• 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 reaction of O2?
• the same
• twice as great
• half as large
• you cannot tell from the given information